DIMM connector with poor solderability

Hello Dears, I'm a small CS Engineering at Taiwan.There are some problem on DIMM connector area with poor solderability at ODM. It was random on Dimm connector area and the SMT area is fine. At the same time, there were other PCB vendor's product which didn't find the same problem.

But the customer won't tell us did they change the flux or not at wafesolder. The flux's name is SM-816. The OSP we used is Enthone. The PCBA couldn't be reworked by many ways we did every thing we could. The NPL is about 10~20%.

After this customer complain, the PCB returned and we reworked it with F2(LX) and sent back to customer. At first, they tried 50pcs and found 1pcs is not OK. The nonOK product can be reworked. So the customer keep produced. The nonOK PCBA numbers is increasing and can't be reworked by the many ways. There's no message we could found the X-section PCBA: No crack in the hole. The PTH copper thickness is enough. The drilling is ok. The EDS just found C,O and Cu.

The customer said they adjusted the temperature of wavesolder machine and the speed of the line and every thing they could but it still happened.

Would you like give us some advise except change the customer..........? Thanks.

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