1. PCB surface treatment
Anti-oxidation, tin spray, lead-free spray tin, immersion gold, immersion tin, immersion silver, hard gold plating, full board gold plating, gold finger, nickel palladium gold OSP: low cost, good solderability, harsh storage conditions, time Short, environmentally friendly process, well welded and flat. Tin spraying: The tin spraying board is generally a multi-layer (4-46 layer) high-precision PCB template, which has been used by many large domestic communication, computer, medical equipment and aerospace enterprises and research units. ) is the connection part between the memory stick and the memory slot, and all signals are transmitted through the gold finger.
The gold finger is composed of many golden-yellow conductive contacts, which are called "gold fingers" because the surface is gold-plated and the conductive contacts are arranged like fingers. The gold finger is actually covered with a layer of gold on the copper clad laminate through a special process, because gold has strong oxidation resistance and strong conductivity. However, due to the high price of gold, most memory is currently replaced by tin plating. Since the 1990s, tin materials have become popular. At present, the "gold fingers" of motherboards, memory and graphics cards are almost all used. Tin material, only some high-performance server/workstation accessories contact points will continue to use gold plating, which is naturally expensive.
2. Why use gold-plated plates?
As the integration level of IC is getting higher and higher, the more and denser IC pins are. The vertical tin spray process is difficult to flatten the thin pads, which brings difficulties to the SMT placement; in addition, the shelf life of the tin spray board is very short. The gold-plated plate just solves these problems:
2.1. For the surface mount process, especially for the 0603 and 0402 ultra-small surface mount, 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 subsequent reflow soldering. Therefore, the whole board Gold plating is common in high-density and ultra-small surface mount processes.
2.2. In the trial production stage, affected by factors such as component procurement, it is often not that the board will be welded immediately after it arrives, but it often takes several weeks or even a month to use it. The shelf life of the gold-plated board is longer than that of lead. Tin 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. Therefore, the problem of short-circuiting of the gold wire is brought about: as the frequency of the signal becomes higher and higher, the transmission of the signal in the multi-layer coating caused by the skin effect has a more obvious impact 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 frequency dependent.
3. Why use immersion gold plate
In order to solve the above problems of gold-plated boards, PCBs using immersion gold boards mainly have the following characteristics:
3.1. Because the crystal structure formed by immersion gold and gold plating is different, immersion gold will be more golden yellow than gold plating, and customers are more satisfied.
3.2. Because the crystal structures formed by immersion gold and gold plating are different, immersion gold is easier to weld than gold plating, and it will not cause poor welding and cause customer complaints.
3.3. Since the immersion gold board only has nickel-gold on the pad, the transmission of the signal in the skin effect is in the copper layer and will not affect the signal.
3.4. Compared with gold plating, immersion gold has a denser crystal structure and is not easy to produce oxidation.
3.5. Since the immersion gold plate only has nickel-gold on the pad, it will not produce gold wire and cause slight shortness.
3.6. Since the immersion gold board only has nickel-gold on the pad, the combination of the solder mask on the circuit and the copper layer is stronger.
3.7. The project will not affect the spacing when making compensation.
3.8. 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 immersion gold is softer than gold plating, so the gold finger of immersion gold plate is not wear-resistant.
3.9. The flatness and standby life of the immersion gold plate are as good as those of the gold-plated plate.
4. Immersion gold plate VS gold-plated plate In fact, the gold plating process is divided into two types: one is electroplating gold, and the other is immersion gold
For the gold plating process, the effect of tin is greatly reduced, and the effect of immersion gold is better; unless the manufacturer requires binding, most manufacturers will now choose the immersion gold process! Generally common In the case of PCB surface treatment, the following types are: gold plating (gold electroplating, immersion gold), silver plating, OSP, tin spray (lead and lead-free), these types are mainly for FR-4 or CEM-3 and other boards In other words, the paper base material also has a surface treatment method of rosin coating; if the tin is not good (poor tin eating), if the production and material process of the solder paste and other chip manufacturers are excluded.
Here only for PCB problems, there are several reasons:
a. During PCB printing, whether there is an oil seepage film surface on the PAN position, it can block the effect of tin; this can be verified by a tin bleaching test.
b. Whether the moistening of the PAN bit meets the design requirements, that is, whether the pad design can sufficiently ensure the support of the parts.
c. Whether the pad is contaminated, which can be obtained by ion contamination test;
The above three points are basically the key aspects that PCB manufacturers consider. Regarding the advantages and disadvantages of several methods of surface treatment, each has its own advantages and disadvantages! In terms of gold plating, it can make the PCB storage time for a long time, and the temperature and humidity of the external environment change less (compared to other surface treatments), and it can generally be stored for about a year; the tin spray surface treatment is second, and OSP is again. A lot of attention should be paid to the storage time of the two surface treatments at ambient temperature and humidity. In general, the surface treatment of immersion silver is a bit different, the price is also high, the storage conditions are more severe, and it needs to be packaged with sulfur-free paper! And the storage time is about three months! In terms of tinning effect, immersion gold, OSP, In fact, spray tin, etc. are almost the same, the manufacturer mainly considers the cost performance aspect!
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