Must-know FPC surface treatment process

FPC is a name given to flexible circuit boards by industry insiders, also known as "flex boards". The purpose of the FPC surface treatment process is to ensure good solderability and electrical properties. Because copper is exposed to the air or under the action of water, it is easy to cause oxidation of copper, and it is unlikely to remain in the state of original copper for a long time. At this time, a special treatment for copper is required, that is, surface treatment process. The following are some surface treatment processes about FPC compiled by Xiaobian:
1. Hot air leveling

Hot air leveling is hot air solder leveling, commonly known as: spray tin. Refers to the process of coating molten tin (lead) solder on the surface of the PCB and flattening (blowing) it with heated compressed air, so that the circuit board forms a layer that resists copper oxidation and provides a good solderability coating. The following points should be paid attention to when hot air leveling the circuit board:

1) The pcb board should be immersed in molten solder;

2) Blow out the liquid solder before the solder solidifies;

3) The air knife can minimize the meniscus of the solder on the copper surface and prevent the solder from bridging.

2. Immersion Tin

Because all current solders are based on tin, the tin layer can be matched to any type of solder. The immersion tin process can form a flat copper-tin intermetallic compound, which makes the immersion tin have the same good solderability as the hot air leveling without the headache of the flatness problem of the hot air leveling; the immersion tin board cannot be stored for too long , the assembly must be carried out according to the order of immersion tin.

3. Immersion Gold

Immersion gold is a thick layer of nickel-gold alloy with good electrical properties wrapped on the copper surface, which can protect the circuit board for a long time; in addition, it also has environmental tolerance that other surface treatment processes do not have. In addition, immersion gold can also prevent the dissolution of copper, which is beneficial for lead-free assembly.

4. Electroless nickel palladium gold

Compared with immersion gold, chemical nickel palladium gold has an additional layer of palladium between nickel and gold. Palladium can prevent corrosion caused by replacement reaction and be fully prepared for immersion gold. Gold covers the palladium tightly, providing a good contact surface.

5. Shen silver

The immersion silver process is between organic coating and electroless nickel / immersion gold, the process is relatively simple and fast; even when exposed to heat, humidity and pollution, silver can still maintain good solderability, but will lose its luster . Immersion silver does not have the good physical strength of electroless nickel/immersion gold because there is no nickel under the silver layer.

6. Plating hard gold

In order to improve the wear resistance of the product and increase the number of insertion and removal, hard gold is plated.

7. Nickel gold plating on the whole board

Nickel-plated gold means that the conductor on the surface of the PCB is first plated with a layer of nickel and then plated with a layer of gold. The purpose of nickel plating is to prevent the diffusion between gold and copper. Now there are two types of electroplated nickel gold: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (the surface is smooth and hard, wear-resistant, contains other elements such as cobalt, and the gold surface looks brighter). Soft gold is mainly used for gold wire in chip packaging; hard gold is mainly used for electrical interconnection in non-soldering places.

8. OSP

OSP is the abbreviation of Organic Solderability Preservatives, which is translated into Chinese: organic solder protection film, also known as copper protection agent. It is a process for the surface treatment of PCB copper foil that meets the requirements of the RoHS directive. Simply put, it is to chemically grow a layer of organic film with anti-oxidation, thermal shock and moisture resistance on the clean bare copper surface. This protective film can protect the copper surface from rusting (oxidation or vulcanization, etc.) in the normal environment. And in the subsequent high temperature soldering, this protective film must allow the flux to be easily and quickly removed, so that the exposed clean copper surface can be immediately combined with the molten solder to form a solid solder joint in a very short time.
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