Although the correspondence under this title has been very interesting and 
informative this string drifted somewhat from the original question,  which was:

From: "Kane, Amol (349)" <[log in to unmask]>
Subject: SN100 for Reflow Application
X-To: TechNet E-Mail Forum <[log in to unmask]>,
Dear technetters,
Is anybody using SN100 (or other high Sn alloys) for SMT reflow soldering 
applications? what have your experiences been with this alloy?

I have a PCB with a BGA, that is cracking due to mechanical stress (that is 
not SMT process related), we have a combination of solder cracking, and pad 
cratering. The PCB currently uses SAC305. We also build a SnPb variant of the 
same exact board with no issues (the bare board material is different for SnPb 
and LF versions). I was therefore wondering whether SCA305 alloy properties 
could be a contributing factor this defect. How does the ductility and shear 
strength of SN100C compare to SAC305 and 67-37SNPb alloy?

To return to the original question, since it has not been clear from the 
correspondence exactly what the nature of the crack was I cannot say with any 
certainty whether the result would be any better with SN100C than with SAC305.   
However, to answer the specific question on the ductility and shear strength of 
SN100C, it is a more ductile alloy than SAC305 and has a lower flow stress 
and in that regard is more like 63/37 SnPb.   If the cracking occurred in a 
situation where the joint was subjected to substantial strain then the greater 
compliance of the SN100C could mean that the solder would accommodate that strain 
without cracking and without transmitting the stress to the substrate or the 
component.  Cratering can be a symptom of unrelieved stress being transmitted 
into the laminate instead of being absorbed by deformation of the solder 
itself, e.g. the cratering failure with area array devices in dropped cell phones 
with SAC305 joints.   Cracking of chip capacitors can be another consequence of 
strain being transmitted rather than accommodated by the solder.

In regard to a later comment on the poor results obtained in reflow with the 
99C alloy (Sn-0.7Cu), although SN100C is based on that same alloy 
microalloying with Ni and Ge substantially changes the behaviour of the alloy, as has been 
well documented in several peer-reviewed scientific papers.  Reflowed SN100C 
joints are smooth, bright and generally free of shrinkage defects and can be 
difficult to distinguish from 63/37SnPb joints. 

The higher melting point of SN100C (227C/440F compared with the 
215-220C/419-428F of SAC305) has been raised as a concern but the experience of the 
increasing number of lines running with SN100C paste indicates that because of the 
modifying effect of the microalloying additions the alloy needs less superheat 
(excess of the peak temperature over the melting point) than SAC305.   For 
reflow lines reflowing SAC305 with a peak temperature around 240-245C/464-473F, 
which seems to be a profile very commonly used with that alloy, SN100C has 
proved to a "drop-in-replacement".   However, depending on the Delta T, we would be 
more cautious about suggesting SN100C would be a drop-in-replacement for 
SAC305 for lines running with a peak temperature 230-235C/446-455F.   However, 
results obtained with the new generation of vapour phase reflow systems indicate 
that good results can be achieved with SN100C with a liquid with a boiling 
point of 235C/455F.

Keith Sweatman
Nihon Superior Co., Ltd
    
    
    



    
    
    





   

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