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Information on the effect of Silver in Common Pb-Free Solder Alloys.
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an Alent plc Company
Effect of Silver in Common Pb-Free
Solder Alloys
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Silver (Ag) in Electronic Solder
• Used in electronic solder for over 50 years
• Reduces Ag scavenging from Ag plated parts
• Improves Thermal Fatigue Resistance
• Good electrical and thermal conductivity
properties
• Wets well to most common PCB surface finishes
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Ag in Pb-free Solder
• Key Advantages Low liquidus temperatures
Lower overall operating temps
Higher Ag results in lower surface tension
Better overall soldering performance vs. low and no Ag alternatives
Better thermal fatigue resistance vs. Ag-free bearing alloys (ie SN100C)
• Disadvantages • Ag has the greatest impact on
cost of the solder
– Price fluctuations of 30%+ in ’08
• Higher Cu Dissolution rates
• Higher Drossing rates
• Reduced high strain or “drop shock” reliability – (critical to SMT devices)
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Melting and freezing characteristics of
SnAgCu alloys
• Study by NIST
– SAC305 and SAC405
are near eutectic
– 217ºC eutectic
reported for
SN3.7Ag0.9Cu
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Melting and freezing characteristics of
SnAgCu alloys
• DSC plots of SAC305, SAC105 and
SAC0307 indicate single high peak
for SAC305 but lower “transition”
peaks for lower silver Pb-free alloys
• This is verified by a phase diagram for an
isothermal section set between the two
transition peaks (223ºC) showing a small
liquid only window at lower Ag levels and
a higher presence of solids
200 210 220 230 240 250
10
20
30
40
50
60
70
80
SAC305
SAC105
SAC0307
Heat
Flo
w
Temperature (oC)
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Melting and freezing characteristics of
SnAgCu alloys
• Among the Ag bearing Pb-free alloys there is a significant differences in microstructure of solid soldered joints even though alloy compositions are not that different
• Coarse Sn dendrites for
SAC305 are different from fine
Sn dendrites for Sn3.9Ag0.6Cu
• Sn3.7Ag0.9Cu shows an Sn
dendrite pattern similar to
SnAg eutectic while
Sn3.6Ag1.0Cu microstructure
does not appear dendritic at all.
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
0.2oC/min 1.2oC/min 3.0oC/min
Sn3.8
Ag
0.7
Cu
Sn2.5
Ag
0.9
Cu
0.2oC/min 1.2oC/min 3.0oC/min
Sn3.8
Ag
0.7
Cu
Sn2.5
Ag
0.9
Cu
Below are micrographs of cross-sectioned solder joints
formed with two SAC alloys. Reflow temp was 240ºC. 3
different cooling rates were used
A combination of
high silver content
and slow cooling
rate results in the
growth of large
size Ag3Sn
platelets
Melting and freezing characteristics of
SnAgCu alloys
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Melting and freezing characteristics of
SnAgCu alloys
• Ag3Sn has a higher melting temperature, thus these platelets start
precipitating and growing while solder is still in the liquid state.
Sometimes the Ag3Sn platelets can grow so large in the liquid stage
that when solder shrinks during freezing, the Ag3Sn platelet
protrudes outwards severely deforming the solder interconnection.
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Wetting Behavior of Pb-free Alloys
• An optimized balance of Ag and other additives helps to lower the surface tension of solder alloys
• Ag-free alloys have a higher surface tension at standard operating temperatures
Lower surface tension results
in faster wetting
contributing to better
soldering performance
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Wetting Behavior of Pb-free Alloys
• There is a significant
difference in the wetting times
of alloys with different levels of
Ag content when the test is
carried out at a 250oC pot
temperature
• The difference in wetting
times of these alloys at 260ºC
is small in absolute values but
the trend is the same
Another wetting balance test was run with SAC alloys with the Ag level
varying from 0% to 1%. The test was run at two pot temperatures,
250ºC and 260ºC.
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Effect of Ag on Copper Dissolution
Time required for 50µm
copper wire to dissolve
in 260ºC molten alloy
bath.
• Ag increases Cu erosion rate
• Cu, Ni and Co reduce the erosion
rate
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Effect of Ag on Wave Soldering
Temperatures
• As mentioned earlier, Ag lowers the surface tension of Pb-free alloys – This allows faster wetting at lower
operating temperatures
Higher Ag alloy
exhibits much better
overall hole fill
0.3% Ag
SAC alloy
3% Ag SAC alloy • 6 layer, 2.4mm
thick board w/ OSP
pad finish
• 260º C pot
temperature
• 3.9 second contact
time
• 3 different alcohol
based fluxes
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Ag bearing SAC alloys have lower liquidus temperatures than Ag-free
alloys and require overall lower operating temperatures
Higher operating
temperatures can
damage PCB
laminates
Weight loss due to thermal decomposition of laminate materials
Source: Isola
PCB’s absorb moisture and can be damaged
during processing by the release of high
pressure water vapor.
Lower Operating Temperatures Reduce these Risks!
Vapor pressure of water
increases exponentially
above 250ºC.
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Effect of Adding Bismuth (Bi) to a
Ag-bearing Pb-free Alloy
• Bi contributes to a refinement of grain structure in SnAgCu alloys reducing stress at grain boundaries
• This reduces stress build-
up along grain boundaries
during temperature cycling
• It also reduces grain
coarsening
Both of these would result
in improved thermal fatigue
resistance
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
Summary of the Effects of Ag in Pb-free
Alloys
• High-silver SnAgCu alloys perform poorly in high-strain rate situations (i.e. drop shock)
• High-silver SnAgCu alloys exhibit better thermal fatigue resistance than lower or no Ag alternatives.
• Silver lowers the required operating temperature for a given soldering application
• A small amount of silver shows noticeable improvement in wetting characteristics of the solder.
• Addition of grain refining elements such as Bi changes the microstructure and improves the thermal fatigue resistance of low silver alloys.
HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
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