Wetting generally describes the behavior of liquids on solid surfaces. This plays a role in applying the PCB surface finish to the copper. Later, the behavior of solder on the surface determines the proper PCB assembly.
It is desirable to have complete and uniform wetting. While the surface of chemical gold (ENIG) is very flat, the tin in the HAL process tends to drip or run along the edges of drilled holes (see HAL vs. chemical gold).
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