As you know, the HASL is the most popular PCB surface treatment,exposed copper maybe be coursed by this method.
HASL is that input the PCB in molten solder (63SN/37PB), then use hot air to blows off the excess solder in the surface and metal-based inner hole of printed circuit board to get a smooth, uniform and also bright solder plated layer. After HASL surface treatment of printed circuit board the plated layer should be bright and uniform integrity, good weldability, without nodules and half-wetting,plated layer is complete no exposed copper.
After HASL, pad surface and the metal-based inner hole are exposed copper is an important test of finished product defects, also is the one of main reasons of HASL rework. please see the below several reasons for exposed copper:
1. Pad surface dirty, contaminated with residual solder pad.
2. Pre-treatment is not enough, coarsening bad.
3. Flux activity is not enough.
We should control the technology of exposed copper of HASL surface treatment, reduce the Rework and poor circuir boards, ensure the quality of products.
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