Bev

- Grain size is a bit difficult to characterize since we don't all agree what the grain size is anyway (or at least I don't agree with many others). As much as I saw so far grain size depends on the production parameters and on the temperature, strain, time history a solder joint has seen after soldering.
- Tin and lead dendrites. Quite the same as above. The presence of dendrites tells part of the story of the production process but who cares?
- Structure. Same again.
- Regarding the voids I have always problems. They can be an indicator that something in the soldering process ( parameters , materials) changes if their size and amount change over time. But it is quite difficult for me to look at a solder joint and say this is too much voids. Especially if the customer asks me why.

Since the structure ( grain size, dendrites secondary segregation etc.) are determined by the solidification process you will find various structures in solder joints on the same PCB because of the different thermal mass of the components. But, as you said, the structure will become similar in the joints after a few thermal cycles or due to diffusion. 
So I believe that it is most important to show that a metallurgical bond took place ( intermetallics), that the requirements of the amount of solder have been met, that the connector caps of some critical components ( AgPd ) haven't dissolved completely and that no cracks are in the joint (after soldering).


Best regards


Guenter

.

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Dipl. Eng. Guenter Grossmann

8600 Duebendorf
Switzerland

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

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