PCB surface treatment

Since the copper surface of the PCB is easy to oxidize in the environment, it must be reprocessed and covered with a layer of coating in the exposed place where the solder-proof ink is not covered to protect the process from oxidation. In response to various subsequent processing needs, a variety of surface treatment processing methods with different materials, prices, and different levels of protection have been developed.

Common PCB surface treatments include the following：

Bare copper：

As the name suggests, the exposed copper foil is not covered by any treatment at all.

The advantage is that the cost is the lowest, and the weldability and flatness are good.

Disadvantages Because there is no protection, it is easy to oxidize after contact with air.

Tin spray (HASL/HAL)：

Dip a layer of tin on the surface of the copper foil and use a hot air knife to flatten the surface. Its equipment is divided into two types: horizontal tin spraying and vertical tin spraying.

Tin spraying is the most widely used surface treatment method for PCB, which has the advantages of good weldability, long storage time and low cost. However, its disadvantage is that the surface flatness is poor, and it is easy to cause poor welding due to inconsistent tin content during SMT, and if the distance between the contacts is too close, it is easy to cause a short circuit due to tin balls.

Since the tin sprayed with tin is a tin-lead alloy, in recent years, countries have vigorously promoted environmental protection, and the demand for lead-free processes has become higher and higher. Therefore, there is another lead-free tin spraying process to choose from.

Lead-free tin spray (Lead-Free HASL)：

It is the same as the tin spraying process, the difference is only in the lead-free composition of its alloy. There are two common alloys: tin, copper, nickel and tin, silver and copper.

Gold ENIG：

(Electroless Nickel Immersion Gold, nickel Immersion Gold), here refers to the nickel immersion process of PCB without external current. A layer of chemical nickel is chemically deposited on the surface of bare copper, and the gold layer is deposited on the upper layer of nickel through the chemical replacement of the tank liquid until the nickel surface is completely covered. Because chemical substitution is used, the thickness of gold cannot be as thick as electroplated gold. The thickness of the gold layer is generally about 1μ"~5μ", and the thickness of the nickel layer is about 100μ"~200μ". The advantage is that the PCB does not need to pull additional wires, and the electroplating design is relatively simple, and because the area to be welded is relatively flat and uniform, the denser areas of the parts (such as BGA) are not prone to poor welding or short circuit due to tin bridges. The disadvantage is the high cost and the deviation of welding strength after SMT.

Electroplated gold：

Electroplating nickel gold, most of them use electroplating hard gold. The nickel layer and the gold layer are plated on the exposed copper surface of the PCB by energizing the wire. Most common are the contact areas where the gold is partially electroplated at the edge of the PCB that need to be repeatedly inserted and unplugged, such as the position commonly known as the gold finger, but there are also a few products that choose the whole board to be electroplated with gold, or the local area of the board to be electroplated with gold. Generally, the thickness of the gold-plated layer is about 5~30μ". The advantage is that the surface of the electroplated gold is hard and wear-resistant, and it can be repeatedly rubbed without easy loss and oxidation. The disadvantage is that the wire to be plated needs to be connected and the surface flatness is poor, and the price is extremely expensive.

Immersion Silver (Immersion Silver)：

PCB silver plate uses dip plating to use the different oxidation potentials of copper and silver to replace the copper atoms on the surface of the copper with silver, thereby achieving the purpose of surface modification. The general thickness is about 6μ"~10μ". The advantage is that the surface is flat and the welding effect is good. The disadvantage is that the price is slightly higher, and it is easy to oxidize/vulcanize after contact with air. The PCB must be used as soon as possible after unpacking, and the storage time is short.

OSP（Organic Solderability Preservative):

OSP is chemically coated with a layer of organic protective film on the exposed copper surface of the PCB. This film can protect the bare copper of the PCB from oxidation due to contact with air. The advantage is that the cost is low, the processing process is fast, and it has the advantages of good welding and leveling of bare copper plates. The disadvantage is that it cannot be directly electrically tested after processing, and the appearance inspection is difficult.