PCB Tech - What is the difference between gold plating and immersion gold process for PCB board surface treatment

1. PCB surface treatment

The surface treatment process of PCB board includes: anti-oxidation, tin spray, lead-free tin spray, immersion gold, immersion tin, immersion silver, hard gold plating, full board gold plating, gold finger, nickel palladium gold OSP, etc.

The main requirements are: low cost, good weldability, harsh storage conditions, short time, environmentally friendly technology, good welding, and flatness. Tin spray: The spray tin board is generally a multi-layer (4-46 layer) high-precision PCB model, which has been adopted by many domestic large-scale communication, computer, medical equipment and aerospace enterprises and research units.

What is the difference between gold plating and immersion gold process for PCB board surface treatment

The golden finger (connecting finger) is the connecting part between the memory module and the memory slot, and all signals are transmitted through the golden finger.

The gold finger is composed of many golden conductive contacts. Because the surface is gold-plated and the conductive contacts are arranged like fingers, it is called "golden finger". The gold finger board needs gold plating or immersion gold.

The gold finger is actually coated with a layer of gold on the copper clad board through a special process, because gold has strong oxidation resistance and strong conductivity. However, due to the high price of gold, most memories are now replaced by tin plating, and tin materials have become popular since the 1990s.

At present, the "gold fingers" of motherboards, memory, and graphics cards are almost all tin materials. Only some high-performance server/workstation accessory contact points will continue to use gold plating, which is naturally expensive.

Second, the difference between gold plating and immersion gold process

Immersion gold adopts a chemical deposition method, which generates a layer of plating through a chemical oxidation-reduction reaction method, which is generally thicker. It is a kind of chemical nickel-gold layer deposition method, which can reach a thicker gold layer.

Gold plating uses the principle of electrolysis, also called electroplating. The surface treatment of other metals is mostly electroplating.

In actual product applications, 90% of the gold plate is immersion gold plate, because the poor weldability of the gold plate is his fatal shortcoming, and it is also the direct reason why many companies abandon the gold plating process!

Immersion gold process deposits a nickel-gold coating with stable color, good brightness, flat coating and good solderability on the surface of the printed circuit. Basically, it can be divided into four stages: pre-treatment (degreasing, micro-etching, activation, post-dipping), immersion of nickel, immersion of gold, and post-treatment (washing waste gold, washing with DI water, and drying). The thickness of immersion gold is between 0.025-0.1um.

Gold is used in the surface treatment of circuit boards. Because gold has strong conductivity, good oxidation resistance, and long life, it is generally used in key boards, gold finger boards, etc. However, the most fundamental difference between gold-plated boards and immersed gold boards is that gold-plated boards are hard Gold (wear-resistant), Immersion Gold is soft gold (not wear-resistant).

1. Immersion gold is different from the crystal structure formed by gold plating. Immersion gold is much thicker than gold plating. Immersion gold will be golden yellow, which is more yellow than gold plating (this is one of the ways to distinguish between gold plating and immersion gold. 1), the gold-plated ones will be slightly whitish (nickel color)

2. Immersion gold and gold plating have different crystal structures. Immersion gold is easier to weld than gold plating and will not cause poor welding. The stress of the immersion gold plate is easier to control, and for products with bonding, it is more conducive to the processing of bonding. At the same time, it is precisely because the immersion gold is softer than the gold plating, so the immersion gold plate is not wear-resistant as the gold finger (the disadvantage of the immersion gold plate).

3. The immersion gold board only has nickel and gold on the pads, and the signal transmission in the skin effect is on the copper layer without affecting the signal.

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

5. As the precision requirements for circuit board processing are getting higher and higher, the line width and spacing have been below 0.1mm. Gold plating is prone to short circuit of gold wire. The immersion gold board only has nickel gold on the pad, so it is not easy to produce gold wire short circuit.

6. The immersion gold board only has nickel and gold on the pads, so the solder mask on the circuit and the copper layer are more firmly combined. The project will not affect the spacing during compensation.

7. For high-demand boards, the flatness requirements are better. Generally, immersion gold is used, and immersion gold generally does not appear as a black pad after assembly. Immersion gold plate has better flatness and service life than gold plate.

Third, why use gold-plated plates

As the integration of ICs becomes higher and higher, IC pins become more dense. 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 has a decisive influence on the quality of the subsequent reflow soldering, so the whole board Gold plating is common in high-density and ultra-small surface mount processes.

2. In the trial production stage, due to factors such as component procurement, it is often not that the board is soldered immediately when 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 alloy is many times longer, so everyone is happy to use it. Besides, the cost of the gold-plated PCB in the sample stage is almost the same as that of the lead-tin alloy board.

But as the wiring becomes denser, the line width and spacing have reached 3-4MIL. Therefore, the problem of gold wire short circuit is brought about: As the frequency of the signal becomes 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, the skin depth is related to frequency.

Fourth, why use 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.

5. Immersion gold board VS gold-plated board. Actually, there are two types of gold plating process: one is electroplating gold, the other is immersion gold

For the gold plating process, the effect of tinning is greatly reduced, and the tinning effect of immersion gold is better. Here is only for the PCB problem. There are several reasons:

1. During PCB printing, whether there is an oil-permeable film surface on the PAN position, which can block the effect of tinning; this can be verified by a tin bleaching test.

2. Whether the lubrication position of the PAN position meets the design requirements, that is, whether the pad design can sufficiently ensure the support of the parts.

3. Whether the pad is contaminated, this can be obtained by the ion contamination test