PCB Tech - Why do some PCB need gold plating

1, PCB surface treatment:

Oxidation resistance, tin spraying, lead-free HASL, gold deposition, tin deposition, silver deposition, hard gold plating, full plate gold plating, gold finger, nickel palladium OSP: low cost, good solderability, harsh storage conditions, short time, environmental protection process, good welding, smooth welding.

Tin spraying: tin spraying board is generally a multi-layer (4-46 layers) high-precision PCB template, which has been used by many domestic large-scale communication, computer, medical equipment, aerospace enterprises and research institutions. The connecting finger is the connecting part between the memory module and the memory slot, and all signals are transmitted through the golden finger.

The golden finger is composed of many golden yellow conductive contacts. It is called "golden finger" because its surface is gilded and the conductive contacts are arranged like fingers.

The gold finger is actually coated with a layer of gold on the copper clad laminate by a special process, because the gold has strong oxidation resistance and strong conductivity.

However, due to the high price of gold, at present, more memory is replaced by tin plating. Since the 1990s, tin materials have been popularized. At present, almost all the "gold fingers" of mainboard, memory and graphics card are made of tin material. Only some high-performance server / workstation accessories contact points will continue to use gold plating, which is naturally expensive.

2, Why gold plate

With the increasing integration of IC, IC pins are more and more dense. However, the vertical tin spraying process is difficult to smooth the fine solder pad, which brings difficulties to SMT mounting; in addition, the shelf life of tin spray plate is very short.

The gold plate solves these problems

1. For surface mount process, especially for 0603 and 0402 ultra-small surface mount, because the pad flatness is directly related to the quality of solder paste printing process, and plays a decisive role in the subsequent reflow welding quality, so the whole board gold plating is often seen in high-density and ultra-small surface mounting process.

2. In the trial production stage, due to the factors such as component procurement, it is not often that the boards are welded immediately when they come, but that they often have to wait for weeks or even months to use. The shelf life of gold-plated plates is many times longer than that of lead tin alloys, so everyone is willing to adopt them.

Besides, the cost of gold-plated PCB in the sample stage is almost the same as that of lead tin alloy plate.

But as the wiring becomes more and more dense, the line width and spacing have reached 3-4mil.

Therefore, the problem of gold wire short circuit is brought: with the increasing frequency of the signal, the transmission of the signal in the multi coating caused by skin effect has a more obvious impact on the signal quality.

The tendency of alternating current to flow on the surface of a wire. According to the calculation, the skin depth is related to the frequency.

In order to solve the above problems of gold plating board, the PCB with gold plate has the following characteristics:

1. Due to the different crystal structure formed by gold deposition and gold plating, the precipitated gold will be golden yellow, which is more yellow than gold plating, and customers are more satisfied.

2. Compared with gold plating, the deposition of gold is easier to weld, which will not cause poor welding and cause customer complaints.

3. Because only nickel and gold exist on the pad, the signal transmission in skin effect is in copper layer, which will not affect the signal.

4. Because the crystal structure of deposited gold is more compact than that of gold plating, it is not easy to produce oxidation.

5. Because only nickel and gold are found on the pad, the gold wire will not be produced, resulting in a short time.

6. Because only nickel and gold are found on the pad, the bonding between the solder mask and the copper layer is stronger.

7. The project will not affect the spacing when making compensation.

8. Because of the different crystal structure formed by gold deposition and gold plating, the stress of gold plate is easier to control, which is more conducive to bonding processing for products with bonding. At the same time, because the sinking gold is softer than the gold plating, the gold plated plate is not wearable.

9. The flatness and service life of the gold plate are as good as those of the gold plate.

For gold plating process, the effect of tin plating is greatly reduced, while that of gold deposition is better. Unless the manufacturer requires binding, most manufacturers will choose the process of gold deposition. Generally, PCB surface treatment is as follows:

Gold plating (electroplating gold, sinking gold), silver plating, OSP, tin spraying (lead and lead-free).

These are mainly for FR-4 or CEM-3 boards, and the surface treatment of paper base materials is also coated with rosin; if the reasons of solder paste and other patch manufacturers' production and material technology are excluded, poor tin coating (poor tin eating) is considered.

There are several reasons for PCB problems

1. In PCB Printing, whether there is oil leakage film on pan position can block the effect of tin coating; this can be verified by tin bleaching test.

2. Whether the pan position meets the design requirements, that is, whether the pad design can ensure the supporting role of the parts.

3. The results can be obtained by ion contamination test. The above three points are basically the key aspects considered by PCB manufacturers.

About the advantages and disadvantages of several ways of surface treatment, is each has its own advantages and disadvantages!

In terms of gold plating, PCB can be stored for a long time, and the temperature and humidity of the external environment change little (compared with other surface treatment), which can be stored for about a year; tin spraying surface treatment, followed by OSP again, these two kinds of surface treatment should pay attention to the storage time of environmental temperature and humidity.

Generally speaking, the surface treatment of silver precipitation is a little different, the price is also high, and the storage conditions are more stringent, so it needs to be packaged with sulfur-free paper! And the storage time is about three months! In terms of tin effect, the sinking gold, OSP, tin spraying and so on are actually similar, and the ipcb-pcb manufacturers mainly consider the cost-effectiveness aspect!