About gold, silver and copper in PCB boards
Printed Circuit Board (PCB) is a basic electronic component that is widely used in various electronic and related products. PCB is sometimes also called PWB (Printed Wire Board, printed circuit board). It was used more in Hong Kong, China and Japan, but now it is less and less (in fact, there is a difference between PCB and PWB).
In Western countries and regions, it is generally called PCB. In the East, the name varies depending on the country and region. For example, in mainland China, it is now generally called printed circuit board (previously called printed circuit board), and in Taiwan, it is generally called PCB. Circuit boards are called electronic (circuit) substrates in Japan and substrates in Korea.
PCB is the support body of electronic components and the carrier for electrical connection of electronic components. It mainly plays the role of support and interconnection. Simply looking at the appearance, the outer layer of the circuit board mainly comes in three colors: gold, silver, and light red. Classified by price: gold is the most expensive, followed by silver, and light red is the cheapest. However, the circuits inside the circuit board are mainly pure copper, that is, bare copper boards.
It is said that there are many precious metals on the PCB. It is reported that on average, each smartphone contains 0.05g of gold, 0.26g of silver, and 12.6g of copper. The gold content of a laptop is 10 times that of a mobile phone!
Why are there precious metals on PCB?
As a support for electronic components, PCB requires soldering components on its surface, which requires a part of the copper layer to be exposed for soldering. These exposed copper layers are called pads. The pads are generally rectangular or circular with a small area. Therefore, after the solder resist is applied, the only thing exposed to the air is the copper on the pad.
The exposed pads on the PCB have the copper layer directly exposed. This part needs to be protected from oxidation.
The copper used in 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, seriously affecting the performance of the final product. Therefore, the pad is plated with inert metal gold, or its surface is covered with a layer of silver through a chemical process, or the copper layer is covered with a special chemical film to prevent the pad from contacting the air. Prevent oxidation and protect the pad to ensure yield in the subsequent welding process.
Gold, silver and copper on PCB
1. PCB copper clad board
Copper-clad laminate is a plate-shaped material made by impregnating glass fiber cloth or other reinforcing materials with resin and covering one or both sides with copper foil and hot pressing.
Taking fiberglass cloth-based copper-clad laminate as an example, its main raw materials are copper foil, fiberglass cloth, and epoxy resin, accounting for approximately 32%, 29%, and 26% of the product cost respectively.
Copper clad laminate is the basic material of printed circuit board, and printed circuit board is an indispensable main component for most electronic products to achieve circuit interconnection. With the continuous improvement of scientific and technological level, in recent years, some special electronic copper clad laminates can be used Direct manufacturing of printed electronic components. The conductors used in printed circuit boards are generally made of thin foil-shaped refined copper, which is copper foil in a narrow sense.
2. PCB immersed gold circuit board
If gold and copper are in direct contact, there will be a physical reaction of electron migration and diffusion (relationship between potential differences), so a layer of "nickel" must be electroplated first as a barrier layer, and then gold is electroplated on top of the nickel, so what we generally call Electroplated gold, its actual name should be called "electroplated nickel gold".
The difference between hard gold and soft gold is the composition of the final layer of gold that is plated. When gold plating, you can choose to electroplat 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 specifically require the thickness of this layer of pure gold when making aluminum wires. In addition, if you choose electroplated gold-nickel alloy or gold-cobalt alloy, because the alloy will be harder than pure gold, it is also called "hard gold".
The gold plating layer is widely used in component pads, gold fingers, connector shrapnel, etc. of circuit boards. The main boards 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 thin layer is plated, it already accounts for nearly 10% of the cost of the circuit board. Gold is used as a plating layer, firstly to facilitate welding, and secondly to prevent corrosion. Even if the gold finger of a memory stick has been used for several years, it still shines as bright as ever. If copper, aluminum, or iron are used, they will quickly rust into a pile of waste. In addition, the cost of gold-plated plates is high and the welding strength is poor. Because of the electroless nickel plating process, black plates are prone to occur. The nickel layer will oxidize over time, and long-term reliability is also an issue.
3. PCB immersed silver circuit board
Immersion silver is cheaper than immersion gold. If the PCB has functional requirements for connection and needs to reduce costs, immersion silver is a good choice. In addition, immersion silver has good flatness and contact, so the immersion silver process should be chosen.
