Hi,
I believe you have classic gold imbrittlement here. With 80 microinches of gold you are way over the limit. You can try to reflow longer time and at a higher temperature. The gold does not melt into the solder joint. The gold dissolves into the solder joint. By raising the reflow temperature or increasing the reflow time above liquidus, the dissolution rate of the gold increases. You want to dissolve most of the gold into the solder joint and reflow or attach to the nickel below the gold. You do in essence create sort of a new alloy. You have a solder joint embrittled with tin/gold needles. I suspect that your customer will have a difficult time reflowing the solid solder deposits. No matter what you do, you will have gold embrittled solder joints. In fact the solder joints will probably look even worse if you raise the temperature or increase the reflow time. This will dissolve more gold into the solder joint and gold embrittle the solder joint even more.
If you print more solder paste or increase your stencil thinkness, your solder joints will look better. This is because you decrease the gold content dissolved into the 63/37 paste by increasing the 63/37 volume. This is also the reason ENIG (thin gold) looked better. The thin gold was probably under 10 microinches thickness.
If you want to create a good quality solder joint that looks good and performs well, decrease the gold thickness. Go to selectively plated gold. Use Indium / Lead alloys if you must solder to thick gold. That presents an entire new set of problems and processes.
Good luck,
Chris
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