PCB News - The surface treatment process of PCB board and its advantages and disadvantages and applicable scenarios

The surface treatment process of PCB board and its advantages and disadvantages and applicable scenarios

With the continuous development of electronic science and technology, PCB technology has also undergone tremendous changes, and the manufacturing process also needs to be improved. At the same time, the process requirements for PCB circuit boards in each industry have gradually improved. For example, in the circuit boards of mobile phones and computers, gold and copper are used, which makes it easier to distinguish the advantages and disadvantages of circuit boards.

Today, I will take you to understand the surface technology of PCB boards and compare the advantages and disadvantages and applicable scenarios of different PCB board surface treatment processes.

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. In fact, it is easy to judge from the color whether hardware manufacturers are cutting corners. However, the wiring inside the circuit board is mainly pure copper, that is, bare copper board.

1. Bare copper plate

The advantages and disadvantages are obvious:

Advantages: low cost, smooth surface, good weldability (in the absence of oxidation).

Disadvantages: It is easy to be affected by acid and humidity and cannot be stored for a long time. It must be used up within 2 hours after unpacking, because copper is easily oxidized when exposed to the air; it cannot be used for double-sided boards because the second side after the first reflow soldering It's already oxidized. If there is a test point, solder paste must be printed to prevent oxidation, otherwise it will not be in good contact with the probe.

Pure copper is easily oxidized if exposed to the air, and the outer layer must have the above-mentioned protective layer. And some people think that the golden yellow is copper, which is wrong because it is the protective layer on the copper. Therefore, it is necessary to plate a large area of gold on the circuit board, which is the immersion gold process that I have taught you before.

Second, the gold-plated board

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. In Shenzhen, there are many merchants who specialize in buying waste circuit boards. They can wash out gold through certain means, which is a good income.

Use gold as a plating layer, one is to facilitate welding, and the other is to prevent corrosion. Even the gold finger of the memory stick that has been used for several years still flickers as before. If copper, aluminum, and iron were used in the first place, they have now rusted into a pile of scraps.

The gold-plated layer is widely used in the component pads, gold fingers, and connector shrapnel of the circuit board. If you find that the circuit board is actually silver, it goes without saying. If you call the consumer rights hotline directly, the manufacturer must be cutting corners, failing to use materials properly, and using other metals to fool customers. 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.

The advantages and disadvantages of immersion gold technology are actually not difficult to draw:

Advantages: It is not easy to oxidize, can be stored for a long time, and the surface is flat, suitable for welding small gap pins and components with small solder joints. The first choice for PCB boards with buttons (such as mobile phone boards). Reflow soldering can be repeated many times without reducing its solderability. It can be used as a substrate for COB (Chip On Board) wire bonding.

Disadvantages: high cost, poor welding strength, because the electroless nickel plating process is used, it is easy to have the problem of black disk. The nickel layer will oxidize over time, and long-term reliability is a problem.

Now we know that gold is gold and silver is silver? Of course not, it is tin.

Three, spray tin circuit board

The silver board is called the spray tin board. Spraying a layer of tin on the outer layer of the copper circuit can also help soldering. But it cannot provide long-term contact reliability like gold. It has no effect on the components that have been soldered, but the reliability is not enough for the pads that have been exposed to the air for a long time, such as grounding pads and pin sockets. Long-term use is prone to oxidation and corrosion, resulting in poor contact. Basically used as the circuit board of small digital products, without exception, the spray tin board, the reason is that it is cheap.

Its advantages and disadvantages are summarized as:

Advantages: lower price and good welding performance.

Disadvantages: Not suitable for welding pins with fine gaps and components that are too small, because the surface flatness of the spray tin plate is poor. Solder bead is easy to produce in PCB processing, and it is easy to cause short circuit to fine pitch components. When used in the double-sided SMT process, because the second side has undergone a high-temperature reflow soldering, it is easy to spray tin and re-melt to produce tin beads or similar water drops into spherical tin dots that are affected by gravity, which makes the surface even worse. Flattening affects welding problems.

Before talking about the cheapest light red circuit board, that is, the miner’s lamp thermoelectric separation copper substrate

Four, OSP craft board

Organic soldering film. Because it is organic, not metal, it is cheaper than tin spraying.

The advantages and disadvantages are

Advantages: It has all the advantages of bare copper plate welding, and the expired board can also be surface treated again.

Disadvantages: easily affected by acid and humidity. When used in the secondary reflow soldering, it needs to be completed within a certain period of time, and usually the effect of the second reflow soldering will be relatively poor. If the storage time exceeds three months, it must be resurfaced. It must be used up within 24 hours after opening the package. OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer before it can contact 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. This layer of film evaporates as soon as it is heated during welding. The solder can weld the copper wire and the components together.

But it is not resistant to corrosion. If an OSP circuit board is exposed to the air for ten days, the components cannot be welded.

Many computer motherboards use OSP technology. Because the area of the circuit board is too large, it cannot be used for gold plating.