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October 30, 2013

What is a Soft Solder Joint?

We place many surface mount connectors. One of the methods of inspection is testing the sheer strength of the formed joint.   

Where can we find specifications to determine if the joint meets any standard for sheer strength.  

When force is applied to a pin using a metal probe and the pin moves, our customer rejects it calling it a "soft solder" joint.  

Is there such a thing as "soft solder." And if so what is the method of determining soft solder?

M.E.

Experts Comments

According to Howard Manko, Solders and Soldering, page 142, the Joint strength of a lap solder joint is ((Length of the joint x yield strength of the solder x the Width of the lead) or stated differently Strength = s x critical area where s is in either shear or tension according to the configuration.  

From table 5-3 the shear strength of 63/37 is 5400 psi, the shear strength of 95Sn5AG is 6000 psi therefore using a lead of .020" wide and .10" long the formula from above the answer for tin/lead is 10.8# and for the lead free is 12#.  

When applying forces with a probe, consider the size of the probe and size of your hand and the size of your forearm, and you are probably applying over 1000# of force to that solder joint, no wonder it moves.  

The strength of the solder is only that ,the strength of the joint, and it is good for comparative purposes only. Solder joints don't fail that way, they fail by fatigue and stress loading, elastic failures. These elements are tested using thermal cycling and or comparison of Coefficient of thermal expansion and such.  
Soft solder is any solder that melts below 800F, above that temperature the process is brazing and above those temperature the process is welding.  

Probing the solder joint is not the way to test whether a solder joint is good or bad as this is a bad practice and should be avoided.  

I hope this information is useful and anyone wants to discuss this further, please contact me at your convenience.
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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
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