1. PCB board surface treatment
PCB board surface treatment processes include: anti-oxidation, spray tin, lead-free spray tin, sink gold, sink tin, sink silver, hard gold plating, full-board gold plating, gold finger, nickel-palladium-gold OSP, etc.. Requirements are mainly: lower cost, good solderability, harsh storage conditions, short time, environmentally friendly processes, good welding, flat . Tin spraying: Tin spraying boards are generally multi-layer (4-46 layers) high precision PCB prototype boards, which have been used by many large domestic communication, computer, medical equipment and aerospace companies and research units.
The connecting finger is the component on the memory stick that connects to the memory slot, and all signals are transmitted through the gold finger. Gold fingers are composed of numerous gold-colored conductive contacts, which are called "gold fingers" because of their gold-plated surface and the finger-like arrangement of the conductive contacts, and gold fingerboards need to be gold-plated or gold-sunk. Gold finger is actually in the copper laminate through a special process and then covered with a layer of gold, because the gold is extremely strong resistant to oxidation, and conductivity is also very strong. However, because of the expensive price of gold, more memory is currently used instead of tin plating, from the 1990s tin material began to popularize, the current motherboard, memory and graphics cards and other equipment "gold finger" is almost all used in the tin material, only some high-performance server / workstation accessories contact points will continue to use gold-plated practice, the price is naturally not expensive.
2. the difference between gold plating and sink gold process
Immersion gold uses the method of chemical deposition, through the method of a chemical redox reaction to generate a layer of plating, generally thicker, is a chemical nickel gold layer deposition method, can achieve a thicker layer of gold.
Gold plating uses the principle of electrolysis, also called the electroplating method. Most other metal surface treatments are also used in the electroplating method.
In the actual product application, 90% of the gold board is sink gold board, because the poor solderability of the gold-plated board is his fatal flaw, and is the direct cause of many companies to abandon the gold-plating process!
Immersion gold process in the printed circuit surface deposition of stable color, good brightness, flat plating, good solderability of nickel-gold plating. It can be basically divided into four stages: pre-treatment (oil removal, micro-etching, activation, post-dipping), nickel sink, gold sink, and post-treatment (waste gold wash, DI wash, drying). Immersion gold thickness between 0.025-0.1um.
Gold is used in the surface treatment of circuit boards, because of the strong electrical conductivity of gold, good resistance to oxidation, long life, general applications such as keypads, gold fingerboards, etc., and gold-plated board and the most fundamental difference is that the gold-plated board is hard gold (wear-resistant), sink gold is soft gold (not wear-resistant).
Sink gold and gold plating formed by the crystal structure is different, sink gold for gold thickness than gold plating to be much thicker, sink gold will be golden yellow, more yellow than gold plating (this is one of the ways to distinguish between gold plating and sink gold), gold plating will be slightly white (the color of nickel).
Sink gold and gold plating formed by the crystal structure is not the same, sink gold compared to gold plating is easier to weld, will not cause poor welding. The stress of the sunken gold plate is easier to control, and for products with bonding, it is more conducive to the processing of bonding. At the same time is also because sink gold than gold plating soft, so sink gold plate to do gold finger is not wear-resistant (the disadvantage of sink gold plate).
l Sink gold board only pads with nickel gold, skin effect in the signal transmission is in the copper layer will not have an impact on the signal.
l Sink gold than gold-plated crystal structure is more dense, not easy to produce into oxidation.
l With the board processing precision requirements are increasingly high, line width, spacing has been to 0.1mm or less. Gold plating is easy to produce gold wire short circuit. Immersion gold board only pads with nickel gold, so it is not easy to produce gold wire short circuit.
l Sink gold board only pads with nickel gold, so the line of solder resistance and copper layer of the combination is more solid. Engineering in making compensation will not have an impact on the pitch.
l For the higher requirements of the board, flatness requirements to be good, generally use sink gold, sink gold generally will not appear after the assembly of the black pad phenomenon. The flatness and service life of the sunken gold plate is better than the gold-plated plate.
3. Why use the gold-plated board？
As the integration of IC is getting higher and higher, IC feet are denser and denser. And vertical spray tin process is difficult to become fine pad blow flat, which brings difficulty to SMT placement; In addition, the spray tin board's life expectancy (shelf life) is very short. And gold plated board just solves these problems: 1, for the surface mount process, especially for 0603 and 0402 ultra small table stickers, because the pad flatness is directly related to the quality of the solder paste printing process, to play a decisive influence on the quality of the later re-flow welding, so, the whole board gold plating in high density and ultra-small table sticker process is often seen. 2, in the trial stage, subject to the influence of factors such as component procurement often Not the board to immediately solder, but often wait for weeks or even months to use the gold-plated board to be used (shelf life) than lead-tin alloy many times longer so we are happy to use. Besides, the cost of gold-plated PCBs in the sample stage is comparable to that of lead-tin alloy boards. But with the wiring more and more dense, line width, spacing has been to 3-4MIL. therefore brings the problem of gold wire short circuit: With the signal, frequency is getting higher and higher, due to the skin effect caused by the signal transmission in multiple layers of the signal quality of the more obvious impact. Skin effect is: high frequency of alternating current, the current will tend to concentrate on the surface of the wire flow. According to the calculation, the skinning depth is related to the frequency.
4. Why use sink gold plate？
In order to solve the above problems of gold-plated board, the use of gold-plated PCBs mainly have the following characteristics:
l Because the crystal structure formed by sink gold and gold plating is not the same, sink gold will be more yellow than gold plating, the customer is more satisfied.
l Because the crystal structure formed by the sunken gold and gold plating is different, sunken gold is easier to weld than gold plating, will not cause poor welding, causing customer complaints.
l Because of the sink gold board only pads have nickel gold, skin effect in the signal transmission is in the copper layer will not have an impact on the signal.
l Sink gold is more dense than gold-plated crystal structure, not easy to produce into oxidation.
l Because the sink gold board only has nickel gold on the pad, so it will not produce gold wire caused by micro short.
l Because the sink gold board only has nickel gold on the pad, so the line of solder resist and copper layer of the combination is more solid.
l The project will not have an impact on the pitch when making compensation.
l Because the crystal structure formed by sink gold and gold plating is not the same, the stress of its sink gold board is easier to control, for products with bonding, more conducive to the processing of bonding. At the same time is also because sink gold than gold plating soft, so sink gold plate to do gold finger not wear-resistant.
l The flatness of the sunken gold plate and the life to be used and gold-plated plate as good.
So at present most factories have adopted the sinker gold process to produce gold plates. But sink gold process than gold plating process cost more expensive (higher gold content), so there are still a large number of low-cost products using gold plating process (such as remote control board, toy board).
5.In fact, there are two kinds of gold plating process: one is electroplating, one is sinking gold
For gold plating process, the effect of the tin on the greatly reduced, while the effect of the tin on the sunken gold is a little better; unless the manufacturers require binding, most manufacturers will now choose the sunken gold process! The general common case of PCB surface treatment for the following kinds of: gold plating (gold plating, sinking gold), silver plating, OSP, spray tin (lead and lead-free), these are mainly for FR-4 or CEM-3 and other plates, paper-based materials and coated rosin surface treatment; on the tin bad (eat tin bad) this piece if you exclude the paste and other SMD manufacturers production and material process reasons.
Here only for the PCB problem said, there are several reasons.
l In the PCB printing, PAN bit on whether there is oil seepage film surface, it can block the effect of tin; this can do bleaching tin test to verify.
l PAN bit on the wetting bit whether to meet the design requirements, that is, whether the pad design can be sufficient to ensure that the role of parts support.
l The pad has not been contaminated, which can be used to ion contamination test results.
On the advantages and disadvantages of several ways of surface treatment, is each has its own strengths and weaknesses! Gold plating, it can make the PCB stored for a longer period of time, and by the outside environment temperature and humidity changes less (relative to other surface treatments), generally can be saved for about a year; spray tin surface treatment followed by OSP again, these two surface treatments in the environmental temperature and humidity of the storage time to pay attention to many. In general, the sinking silver surface treatment is a little different, the price is also high, preservation conditions are more demanding, the need to use sulfur-free paper packaging treatment! And the preservation time is about three months! In terms of tin effect, sink gold, OSP, spray tin, etc. is actually about the same!
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