PCB Technical - Several commonly used processes for PCB surface treatment and their advantages and disadvantages

Several commonly used processes for PCB surface treatment and their advantages and disadvantages

PCB surface treatment technology refers to the process of artificially forming a surface layer that is different from the mechanical, physical and chemical properties of the substrate on the PCB components and electrical connection points (pad surface). Its purpose is to ensure good solderability or electrical properties of the PCB. Since copper tends to exist in the form of oxides in the air, it will seriously affect the solderability and electrical performance of the PCB, so surface treatment of the PCB pads is required.

The commonly used surface treatment methods are as follows:

1, hot air leveling (HASL)

Hot air leveling, also called tin spraying, is the process of coating molten tin-lead solder on the surface of the PCB pad and leveling (flattening) with heated compressed air to form a layer that is resistant to copper oxidation and provides good solderability Sexual coating. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction, and the thickness is about 1 to 2 mils;

advantage

(1) Good solderability

(2) Simple process

(3) Low cost

shortcoming

(1) The surface of the pad is not flat

(2) Lead is harmful to the human body

(3) Large impact on the board and easy to deform

2. Anti-oxidation (OSP)

On the clean bare copper surface, a layer of organic film is grown chemically. This layer of film has anti-oxidation, heat shock resistance, and moisture resistance to protect the copper surface from rusting (oxidation or sulfidation, etc.) in a normal environment; at the same time, it must be easily assisted in the subsequent welding high temperature The flux is quickly removed to facilitate welding;

advantage

(1) Excellent solderability

(2) Low cost

shortcoming

(1) Short shelf life

(2) Easy to scratch

(3) Easy to oxidize

3, chemically immersed nickel gold (ENIG)

A thick layer of nickel-gold alloy with good electrical properties is wrapped on the copper surface and can protect the PCB for a long time. Unlike OSP, which is only used as an anti-rust barrier layer, it can be useful and achieve good electrical performance during long-term use of PCB boards. In addition, it also has tolerance to the environment that other surface treatment processes do not have;

advantage

(1) Excellent solderability

(2) Good flatness

shortcoming

(1) High cost

4. Chemical Immersion Silver

is between OSP and electroless nickel/immersion gold, the process is simpler and faster. When exposed to heat, humidity and pollution, it can still provide good electrical performance and maintain good solderability, but it will lose its luster. Because there is no nickel under the silver layer, immersion silver does not have the good physical strength of electroless nickel/immersion gold;

advantage

(1) Excellent solderability

(2) Good flatness

shortcoming

(1) High cost

(2) Easy to oxidize

(3) Difficult to save

5. Electroplating nickel gold

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

advantage

(1) General solderability

(2) High cost

(3) High hardness

shortcoming

(1) Not easy to scratch

(2) No processing

(3) The combination rate with ink is not good