PCB Tech - Why should gold be used for surface treatment?

PCB boardsurface treatment

Anti-oxidation, tin spray, lead-free tin spray, immersion gold, immersion tin, immersion silver, hard gold plating, full board gold plating, gold fingers, nickel palladium gold OSP: lower cost, good solderability, harsh storage conditions, time Short, environmentally friendly technology, good welding and smooth.

Tin spraying: The spray tin plate is generally a multi-layer (4-46 layer)high-precision PCB model. It has been used by many large domestic communication, computer, medical equipment, and aerospace companies and research units. ) Is the connecting part between the memory bar and the memory slot, all signals are transmitted through golden fingers.

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

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 of the memory is now replaced by tin plating. Since the 1990s, tin materials have been popularized. At present, the "golden fingers" of motherboards, memory and graphics cards are almost all used. Tin material, only part of the contact points of high-performance servers/workstations will continue to be gold-plated, which is naturally expensive.

Why use 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:

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 impact on the quality of the subsequent reflow soldering. Therefore, the entire board is gold-plated. It is common in high-density and ultra-small surface mount processes.

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 the lead-tin alloy. Many times longer, so everyone is happy 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.

Brings the problem of gold wire short circuit: 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.

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:

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.

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.

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.

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

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.

The project will not affect the spacing during compensation.

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.

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

Immersion gold board VS gold-plated board

In fact, the gold plating process is divided into two types: one is electroplating gold, the other is immersion gold.

For the gilding process, the effect of tinning is greatly reduced, and the tinning effect of immersion gold is better; unless the manufacturer requires binding, most manufacturers will now choose the immersion gold process! Generally, PCB surface treatments are as follows: gold plating (electroplating gold, immersion gold), silver plating, OSP, tin spraying (lead and lead-free), these types are mainly for FR-4 or CEM-3 For the board, the paper base material also has the surface treatment method of coating rosin; the poor tin application (bad tin eating) is considered if the production and material technology reasons of the chip manufacturer such as solder paste are excluded.

Here is only for the PCB problem, there are the following reasons:

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

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.

Whether the pad is contaminated or not, this can be obtained by the ion contamination test; the above three points are basically the key aspects considered by PCB manufacturer.

Regarding the advantages and disadvantages of several methods of surface treatment, each has its own strengths and weaknesses!

In terms of gold plating, it can keep PCBs for a long time, and the temperature and humidity of the external environment are less changed (compared to other surface treatments), and generally can be stored for about one year; secondly, the surface treatment of tin spraying, OSP again, this A lot of attention should be paid to the storage time of the two surface treatments at ambient temperature and humidity.

Under normal circumstances, the surface treatment of immersion silver is a bit different, the price is also high, and the storage conditions are more demanding, so it needs to be packaged in sulfur-free paper! And the storage time is about three months! In terms of the effect of tinning, immersion gold, OSP, tin spraying, etc. are actually the same. Manufacturers mainly consider cost-effectiveness!