With the rapid development of today's science and technology, the production of PCB technology has also undergone great changes, with the precision of the grasp, the manufacturing process also needs to improve. At the same time, the process requirements for PCB boards in each industry have gradually improved, such as the use of gold and copper in the circuit board of today's mobile phones and computers, resulting in the advantages and disadvantages of the circuit board has gradually become easier to distinguish.
Today, I will take you to understand the surface process of PCB board, and compare the advantages and disadvantages of different PCB board surface treatment processes and applicable scenarios.
Simply from the appearance, the outer layer of the circuit board has three main colors: gold, silver, light red. According to the price classification: gold is the most expensive, silver is second, light red is the cheapest, from the color is actually easy to determine whether there is a hardware manufacturer jerry-cutting behavior. However, the circuit inside the circuit board is mainly pure copper, that is, bare copper.
The advantages and disadvantages of bare copper plate are obvious, advantages: low cost, flat surface, good weldability (in the case of not being oxidized). Disadvantages: easy to be affected by acid and humidity, can not be long, after unpacking need to be used up within 2 hours, because copper exposed to the air is easy to oxidize; It cannot be used with dual panels because the second side has oxidized after the first reflow. If there is a test point, the solder paste must be printed to prevent oxidation, otherwise it will not be able to make good contact with the probe later.
Pure copper is easily oxidized if exposed to air, and the outer layer must have the above protective layer. And some people think that gold is copper, that is not the idea, because it is the protective layer above copper. Therefore, it is necessary to gold-plate a large area on the circuit board, which is the gold sinking process that I have taken you to understand before.
The second is the gold-plated plate, as the name suggests is to plate a layer of gold, of course, is real gold, even if only plated a very thin layer, it has accounted for nearly 10% of the cost of the circuit board. In Shenzhen, there are many businessmen who specialize in buying used circuit boards, and washing out gold by certain means is a good income.
PCB manufacturers use gold as a coating, one is to facilitate welding, and the other is to prevent corrosion. Even if the gold finger of the memory has been used for several years, it is still flashing as before, if the copper, aluminum, and iron were used, it has now been rusted into a pile of waste.
Gold-plated plate is widely used in the parts of the circuit board solder pad, gold finger, connector shrapnel and other positions. If you find that the circuit board is actually silver, it does not need to be said, call the consumer rights hotline directly, it must be the manufacturer cut corners, did not use the material well, and used other metals to fool customers. The most widely used mobile phone circuit board motherboard is mostly gold-plated, gold-plated, computer motherboards, audio and small digital circuit boards are generally not gold-plated.
Through the above introduction, it is not difficult to find the advantages and disadvantages of gold-plated plate, advantages: not easy to oxidize, can be stored for a long time, the surface is flat, suitable for welding fine-gap pins and components with small solder joints. Preferred PCB board with buttons (such as mobile phone board). Reflow welding can be repeated multiple times without much loss of weldability. It can be used as the base material for COB(Chip On Board) wiring. Disadvantages: high cost, poor welding strength, because the use of non-electroplated nickel process, easy to have black disk problems. The nickel layer oxidizes over time, and long-term reliability is an issue.
Now many small partners will ask, the yellow is gold, then the silver is silver? Of course not. The correct answer is: tin.
This board is called a tin-spray circuit board. Spraying a layer of tin over the copper wire can also help with welding. But it doesn't offer the same long-term contact reliability as gold. It has no effect on the components that have been welded, but the reliability is not enough for the pads exposed to the air for a long time, such as the ground pad, the pin socket and so on. Long-term use is easy to oxidize and rust, resulting in poor contact. Basically used as a circuit board for small digital products, without exception is the spray tin plate, the reason is cheap.
Its advantages and disadvantages are not difficult to summarize, advantages: low price, good welding performance. Disadvantages: It is not suitable for welding pins with fine gaps and too small components, because the surface of the tinspray plate is poor. In PCB processing, it is easy to produce solder bead, which is more likely to cause short circuit to fine pitch components. When used in double-sided SMT process, because the second side has undergone a high temperature reflow welding, it is easy to spray tin and re-melt, resulting in tin beads or similar water beads affected by gravity into dripping ball tin points, resulting in more uneven surface and thus affecting welding problems.
The last one is the OPS process, in simple terms, is the organic welding film. Because it is organic, not metal, it is cheaper than the tin-spraying process. Its advantage is that it has all the advantages of bare copper plate welding, and the expired board can also be re-finished. Disadvantages: Easily affected by acid and humidity. When used in secondary reflow welding, it needs to be completed within a certain time, usually the second reflow welding effect will be relatively poor. If the storage time exceeds three months, it must be resurfaced. It should be used within 24 hours after opening the package. OSP is an insulating layer, so the test point must be stamped with solder paste to remove the original OSP layer before touching the pin point for electrical testing.
The only function of this organic film is to ensure that the inner copper foil will not be oxidized before welding. The film evaporates when it's heated during welding. The solder can be used to weld the copper wire and components together. However, there is a problem that has not yet been solved, that is, it is very corrosion resistant, an OSP circuit board, exposed to the air for ten days, can not weld components. The OSP process is mainly experienced on the computer motherboard, because the computer circuit board is too large, if the use of gold plating, the cost will be high.
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