PCB Technical - PCB surface coating function and selection principle

Due to its excellent electrical conductivity and physical properties, copper is selected as a conductive material by PCB (Printed Circuit Board). However, when exposed to the air, the copper surface tends to be oxidized, and a solid and thin oxide layer is formed on the surface, which mainly leads to defects in the solder joints, which reduces the reliability of the product and shortens the shelf life. Therefore, it is very necessary to implement protective measures to prevent the copper surface from being oxidized. This is the original reason for the appearance of the surface coating, which should be heat-resistant and solderable. So far, PCB surface coating has developed rapidly, and a large number of classifications have been generated. How to choose the right type is still very important. Therefore, this article will discuss the PCB surface coating function,

What is PCB surface coating?

•The importance of PCB surface coating

In order to prevent the copper surface of the pad on the PCB from being oxidized and contaminated before solderability, it is important to apply a surface coating (also called surface treatment) on the copper to protect it.

Copper has the second best electrical conductivity and physical properties (the best is silver) plus its abundant storage and low cost, so copper was chosen as the conductive material for PCBs. However, as an active metal, copper is easily oxidized, and an oxide layer (copper oxide or cuprous oxide) is prone to appear on the surface, leading to solder joint defects, reducing product reliability and shortening the shelf life.

According to statistical data, 70% of the defects on the PCB are from solder joints for the following reasons:

Reason #1: Pollution and oxidation of the pads on the PCB will cause incomplete soldering and cold solder joints.

Reason #2: Because the diffusion between silver and copper tends to produce a diffusion layer, and the intermetallic compound (IMC) tends to produce between tin and copper, the interface is loose and fragile.

Therefore, the copper surface to be soldered should realize a protective layer with solderability or isolation function, so that defects can be reduced or avoided.

•Requirements for PCB surface coating

The surface coating on the PCB pad should meet the following requirements:

one. Heat resistance

At the high temperature during the soldering process, the surface finish should also be able to prevent the PCB pad surface from being oxidized and make the solder directly contact the copper.

The heat resistance of organic surface coating refers to the performance of melting point and thermal decomposition temperature. The melting point of the surface coating should be close to or lower than the melting point of tin, and its thermal decomposition temperature should be much higher than the melting point and soldering temperature of the solder. As a result, oxidation does not occur on the copper surface during welding.

Can cover

Basically, the PCB surface treatment can completely cover the surface of the copper pad without being oxidized or contaminated before and during the soldering process. It will not drift, decompose or float on the surface of the solder joints. Therefore, in order to ensure that the molten solder can be completely soldered to the pad, the surface tension of the molten surface coating should be small and the decomposition temperature should be high, so that high solderability can be ensured before and during soldering.

Residue

The residue here refers to the residue left on the surface of the pad or solder joint after soldering. Generally speaking, residues are dangerous and should be eliminated, which is why cleaning measures are usually carried out after soldering.

Corrosive

Corrosion here refers to the corrosion of the surface of the circuit board caused by the residual solder after soldering, such as the PCB substrate material or metal layer.