PCB surface treatment technology

There are various processes for PCB surface treatment. 

1. Bare copper pcb

The pros and cons are obvious:

Advantages: Low cost, flat surface, good weld ability (without being oxidized).

Disadvantages: easy to be affected by acid and humidity, can not be put for a long time, after unpacking need to be used up within 2 hours, because copper exposed to the air easy oxidation; Cannot be used on double panels because the second side is oxidized after the first reflow welding. If there is a test point, solder paste must be printed to prevent oxidation, otherwise there will be no good follow-up contact with the probe.

Pure copper if exposed to air is easily oxidized, the outer layer must have the above protective layer. So it is necessary to carry on surface treatment in circuit board processing.

2.OSP process pcb board

OSP differs from other surface treatment processes in that it acts as a barrier between copper and air. Simply put, OSP is a layer of organic film grown chemically on a clean bare copper surface. Because it's organic, not metal, it's cheaper than tin-spraying.

The only function of this organic film is to ensure that the inner copper foil does not oxidize before welding. The film evaporates when you heat it up during welding. The solder will be able to weld the copper wire and components together. But this layer of organic film is very corrosion resistant, an OSP circuit board, exposed to air for more than ten days, can not be welded components. Many computer motherboards use the OSP process. Because the board area is so large, OSP is more economical.

Advantages: With all the advantages of bare copper plate welding, expired boards can also be re-finished.

Disadvantages: 1.OSP transparent colorless, so it is difficult to check, it is difficult to identify whether the OSP processing. 2. The OSP themselves are insulated and do not conduct electricity, which will affect electrical testing. Therefore, the test point must be opened with steel mesh and solder paste to remove the original OSP layer in order to contact the needle point for electrical testing. OSP cannot be used to handle electrical contact surfaces, such as keyboard surfaces for keystrokes. 3.OSP is easily affected by acid and temperature. When used for secondary reflow welding, it should be completed within a certain time, usually the second reflow welding effect will be poor. If it lasts longer than three months, it must be resurfaced. It should be used within 24 hours after unpacking.

3. hot air solder leveling (HASL)

Hot air leveling, also known as hot air solder leveling, is a process in which the PCB surface is coated with molten tin lead solder and heated compressed air leveling (blowing flat), so that it forms a coating layer that is resistant to copper oxidation and can provide good solder-ability. The solder and copper form copper tin gold compounds at the joint of hot air polishing, the thickness of which is about 1 ~ 2mil.

Wave-crest welding is the best welding method when perforated devices are dominant, and HASL is sufficient to meet the technical requirements of wave-crest welding. Of course, nickel plating/gold plating is often used when the joint strength is required to be high.

Advantages: Low cost

Disadvantages: a.HASL treated pad is not smooth enough, coplanar can not meet the process requirements of fine spacing pad. b. Not environmentally friendly, lead is harmful to the environment.

4. Gilt pcb board

Gilding, which uses real gold, even in a thin layer, accounts for nearly 10% of the board's cost. The use of gold as a coating, one is to facilitate welding, two is to prevent corrosion. Even if the gold finger of the memory is used for several years, it is still shiny. If the copper, aluminum and iron have been used for the same time, they are now rusted into a pile of scrap.

Gold coating is widely used in the parts of the circuit board welding pad, gold finger, connector shrapnel and other positions. The most widely used mobile phone circuit board motherboard is mostly gold plate, gold plate, computer motherboard, audio and small digital circuit board are generally not gold plate.

Advantages: Strong conductivity, good oxidation resistance, long life. Dense coating, relatively wear-resistant, generally used in bonding, welding and plugging occasions.

Disadvantages: High cost, poor welding strength.

5. Gold melting/sinking (ENIG)

Nickel leaching gold (ENIG), also known as nickel gold, nickel nickel gold, referred to as gold and gold precipitation. ENIG is chemically coated with a thick, electrically conductive nickel-gold alloy that protects the PCB for a long time. The thickness of the inner layer of nickel is generally 120~240μin(about 3~6μm), and the thickness of the outer layer of gold is generally 2~4μinch(0.05~0.1μm). Unlike OSP, which is only used as an anti-rust barrier layer, it can be useful in the long-term use of PCB and achieve good electrical performance. In addition, it has environmental tolerance that other surface treatment processes do not have.

Advantages: 1.ENIG treated PCB surface is very flat, coplanar is very good, suitable for key contact surface. 2.ENIG has excellent solder-ability. Gold will quickly blend into the melted solder, which forms Ni/Sn metal compounds with Ni.

Disadvantages: The process flow is complex, and to achieve good results need to strictly control the process parameters. Most troubling of all, EING treated PCB surfaces are prone to black-disc benefits during ENIG or welding. The direct performance of the black disk is excessive Ni oxidation, too much gold, will make the solder joint embrittlement, affect the reliability.

6. Electroless nickel palladium plating (ENEPIG)

Compared with nickel gold, ENEPIG has an extra layer of palladium between nickel and gold. In the deposition reaction of replacement gold, the electroless palladium layer can protect the nickel layer from excessive corrosion by the cross replacement gold. While preventing the corrosion caused by the replacement reaction, palladium is fully prepared for gold leaching. The thickness of nickel deposit is generally 120~240μin(about 3~6μm) and palladium deposit is 4~20μin(about 0.1~0.5μm). Gold deposition thickness is generally 1~4μin(0.02~0.1μm).

Advantages: It has a very wide range of applications, and the chemical nickel palladium surface treatment relative to nickel gold surface treatment can effectively prevent the connection reliability problems caused by Black Pad defects, can replace nickel gold.

Disadvantages: While ENEPIG has many advantages, palladium is expensive and a scarce resource. At the same time, like nickel gold, its process control requirements are strict.