The PCB is support for electronic components and is a carrier for electrical connection of electronic components, mainly for supporting and interconnecting. From the outside, the outer layer of the board has three main colors: gold, silver, and light red. Classified by price: gold is the most expensive, silver is second, light red is the cheapest. However, the circuit inside the board is mainly pure copper, which is bare copper.
Why are there precious metals on the PCB?
PCB as a support for electronic components, the surface of which requires soldering components, requires a portion of the copper layer to be exposed for soldering. These exposed copper layers are called pads. The pads are generally rectangular or circular, and the area is small. Therefore, after the solder resist is applied, the only copper exposed on the pad is exposed to the air.
The exposed pad on the PCB, the copper layer is directly exposed. This part needs protection to prevent it from being oxidized.
The copper used in the PCB is easily oxidized. If the copper on the pad is oxidized, it is not only difficult to solder, but also the resistivity is greatly increased, which seriously affects the performance of the final product. Therefore, the pad is plated with inert metal gold, or a layer of silver is chemically coated on its surface, or a special chemical film is used to cover the copper layer to prevent contact between the pad and the air. Prevents oxidation and protects the pad from ensuring yield during the next soldering process.
Gold, silver and copper on the PCB
1, PCB CCL
The copper-clad laminate is a plate-like material which is obtained by immersing a glass fiber cloth or other reinforcing material on one side of the resin or double-sided coated with a copper foil and hot pressing.
Taking glass fiber-based copper-clad laminates as an example, the 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.
CCL is the basic material of printed circuit boards, and printed circuit boards are the indispensable main components of most electronic products to achieve circuit interconnection. With the continuous improvement of technology, some special electronic CCLs can be used in direct manufacturing of printed electronic components in recent years. Conductors for 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 is in direct contact with copper, there will be a physical reaction (potential difference) in which electrons migrate and diffuse. Therefore, it is necessary to first plate a layer of "nickel" as a barrier layer, and then plate gold onto the top of 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 this layer of gold that is finally plated. When gold plating, you can choose to plate pure gold or alloy. Because the hardness of pure gold is soft, it is called "soft gold".Because "gold" can form a good alloy with "aluminum", COB will specifically require the thickness of this layer of pure gold when hitting the aluminum wire. In addition, if you choose electroplated gold-nickel alloy or gold-cobalt alloy, because the alloy will be harder than pure gold, it is called "hard gold".
The gold-plated layer is widely used in component pads, gold fingers, connector shrapnel, etc. of the board. The motherboards of the most widely used mobile phone circuit boards are mostly gold-plated boards, and the gold-plated boards, computer motherboards, audio, and small digital boards are generally not gold-plated.
Gold is the real gold. Even if only a very thin layer is plated, it already accounts for nearly 10% of the cost of the board. Gold is used as the coating, one for the convenience of welding and the other for corrosion protection. Even the golden fingers that have been used for several years are still flashing. If copper, aluminum or iron is used, it will soon be rusted into a pile of waste. In addition, the cost of the gold plate is high, and the welding strength is poor because the use of the electroless nickel process makes it easy to have a black disk problem. The nickel layer oxidizes 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 connectivity requirements and needs to reduce costs, immersion silver is a good choice; plus the good flatness and contact of the immersion silver, it should choose the silver process.
There are a lot of applications in the field of communication products, automobiles, and computer peripherals. In the high-speed signal design, immersion silver has also been applied. Since immersion silver has good electrical properties unmatched by another surface treatments, it can also be used in high frequency signals. EMS recommends the use of the immersion silver process because it is easy to assemble and has good checkability. However, due to defects such as tarnishing and solder joint voids, the growth of the silver is slow (but not degraded).
