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The Corrosive Effect of Soldering Fluxes
and Handling on Some Electronic Material!
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A variety of rosin-based liquid soldering fluxes are characterized
as to halide content, and investigation indicates galvanic corrosion dmS
stress corrosion cracking are mechanisms oi material degradation thy
can cause premature failure of inadequately cleaned electronic devicy
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BY B. D. D U N N AND C. CHANDLER
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ABSTRACT. A preliminary survey has
been conducted to assess the variety
of rosin-based liquid soldering fluxes
utilized by European Space Agency
contractors. These fluxes were characterized according to their halide contents, and a limited number of the
more common commercially available
types were further evaluated in order
to assess their performance in terms of
solderability and corrosiveness. The
investigation included both standard
and ad hoc corrosion tests and the
determination of flux halide content
and pH values. The corrosion tests
were performed in a w a r m , damp
atmosphere following the deliberate
contamination by the fluxes of samples of electronic materials.
The corrosive effects of residual flux
on the surfaces of stressed Kovar component leads and silver-plated copper
wires are correlated against the physiochemical properties of these fluxes.
The results were compared with those
obtained from similar control samples
either in clean condition or after deliberate contamination during handling.
Galvanic corrosion and stress corrosion cracking are considered to be
mechanisms of material degradation
which can cause the premature failure
of inadequately cleaned electronic
devices. The acceptance of supposedly
"non-corrosive" liquid soldering fluxes
on the basis of routine standard tests is
unlikely to obviate all the potential
corrosion problems associated with
electronic hardware.
Introduction
Electronic packages intended for
European Space Agency (ESA) space-
craft projects are generally assembled
by hand-soldering methods. Companies which have been contracted to
manufacture such equipment will follow the general soldering requirements specified by ESA1 in order to
obtain an adequate standard oi soider
joint reliability. This is achieved mainly
by the employment of trained and
certified operators and inspectors, but
also by the control of materials and
soldering techniques. The mass assembly of components to printed circuit
boards by wave soldering has b'jen
agreed for the ESA Spacelab project in
view of the large number of identical
circuits utilized by this project and,
following qualification programs, a
limited number of wave-soldering
lines have been approved.
The successful outcome ot all soldering operations will depend on several material factors. The choice of a
suitable soldering flux is very important, because it is the flux medii.m
which will provide for the imlial transfer of heat from the hot-soldering iron,
or liquid wave of solder, *o the surfaces being joined together. The ESA
soldering
specification
limits
the
choice of material finishes which may
be interconnected to those having an
excellent solderability, so that highly
activated fluxes are not needed during
the actual assembly process. r luxes of
high activity, which are potentially
more dangerous from a corrosion
viewpoint, are permitted during the
B. D. DUNN and C CHANDLER are with
the Materials Section, Product Assurance
Group, European Space Research anc Tech*
nology Centre, Noordwijk, Ti:e Netherlands
o
initial pre-tinning
of "diifie-.i**- ,.-".*
ponent leads—particularly m r i s •• of nickel-based alloys—to adW&we h•---•• r
ter solderability. After pre»l?«v . . H , »he
flux residues must be thorotggjMy
I •'•''''
cleaned from the component \e-rn ,*,
face to preclude time-depef***;-- J cor1 •"**
rosive attack.
•
Notwithstanding the tight Sofefe
process controls, several p
associated w i t h the corrosive liquid-soldering fluxes arid If?
dues have cost certain ESA < v
much wasted time and «•-,majority of these problem-! 4 •
•'
as non-conformances to :S
inspection requirements, ••>"';
the formation of corrosic^'. ; r
on the surfaces of both s; .
copper wires and fus
coated printed circuit Lcases, soldering had been
w i t h the additional applicat 1 •:- ii
uid fluxes, supplied by rr-;: ufacturers to both U.S." an.' ; .
'•••••
specifications, purporting .••'••,
v.
dues of a non-hygroscopk', mm
•
sive and non-conducpv- tgtytf Metallographic
examine:
gi
.r
strands and printed wi i
• .•••imported both bright g
•
>-.ybrown corrosion p odi :i
:
reveal reduced co
tional areas; it is as •
'
short term such disc
•.:<.•<< ,.from surface corrosi; >
cosmetic defect.
v ;spacecraft electron:.' . • . •,:
harnesses have long •:; :
•
to launch. Once it .
to assess the long te r
corrosion mechar sim
conductors w i t h . -: ; i
which have beer (I •• ••:. •._••
for at least 10 year
W E E D I N G RESEARCH Si
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Problems of a more serious nature
included the failure of various goldplated Kovar leads on flat-packaged
components. These leads had been
formed and then prepared according
to the ESA requirement 1 for gold
removal prior to soldering. De-golding
and pre-tinning were performed by
dipping the leads into solder baths
fluxed w i t h a mildly activated rosin.
After a few months of storage, the
leads from several batches of flat packages were observed to fracture c o m pletely when exposed to light handling operations. Metallographic investigations strongly indicated that the
failure mechanism of these leads was
one of stress corrosion cracking of the
Kovar alloy due to the combined
effect of residual stresses in the lead
material following the forming operation and the presence of a thin surface
film of supposedly "non-corrosive"
flux residue. Other failures have
involved the fracture of mounted transistor leads following equipment level
vibration testing; fractography revealed that stress corrosion had initiated a crack in the lead material and
this had later propagated by a fatigue
mechanism.
Discussion of these sporadic corrosion-related problems during Project
Material Review Board meetings has
led to the supposition that, although
ESA contractors purchased both cored
and liquid solder fluxes to recognized
specifications, the complex chemical
composition of activators contained in
proprietary rosin-based fluxes may
change from one batch to another
with resultant modification of properties such as corrosiveness and solderability.
Initially, a survey of soldering fluxes
used by some of the major ESA contractors was made; the results are presented in the Appendix. This survey
established that all the liquid soldering
fluxes employed for component assembly work and wire interconnections w o u l d satisfy the corrosion
requirements of recognized flux specifications. 2 " 4 It indicated that even the
strongly activated fluxes, normally
used for the pre-tinning of "poor solderability" component leads, might
not cause extensive corrosion of standard copper mirror test pieces; it was
recommended that the true corrosive
nature of any flux can be realistically
assessed only if the corrosion test
assembly reproduces the essential
characteristics of the individual metals
which make up a soldered connection.
Additional test methods to the
screening tests reported in the Appendix have now been selected in an
attempt to assess the relative effectiveness of different liquid fluxes. Thirteen
flux types have been chosen from the
290-sl OCTOBER 1980
71 previously examined in the flux
survey; they represent typical commerical products ranging from non-activated to fully activated rosin-based
fluxes containing halogenated additives. The copper mirror test procedure2-4 is now re-examined against a
new flux test proposal. 13 Also, certain
special corrosion tests have been
devised to reproduce those material
compositions
and
environments
thought most likely to have promoted
both the previously mentioned spacecraft corrosion problems and the few
problems which have been reported in
the literature. 5 - 10
The ionic content of each flux was
assessed together w i t h the effect of
excessive operator handling contamination on the ensuing corrosion of
component leads. The various flux
types which have been subjected to
this test program are listed in Table 1.
Experimental and Test Procedures
Chemical Analysis
Chemical analysis of solder fluxes is
extremely difficult to perform and is
often found to be inaccurate. Rosin
fluxes can include a vast range of
additives known only to the flux manufacturers themselves. These may include solvents and wetting, foaming
and viscosity agents which have been
chosen to strengthen the fluxing properties of the rosin.
Full chemical analyses have been
attempted. Fluorine, chlorine, bromine, iodine, sulpher and phosphorus
have been detected by emission spectroscopy, X-ray fluorescence spectroscopy and activation analysis. Gas chromatography and infrared absorption
spectroscopy have been used to separate and determine the volatile organic
components.
Non-volatile
components have been identified by both
ionic and non-ionic chromatography
on ion exchange columns. Those
analyzed
compositions
contained
ethanol, methanol, water, fatty acid
monoethanolamide,
alkylbenzenesulphonate and many unidentified
compounds derived from
abietic
acid.
A full chemical analysis of each flux
under evaluation was soon discontinued, particularly as it was thought
unlikely that such details could ever be
related to the effectiveness or corrosive properties of individual fluxes.
The chemical analysis was, therefore,
limited to the t w o simple checks used
in the initial flux survey described in
the Appendix—halide content determination, and pH value determination.
Dynamic Conductivity Monitor (DCM)
The D C M utilizes a solvent pumping
system for the measurement of ionic
conductivity of flux residues. This
equipment, often referred to as the
lonograph,* has been evaluated by
Naval Avionics and pronounced as a
method which provides for the semiquantitative measurement of flux residues on printed circuit boards. 1112
The test is limited to the detection
of ionizable constituents in solder
fluxes which are monitored by the
D C M on a scale based on a known
quantity of sodium chloride dissolved
in either pure water or a 1:1 solvent
mixture of isopropanol and water. Calibration of this system was based on
the conductivity of 1 /ig NaCI per 1 ^l
water.
For the purpose of this test program,
a constant amount of flux (5 /xl) was
added to the equipment's solvent
which was continuously pumped in a
closed loop and then passed through
two conductivity cells. Values of conductivity were measured for each of
the 13 fluxes in their as-received condition. Each flux sample was then
boiled for 1 minute (min) at -l-200 o C
(392°F) in an attempt to simulate flux
composition modifications which may
occur during a soldering operation;
the D C M test was then repeated by
introducing 5 /xl of the boiled flux
concentrate into the solvent.
Effectiveness of Flux Based on
Solderability Tests
The soldering efficiencies of the
various fluxes were assessed by means
of a standard solderability test method
compliant w i t h B.S. 2011, Part 2T (Philips Globule Method). The solderability
test was applied to both gold-plated
copper wire and plain copper wire by
the horizontal immersion of short
lengths of each degreased wire in a
globule of liquid eutectic solder held
at 4-235°C (455°F) on a heated steel
block.
Immediately prior to wire immersion, a standard volume of the flux
under investigation was dripped onto
the molten solder surface. Once
immersed, the wire split the solder
globule into two halves and the time
was taken for each half to wet, flow
and coalesce around the wire surface.
This test was repeated 50 times for
each combination of wire and flux
type. New 200 mg pellets of solder
were applied to the heated block at
the commencement of each test. The
mean soldering times were calculated
for each flux when applied to the
gold-plated wire (x U] ) and the copper
wire (x c „). An arbitrary unit, termed
flux efficiency (FE), was generated to
*Trademark ol Alphametal.
Table 1—Identification of Fluxes
Identification
code
Act vity as described
by manufacturer"'
A1
A2
A3
A4
K1
K2
K3
M1
M2
M3
Z1
Z2
Z3
RA
RA
R
RMA
RA
RA
RA
R
RA
RMA
RMA
RA
R
(a
'R—pure or l o w activity rosin; R M A — m i l d l y
rosin; RA—activated rosin.
rerspex Container
S = l o c a t i o n oi' i tnndard s c r a t c h
Fig. 1—Test configuration for possible corrosion of Kovar component leads under
constant deformation
activated
enable a comparison between the
ability of different fluxes to effect
solder wetting of these particular metallic finishes:
FE = x 0l , + x A u .
Copper Sheet Corrosion Test
A new test which evaluates the corrosiveness of flux residues has recently
been proposed 13 and may become
applicable in the assessment of rosinbased fluxes. The test method involves
melting a piece of solder on a copper
sheet in the presence of the flux under
evaluation, and submitting this test
piece to a humid environment. The
test results are subjective and based on
visual inspection for corrosion or
chemical reaction between the copper
sheet, solder alloy and constituents in
the flux residue.
Square copper sheet test pieces
(50 X 50 mm, i.e., 2 x 2 in.) were
made from 0.5 mm (0.02 in.) thick
material in the half-hard condition. An
indentor and die were applied to each
test piece to form a central 4 mm (0.16
in.) diameter depression.
Before this corrosion test could be
initiated, it was necessary to calculate
the non-volatile content of each flux
so as to enable a constant weight of
solid flux to be transferred into the test
piece depression. The solid content of
each flux was f o u n d from weight-loss
calculations based on the evaporation
of volatiles from flux samples situated
in dried aluminum containers after
three hours in an oven held at
110 ± 2°C (230 ± 3.6°F). Desiccators
containing silica gel were employed
for storage of the containers and solid
fluxes. The copper test pieces were
thoroughly cleaned and pretreated in
line with the recommended procedures 13 and then a sample of each flux
was transferred to the indentations.
The solid flux samples were heattreated at 60°C (140°F) for 10 min, and
this was followed by the addition of a
1 g pellet of 60:40 tin-lead solder to
each depression. By means of tongs,
the test pieces were in turn lowered
onto the surface of a heating bath
containing liquid solder at 235°C
(455°F). Contact between test piece
and bath was maintained until 5 seconds (s) after the initial melting of the
solder pellet. The test pieces were
removed in the horizontal position
and cooled for 15 min.
They were then transferred to a
humidity chamber and held, in a vertical position, for 21 days at a temperature of between 38 and 42°C (100.4
and 107.6°F) and at a relative humidity
of between 91 and 95%. Assessment of
any corrosion product was made after
the test period w i t h an eyeglass at x 7
magnification. The test pieces were
considered to have failed the test
should any blue-green corrosion product have formed on the copper sheet,
or if discrete white or colored spots
had appeared in the flux residue or on
the surface of the solder alloy.
Copper Mirror Corrosion Test
This test was performed w i t h flux in
both the as-received condition and
after it had been boiled for 1 min at
approximately 200°C (392°F). Laboratory production of the mirrors followed the method prescribed in MILF-14256C4 and the test accept/reject
criteria are as described in the Appendix.
"Ad Hoc" Corrosion Tests
The tests were devised to reproduce
and investigate the corrosion mechanisms which have occurred during the
fabrication of several ESA spacecraft.
The samples consisted of:
1. Insulated silver-plated
copper
wire, partly stripped to expose approximately 8 mm (0.31 in.) of stranded
wire. The materials have been approved for space use; however, the
silver-plated strands were nicked and
scraped w i t h a stripping tool so as to
reveal the underlying copper.
2. High reliability, space quality, 14pin flat-packaged components were
chosen to reproduce service failures as
best as possible. These components
contained glass-to-Kovar seals w i t h
gold-plated Kovar leads and were of a
type k n o w n to pass regularly electronic component environmental qualification tests.14 A jewellers' tooling jig
was used to form a standard scratch on
each component lead. The scratches
were positioned mid-length across the
upper surface of the leads; the jig was
then set such that a diamond cutting
edge passed through the gold-plated
layer and was just deep enough to
penetrate and expose a microscopically thin band of the Kovar base material.
The test configuration was chosen
to represent both residual and applied
stresses which may be expected of a
component lead during service. Examples of the causes of such stresses are
lead bending prior to mounting the
component on a printed circuit board,
the soldering operation which may
constrain the leads, particularly if
plated-through holes are employed,
and differences in coefficient of
expansion of interconnected materials
during thermal cycling.
In order to reproduce these stresses,
small containers were accurately machined from thick perspex sheets. The
inside width of the containers was
slightly less than the measured length
of each component—as sketched in
Fig. 1^so that all leads required to be
deflected at their ends as they were
slipped between the perspex walls.
Once released, each component was
thus suspended within the container
sides by the spring properties of its
Kovar leads. The dimensional mismatches were calculated to produce a
constant tensile strain on the lead top
surfaces approaching that of the Kovar
material's yield strength.*
The silver-plated copper wire and
flat-packaged component test pieces
* Metallography had revealed that the component leads were in the fully annealed
condition. Mechanical testing of individual
leads produced the following results: 29.2
kg/mm 2 yield strength; 43.4 kg/mm 2 ultimate tensile strength; 14 X 104 kg/mm 2
Young's modulus; 43% elongation at fracture and 150 VPN micro-hardness
W E L D I N G RESEARCH S U P P L E M E N T I 291-s
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WELDING RESEARCH SUPPLEMENT | 295-s
35
cl
©
Ml
= R
• = RMA
SLOW
• =
RA
KEY MANUFACTURERS'
DISCRIPTION OF FLUX ACTIVITY
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100
1000
100 000
10000
B O I L E D F L U X , D C M. T E S T R E S U L T ( r j g Net C l / j j l F L U X )
Fig. 4—Relationship
between
flux efficiency
and the ionic content
of boiled
fluxes
RMA and R-type fluxes on gold wires
are observed to be 0.14, 0.25 and 0.35 s,
respectively. Repetition of these tests
on plain degreased copper wires
proved to be more discriminatory
owing to the additional time taken by
the flux to remove an adherent surface
film of tarnish. Typical mean wetting
times for RA, RMA and R-type fluxes
are 0.5, 1.3 and 2.0 s, respectively.
split solder globule to flow and coalesce around the wire. It should be
noted that, before molten solders can
wet and spread, the metallic surfaces
must be free of any tenacious nonmetallic films such as oxides or sulphides. One purpose of the applied
flux is to remove these barrier films
either by reducing them to the metallic state or by chemically dissolving
them.
Two c o m m o n , easily reproducible
metal surfaces were chosen to represent typical material finishes that are
frequently interconnected by soldering techniques in the electronics
industry. Gold-plated (extremely thin
to preclude the formation of brittle
tin-gold intermetallics) copper wire
represented a barrier-free surface of
high solderability: degreased plain
copper wire represented a surface
which, due to long atmospheric exposure, supported a thin film of oxidation
products.
Results of Standard Corrosion Tests
The results of the solderability tests
are presented in Fig. 3. It is seen from
the solderability distribution curves for
the gold-plated wires that most of the
soldering times are very short, particularly when the fully activated fluxes
were applied. In these instances when
the fluxes are not expected to undergo
any surface chemical reactions, it is
only just possible to differentiate
between the respective wetting abilities of the various fluxes.
Typical mean wetting times for RA,
The flux corrosiveness performance
results show good agreement between
the standard copper mirror test and
the proposed copper sheet test, as
shown in Table 2. However, the
inspectors performing the visual examination of both types of humiditytested samples regarded the specified
classes of acceptance and rejection
criteria to be very subjective.
One inspector considered that the
"marginal passes" attributed to the
copper mirror test identified as Class 2
296-sl OCTOBER 1980
The arbitrary unit of flux efficiency
(FE) which is ascribed to each flux
listed in Table 2 has been plotted
against the ionic content, as indicated
by the D C M test result for boiled flux,
in Fig. 4. The fast acting fluxes are
noted to be of the RA-type and generally contain a high ionic concentration. However, the remaining R and
RMA fluxes show no particular relationship between FE and ionic content.
(see Table 2, footnote c) ought to have
been recorded as "failures" according
to his interpretation of the specification.' All inspectors agreed that the
Class 3 failures, evident as corrosion
and flux penetration of the copper,
were immediately apparent and that
quality checks of the "goods i n w a r d "
type, for this classification of corrosive
flux, w o u l d probably be both rapid
and straightforward to control.
It is also noticed from the copper
mirror results in Table 2 that identical
results were obtained for both the
as-received fluxes and the same samples after boiling. This is somewhat
surprising since the D C M results indicated that, once boiled, the ionic content of these fluxes increased; this
would be expected to create a more
corrosive environment. It is probable
that the copper mirrors have not been
greatly affected by this increase in
ionic content because stable basic
chlorides, such as those green patinas
which form on copper, will reduce the
Ck activity and may form barrier layers
that retard corrosion. In fact, only
fluxes which contain very high (greater
than 4.45%) halide concentrations and
extremely high ionic contents were
seen actually to penetrate the thin
copper film. The copper mirror test is,
however, limited in that it does not
indicate the danger of electrolyte corrosion accelerated by direct contact
between different metals.
Under actual soldering conditions, a
liquid flux will flow to surround the
molten front of solder and will be in
contact w i t h both hot metallic and
non-metallic materials that may substantially modify the chemical composition and activity of the flux. Once
AiB
AIR
cooled, some fluxes are extremely
hygroscopic and may promote galvanic corrosion between the various
interconnected materials. The proposed copper sheet test method may
go some u a \ to assess this problem of
A2R
bi-metallic corrosion resulting from
the applied solder.
Cooper is more noble than solder.
When these are in contact in the presence of a chloride concentration, this
factor will cause galvanic corrosion of
IT"*
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Fig. 5—Overall view of samples after 56 days exposure
W E L D I N G RESEARCH S U P P L E M E N T I 297-s
or.
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Table 3—Detailed Results of the "Ad Hoc" Testing
Stress
corrosion
attributed
by flux
from
microsections 0
Corrosivity of flux on gok -plated
Kovar c o m p o n e n t leads based
on visual ins section; I d )
Corrosivity of f ux on scratched
silver-pl ated copper wire based on
visual ins aection b l
u
b
t
L
O
t
a
I
Total
corrosivity
index""
Duration of exposure (days)
Flux
type""
4
10
21
30
40
56
4
10
21
30
40
56
56
A1a
A1b
A2a
A2b
A3a
A3b
A4a
A4b
K1a
K1b
K2a
K2b
K3a
K3b
M1a
M1b
M2a
M2b
M3a
M3b
21a
Z1b
Z2a
Z2b
Z3a
Z3b
3
5
3
5
1
1
1
1
3
3
3
3
3
5
1
1
1
1
1
1
1
1
3
1
1
1
5
5
5
5
1
1
1
1
3
3
3
3
5
5
1
1
1
3
3
3
1
1
3
3
1
1
5
5
5
5
1
1
1
1
3
3
3
3
5
5
1
1
1
3
3
3
1
1
3
3
1
1
5
5
5
5
3
1
1
I
3
3
3
3
5
5
1
1
3
3
3
3
1
1
3
3
1
1
5
5
5
5
3
1
1
1
3
3
3
3
5
5
1
1
3
3
3
3
1
1
3
3
1
1
5
5
5
5
3
1
I
1
3
3
3
3
5
5
1
1
3
3
3
3
1
1
3
3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
5
5
5
5
3
1
1
1
3
3
1
1
5
5
1
1
1
3
3
3
1
1
3
3
1
1
5
5
5
5
3
1
1
1
3
3
3
3
5
5
2
2
3
3
3
3
1
1
3
3
2
2
5
5
5
5
3
1
1
1
3
3
3
3
5
5
2
2
3
3
3
3
1
1
3
3
3
3
5
5
5
5
3
1
2
2
3
3
3
3
5
5
3
3
3
3
3
3
1
1
3
3
3
3
5
5
5
5
3
1
2
2
3
3
3
3
5
5
3
3
4
4
3
3
1
1
3
3
3
3
10
10
10
4
10
4
4
0
10
10
10
10
10
10
4
4
10
10
6
10
0
6
4
10
4
8
64
66
64
58
38
16
18
14
44
44
52
52
64
66
22
22
36
43
38
42
12
18
38
42
23
27
1
1
2
1
3
1
3
1
3
1
3
1
10
0
—
Contamin ated control
Clean control'
-
el
130
122
54
32
88
104
130
44
79
80
30
80
50
—
—
' s ' C o d e of flux types (see Table I for details), a) represents sample of fresh, u n c o n t a m i n a t e d flux, as-received and b) represents same flux b o i l e d for o n e m i n u t e at 200 ± 5 ° C t o s i m u l a t e a
soldering o p e r a t i o n w h i c h may m o d i f y flux properties.
lb
' A t regular intervals the samples w e r e visually inspected a n d classified
1 = no c o r r o s i o n , n o d i s c o l o r a t i o n or tarnish
3 = less t h a n 50% o f surface c o r r o s i o n
5 = extensive surface corrosion w i t h severe d i s c o l o r a t i o n .
"'Classification of d e p t h of stress-corrosion cracking based o n data appearing in Table II.
" " T h e 'Corrosivity Index' presupposes t h e presence of b o t h u n h e a l e d and b o i l e d flux o n these samples and represents t h e s u m m a t i o n of all classifications for a particular flux type.
,e
'These samples consist of stressed g o l d - p l a t e d Kovar leads either d e l i b e r a t e l y c o n t a m i n a t e d w i t h h a n d perspiration or as-cleaned i n I.P.A. a n d d e - i o n i z e d w a t e r ; n o flux had b e e n
applied.
the electronegative solder metal. The
potential difference which will arise is
difficult to estimate accurately, but as
a guide1"' is taken to be 0.3 volts (V).
(Some fluxes can contain the same
chloride concentration as seawater, 2.2
g / l , the basis of this guide.) As far as
electronic circuit materials are concerned, copper and solder are relatively "compatible," and this copper sheet
test w o u l d probably be more selective
if thin platings of a noble metal such as
silver or gold were applied to the
copper, these metals being separated
from solder by a potential of 0.5 and
0.65 V, respectively.
Results of the Ad Hoc Corrosion Tests
It is believed that the standard tests
for establishing the corrosiveness of
liquid soldering fluxes bear little
resemblance to those conditions in
which electronic materials may be-
298-sl O C T O B E R 1980
come contaminated by a flux and subsequently corrode. Assimilation of the
conditions which are considered to
have promoted ESA spacecraft electronics corrosion problems are likely
to have been achieved by the "ad hoc"
corrosion tests of this program.
Visual Inspection Results. A general
overall view of the component and
stranded copper wire samples is seen
in Fig. 5 which depict the corroded
appearances of many items after 56
days exposure to 95% RH at 40°C
(104°F). It is noted that the control
specimens show no evidence of corrosive attack at the end of the test
period. The progressive inspection
results and a description of the numerical classification system employed
throughout this evaluation are listed in
Table 3.
It is important to note that, after
only 10 days subjection to the test
environment, both the damaged sil-
ver-plated wires and the scratched
flat-pack leads have invariably attained their worst visual appearance.
Samples supporting a typical RA type
flux and one RMA type flux have been
detailed in Figs. 6 and 7 after 21 days of
humidity exposure. These photographs are captioned according to the
inspectors' observations.
It is to be observed that the fluxes
containing the higher ionic content
and having higher activity (RA) produce the most severe forms of surface
corrosion. It is also observed that, despite the claims made by certain flux
manufacturers, many products are hygroscopic. Such fluxes are likely to
have become electrically conductive
gels during the test period and are able
to initiate and sustain various forms of
corrosive attack that are dependent on
electrochemical reactions.
Effect of Fluxes on
Silver-plated
Wires. The deliberately damaged si I -
.4.1 .
lu»
f/g. 6—Flux type Al (RA) after 27 days exposure. Corrosion products appear on all metallic surfaces, the wires support a green product, and the
leads have several micro pits in their plating. A—as-received flux; B—boiled flux
•iri j I MI
in
Fig. 7— Flux type Z1 (RMA) after 27 days exposure. A non-hygroscopic flux showing absolutely no sign of corrosion on metallic parts
A—as-received flux; B~boiled flux
ver-plated wires show varying degrees
of surface corrosion products after
exposure to flux and humidity; the
results are listed in Table 3. Unfortunately, it was not possible to quantify
the extent or depth of corrosion by
means of either surface scanning electron microscope or metallographic
examination of individual samples.
The depth of the deliberate score
varied from one wire strand to another. While the non-surface-corroded
samples failed to reveal any metal
wasting, it was extremely difficult to
establish the internal corrosive depth
of attack due to the presence of preferential sites of corrosion and the nonuniform cross-sectional areas of the
damaged strands. The more reactive
RA fluxes were observed to cause rather extensive corrosion in locations
where the copper substrate had been
exposed and only slight corrosion
beneath the layer of supposedly intact
and pore-free silver plating.
The worst case of wire strand corrosion produced by one of the stronger
RA fluxes is shown in Fig. 8. It is noted
that, once the flux and its residue had
been cleaned from the strands during
the preparation of samples for SEM
and metallographic inspection, the
remaining adherent corrosion product
appeared to have a green-brown coloration. It is believed that this debris is
a mixture of so-called red and green
plagues which have been previously
reported in the literature. 1 ' 1 Red plague
is the oxidation product, Cu.O, which
forms because of galvanic action
between silver and copper in the presence of the electrically conductive flux
gelFigure 8 illustrates how the more
noble silver does not enter into the
corrosion reaction. This galvanic attack
is believed to be intensified by the
presence of active chloride ions, Cl ,
and abietic acid present in the flux
which form a green product (green
plague) which can be washed safely
away during post-soldering cleaning.
A potential danger arises, however,
when strong RA-type fluxes and residues are entrapped under the wire
insulation material by capillary action
during soldering or by wicking of contaminated cleaning fluid along the
stranded wires.
On the basis of these results, it is not
considered possible to predict the
long-term reliability of spacecraft
wires which turn green shortly after
the introduction of flux during soldering operations. The greatest danger
will occur when the flux is hygroscopic, contains a high ionic concentration
and is present on wires which will be
periodically subjected to a humid
environment.
Stress Corrosion
of
Component
Leads. Probably the most revealing
results to have been produced by this
evaluation program are those which
indicated the low stress corrosion
cracking resistance of gold-plated
Kovar leads when stressed close to this
material's yield point, then subjected
to the soldering fluxes under evaluation. These results are listed in the last
two columns of Table 2 and, for full
details, the photomicrographs and
captions presented in Figs. 9-21 must
be reviewed.
WELDING RESEARCH SUPPLEMENT I 299-s
The extensive degree of general and
stress corrosion tracking (SCC) shown
by these longitudinal sections is quite
surprising because close visual inspection of the lead surfaces does not
always indicate any sign of surface
corrosion (e.g., see Fig. 7, flux Z1, type
RMA). The photomicrographs (Figs.
9-21) depict longitudinal sections
made through the lead mid-planes
under the "standard scratch" and in
any region away from the scratch
which possesses a particularly severe
corrosion site. The results of applying
either as-received or boiled flux to the
stressed Kovar leads may be compared
and, although not consistent, most
fluxes appear to have an enhanced
corrosiveness once boiled.
It should be noted that several of the
component samples suffered lead
breakages as they were being carefully
removed from their containers at the
end of the 56-day test period. Many of
these fractured items were viewed by
scanning electron microscope. With
one typical example, the lead was
microsectioned, polished and—at a
later stage—lightly etched to reveal a
transgranular mode of SCC propagation.
As in the case of the example cited,
several photographed SCC paths did
not begin at the standard scratch. It is
thought that the gold plating on the
lead surfaces is porous and capable of
initiating and sustaining SCC growth
by diffusion of chloride ions from the
residual flux to the crack tip. The electrochemical mechanism of SCC in
alloy steels is accelerated by the application of anodic currents, and a similar
situation is thought to exist in the case
of these gold-plated Kovar leads,
anodic currents being set up between
the porous plating and the less noble
Kovar alloy. It is to be noted that the
corrosion products occupy a larger
volume than the Kovar from which
they are formed; these products will
tend to increase the SCC propagation
rate by creating a wedging action and
additional stress concentrations at the
crack tip.
of
The fracture of Kovar alloy as a result
exposure to SCC environmental
Fig. 8—One of the strongest and hygroscopic RA-type fluxes produced severe galvanic
corrosion of the copper conductors from these stranded wires (A). The SEM photograph (B)
of one strand shows a silver "shell" whin h, when microsectioned (Cand D) reveals extensive
wasting of copper. (The original form of silver plate is marked on C, but this was crushed by
the mounting media; the polished sections have been lightly etched in ammonium peroxide
to highlight the copper grain structure.) A—optical photograph; B—SEM photograph, X350;
C-microsection, X400; D—detailed photomicrograph, X900 (reduced 23% on reproduction)
300-sl O C T O B E R 1980
conditions have not been widely
reported in the literature.' I " i , s Effective protection of Kovar component
leads from SCC has been achieved 1 " by
firstly chemically removing the workdamaged (from lead-stamping operations) surface of the lead prior to thin
gold plating. The thin plate is designed
to protect the lead from oxidation
during component manufacture, but is
finally removed by dip-coating with a
ductile pore-free finish of eutectic tinlead solder, and any gold plate remaining adjacent to the component-tolead glass-to-metal seal is additionally
protected by a silicone varnish.
Another method" to minimize the
SCC failure of leads is to plate the
Kovar with 12.5 microns of nickel prior
to gold-plating. However, even this
process will not overcome chlorideassisted SCC if the nickel is mechanically cracked during either component
manufacture or subsequent lead-forming operations. 17
It has been suggested that it is
impossible to avoid the SCC failure of
Kovar leads by solely increasing the
thickness of the gold finish. One
report 1 '' states that many thick goldplated transistor leads containing residual stresses induced by 90 deg angle
bends, in the presence of a commercial soldering flux and a humid environment, were observed to fracture
after only 23 days.
The halide concentration necessary
to promote SCC in Kovar has not been
previously studied. In the chemical
industry, where these corrodents are a
serious problem, it has been observed111 that an extremely small concentration of 0.02% aqueous NaCI will
readily cause SCC of high alloy stainless steels, including AISI 316 stainless
steel.
Based on the flux evaluation results
presented in Table 3, an attempt has
been made to index the various fluxes
according to their ability to cause corrosion. This "corrosivity index" appears as the summation of all the
numerically classified results for general surface corrosion and SCC.
A comparison between this index
and the ionic and halide contents of
the various fluxes is presented in Figs.
22 and 23. These show some correlation between a high corrosivity and
either a high halide or high ionic content. There is, however, no relationship between the SCC susceptibility of
Kovar leads and either halide or ionic
content of individual fluxes. In the
case of one as-received R-type flux,
designated A3, it is seen that a halide
concentration as low as 0.0011% may
initiate and propagate sufficient SCC
to cause lead fracture (Fig. TIB).
Effect of Skin Secretions on Stressed
Component
Leads. Spacecraft elec-
Q.
o
X
o
<
LLI
a.
O
Fig. 9-Flux A1. Pitting corrosion seen under
scratch (A), with general stress corrosion at
sites B, C and D
Fig. 70— Flux A2. Surprisingly little
corrosion
under scratch (A, Q, but severe lead embrittlement in one selected region (B)
Fig. 11—Flux A3. Only one site of stress
corrosion (B) with cracks propagating
to
75% of lead thickness
I
O
<
LLI
IMPORTANT
NOTE
Figures 9 - 2 1 c o n s i s t of t o u r v i e w s
each laid o u t in q u a d r a n t f o r m
w h e r e i n t h e q u a d r a n t s are to be
identified according to the f o l l o w ing n o t a t i o n :
A
(As-received
(As-received
flux; r e g i o n
flux; s e l e c t e d
u n d e r scratch)
regions)
C
(Boiled flux;
region under
scratch)
Fig. 12— Flux A4. Only very slight
corrosion adjacent to scratch on (A)
Z
LU
Q.
o
>
LU
Q
I
O
<
D
(Boiled flux;
selected
regions)
pitting
LU
tn
Fig. 13—Flux Kl. Extremely severe form of
stress corrosion cracking through the lead
thickness (A-D) with some exfoliation corrosion (D). In one region (D) the gold plate
is blistering due to the buildup of corrosion
products
a.
o
_J
LU
>
o
<
LU
tf>
LU
tr
z
LU
o
_i
LU
>
LU
o
I
o
<
LU
tf>
LU
Fig. 14—Flux Kl. Appearance of stress corrosion cracking similar to Fig. 16
Fig. 15—Flux K3. Leads have fallen apart due
to the presence of a fine network oi hairline
stress corrosion crac ks
Fig. 76—Flux Ml. Pitting corrosion
adjacent
to scratches (A,C) with some surface corrosion and blistering of gold plating (D)
WELDING
R E S E A R C H S U P P L E M E N T ; 301-s
Ml
Fig. 17—Flux M2. Severe case of stress corrosion cracking and associated blistering (BO)
Fig. 18-Flux M3. An extraordinary mixture
of corrosive classes, general Kovar corrosion
and blistering due to corrosion product (A),
slight intergranular and pitting corrosion
(B), zero corrosion (C) and severe stress
corrosion (D)
tronic units are generally handled by
operators under "clean r o o m " conditions. The preferred codes of practice
for component assembly by hand or
wave soldering to printed circuit
boards recommend that operators
wear finger cots or lint-free gloves
when they bend, straighten and insert
component leads into pcb termination
areas. These practices are certainly not
universal since some operators feel
restricted when wearing hand covers
for deiicate soldering operations or tor
reworking incorrect joints on high
density boards. The use of bare hands
in these instances may be justified
provided the boards are thoroughly
solvent-cleaned by approved methods1 immediately after assembly.
The effect of severe handling, which
302-sl O C T O B E R 1980
Fig 19— Flux Zl. Slight intergr'anular corrosion in one region of stressed lead
(D—boiled flux)
causes skin secretions to be deposited
upon stressed component leads, was
evaluated by means of two clean flatpackaged components supporting the
"standard scratch." Once contaminated by perspiration, these samples
were subjected to the stress and
humidity environment in parallel w i t h
cleaned control samples and the previously' described fluxed samples. All
devices were inspected at regular
intervals and classified according to
the degree of observed surface corrosion. The results are listed in Table 3.
After 21 days, the contaminated
sample had become surface-corroded
to an extent less than 50"o of its lead
surface area. The cleaned control sample remained uncorroded. At the end
of the 56-day period, the contaminated and control samples were
microsectioned;
these
as-polished
photomicrographs appear in Figs. 24
and 25, respectively. Despite the
somewhat innocuous surface appearance of the contaminated leads, the
sections reveal highly branched stress
corrosion cracks beneath the stressraising scratch as well as in selected
regions beneath the gold plate. Many
of the fine cracks which penetrated
more than 50% of the lead thickness
occurred in regions well away from
surface blisters and w o u l d not have
been apparent from visual inspection
alone.
The component control samples
showed absolutely no signs of surface
or stress corrosion cracking f o l l o w i n g
the 56-day exposure to temperature
and pure water humidity.
Measurements have been made1'1 on
the salt contributed by fingerprints on
handled printed circuit boards. Individual prints were found to contribute
as much as 30 micrograms of NaCI as
measured on the D C M . It may be
Fig. 20—Flux Z2. Surface pitting corrosion
(A,B,D) and severe stress corrosion (C)
Fig. 2'1-Flux Z3. Surface pitting corrosion
(A,B,C) and severe stress corrosion (D)
difficult to remove completely such
high concentrations of salt from the
vicinity of similar fingerprints when
they are present on component surfaces due to the porous nature of the
gold plating on Kovar leads.
An induction period prior to Kovar
cracking may be dependent on the
buildup and penetration of a local
corrosive solution somewhere along
the lead surface and SCC, in the worst
case promoting lead fractures, may be
observed only after a long period of
time when the environmental conditions are conducive to crack propagation.
These test results emphasize the
need to preclude unnecessary handling of Kovar and probably other
iron-nickel-cobalt alloy lead materials
by bare hands. When circumstances
permit handling, every precaution
must be taken to ensure complete
© =R
AS-RECEIVED TO
BOILED CONDITION
• = RMA
• = RA
KEY MANUFACTURERS'
Dl SCRIPT I ON OF FLUX ACTIVITY
130
A1
K3
Q.
o
120
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A2
B 110
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K2
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90
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tr
u.
80
X
UJ
Q
UJ
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100
X
K1
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Z2
M3
UJ
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M2
70
60
A3
Z3
50
Q.
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20
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0
100
10
10000
1000
100 000
UJ
QL
D.C.M. TEST R E S U L T S ( j j g N a d /j_tI F L U X )
Fig. 22—Relationship between the ionic content of a flux and its corrosive index
0L
o
_l
UJ
>
® = R
•
= RMA
I
•
= RA
QL
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KEY MANUFUCTURERS
DISCRETION OF FLUX ACTIVITY
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UJ
QL
Q.
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00001
0 001
0 01
UJ
{A
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QL
01
HALIOE CONTENT OF AS -RECEIVED FLUX, °l. BY WEIGHT
Fig. 23-Relationship between the halide content of the as-received fluxes and their corrosivity. based on ad hoc tests
UJ
Q.
removal of potential corrosive films by
adequate post-assembly cleaning procedures.
Conclusions
1. The flux survey and subsequent
program of testing and evaluation of
specific types of commercial liquid
soldering fluxes may serve as a rough
guide to flux selection for application
during the soldering of spacecraft
hardware. The surprisingly strong corrosive nature of many commercial
fluxes precludes their use whenever
there is the slightest chance that a
trace of their residues may remain on
delicate electronic materials w h i c h , for
the new generation of ESA spacecraft,
may require long storage times prior to
launch and may have, in the case of
communications satellites and Spacelab, operating lives of 10 years.
2. The mean solder wetting times
for the various classes of fluxes examined in the program are as follows: RA
0.14 s for gold-plated and 0.5 s for
copper wires; RMA 0.25 s for goldplated and 1.3 s for copper wires; R
0.35 s for gold-plated and 2.0 s for
copper wires.
The fully activated RA-type fluxes
generally contain a high ionic concentration and possess a high fluxing efficiency (FE) whereas no particular relationship could be established between
W E L D I N G RESEARCH S U P P L E M E N T : 303-s
o
_l
UJ
>
X
o
QL
<
UJ
CO
Fig. 24—Section across handled
specimen—severe stress corrosion
cracking.
A—under scratch; B-selected
region
Fig. 25—Section across clean control sam
pie—no evidence oi any form of corrosion
A—under scratch; B-selected
region
FE a n d i o n i c c o n c e n t r a t i o n f o r t h e
R M A a n d R-type fluxes. T h e a p p l i c a t i o n of heat w a s f o u n d t o increase t h e
i o n i c c o n t e n t of m o s t tluxes.
6. C o n c e r n i n g
gold-plated
Kovar
c o m p o n e n t leads, t h e s e m a y b e c o m e
slightly surface-corroded, but degradation
by
stress c o r r o s i o n
cracking
(SCC), as c a u s e d by t h e m a j o r i t y o f t h e
t e s t e d f l u x e s , is n o t a l w a y s v i s i b l e u n t i l
s u c h leads f r a c t u r e in t w o . T o a v o i d
SCC i n i t i a t i o n a n d p r o p a g a t i o n , t h e
f o l l o w i n g ESA s o l d e r i n g r e q u i r e m e n t s '
m u s t be e n f o r c e d :
3. A l t h o u g h s o m e w h a t s u b j e c t i v e ,
t h e s t a n d a r d c o p p e r m i r r o r test a n d
t h e p r o p o s e d c o p p e r sheet test p r o v i d e d c o m p a r a b l e results. B o i l i n g i n d i v i d u a l fluxes d i d not p r o d u c e d i f f e r e n t
results. N e i t h e r test m e t h o d is c o n s i d ered to be p a r t i c u l a r l y s e l e c t i v e in
assessing t h e c o r r o s i v e n e s s of f l u x e s
u n d e r service c o n d i t i o n s .
4. O n l y t h e " a d h o c " c o r r o s i o n tests
are c o n s i d e r e d l i k e l y t o s h e d l i g h t o n
t h e c o m p o n e n t lead a n d w i r e p r o b lems e n c o u n t e r e d o n ESA s p a c e c r a f t
projects; they reproduce the material
characteristics and the
interrelated
m e c h a n i s m s of g a l v a n i c c o r r o s i o n a n d
stress c o r r o s i o n c r a c k i n g .
5. C o n c e r n i n g s i l v e r - p l a t e d c o p p e r
w i r e , fluxes of t h e R A - t y p e s h o u l d n o t
be used. If t h e r e is any l i k e l i h o o d that
t h e p l a t i n g is d a m a g e d or p o r o u s , t h e n
c e r t a i n of t h e R M A a n d R - t y p e fluxes
listed in T a b l e 3 w o u l d n o t be r e c o m m e n d e d , p a r t i c u l a r l y w h e n t h e ingress
of flux or c o n t a m i n a t e d c l e a n i n g s o l u t i o n s c a n n o t be p r e v e n t e d . In case of
doubt, the c l e a n i n g / v a c u u m
bake
r e m e d y o u t l i n e d in t h e l i t e r a t u r e " m a y
eliminate corrosion.
• Flux a n d r e s i d u e are r e m o v e d i m m e d i a t e l y after s o l d e r i n g .
• Stress relief b e n d s m u s t be p r o v i d e d
b e t w e e n c o m p o n e n t b o d y a n d part
termination.
• Leads must n o t b e s h a r p l y b e n t ; use
the m i n i m u m lead-bending requirements.
• Leads m u s t not be f o r c e d t o lie flat
d u r i n g s o l d e r i n g , a n d t h e y m u s t be
f o r m e d accurately to prevent residual
stresses, e.g., c o m p o n e n t s m u s t n o t be
h e l d i n t o PCB p l a t e d - t h r o u g h h o l e s by
t h e s p r i n g p r o p e r t i e s of t h e i r leads;
this c o u l d p r o v i d e ideal c o n d i t i o n s f o r
c a t a s t r o p h i c SCC f a i l u r e .
• Non-authorized
fluxes,
solvents,
etc., m u s t be r e m o v e d f r o m t h e w o r k
area.
• Also, m o u n t i n g
pads, c o n f o r m a l
c o a t i n g s , etc., s h o u l d be d e s i g n e d t o
l i m i t stresses d u e t o d i f f e r e n t i a l t h e r mal expansion.-"
Fig. 2b—Photograph showing the deleterious eiiect oi Class 2 top-marginal
pass)
and Class 3 (bottom-tail)
(luxes
following
the copper mirror test
7. T h e leads o f e l e c t r o n i c c o m p o n e n t s m a y b e c o m e severely d e g r a d e d
by t h e t r a n s f e r of c o n t a m i n a t i o n , s u c h
as p e r s p i r a t i o n f r o m an o p e r a t o r ' s bare
hands.
Such
practices
must
be
a v o i d e d , a n d it is an ESA r e q u i r e m e n t '
t h a t c l e a n w h i t e g l o v e s or f i n g e r c o t s
be w o r n in o r d e r to a v o i d t h e f o r m of
c a t a s t r o p h i c SCC d e p i c t e d in Fig. 27.
Acknowledgment
T h e a u t h o r s w i s h t o t h a n k M r . D.S.
C o l l i n s f o r his assistance w i t h t h e m e t a l l o g r a p h y a n d M r . H. S m i t h of t h e
F u l m e r Research I n s t i t u t e , E n g l a n d , f o t
the halide content and pH determinations.
References
1. ESA-PSS-14/QRM-08, "The
Manual
Soldering of High Reliability E lectrical Connections," 1918.
2. QQ-S-571,
"Non-corrosive
Rosincored Solder W i r e , " 1963.
3. DTD-599A, "Non-corrosive Flux for
Soft Soldering," March 1961.
4. MIL-F-14256C, "Flux Solutions of Rosin or M i l d l y Activated Rosin," 1963.
5. Peters, S.T., and Wesling, N., "Corrosion of Silver-plated Copper Conductors,"
SAMPE 13th National Symposium, May
MARGINAL PASS
. . . ..
_|_
0 001
0.01
HALIDE CONTENT
27-Effect
oi halide content
304-sl O C T O B E R
1980
on copper mirror
test results
01
OF AS RECEIVED FLUX , 7. BY WEIGHT
_l_
1.0
•
1968.
6. Peters, S.T., "Review and Status of Red
Plague Corrosion of Copper Conductors,"
Insulation/Circuits,
May 1970, p. 55.
7. Reich, B., "Stress Corrosion Cracking
of Cold-plated Kovar Transistor Leads," Solid State Technology, April 1969, pp. 36-38.
8. Studnick, W.R., and Foune, C.C,
"Testing for Corrosivity in Activated Liquid
Soldering Fluxes," The Western
Electric
Engineer, |an. 1973, Vol. XVII, No. I, pp.
3-8.
9. Weirick, L.J., "A Metallurgical Analysis
of Stress Corrosion Cracking of Kovar Package Leads," Solid State Technology,
1975,
Vol. 18, No. 3, pp. 25-30.
10. D u n n , B.D., "Reliable loints for
Spacecraft, Part 2," Electronic
Production,
1978, Vol. 7, No. 4, p. 23.
VI. Brons, )., "Evaluation of Post-solder
Flux Removal," Welding journal,
54(12),
Research Suppl. Dec. 1975, pp. 444-s to 448s.
12. Tautscher, C.)., "Printed Wiring Board
Cleanliness Testing," Circuit World, Vol. 4,
No. 2, p. 30.
13. B.S. Draft Specification No. 77/78244
intended to replace B.S. No. 41 1, 1954.
14 MIL-S-19500D, "General Specifica-
tion tor Semiconductor Devices—Group B
Tests."
15. D u n n , B.D., "Product Assurance and
Choice of Materials for Satellite Construct i o n , " Metall, August 1976, 30 (9), pp.
711-720.
16. Elkind, M.|., and Hughes, H.E., "Prevention of Stress Corrosion Failure in FeNi-Co Alloy Semiconductor Leads," Bell
Telephone Laboratory Report in Physics oi
Failure
in Electronics,
5 (1967), pp.
447-495.
17. Harboe, R., and Adams, L., " A n Investigation of the C M O S Lead Corrosion Problem." ESTEC-Working Paper No. 1023 (confidential).
18. Reich, B., "SCC of Gold-plated Kovar
Transistor Leads," Solid State
Technology,
1969, Vol. 12. No. 4, pp. 36-38.
19. Spa'hn, H., "Performance Requirements tor Stainless Steels in the Chemical
Process Industry," Proceedings of the Stainless Steel 1977 Conference,
London, p.
161.
20. D u n n , B.D.. "The Resistance of
Space-quality Solder Joints to Thermal Fatigue," Circuit World, Part I, Vol. 5, No. 4,
1979, pp. 11-17; Part 2, Vol. 6, No. 1, 1979,
pp. 16-27.
A p p e n d i x : S u m m a r y of Survey C o n d u c t e d t o Establish S o l d e r i n g
Fluxes
U t i l i z e d by E u r o p e a n M a n u f a c t u r e r s of E l e c t r o n i c E q u i p m e n t f o r
Space
Application
T h i r t y of t h e m a j o r E u r o p e a n c o m panies e n g a g e d in t h e m a n u f a c t u r e o f
e l e c t r o n i c h a r d w a r e f o r ESA s p a c e c r a f t
applications were approached. T w e n t y - f i v e c o m p a n i e s r e s p o n d e d by f o r w a r d i n g f o r analysis a n d t e s t i n g o n e or
m o r e s a m p l e s of fresh u n c o n t a m i n a t e d f l u x as u s e d at t h e i r p l a n t s . T h e
s a m p l e s w e r e a c c o m p a n i e d by c o m plete i n f o r m a t i o n about batch identity, manufacturer's date of purchase
a n d i n t e n d e d usage.
It w a s e v i d e n t f r o m t h e r e p l i e s
received that the majority of c o m p a nies d o n o t carry o u t any f o r m of
i n c o m i n g i n s p e c t i o n tests or c h e m i c a l
c o n t r o l s o n any of t h e i r l i q u i d flux
p u r c h a s e s . In g e n e r a l , it w a s f o u n d
t h a t c o m p a n y p u r c h a s e o r d e r s referenced only that procurement should
be against c e r t a i n n a t i o n a l s p e c i f i c a tions and that acceptance testing was
a s s u m e d t o h a v e b e e n p e r f o r m e d at
t h e s u p p l i e r ' s p l a n t p r i o r t o b a t c h release.
In t o t a l , 71 f l u x s a m p l e s
wore
r e c e i v e d — T a b l e 4. T h e m o s t c o m m o n
flux p r o d u c t s o r i g i n a t e d f r o m British,
U.S. a n d G e r m a n m a n u f a c t u r e r s . A f e w
were received w i t h D u t c h and Belgian
brand names. All samples were subj e c t e d to a halide content
determination
according to the m e t h o d des c r i b e d in t h e l i t e r a t u r e 1 , h e r e , t h e
p e r c e n t a g e by w e i g h t of h a l i d e is c a l c u l a t e d as c h l o r i n e o n t h e w e i g h t of
t h e n o n - v o l a t i l e p o r t i o n of flux.
A pH
value
determination
was
attempted
by
placing
chemically
t r e a t e d p H p a p e r s in t h e a s - r e c e i v e d
flux for o n e m i n u t e ; color changes of
t h e papers w e r e d e p e n d e n t o n t h e
h y d r o g e n - i o n c o n t e n t of each flux. All
fluxes w e r e a c i d i c , h a v i n g a p H of less
t h a n 7. H o w e v e r , t h e d y e in t h e p a p e r s
d i d n o t r e s p o n d s t r o n g l y t o all f l u x e s
a n d t h e s e results are s o m e w h a t s u b jective.
Each f l u x w a s s u b j e c t e d t o a copper
mirror
test. T h i s test w a s p e r f o r m e d
Table 4—Summary of Various Flux Types Tested, User Companies and Laboratory Results
Laboratory test results
Flux
code
no.
1
2
3
4""
Country and
no. of user
comp any
GB
S
NL
GB
4
11
13
24
GB
25
D
DK
NL
B
NL
DK
9
10
13
14
18
26
D
GB
DK
B
NL
E
1
3
10
14
18
19
B
DK
NL
S
5
7
13
27
Batch
identity
of flux
a
a
j
a
b
c
c
a
a
c
b
b
b
a
a
b
a
a
a
b
b
d
a
Halide content
of as-received
sample, %
Copper mirror test
pH of original
flux
24 h at 30 C and 50% RH
As-receiv ;d
3oiled
5.1
4.5
4.8
4.8
4.5
4.8
4,8
(1)
(0)
(1)
(1)
(1)
(0)
(1)
(1)
(1)
(1)
(1)
(I)
0.0012
0.0003
0.0011
0.0064
0.0003
< 0.0004
4.0
4.8
4.8
4.5
3.9
4.8
(1)
(0)
(0)
(0)
(I)
(0)
(I)
(0)
(0)
(0)
(I)
(0)
0.03
0.051
0.038
0.044
0.032
0.024
5.1
5.0
5.1
4.5
5.0
5.1
(1)
(0)
(0)
(0)
(1)
(I)
(1)
(0)
(0)
(0)
(I)
0.01
0.0095
0.012
0.01
4.5
4.8
4.8
4.8
(0)
(1)
(0)
(1)
(0)
(1)
(0)
(I)
0.069
0.037
0.022
0.016
0.054
0.028
0.053
0)
0)
(Continued
WELDING
CM
on next page)
RESEARCH SUPPLEMENT
305-s
using flux in the as-received condition
and after boiling a standard volume of
flux for one minute at approximately
200°C (392°F) in order to simulate the
soldering operation. The copper mirrors were prepared in the laboratory by
vacuum-depositing 5000 A of pure
copper onto treated vapor-degreased
glass slides as required by MIL-F14256C" Approximately 0.05 ml of flux
was placed on the copper side of the
mirrors which were then stored in the
horizontal position inside a humidity
chamber at 30 ± 2°C (86 ± 3.6°F) and
50% relative humidity for 24 h. Each
slide was then visually examined for
corrosive attack on the copper.
The pass/fail criteria of this test are
given in the footnote to Table 4. Any
penetration of the copper thickness
could be readily seen by holding each
mirror against a light source. Such corrosive attack was classified as a class 3
flux failure. Attack o n , or modification
Table 4—Summary of Various Flux Types Tested, User Companies and Laboratory Results (Continued)
Laboratory test results
Flux
code
no.
Country and
no. of jser
company
Batch
identity
of flux
Halide content
of as-received
sample, %
pH of original
flux
Copper mirror test,
24 h at 30°C and 50% RH
As-received
Boiled
E
NL
B
D
6
13
14
22
a
w
c
a
0.46
0.48
0.48
0.46
4.5
(0)
(0)
-
-
-
4.5
4.5
(0)
(1)
(0)
(I)
DK
S
NL
7
11
13
a
b
k
0.83
0.28
0.59
4.5
3.9
4.8
d)
(1)
(1)
(2)
DK
NL
F
7
13
31
c
e
a
0.067
0.011
0.0009
3.9
4.5
4.8
(I)
(2)
(2)
NL
CB
CB
13
15
25
t
0.42
0.40
0.36
-
-
-
b
b
4.5
4.8
(0)
(0)
(0)
(0)
DK
GB
10
30
d
a
2.31
2.32
4.5
4.8
(2)
(1)
(2)
(1)
\l
GB
13
25
q
q
0.0002
0.0018
-
-
-
4.5
(2)
(2)
11
DK
GB
10
15
e
a
0.70
0.44
3.9
3.9
(0)
(2)
(0)
(2)
12
B
5
q
0.0035
4.8
(2)
(2)
13
14
15
16
17
D
D
D
D
D
8
8
8
8
9
< 0.0006
5.84
< 0.0006
0.0009
0.0019
4.8
3.9
4.5
4.2
4.5
(2)
(2)
(0)
(0)
(2)
(2)
(2)
(0)
(0)
(2)
5
6""
7""
8
9
10
(1)
(2)
0)
(2)
(2)
18
DK
10
0.88
3.9
(1)
(1)
19
20
21
22
23
NL
NL
NL
NL
NL
13
17
17
17
18
0.0009
0.004
0.0028
0.68
0.012
4.2
4.8
4.5
4.5
5.1
(0)
(1)
(0)
(2)
(2)
(0)
24
I
F
32
21
a
a
0.078
0.048
5.0
4.5.
-
-
(1)
(1)
25
26
D
DK
22
26
b
0.019
12.95
4.8
2.0
(1)
(3)
d)
q
27
28
29
30"
NL
NL
NL
NL
13
13
13
13
(0)
(0)
f
0.0029
4.5
(2)
(2)
31
D
8
e
< 0.0004
4.2
(2)
(2)
33""
34""
35""
36""
37""
38""
\l
NL
NL
NL
NL
NL
NL
13
13
13
13
13
13
13
7.93
4.45
0.0011
0.0013
0.73
0.75
62.50
3.4
3.6
4.8
4.8
4.8
4.8
1.0
(3)
(3)
(0)
(1)
(3)
(3)
(0)
(I)
(I)
(2)
(3)
39
I
32
< 0.0002
10.0
3 2
«b
ta,
0.006
0.0046
0.008
(D
(2)
(3)
0)
(0)
(2)
(3)
The copper mirror test was p e r f o r m e d o n flux samples in t h e as-received c o n d i t i o n , in the same c o n c e n t r a t i o n as used by t h e p a r t i c i p a t i n g ESA contractors The test was repeated after
each sample had been b o i l e d for 1 m m at 200 ± 5°C t o simulate the soldering o p e r a t i o n . The pass/fail criteria are classified as f o l l o w s : Class 0—Pass: no d i s c o l o r a t i o n of flux or c o p p e r ; class
1 - M a r g i n a l pass: strong discoloration of flux, but no attack o n copper, class 2 - M a r g i n a l pass: slight d i s c o l o r a t i o n of flux w i t h salmon p i n k ' e t c h ' o n copper, class 3—Fail: excessive e t c h i n g
w i t h penetration of copper Ihickness
' h l Huxes chosen for further testing
306-si O C T O B E R 1980
of, the copper surface was not classified as a failure, but as a marginal pass
(either class 1 or 2). Examples of
classes 2 and 3 are shown in Fig. 26.
The results of the copper mirror test
and the halide content analysis are
compared in Fig. 27. The following
observations are made f o l l o w i n g the
results of the tests:
1. Based on the copper mirror test
results, the extremely corrosive fluxes
have a halide content in excess of 4.0%
and pH values of less than 3.5. Such
fluxes are rarely employed by ESA con-
tractors and used only for the pretinning of "difficult" metals.
2. W i t h the exception of the extremely corrosive fluxes, there appears
to be no relationship between the
resistance to copper mirror corrosion
and the individual flux halide concentrations (from Fig. 27). However, this
is unlikely to be true for bi-metallic
corrosion or in the presence of alloys
susceptible to stress corrosion. For a
realistic assessment of the corrosive
nature of any flux, it is therefore recommended that the corrosion test
assembly reproduce accurately the
physical makeup of each particular
solder joint configuration.
3. Slight variations are observed in
the halide content and pH values of
different batches of the same flux type
when purchased in different countries.
This indicates that flux manufacturers
may change the chemical formulation
of their brand name products. However, this has no marked effect on the
copper mirror test.
4. Flux types as-received and after
boiling produce identical copper mirror test results.
WRC Bulletin 258
May 1980
International Benchmark Project on Simplified Methods for Elevated Temperature
Design and Analysis: Problem I—The Oak Ridge Pipe Ratchetting Experiment;
Problem II—The Saclay Fluctuating Sodium Level Experiment
by H. Kraus
Problem I—The Oak Ridge Pipe Ratchetting Experiment is analyzed by general purpose finite element
computer programs and by approximate analytical techniques. The methods are described and the results are
compared to the experimental data.
Problem II—The Saclay Fluctuating Sodium Level Experiment is analyzed by special purpose computer
programs and by approximate analytical techniques. The Methods are described and the results are
compared. Experimental data are not yet available.
Publication of these reports was sponsored by the Subcommittee on Elevated Temperature Design of the
Pressure Vessel Research Committee of the Welding Research Council.
The price of WRC Bulletin 258 is $10.00 per copy, plus $3.00 for postage and handling. Orders should be
sent with payment to the Welding Research Council, 345 East 47th St.. Room 801. New York, NY 10017.
WRC Bulletin 259
June 1980
Analysis of the Radiographic Evaluation of PVRC Weld Specimens 155, 202, 203,
and 251J
by E. H. Ruescher and H. C. Graber
This report is one of a series of analyses of nondestructive examination data obtained from heavy-section
steel weldments with intentionally introduced flaws prepared by the PVRC Subcommittee on Nondestructive
Examination of Pressure Components.
The primary objective in this work area was to determine the radiographic detectability of deliberately
induced flaws. A group of evaluation teams without prior knowledge of the number, type or location of the
intentional discontinuities independently examined each specimen in accordance with identical instructions.
The results of these examinations were used as the basis for decisions regarding the flaws.
This report describes the evaluation techniques used to reduce the data from the detectability of the
deliberately induced and naturally occurring flaws in the weld specimen.
Publication of this report was sponsored by the Subcommittee on Nondestructive Examination of Pressure
Components of the Pressure Vessel Research Committee of the Welding Research Council
The price of WRC Bulletin 259 is $11.00 per copy, plus $3.00 for postage and handling. Orders should be
sent with payment to the Welding Research Council, 345 East 47th St., Room 8 0 1 , New York, NY 10017.
W E L D I N G RESEARCH S U P P L E M E N T , 307-s
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