•Significance of multi-layer PCB Surface Coating

To stop the copper surface of pad on multi-layer PCB from being oxidized and polluted prior to solderability, it’s of dramatic significance to apply surface coating (also called surface finish) on copper to carry out protective function.

Copper features the second best conductivity and physical properties (the top best is silver) plus its rich storage and low cost, so copper has been selected as conducting material for multi-layer PCBs. Nevertheless, as a type of active metal, copper becomes oxidized so easily that oxide layer (copper oxide or cuprous oxide) tends to be generated on the surface, causing defects on solder joints that will reduce product’s reliability and shorten shelf life.

Based on statistics, 70% of defects on multi-layer PCB boards derive from solder joints due to the following two reasons:

Reason#1: The pollution and oxidization occurring to pad on multi-layer PCB tend to lead to incomplete soldering and cold solder joints.

Reason#2: Diffusion layer tends to be generated due to diffusion between silver and copper while inter -metallic compound (IMC) tends to be generated between tin and copper, leading interface to be loose and fragile

Therefore, the copper surface ready to be soldered should implement protective layer with solder ability or isolating function so that defects can be eased or avoided.