effect of silver in common pb-free solder alloys

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


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)


Melting and freezing characteristics of

SnAgCu alloys

• Study by NIST

– SAC305 and SAC405

are near eutectic

– 217ºC eutectic

reported for

SN3.7Ag0.9Cu



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)



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.


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


SnAgCu alloys



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.


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


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.


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


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


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.


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


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.


For more information on ALPHA

products click the link below:

ALPHA Products

http://blog.cooksonelectronics.com/how-do-you-add-value-to-your-assembly-process/?utm_campaign=All-Products

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