Requirements for PCB surface coating (plating)

In order to ensure that the copper surface of the solder pad on the PCB is not oxidized and polluted before soldering, electric circuit board must be protected by a surface coating (plating) coating, and the surface coating (plating) coating must meet the necessary and sufficient conditions to achieve the purpose.

From the process of soldering components on the PCB pad and the inspection results, the requirements for coating (plating) the top surface of the PCB pad are mainly as follows.

Heat resistance

At the high temperature of soldering, the surface coating (plating) coating can still protect the copper surface of the green solder mask PCB pad from being oxidized and allow the solder to enter the copper (or metal) surface to achieve the connection. The heat resistance of the organic surface coating (plating) coating refers to its melting point and thermal decomposition (evaporation) temperature performance. Its melting point should be close to or slightly lower than that of the solder (tin), but its thermal decomposition temperature (≥ 350 ℃) should be much higher than the melting point and soldering temperature of the solder to ensure that the copper surface does not oxidize during soldering. The heat resistance of the metal surface coating does not have this problem.

Coverage

The organic heat-resistant solderable coating (including flux) can be completely covered on the surface of the copper pad before welding and during the welding process without being oxidized and contaminated, and only after the molten solder is soldered to the surface of the copper pad. Swim away, decompose and evaporate, and float (cover) on the surface of the solder joint. Therefore, in order to ensure that the molten solder is completely welded on the connection pad, the surface tension of the molten organic surface coating layer must be small and the decomposition temperature must be high to ensure good coverage before and during welding. At the same time, its specific gravity is much smaller than that of molten solder (tin) to ensure that the molten solder is squeezed and penetrated onto the copper surface, so the coverage of the organic surface coating layer refers to its surface tension, specific gravity, etc. at the soldering temperature. Performance. The metal surface plating layer is partially melted into the solder during soldering or on the surface of the barrier layer to achieve the connection.

High Frequency PCB

The remains

The residue of the organic heat-resistant solderable coating (plating) coating refers to the residue on the solder pad or the solder joint after soldering. Generally, these residues are harmful (such as organic acids or halides, etc.) and should be removed, so cleaning measures should be used after welding. Now there is no-clean welding technology, that is because the residue of the organic surface coating (plating) coating after welding is very small (most of them have been decomposed and volatilized).

Corrosive

The corrosion resistance of the organic heat-resistant solderable coating (plating) coating refers to the corrosion phenomenon on the high TG PCB surface after soldering, such as corrosion to the surface of the PCB substrate and the metal layer. This is because there are more or less halides or organic acids in the organic heat-resistant solderable coating (plating) coating (mainly to further remove residual oxides and pollutants on the copper pads), but these acidic The presence of substances after welding is harmful. In addition to decomposition and volatilization, the deficiency must be cleaned and removed.

Environmental protection

The environmental protection of the surface coating (plating) coating means that the waste water generated during the formation of the coating layer and the waste liquid for cleaning after welding should be substances that are easy to handle, low in cost and not pollute the environment.