Jonathan

Here what I can say from your picture:

Lead made of copper.
Clipping operation pretty rough. We already had cases, where the clipping operation by hand caused the kind of cracks ( in the intermetallics) as seen on your picture.
However, I believe that you have the cracks before you clip the leads. In this case it looks to me like no wetting.
This could mean:
- Insufficient tinning of the leads --> oxidised copper
- Thin tinning --> Intermetallics
- Old components --> Intermetallics
- Contamination

I suggest you let someone have a look into the solder joint ( microsection ) to find the cause of the defect. Just changing the preheat temperature doesn't seem a good idea to me.

Have a great day

Guenter

Guenter Grossmann
Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland

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

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