I received some PCBA assembled at one of our contract manufacturer This is
double sided / mixed tech assembly. There are some locations where one
could either use a BGA or a QFP. Currently we are using the BGA version.
The stencil does not have aperture opening to print QFP ( they use
different stencil ), however on the QFP pads, I see some purple / brownish
pads, almost exhibiting corrosive traits. The PCB are supposed to immersion
silver. I see some small solder balls here and there. Test via pads look
good. The SMT solder joint has transparent residue typical of No clean paste

The contract manufacterer should be using eith AIM NC 253 or 293 or 291 or
Indium NC SMQ 92J.

I can share some photos. Any ideas as to what could have happened /
chemistry guess as to the flux / paste ? I am still in the preliminary data
collection mode

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