How to classify PCB surface coating (plating)?

According to manufacturing technology methods, it can be divided into two types: surface coating layer metal surface plating layer.

①Surface coating layer

The surface coating layer refers to a physical coating on the surface of a fresh copper connection disk with a heat-resistant solderable covering layer. Such as the earliest use of natural rosin, various synthetic rosin-like substances (including a variety of flux) to OSP (organic solderability protection agent). Their main feature is the ability to protect form fresh (non-polluting non-oxidizing) copper surfaces before during soldering to provide a direct solder connection. Also hot air solder leveling (HASL) is also applied, but it begins to form a "temporary steady state" CuxSny intermetallic compound (IMC) during the HASL process. The solder is soldered to the CuxSny IMC.

② Surface plating layer

Surface plating refers to the formation of a thin layer of heat-resistant solderable metal on the surface of fresh copper land by electroless plating electroplating, such as electroplated gold, electroless tin, electroless silver, electroless nickel-gold , Electroless nickel-palladium-gold, electroless nickel-palladium, electroless palladium, etc. Their main feature is that they can protect form fresh (non-polluting non-oxidizing) copper surfaces metal barrier layers before during soldering to ensure that the solder can be soldered to the copper surface barrier surface.

Classified by application (welding) results, these surface coatings (plating) can be divided into three categories:

①Surface coating (plating) coating on the barrier-free layer by soldering

The main feature of this type of surface coating (plating) is that it is squeezed away the copper surface by the molten solder during high temperature soldering floats on the solder surface is thermally decomposed both, but the connection interface of the solder joint will form The temporarily stable intermetallic compound (IMC), which causes hidden troubles during application, such as natural rosin, synthetic rosin-like substance (including various fluxes), OSP (organic solderability protection agent) , Electroless tin plating, electroless silver plating, etc.

②Surface plating layer soldered on the diffusion layer

In order to eliminate the temporary stable intermetallic compounds (IMC), copper was first plated with thick gold as the surface coating layer, but practice application have shown that: (1) diffusion between gold copper is prone to occur, that is, gold atoms will diffuse Into the copper crystal structure, copper atoms will also diffuse into the gold crystal structure. This is because gold copper are both face-centered cubic crystals, the melting point atomic radius are very close, so diffusion easily occurs; (2) gold -The diffusion layer between the copper interfaces is prone to internal stress. This is because the thermal expansion coefficient of copper is greater than the thermal expansion coefficient of gold, the crystalline structure of the diffused gold-copper interface will inevitably be deformed due to internal stresses extruding each other, causing looseness brittleness Problems such as cracks, then cause line failures, this situation has had profound severe historical lessons

③Surface metal plating layer soldered on the barrier layer

 The main feature of this type of surface coating is that during the high temperature welding process, the solder is welded on the surface of the metal barrier layer instead of directly on the copper surface, so the connection interface of the solder joint will form an unstable Intermetallic compounds do cause diffusion between metals: such as electroplated nickel-gold, electroless nickel-gold, electroless nickel-palladium-gold, electroless nickel-palladium, electroless palladium, etc. Since the barrier layer is metal, all are formed by electroless plating electroplating, it can also be called a metal surface plating layer. pcb customization

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