Patty

It is close to impossible to say anything without seeing the solder joints. Possible reasons for the joint failure have already been named
- Low cycle fatigue due to thermal cycling
- Fatigue due to vibrations
Or a combination of both. What I found is, that if the fractures are to a considerable amount due to vibrations one finds damage in the leads in the upper bend towards the package as well.

The failure due to residual stress in the joints due to the movement of components during the reflow process sounds somewhat out of the way to me. Solder creeps fast enough at room temperature that nearly no stress will be in the joint after 1 or 2 hours.

Using a more ductile solder doesn't help a lot. Degradation is strain driven. So, if you have a ductile solder or a stiff one doesn't change much if there is enough time to relax the stress. Try to think in strain, forget stress. What you need is a material that is fatigue resistant, however I couldn't name you one. SnPb solders are already very good.

Maybe you could give some more hints about the mission profile of the PCB's. Or you might even have pictures (Microsections, Stereo microscope, SEM) to set on Steves homepage?


Guenter



EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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