A Guiding Influence in the Electronics Industry
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SCREEN PRINTING SURFACE MOUNT ADHESIVE
- THE COMPLETE SOLUTION.
Author:
Alan Hobby
DEK Printing Machines Ltd, 11 Albany Road, Granby Industrial Estate, Weymouth
Dorset DT4 9TH, England
Tel: (01305) 760760 Fax: (01305) 760123
INTRODUCTION
Solder paste printing is very widely used
throughout the surface mount industry yet the great
majority of manufacturers who use wave soldering
still use dispensers when applying glue for bottom
side chip attachment. With the increasing use of
high speed chip shooters and ever more complex
boards, even the fastest (and most costly) glue
dispensers struggle to keep pace. Screen printing
provides a cheap and extremely fast alternative.
Whereas dispensing is a serial process, virtually
any number of glue dots can be deposited
simultaneously with a screen printing machine.
There have been objections to using screen
printing, principally that only a single thickness of
adhesive can be printed. While that was true until
quite recently, this single thickness is suitable for
the very great majority of components that are
commonly wave soldered. This paper will present
guidelines and recommendations for such printing.
However, further information will be presented on
advanced techniques which allow a wide range of
thickness of adhesive to be printed, using standard
printing machines, allowing both the smallest chip
components and those with the largest stand-offs to
be assembled. In addition, boards can be printed
which already contain pre-inserted axial and radial
components, something which would
be
impossible with ordinary stencilling techniques.
Additionally, guidelines will be offered for those
who wish to print conductive adhesives, both
conventional and “ Z-axis”, to replace solder paste.
CONVENTIONAL GLUE PRINT FOR
BOTTOM S IDE CHIP ATTACH.
Mass production of surface mount assemblies
generally takes one of two forms. One major
technique comprises printing a relatively thin, well
controlled deposit of solder paste onto the lands of
the circuit boards, placing components onto/into
the paste, then passing the assembly through an
oven so that the solder melts and forms the
electrical and mechanical joint between the board
and the components. The alternative technique
comprises dispensing dots of a non-conductive
adhesive onto the board but between the lands. The
components are placed onto the glue which is cured
in an oven. Sometimes, conventional leaded
components are inserted through holes in the board
as well. The inverted board is then passed over a
wave of molten solder. The solder bridges between
the lands and the component terminations and,
once again, forms the joints between board and
component. The former method invariably uses
screen printing as the method of depositing the
solder paste, whereas, traditionally, glue is
deposited by using pressurised, hollow needles to
squirt individual dots onto the board surface.
Glue dispensers are as highly developed, reliable,
versatile and repeatable as any other part of a
modern assembly line. Screen printing of glue onto
circuit boards has been available as a technique for
many years and there have been a few companies
who have used it successfully. However, rather like
underwater football, it has never become a
widespread, mainstream activity. The successful
users clearly derived benefit from it but the majority
saw limitations, not only in underwater football but
in screen printing as well. It was possible only to
make a single thickness deposit so there could be
difficulties in assembling components with large
and small underside clearances. It might take 30 or
45 seconds to print even a small board but glue
could be dispensed for a few tens of components
very much quicker. In any case, screen printing is
messy, isn’t it?
Over the last 12 to 18 months, however, there has
been more widespread interest in glue printing than
in the previous 12 to 18 years. This is due almost
entirely to the need for speed. Chip shooters throw
components at boards at frightening rates and,
dollar for dollar, the glue dispensers are always
slower. Glue dispensing, therefore becomes the
bottleneck in the line. Printing machines have
become a little faster over the years but they have
the very big advantage that, whether you print 1
dot or 1000 dots, they do it in substantially the
same time. Whereas dispensing is a serial process,
virtually any number of glue dots can be deposited
simultaneously when screen printing. Taking a
ludicrous example, a board 500 mm square
requiring dots at 1 mm pitch can be printed with
the simplest machines in around 30 seconds
whereas a dispenser running at 100,000 dots per
hour, far faster than anything presently available,
would take 2½ hours. More realistically, a board
requiring 1000 glue dots would still need 30
seconds in the printer but 2½ minutes in a glue
dispenser running at 25,000 dots per hour. They
can also print any required shape of deposit with
ease, so single, double and multiple dots, squares
or stripes are easily produced. The very great
majority of wave soldered, surface mount
components placed are chip resistors, capacitors and
other small devices with either a very small
underside clearance or none, so the single print
thickness is rarely an issue. The glue manufacturers
have worked away quietly and all have adhesives
which are readily and reliably printable.
Traditionally, the operator of a solder paste screen
printing machine would return to it frequently,
wipe the underside of the stencil, talk to the
machine and tell it how well it was performing and
generally encourage it to keep on producing. The
glue print operator will, more probably, simply
throw some glue into the machine, press the “ Go”
button and then wander off to more difficult things,
perhaps returning at the end of the shift to clean
down.
S tencil recommendations.
An etched metal mask 0.25 mm thick will
normally be used. This gives excellent definition
and a long life and can be used to assemble chip,
SOT23 and, with many adhesives, SOIC devices.
The mask could equally well be electro-formed or
laser cut but since the apertures are not particularly
small, the cheapness of etching is normally
preferred.
The glue dots must not spread onto the board
lands, so typical dot diameters will be 0.75 mm for
0805 chips and SOT 23s, 1 mm for 1206s and
above, and multiple 2 mm dots for SOICs. If
cylindrical components (MELFs) are to be
assembled, two dots, each 1 mm diameter and at 1
to 1.5 mm centres can be used to help to prevent
the component from rolling. For any of the
components, the deposits need not be dots,
although operators may initially feel more
comfortable with them, as they are used to the
shape. Some assemblers prefer to use a line some
0.4 - 0.5 mm wide and of the same length as the
width of the component. Thus the line length
would be 1.25 mm for an 0805 capacitor. Lines can
be superior to dots because, for the same volume of
adhesive, there is less chance of it creeping onto the
board lands. In addition, there is a better height to
width ratio, which reduces the probability of the
component sliding on the adhesive during
placement.
Printer parameters
The normal rules of screen printer set-up apply.
The squeegee can be metal or reasonably hard
polyurethane, depending on any individual
preference. A 45° angle of attack will probably fill
stencil apertures more efficiently than the more
normal 60°. If there are any fears, (probably
unfounded), of material contamination should the
squeegees be not properly cleaned, this very
obvious difference makes it easy for operators to
distinguish between squeegees for glue and solder.
Squeegee pressure should be sufficient to clear paste
from the stencil surface and speed can be as slow as
the production line will permit. If the stencil/board
separation speed can be controlled, slow speed will
usually yield a more “ peaky” print which is good
for secure component placement.
These recommendations for the standard technique
of bottom side chip attach glue printing should
form a good starting point for most glues. As for all
general screen printing recommendations, they
should not be taken as the definitive, final word but
should be modified to optimise the process for
particular, local conditions.
MULTI-THICKNES S PRINTING FOR
BOTTOM S IDE ATTACH.
One of the other developments which has fuelled
interest in glue printing has been the introduction
of techniques for varying the height of the glue
dots. If it becomes necessary to assemble
components which range from small chip resistors
up to J-leaded PLCCs, the stand-off (or distance
from the top of the board to the underside of the
component) will vary from zero to around ¾ mm,
well beyond the range of heights of a normal
stencil. (From personal experience, the number of
factories assembling PLCCs by wave soldering is
very small. The height of such components,
coupled with the distribution of leads on all four
sides, these leads being tucked under the body of
the component, seems to make wave soldering a
technique which is best avoided. Conversations
have indicated that the component should be placed
at 45° rather than the normal 90°, that quite
elaborate solder thieves are necessary and, in any
case, there is a high risk of these large plastic
mouldings outgassing dramatically (“ pop-corning”)
as they pass through the wave). Table 1 indicates
typical dot heights required for the normal range of
surface mount components.
Component
0402
0603
0805
1206
SOIC
PLCC
Typical stand-off
(mm)
.050
.075
.075
.100
.375
.750
Table 1
Suggested glue
dot height (mm)
.075
.115
.115
.150
.500
.875
“PumpPrinting”.
The technique developed at DEK Printing
Machines in England, called PumpPrint , uses a
relatively thick (1 mm) plastic stencil which is
drilled by normal PCB manufacturing techniques to
form the apertures. Laser cutting and chemical
etching are not suitable techniques for fabricating
plastic stencils. The holes drilled in the stencil are
of different diameters ranging typically from 1 to 2
mm. The smaller diameter holes produce small,
low dots and the large holes produce large, tall
dots. This effect is well known in the field of solder
paste printing where paste remains blocked in small
apertures, resulting in insufficient paste being
deposited on fine pitch lands. When there is more
paste sticking to the walls of the apertures than to
the board land, most paste will remain in the
aperture. Conversely, when this situation is
reversed, most paste will transfer onto the board.
While classically, this shows very poor screen
printing technique, it is highly beneficial for glue
printing. Small components always have small or
negligible stand-offs, so require small, low dots
whereas large components have high stand-offs and
require large or several large, tall dots.
fact, many were so high that they fell over and gave
the appearance of tailing . The prints then collapsed
to about one third of the height of other glues and
spread by about a third in diameter.
An additional advantage of this technique is that
pockets can be machined into the underside of these
very thick stencils, thus protecting anything already
on the board surface, while allowing prints to be
made between them. Such “ things already on the
board surface” include wet solder paste prints and
the clinched leads of previously inserted
components.
Printer parameters
Several glues from four manufacturers were test
printed and Figure 1 shows the profile of four
typical examples. The effect of diameter on height
is clearly shown. Figures 2 - 9 show this
relationship between stencil hole diameter and glue
dot height and diameter. It will be seen that most
adhesives behave quite similarly. The dot
diameters are essentially those of the holes through
which the glue was printed and the profiles are
concave-sided cones. There is essentially no change
in height or profile with time after printing, at least
for several hours. The separation of the stencil and
board is vertical so there are none of the tails which
can be a problem with poorly set dispensers.
However, one glue was very poor and, to avoid
embarrassment, is not identified. Immediately after
printing, the prints were very high and peaky. In
The ratio of dot diameter to height appears not to
be truly linear but for practical, design-rule
purposes, an assumed linear relationship is accurate
enough. The linear trendline is,
thus,
superimposed.
In the unlikely event that glue dot heights greater
than 0.8 or 0.9 mm are required, some care in
selection of materials is necessary. As the hole
diameter increases, so does the dot height but the
process can become unreliable. The first one or two
prints can often produce large heights but this is
not reproducible. The glue simply does not enter
the apertures above some 2.5 mm dia. It is thought
that the first squeegee passes fill the apertures in the
normal way, air in the bottom of the apertures
being pushed out by the descending glue. However,
on subsequent prints, the adhesive residues form an
air tight seal between the stencil and the board so
an air bubble is trapped in the bottom of the
aperture. Even with multiple passes, no print is
achieved. Three glues were found which gave
results which reliably exceeded 1 mm in height at 3
mm dia. These were Amicon D 125 F3, Multicore
SA 35 and Multicore SA 335. (Fig 10)
The process for all normally wave-soldered
components is very robust and a wide range of
parameters can be use. When printing solder paste
with stencils of normal thickness (~ 0.15 mm), it is
best to apply sufficient pressure to clear paste from
the stencil surface in order to avoid blocking paste
in the apertures but not so much that paste is
scooped from the them. In PumpPrinting, the
apertures are always partially blocked with wet glue
so a rather low pressure has little effect. Conversely,
if some glue is scooped from the top of the
apertures, there is still sufficient below it to achieve
a sound print.
However, a controlled, slow
separation speed improves print uniformity and 45°
squeegees ensure well-filled apertures. The
squeegees can be of metal or polyurethane, even the
metal squeegees causing no damage to the plastic
stencils if the stock material is chosen correctly. It
takes a few passes, (typically three to five), to
“ pump” glue down to the bottom of the finer
apertures but for the normal production run, a
single pass is sufficient. Under normal
circumstances, there is no need to wipe the
underside routinely. The glue does not normally
spread and there are no apertures close together
which might bridge. If the stencil should become
blocked by, say, debris picked up from the board
surface, a dry wipe will normally restart the
process. If the stencil is thoroughly cleaned, the
glue being removed from the apertures, it will be
necessary to perform the initial multiple passes to
re-pump the stencil.
Stencil cleaning at the end of the run can prove
troublesome by normal techniques. Simply wiping
the stencil with a solvent-soaked tissue does not
clear the apertures. Spray cleaning in a cleaning
machine may work but the solvents generally
simply bounce off the stencil without penetrating
the apertures. Blowing the apertures clear with a
compressed air line can be effective but time
consuming when there are many apertures and
creates a spray of fine glue spots. The most efficient
method to clear the apertures is to use a vacuum
under-screen cleaner, followed by a solvent rinse.
Suitable solvents can be obtained from the adhesive
suppliers. Alternatively, the “ universal” screen
cleaners manufactured for the industrial screen
printing industry, such as Sericol’s ZT 639, are
very effective. Common solvents such as isopropyl
alcohol are not normally sufficiently powerful.
If the printing machine requires fiducials in the
stencil for vision alignment, there can be difficulties
in making the fiducials visible and permanent. The
normal way to achieve this is by filling them with
a black epoxy material which contrasts well with
the stencil material. Unfortunately, the adhesion of
the fill to the stencil is not especially good and it
can fall out. One way to overcome this difficulty is
to make the fiducials small in diameter. (There is
no reason why the fiducials in the stencil need
match those on the board, except in position. Thus
a 3 mm cross on the board can be matched to a 0.5
mm dia circle in the stencil).
“S tars and S tripes”
A complementary technique, developed by DEK
USA, uses conventional laser cut stencils 0.25 mm
thick. The apertures for printing glue for the
normal, small chip components are rectangular
stripes, typically 40 to 50% of the component land
spacing in width and similar to the component
width in length. The apertures for the IC
components which require higher than normal
stand-offs are shaped rather like a clover-leaf or star
and laser cutting is used in the stencil manufacture
to ensure that these star shapes have sharp internal
spikes. These spikes lift the centre of the print by
some 50%, sufficiently high to give good contact to
the underside of SOICs.
If very tall deposits are required for large stand-off
components, the process is modified slightly to
include a second squeegee sweep. The first is at
normal pressure to fill the stencil apertures while
the second is at zero pressure to flood glue over the
stencil surface again. This has no effect on the
stripes for the standard components but the height
of the star prints can be increased by up to six
times the stencil thickness.
CONDUCTIVE
ADHES IVES
S OLDER REPLACEMENT.
FOR
There are, perhaps three reasons for replacing solder
paste with conductive adhesive,
1. the environmental issue of removing toxic lead,
2. because you have been told to,
3. because for a particular application it provides a
better solution than solder.
Concerning the first, as a colleague from exYugoslavia said, the lead in bullets is far more
dangerous and no-one is replacing that. The second
is difficult to argue against while the third is the
most sound. The assembly of non-solderable
silicon in a chip-on-board (COB) application would
be a good example of the latter.
Two basic types of conductive adhesive are
available, isotropic, conducting both horizontally
and vertically, and anisotropic, conducting only in
the Z axis. Anisotropic adhesive comprises,
essentially, a small number of conducting particles
mixed into a non-conducting adhesive. The
conducting phase is, thus, widely but thinly
dispersed. When applied to a component footprint,
the probability of sufficient particles forming a
conducting path between adjacent lands is
extremely small. Conversely, a component pressed
onto the lands is certain to trap particles between
each lead and the corresponding land and so form a
connection. Anisotropic adhesives should be very
easy to print. A thin layer of adhesive is printed
over the entire footprint area of each device. No
great alignment accuracy is required and as long as
the thickness is about correct, little can go wrong.
Because the area of each block of adhesive is
relatively large, a meshed screen is normally used
rather than an open stencil, the thickness of the
mesh preventing the adhesive being scooped from
the centre of the print. However, a number of
examples have been test printed which exhibited
very poor print qualities, tending to dry very
rapidly in the screen mesh. Some care should be
taken in the adhesive selection, therefore. The
screen will require relatively large apertures for the
conducting particles to pass through, the standard
recommendation being that the apertures should be
at least three, preferably five times the particle size.
Screens of 80 to 100 tpi of ultra fine mesh, (which
was specially made for solder paste printing), are
ideal, therefore, with an addition of 0.05 - 0.1mm
of emulsion.
The more normal, fully conducting adhesive is
widely used in thick film assemblies, going back at
least to the 1960s. It is the preferred method if chipand-wire hybrids are produced. It can also be used
for assembling components onto PCBs, especially
for COB. The conductive adhesive is printed onto
the track terminations, the components are placed
on/in it and the adhesive is cured. This will almost
certainly be at a lower temperature than solder
reflow but the time may be longer. Unlike solder,
none of these materials pull back onto the
terminations during "reflow" so their use at fine
pitches requires care. In addition, they are much
more likely to be sensitive to moisture than are
soldered joints. This becomes especially important
when the components or boards have base metal
terminations, such as tinned rather than palladium
silver terminations on chip components or hot air
levelled solder lands rather than gold over nickel.
Advice should be sought from the adhesive
suppliers on the compatibility of components,
materials and environmental conditions.
S creen and stencil recommendations
for regular, conductive adhesives.
For COB and similar applications, conductive
adhesives are printed quite thinly through screens of
around 150 to 200 tpi stainless steel or polyester,
with an emulsion thickness of 12 - 25µ. Screens
rather than stencils are chosen because a thin print
is required. It is clearly important not to drown the
thin silicon in adhesive so a deposit thickness of
around 0.05 mm is fairly typical. A stencil this
thin would be very fragile. It is important that a
reasonably thick deposit be laid down for solder
replacement applications, however, in order to
maintain satisfactory aged adhesion, so a metal
mask 0.1 mm thick would be a good choice.
As a design guide for COB and similar
applications, the print should be some 0.25 mm
bigger all round than the terminations of packaged
components, but equal in size to any bare silicon.
Note that all of these refer to the component
dimensions, not the board land dimensions.
For solder replacement applications, it is important
to remember that misaligned or spreading adhesive
will not pull back onto the lands during “ reflow”
so no excess adhesive can be tolerated. Unlike
solder paste, which contains around 50% volatile
material, adhesives do not reduce in volume on
curing. Finally, adhesives do not pull up the sides
of component terminations so no material need be
supplied to form these fillet volumes. Hence the
volume of adhesive printed will be considerably
less than the volume of solder paste which it
replaces. QFP and other IC aperture widths should,
therefore, be some 0.1 mm narrower than half of the
component lead pitch. For example, a 0.5 mm
pitch QFP would use stencil apertures [(0.5÷2)-0.1]
mm wide, i.e. 0.15 mm. Other components should
be reduced typically by 0.1 mm below the
matching board land dimensions.
Printer parameters.
For those applications using a metal stencil, the
normal printer parameters apply. The squeegee
should be of hard polyurethane or metal and, as the
stencils are not particularly thick, a 60° angle of
attack will be suitable. The pressure should, as
always, be set to clear the adhesive from the stencil
surface. The fluid nature will allow quite fast print
speeds to be used but there is little point in
printing faster than the rest of the production line
requires.
If a meshed screen is used for those applications
which require a thin print, the squeegee will
probably be of soft (60 - 75 shore hardness),
polyurethane at an angle of 60°. A metal squeegee
would damage a screen. Once again, pressure and
speed will be selected to clear adhesive from the
screen surface. Higher than normal pressure can be
used because the mesh in the apertures defines a
minimum print thickness. Unlike metal masks,
meshed screens should always be set with a small
gap (“ snap-off”), between the screen and the board.
This gap should be set in conjunction with the
squeegee speed to allow the screen to peel, not
snap, away from the board surface immediately
behind the squeegee. Typically, the gap will be
approximately equal to the screen width multiplied
by 0.005, but this will vary with print speed, print
area, adhesive tackiness, etc. This control takes the
place of the separation speed which is normally set
for metal masks.
The traditional objection to the use of screen
printing, - that only a single thickness can be
achieved - , has been overcome.
Design guides have been offered for such printing
and for solder replacement with conductive
adhesives.
CONCLUS IONS
The use of screen printing for surface mount glue
applications is a proven, long established but not
widely used production technique.
The increasing use of high speed chip placing
equipment calls for ever faster glue deposition. This
is easily achieved by screen printing.
Acknowledgements
Much initial work and characterisation of
PumpPrinting was performed at DEK by Colin
MacKay and Mark Whitmore. All of the work on
“ Stars and Stripes” was performed at DEK USA by
Ricky Bennett and Richard Lieske, to all of whom
grateful thanks.
Fig 1 Typical glue dots for holes 1 - 2 mm diameter
Fig 2 PumpPrint data for Amicon D 125 F3
Glue dot height/dia mm
2.5
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
0.5
0
0.6
0.8
1
1.2
1.4
Hole dia mm
1.6
1.8
2
Fig 3 PumpPrint data for Heraeus PD 922
Glue dot height/dia mm
2.5
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
0.5
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0
Hole dia mm
Fig 4 PumpPrint data for Heraeus PD 945
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
Hole dia mm
2
1.8
1.6
1.4
1.2
1
0
0.8
0.5
0.6
Glue dot height/dia mm
2.5
Fig 5 PumpPrint data for Heraeus 860002 S A
Glue dot height/dia mm
2.5
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
0.5
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0
Hole dia mm
Fig 6 PumpPrint data for Loctite 3612
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
Hole dia mm
2
1.8
1.6
1.4
1.2
0.8
0
1
0.5
0.6
Glue dot height/dia mm
2.5
Fig 7 PumpPrint data for Multicore S A 33 PS
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
2
1.8
1.6
1.4
1.2
1
0
0.8
0.5
0.6
Glue dot height/dia mm
2.5
Hole dia mm
Fig 8 PumpPrint data for Multicore S A 35
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
0.5
Hole dia mm
2
1.8
1.6
1.4
1.2
1
0.8
0
0.6
Glue dot height/dia mm
2.5
Fig 9 PumpPrint data for a poor glue
2
Glue dot height mm
1.5
Glue dot dia. mm
1
Linear (Glue dot
height mm)
2
1.8
1.6
1.4
0.8
0.6
0
1.2
0.5
1
Glue dot height/dia mm
2.5
Hole dia mm
Fig 10 Extended PumpPrint data
1.8
1.6
Glue dot height (mm)
1.4
1.2
1
0.8
0.6
0.4
Amicon D 125 F3
0.2
Multicore SA 35
Multicore SA335
0
1
1.5
2
2.5
Hole dia (m m)
Note
The following glues have been found unsuitable
Heraeus 944
(a high speed dispensing glue, tending to form “ dough-nuts”)
Loctite 3611
(a pin transfer glue, stringing and slumping badly)
3