Printed Circuit Board (PCB) is a basic electronic component widely used in various electronic and related products. PCB is sometimes called PWB (Printed Wire Board). It used to be more in Hong Kong and Japan before, but now it is less (in fact, PCB and PWB are different). In Western countries and regions, it is generally called PCB. In the East, it has different names due to different countries and regions. For example, it is generally called printed circuit board in mainland China (previously called printed circuit board), and it is generally called PCB in Taiwan. Circuit boards are called electronic (circuit) substrates in Japan and substrates in South Korea.
PCB is the support of electronic components and the carrier of the electrical connection of electronic components, mainly supporting and interconnecting. Purely from the outside, the outer layer of the circuit board mainly has three colors: gold, silver, and light red. Classified by price: gold is the most expensive, silver is second, and light red is the cheapest. However, the wiring inside the circuit board is mainly pure copper, which is bare copper.
It is said that there are still many precious metals on the PCB. It is reported that, on average, each smart phone contains 0.05g gold, 0.26g silver, and 12.6g copper. The gold content of a laptop is 10 times that of a mobile phone!
As a support for electronic components, PCBs require soldering components on the surface, and a part of the copper layer is required to be exposed for soldering. These exposed copper layers are called pads. The pads are generally rectangular or round with a small area. Therefore, after the solder mask is painted, the only copper on the pads is exposed to the air.
The copper used in the PCB is easily oxidized. If the copper on the pad is oxidized, it will not only be difficult to solder, but also the resistivity will greatly increase, which will seriously affect the performance of the final product. Therefore, the pad is plated with inert metal gold, or the surface is covered with a layer of silver through a chemical process, or a special chemical film is used to cover the copper layer to prevent the pad from contacting the air. Prevent oxidation and protect the pad, so that it can ensure the yield in the subsequent soldering process.
1. PCB copper clad laminate
Copper clad laminate is a plate-shaped material made by impregnating glass fiber cloth or other reinforcing materials with resin on one side or both sides with copper foil and hot pressing.
Take glass fiber cloth-based copper clad laminate as an example. Its main raw materials are copper foil, glass fiber cloth, and epoxy resin, which account for about 32%, 29% and 26% of the product cost, respectively.
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Copper clad laminate is the basic material of printed circuit boards, and printed circuit boards are the indispensable main components for most electronic products to achieve circuit interconnection. With the continuous improvement of technology, some special electronic copper clad laminates can be used in recent years. Directly manufacture printed electronic components. The conductors used in printed circuit boards are generally made of thin foil-like refined copper, that is, copper foil in a narrow sense.
2. PCB Immersion Gold Circuit Board
If gold and copper are in direct contact, there will be a physical reaction of electron migration and diffusion (the relationship between the potential difference), so a layer of “nickel” must be electroplated as a barrier layer, and then gold is electroplated on top of the nickel, so we generally call it Electroplated gold, its actual name should be called “electroplated nickel gold”.
The difference between hard gold and soft gold is the composition of the last layer of gold that is plated on. When gold plating, you can choose to electroplate pure gold or alloy. Because the hardness of pure gold is relatively soft, it is also called “soft gold” . Because “gold” can form a good alloy with “aluminum”, COB will particularly require the thickness of this layer of pure gold when making aluminum wires. In addition, if you choose to electroplated gold-nickel alloy or gold-cobalt alloy, because the alloy will be harder than pure gold, it is also called “hard gold”.
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The gold-plated layer is widely used in the component pads, gold fingers, and connector shrapnel of the circuit board. The motherboards of the most widely used mobile phone circuit boards are mostly gold-plated boards, immersed gold boards, computer motherboards, audio and small digital circuit boards are generally not gold-plated boards.
Gold is real gold. Even if only a very thin layer is plated, it already accounts for nearly 10% of the cost of the circuit board. The use of gold as a plating layer is one for facilitating welding and the other for preventing corrosion. Even the gold finger of the memory stick that has been used for several years still flickers as before. If you use copper, aluminum, or iron, it will quickly rust into a pile of scraps. In addition, the cost of the gold-plated plate is relatively high, and the welding strength is poor. Because the electroless nickel plating process is used, the problem of black disks is likely to occur. The nickel layer will oxidize over time, and long-term reliability is also a problem.
3. PCB Immersion Silver Circuit Board
Immersion Silver is cheaper than Immersion Gold. If the PCB has connection functional requirements and needs to reduce costs, Immersion Silver is a good choice; coupled with Immersion Silver’s good flatness and contact, then the Immersion Silver process should be chosen.
Immersion Silver has many applications in communication products, automobiles, and computer peripherals, and it also has applications in high-speed signal design. Since Immersion Silver has good electrical properties that other surface treatments cannot match, it can also be used in high-frequency signals. EMS recommends using the immersion silver process because it is easy to assemble and has better checkability. However, due to defects such as tarnishing and solder joint voids, the growth of immersion silver has been slow (but not decreased).
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The printed circuit board is used as the connection carrier of integrated electronic components, and the quality of the circuit board will directly affect the performance of intelligent electronic equipment. Among them, the plating quality of printed circuit boards is particularly important. Electroplating can improve the protection, solderability, conductivity and wear resistance of the circuit board. In the manufacturing process of printed circuit boards, electroplating is an important step. The quality of electroplating is related to the success or failure of the entire process and the performance of the circuit board.
The main electroplating processes of pcb are copper plating, tin plating, nickel plating, gold plating and so on. Copper electroplating is the basic plating for the electrical interconnection of circuit boards; tin electroplating is a necessary condition for the production of high-precision circuits as the anti-corrosion layer in pattern processing; nickel electroplating is to electroplate a nickel barrier layer on the circuit board to prevent copper and gold Mutual dialysis; electroplating gold prevents passivation of the nickel surface to meet the performance of soldering and corrosion resistance of the circuit board.