The material pads on the PCBA board are black, which may be caused by the following reasons:
1. PAD oxidation
2. The problem of solder paste
3. The problem of the PCB board itself
PAD oxidation will also cause the blackening of the pad, the processing method, try a more active solder paste; or, adjust the furnace temperature curve, use the RTS furnace temperature curve to shorten the time below 180 degrees, properly increase the reflow temperature ;
Solder paste problem
There is only one solder paste on the solder joints. There is only one reason: profile wetting time is not enough (melting time above the melting point). It is not necessarily a profile problem. You have to take some time to find the root cause.
PCB board itself
The cause of the solution used by the PCB manufacturer is that the gold is contaminated. Processing method, PCB production open N2 will be gone.
There may be more reasons for the blackening of the pads on the PCBA board. It can be checked one by one in each process. Maybe the soldering time of the soldering group in the back soldering group is too long? So it is not by everyone's guess. Check it out on your own!