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Sn 119 Pb 207
Alloying is a
term used for
a meltcombination
of two or more
metals…
Pb
Phase
Diagram
Solidus and liquidus lines meet at 183C
Sn
Alloying brings down phase transformation and therefore change the physical properties
like M Pt
The main purpose of alloying is to reduce the melting point through eutectic or near
eutectic phase formation
Literature data shows maximum shear strength of Sn/Pb alloy at 63/37 eutectic
Temperature Profile in Reflow soldering
(IR soldering)
N2 atmosphere Reflow
30-60s typically
Typical times for
Pre-Heat is 90s
(2°C/sec)
Soaking is 90-120s
(0.5°C/sec)
Reflow is 30-60s
(1.5-2°C/sec)
Cool gradually
(3°C/sec)
Flux based soaking times
1. Low activated paste: 90sec
2. Standard: 60sec
3. Highly activated:30 sec
A typical thermal profile for reflow soldering
Figure: Wikimedia Commons
•  Tg of substrate
•  Melting temperature of solder alloy used
•  Component failure temperature, time etc
•  Solder alloy composition
•  Population density of components on PCB
A PCB assembly has to utilize reflow soldering (IR). The following details are available for
the process:
1. This process is NOT done in N2 atmosphere. Temperature in reflow zone can be +/1°C
2. FR-4 substrate is used with a Tg of 160°C & PCB has Sn plating finish
3. Solder paste (with MA flux) is SAC 305 alloy (3% Ag and 0.5% Cu)- MP of 217°C
4. 100% SMD board assembly; 4 high pin-count PBGAs used; others are SOIC,
passives etc.
5. The most vulnerable components among these can withstand 235°C for 60
seconds.
6. It is recommended to have one-step reflow only. BGAs are large area (43x43mm).
7. Draw a graph (time Vs temp) for the thermal profile and answer the questions
below.
Q s: 1) Suggest start and end temperatures from RT to peak reflow.
2) Mention heating rates in each zone including cooling zone.
3) Indicate board dwell times in ALL zones.
This problem will be worked out during this session
Issues with IR Reflow - thermal shock
Solution? - correct thermal profile specific to the paste
Homework problem
Assume a process not done in N2 atmosphere; temperature variation in
reflow zone can be +/- 1°C
High Tg FR-4 substrate with a Tg of 190°C used
Board has HASL finish of 63:37 with MP of 183°C
Solder paste material used is Pb-free with a MP of 218°C
SMD components used; no C4; BGA- 2 nos. large area, high pin-count
The most vulnerable component can withstand 230°C for 30 seconds
Draw a reflow soldering thermal profile for the above
situation. Mention heating rates and dwell times aptly.
[email protected]
• Solder as quickly as possible
• Use lowest possible temperature that
yields acceptable joints
• Avoid repeated soldering; this will
increase the thickness of the IM layer
• IM layer grows at any temperature;
higher the faster
Cu6Sn5 and Cu3Sn
Tin rich
Solderable
Copper rich
Brittle and nonsolderable
•  Intermetallic growth is common
in all solid-state systems.
•  Caused by solid-state diffusion
of one metal into another metal s
lattice.
•  Rate of diffusion depends on
the crystal structure of each of
the metals
Some common electronic solder alloys and their designations
Tin
%
Lead
%
Silver
%
MP
°C
Application areas
62
36*
2
179
SMD
63
37*
--
183
SMD/ Hybrids
60
40*
--
183-191
SMD/ Hybrids
96.3
--
3.7
221
High-melting lead free
10
90
--
275-302
BGAs
3
97
--
314-320
C4 flip chip
5
93.5
1.5
296-301
High temperature
component
manufacture
*Indicates that it contains 0.2 to 0.5% Antimony (Sb)
for improvement in low temperature performance
Best Alternatives for Lead
96.5% Tin; 3.0% Silver and 0.5% Copper adopted by Japanese
95.5% Tin; 3.9% Silver and 0.6% Copper adopted by NEMI
SAC 305 alloy
SAC 405 alloy
• Silver provides mechanical strength; improves resistance to fatigue
from thermal cycles.
• Copper lowers the melting point, improves resistance to thermal
cycle fatigue and improves wetting properties of solder
• Bismuth lowers the melting point and improves wettability
• Indium lowers the melting point and improves ductility
• Zinc lowers the melting point and is low-cost. But is susceptible to
corrosion.
• Antimony is added to increase strength.
Lower Melting Point lead-free alloys
Most lead free Solder alloys melt at temperature higher then Sn-Pb
- To lower the melting point, use Indium or Bismuth
- Possess better rework/repair characteristics
INDIUM Alloys :- 52% In and 48% Sn 118°C MP
•  Useful for temperature sensitive components
•  Cost and Availability is a problem
BISMUTH Alloys :- 42 % Sn and 58% Bi 138°C MP
•  Cost comparative to TIN
•  Creates brittleness.
•  Poor fatigue Resistance
Disadvantages : 1) Reduction in wetting 2) Special Flux required