PCB surface treatment process

- Feb 03, 2018-

Hot air leveling HASL, hot air solder.

Hot air leveling and hot air solder leveling, it is coated on the PCB surface heating molten tin lead solder and compressed air leveling play (flat) process, to form a layer of copper oxidation resistance and can provide good weldability of the coating layer.The solder and copper are formed at the junction of the hot wind and the copper is a compound of copper, and the thickness is about 1~2mil.

The PCB will be immersed in the molten solder during the hot air conditioning. The wind knife will blow the liquid solder before the solder solidifies, and it will be able to minimize the bending of the solder on the copper surface and prevent the welding bridge.

The hot air is evenly divided into vertical and horizontal types, which is generally considered to be good, which is mainly composed of horizontal hot air leveling coating, which can realize automatic production.The general process is: microerosion -> preheating -> coating flux -> spray tin -> cleaning.

Fold organic coating OSP.

OSP is different from other surface treatment processes: it ACTS as a barrier between copper and air;In a nutshell, the OSP is a chemically modified layer of organic skin on the surface of clean, bare copper.This film has anti-oxidation, thermal shock and moisture resistance, which can protect the copper surface from rust (oxidation or vulcanization) in normal environment.At the same time, it must be easily removed by the flux in the subsequent welding heat for welding.

The organic coating process is simple and low cost, which makes it widely used in the industry.The early organic coating molecules are the antirust of imidazole and benzotriazole, and the latest molecules are mainly benzimidazole.In order to ensure that multiple reflow soldering can be performed, only one layer of organic coating on the copper surface is not acceptable, and there must be many layers, which is why the copper solution is usually required in the chemical tank.After coating the first layer, the coated layer adsorbs copper;Then the second layer of organic coating molecules combined with copper, until 20 or even hundreds of organic coating molecules were assembled on the copper surface.

The general process is: degreasing -> microcorrosion -> pickling -> pure water cleaning -> organic coating -> cleaning, process control is relatively easy to process.

Folding chemical nickel plating/leaching gold (also known as chemical gold)

The electroless nickel/dip coating is coated with a thick layer of nickel alloy with good electrical properties and can protect PCB for a long time.Unlike the OSP, which is only a rust-proof barrier, it can be useful in the long term use of PCB and achieve good electrical performance.In addition, it has the tolerance of other surface treatment technologies.

The nickel plating is due to the spread of gold and copper, which can prevent the spread of the metal. Without the barrier of the nickel, gold would spread to copper within hours.Another advantage of electroless nickel plating/leaching is the strength of nickel, which can control the expansion of the Z direction under high temperature with just 5um thickness.In addition, electroless nickel/leaching can also prevent the dissolution of copper, which will be beneficial for lead-free soldering.

The general process is: deacid-washing cleaning -> microerosion -> pre-soaking -> activation -> electroless nickel -> chemical immersion gold;There are six chemical channels in the process, involving nearly 100 chemicals, and the process is complicated.

Fold the immersion silver

The process between OSP and nickel plating is simpler and faster.The dip is not a thick armor for PCB, and even exposed to heat, humidity and pollution, it can provide good electrical performance and good weldability, but it will lose its luster.Because there is no nickel under the silver layer, the silver is not equipped with all the good physical strength of electroless nickel/leaching.

Silver leaching is a displacement reaction, it is almost submicron grade pure silver coating.Sometimes also contains some organic matter, in the process of immersion silver silver corrosion mainly is to prevent and eliminate silver migration issues, generally difficult to quantity measured by a thin layer of organic matter, analysis shows that the weight of the organism is less than 1%.

Fold the immersion tin

Since all solders are based on tin, the tin layer can match any type of solder. From this point of view, the process of immersion tin is very promising.However, the previous PCB is easy to appear tin whiskers after the process of soldering tin, and the solder and tin transfer in the welding process can bring reliability problems, thus limiting the adoption of the tin immersion process.The organic additives were added in the tin solution, and the structure of the tin layer was granular, which overcame the previous problems, and also had good thermal stability and weldability.

Immersion tin process between flat copper tin compounds can be formed, this feature allows the immersion tin has and hot air leveling as good weldability without hot air leveling headache flatness problem;There is also no diffusion problem between electroless nickel/dip metal.It's just that you can't store it for too long,

Other surface treatment processes in this section.

Other applications show that the treatment process has fewer applications, including electroplating nickel plating and electroless palladium plating.

Folding electroplated nickel (electroplating)

Electroplating nickel is the progenitor of PCB surface treatment process. Since PCB appears, it has emerged and gradually evolved other processes.Nickel plating is a layer of gold on the surface of the PCB, which is plated on a layer of nickel. The nickel plating is mainly to prevent the spread between gold and copper.Now there are two types of electroplating nickel gold: soft gold plating (gold, gold surface looks not bright) and hard gold plating, surface smooth, hard, wear-resisting, containing cobalt and other elements, the surface looks more light).Soft gold is mainly used for chip packaging gold wire;The hard gold is mainly used for electrical interconnections in non-welding areas (eg, goldfinger).

Under normal circumstances, welding can lead to a brittle electroplating, which will shorten the service life and thus avoid welding on electroplating.However, the electroless nickel/leaching gold is very thin and consistent, and the brittle phenomenon rarely occurs.

Folding chemical plating palladium.

The process of electroless plating is similar to that of electroless nickel plating.By reducing agent is the main process (like sodium dihydrogen phosphate) make palladium ions on the surface of catalytic reduction of palladium, newborn may be called the push reaction palladium catalyst, thus arbitrary thickness of palladium plating layer can be obtained.The advantages of chemical plating palladium are good welding reliability, thermal stability and surface smoothness.The disadvantage is that palladium is a relatively rare precious metal, so the cost will increase.