PCB Technical - PCB gold plating and immersion gold related comparison

If PCB electroplating is subdivided, it can be divided into many types of electroplating. Let's introduce which electroplating types are divided into:

One: The whole board is gold-plated.

Generally refers to [electroplating gold] [electroplating nickel gold plate], [electrolytic gold], [electric gold], [electric nickel gold plate], there is a distinction between soft gold and hard gold (usually used as gold fingers). The principle is to dissolve nickel and gold (commonly known as gold salt) in chemical water, immerse the circuit board in the electroplating cylinder and pass current to form a nickel-gold plating layer on the copper foil surface of the circuit board. The characteristics of high hardness, wear resistance, and resistance to oxidation are widely used in electronic product names.

Two: Immersion:

A layer of plating is formed by the chemical oxidation-reduction reaction method, which is generally thicker, which is a kind of chemical nickel-gold layer deposition method, which can reach a thicker gold layer, usually called immersion gold.

Three: gold plate

As the integration level of IC becomes higher and higher, IC pins become more denser. The vertical spray tin process is difficult to flatten the thin pads, which brings difficulty to the placement of SMT; in addition, the shelf life of the spray tin plate is very short. The gold-plated board just solves these problems: 1 For the surface mount process, especially for 0603 and 0402 ultra-small surface mounts, because the flatness of the pad is directly related to the quality of the solder paste printing process, it is important for the quality of the subsequent reflow soldering. Therefore, the whole board gold plating is common in the high-density and ultra-small surface mount process. 2 In the trial production stage, due to factors such as component procurement, it is often not that the board is soldered as soon as it comes, but it is often used for several weeks or even months. The shelf life of the gold-plated board is better than that of lead-tin The alloy is many times longer so everyone is willing to use it. Besides, the cost of gold-plated PCB in the sample stage is almost the same as that of lead-tin alloy board.

But as the wiring becomes denser, the line width and spacing have reached 3-4MIL. This brings about the problem of short circuit of gold wire:

As the frequency of the signal gets higher and higher, the signal transmission in the multi-plated layer caused by the skin effect has more obvious influence on the signal quality:

The skin effect refers to: high frequency alternating current, the current will tend to concentrate on the surface of the wire to flow.

According to calculations, skin depth is related to frequency:

Other shortcomings of gold-plated board have been listed in the table of difference between immersion gold board and gold-plated board.

Four: Immersion Gold Board

In order to solve the above problems of gold-plated boards, PCBs using gold-plated boards mainly have the following characteristics:

1. Because the crystal structure formed by immersion gold and gold plating is different, immersion gold will be golden yellower than gold plating, and customers will be more satisfied.

2. Because the crystal structure formed by immersion gold and gold plating is different, immersion gold is easier to weld than gold plating, and will not cause poor welding and cause customer complaints.

3. Because the immersion gold board only has nickel and gold on the pad, the signal transmission in the skin effect will not affect the signal on the copper layer.

4. Because immersion gold has a denser crystal structure than gold plating, it is not easy to produce oxidation.

5. Because the immersion gold board only has nickel and gold on the pads, it will not produce gold wires and cause slight shortness.

6. Because the immersion gold board only has nickel and gold on the pads, the solder mask on the circuit and the copper layer are more firmly bonded.

7. PCB engineering will not affect the spacing when making compensation.

8. Because the crystal structure formed by immersion gold and gold plating is different, the stress of the immersion gold plate is easier to control, and for products with bonding, it is more conducive to bonding processing. At the same time, it is precisely because the immersion gold is softer than the gilding, so the immersion gold plate is not wear-resistant as the gold finger.

9. The flatness and stand-by life of the immersion gold board are as good as the gold-plated board