Special processing techniques for PCB surface

Because copper is easily oxidized in the air, it will cause discoloration, blackening, and even rust. Therefore, the copper oxide layer will have a great impact on welding, and it is easy to form false welding and virtual welding, which will cause welding in severe cases. The disk and the components cannot be soldered. To solve this problem, a process must be set up to solve this problem in the production of PCB, that is, a layer of substance that can protect the pad from oxidation is coated (plated) on the surface of the exposed pad.

At present, common and widely used PCB surface treatment processes include: spray tin (HASL), immersion tin, immersion silver, chemical immersion gold, electroplating gold, OSP (anti-oxidation), chemical immersion gold (ENIG), etc. Of course, There are also some special PCB surface treatment processes for special applications. The cost is different for different surface treatment. After all, the occasions used are different. Just like when we buy products, we only choose the right ones and not the expensive ones. In order to consider the cost performance, some merchants will meet the PCB application scenarios at the lowest price. But, naturally there will be a variety of different crafts for us to choose. We can’t say which craft is good or bad, because every craft has good and bad. The most important thing is that we have to find the right one and how to use it well. 
Comparison of advantages and disadvantages of PCB surface treatment process
HASL  Hot Air Solder Leveling, hot air leveling
Advantages: lower price and good welding performance.
Disadvantages: Not suitable for soldering pins with fine gaps and components that are too small, because the surface flatness of the spray tin plate is poor. Solder bead is easy to produce in PCB processing, and it is easy to cause short circuit to fine pitch components. When used in the double-sided SMT process, because the second side has undergone a high-temperature reflow soldering, it is easy to spray tin and re-melt to produce tin beads or similar water drops into spherical tin spots that are affected by gravity, which makes the surface even less Leveling affects welding problems.

OSP  Organic Soldering Preservative, anti-oxidation
Advantages: It has all the advantages of bare copper welding. The expired (three months) board can also be resurfaced, but usually only once.
Disadvantages: easily affected by acid and humidity. When used in the secondary reflow soldering, it needs to be completed within a certain period of time. Usually the effect of the second reflow soldering will be relatively poor. If the storage time exceeds three months, it must be resurfaced. It must be used up within 24 hours after opening the package. OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer before it can contact the pin point for electrical testing.

Immersion Silver  (ENIG, Electroless Nickel Immersion Silver)
Advantages: good flatness, ultra-fine pitch, good conductivity and weldability, simple process control, high production speed, and reworkable
Disadvantages: the storage period is relatively short, can not be welded many times, the surface of silver is easy to turn yellow

Immersion Silver is cheaper than Immersion Gold. If the PCB has connection functional requirements and needs to reduce costs, Immersion Silver is a good choice; coupled with Immersion Silver's good flatness and contact, it is better to choose Immersion Silver technology. Immersion Silver has many applications in communication products, automobile PCB , and computer peripherals. Immersion Silver also has applications in high-speed signal design. Since Immersion Silver has good electrical properties that other surface treatments cannot match, it can be used in high-frequency signals as well. EMS recommends the immersion silver process because it is easy to assemble. However, due to defects such as tarnishing and solder joint voids, the growth of immersion silver is slow.

Immersion Tin (ENIG, Electroless Nickel Immersion Tin)
Advantages: good flatness, ultra-fine pitch, good conductivity and weldability, simple process control, high production speed, and multiple welding
Disadvantages: Thiourea (attack solder mask, carcinogenic), easy to scratch, there is the possibility of tin whiskers

The appearance of Immersion Tin is the result of production automation requirements. Immersion tin does not bring any new elements into the welding place, which is especially suitable for backplanes for communication PCB. Tin will lose its solderability beyond the storage period of the board, so tin sinking requires better storage conditions. In addition, the tin-immersion process is restricted to use because it contains carcinogens.

Immersion Gold (ENIG, Electroless Nickel Immersion Gold)
Advantages: it is not easy to oxidize, can be stored for a long time, and the surface is flat, suitable for welding small gap pins and components with small solder joints. The first choice of PCB board with buttons (such as mobile phone boards). Reflow soldering can be repeated many times without reducing its solderability. It can be used as a substrate for COB (Chip On Board) wire bonding.
Disadvantages: high cost, poor welding strength, because the electroless nickel plating process is used, it is easy to have the problem of black disk. The nickel layer will oxidize over time, and long-term reliability is a problem.

Comparing different PCB surface treatment processes, their costs are different, and the application occasions are also different. Each process has its own merits, and the existence is reasonable. The key is that we must know them and use them well. POE PCB Manufacturing has many years of experience in surface treatment and has won unanimous praise from customers. If you have any questions, you can consult our online customer service at any time, and look forward to your inquiry!