Copper has been selected as conductive material by PCB (Printed Circuit Board) owing to its excellent conductivity and physical performance. However, copper surface in Flexible PCB manufacturer’s factory tends to be oxidized when exposed to air with a solid and thin oxide layer caused on the surface, which mostly leads defects to occur to solder joints and thereafter will reduce product’s reliability and shorten shelf life.

Therefore, it’s of quite necessity to implement protective measures to stop copper surface from being oxidized, which is the original reason for the advent of surface coating that should be heat-resistant and solderable. Up to now, PCB surface coatings in Flexible PCB manufacturer’s factory have rapidly developed with numerous classifications generated and how to select a proper type still matters. Thus, this article will discuss PCB surface coating functions, classifications and introduce some selecting principles.

To stop the copper surface of pad on 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 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 PCB boards derive from solder joints due to the following two reasons:

Reason#1: The pollution and oxidization occurring to pad on 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 solderability or isolating function so that defects can be eased or avoided.