PCB News - Into the gold, silver and copper in the PCB board

Into the gold, silver and copper in the PCB board

Printed Circuit Board (PCB) is a basic electronic component, which is 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 boardmainly 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, that is, bare copper board.

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 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 the PCB?

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 applied, the only copper on the pads is exposed to the air.

For the exposed pads on the PCB, the copper layer is directly exposed. This part needs to be protected to prevent it from being oxidized.

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 pads, so that they 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 on 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.

Copper clad laminates are the basic materials 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-shaped 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 The actual name of electroplated gold 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".

The gold-plated layer is widely used in the component pads, gold fingers, and connector shrapnel of the circuit board. Most of the motherboards of the most widely used mobile phone circuit boards are 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. Use gold as a plating layer, one is to facilitate welding, and the other is to prevent corrosion. Even the gold fingers of the memory sticks that have been used for several years still flicker as before. If copper, aluminum, or iron are used, they 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, it is better to choose Immersion Silver technology.

Immersion Silver has many applications in communication products, automobiles, and computer peripherals, as well as 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 immersion silver's defects such as tarnishing and solder joint voids, its growth is slow (PCB factory).