PCB Technical - Why gold plating on PCB board?

1. PCB board surface treatment:

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

Tin: Tinjet board is usually a multi-layer (4-46 layer) high-precision PCB sample, which has been used by many large communication, computer, medical equipment and aerospace enterprises and research units in China. Goldfinger is a connecting part between memory and memory slot, through which all signals are transmitted.

The gold finger is made up of a number of golden conductive contacts, which are gold-plated and arranged in a finger-like manner.

Gold fingers are actually coated with a special layer of gold on top of copper-clad plates, because gold is highly resistant to oxidation and

highly conductive.

Because the price of gold is expensive, however, the current tin plating is used to replace the more memory, tin material from the last century 90 s began to spread, the motherboard, memory, and video devices such as "golden finger" is almost always used tin material, only some high-performance server/workstation accessories will contact point to continue the practice of using gold plated, the price expensive.

2. Why to use gold plate

With the higher degree of integration of IC, IC feet are more and more dense. The vertical tin-spraying process is difficult to flatten the thin pad, which brings difficulties to SMT. In addition, the shelf life of tin-sprayed plates is very short.

And the gold plate is a good solution to these problems:

1) for the surface mount process, especially for 0603 and 0402 ultra-small table paste, because the flatness of the pad is directly related to the quality of the paste printing process, the reflow welding quality has a decisive impact on the back, so the whole plate gold plating is often seen in the high-density and ultra-small table paste process.

2) In the trial production stage, under the influence of component procurement and other factors, the boards are not welded immediately after they come, but often wait for several weeks or even a month before they are used. The shelf life of gold-plated plates is many times longer than that of lead-tin alloy, so everyone is willing to adopt them.

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

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

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

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

ing to the calculation, skin depth is related to frequency.

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

1, because of the crystal structure formed by sunken gold and gold plating is not the same, sunken gold will be golden yellow more yellow than gold plating, customers are more satisfied.

2, sunk gold is easier to weld than gold, will not cause poor welding, causing customer complaints.

3. Because there is only nickel gold on the pad of the gold-plated plate, the signal transmission in the skin effect is in the copper layer, which will not affect the signal.

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

5, because the gold plate only has nickel gold on the pad, so it will not produce gold wire resulting in slightly short.

6, because the gold plate only has nickel gold on the pad, so the combination of resistance welding and copper layer on the line is more solid.

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

8, because the crystal structure formed by gold and gold plating is not the same, the stress of the gold plate is easier to control, which is more conducive to the processing of the state. At the same time, because the gold is softer than gold, so the gold plate is not wear-resistant.

9, The smoothness and service life of the sunken gold plate are as good as that of the gold-plated plate.

For the gold-plating process, the effect of tin is greatly reduced, and the tin effect of gold is better; Unless binding is required by the manufacturer, most manufacturers will choose the gold sinking process. Generally, the surface treatment of PCB is as follows:

Gold-plated (electroplated, sunk gold), silver plated, OSP, tin-sprayed (lead and lead-free).

These are mainly for FR-4 or CEM-3 plate, paper base material and rosin coated surface treatment; On tin bad (tin eating bad) this if the exclusion of solder paste and other patch manufacturers production and material technology reasons.

Here only for PCB problems, there are several reasons:

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

2, Whether the PAN position meets the design requirements, that is, whether the pad design can be enough to ensure the support of parts.

3, pad is not contaminated, which can be obtained by ion pollution test results; The above three points are basically the key aspects considered by PCB manufacturers.

The advantages and disadvantages of several ways of surface treatment, are each have their strengths and weaknesses!

Gold-plating, it can make THE PCB storage time is longer, and by the external environment temperature and humidity change is small (relative to other surface treatment), generally can be saved for about a year; Tin spray surface treatment, OSP again, these two kinds of surface treatment in the environment temperature and humidity storage time to pay attention to many.