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Uneven tin plating thickness (MSA base)

2006

HI, I'm doing chip resistor barrel plating (Ni/Sn).
Recently I faced problem for uneven tin thickness. Top faced very thin tin layer ( 0~1 µm) but terminal side extremely high thickness ( 20~30 µm). Bottom is 5~ 7 µm.
Suppose top is HCD area but how come become thin thickness.
Is the imbalance of wetting agent & brightener? Will poor filtration will cause uneven thickness?
Plating temperature ( control at 22°C). Tin control at 12 g/l, acid control at 185 g/l. Barrel speed at 26 rpm.
If during tin plating with stop line, any problem to create de-wetting during solderability?

Thanks,

Vincent Loh
- Malaysia