The surface treatment processes of PCB board

1. Bare copper plate

The advantages and disadvantages are obvious:

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

Disadvantages: easy to be affected by acid and humidity, can not be kept for a long time, need to be used up within 2 hours after unpacking, because copper is easily oxidized when exposed to air; Cannot be used on double panels because the second side is oxidized after the first reflow welding. If there are test points, solder paste must be printed to prevent oxidation, otherwise subsequent contact with the probe will not be good.

Pure copper is easily oxidized if exposed to air and must have the above protective coating. And some people think that gold is copper, which is not true, because that’s the protective layer over the copper. So you need a large area of gold plating on the circuit board, that is, I took you to understand the gold process.

Two, gold plate

Gold is the real gold. Even a thin coating accounts for nearly 10% of the cost of a circuit board. In Shenzhen, there are a lot of merchants specializing in the acquisition of scrap circuit boards, through certain means to wash out the gold, is a good income. The use of gold as a coating, one is to facilitate welding, the other is to prevent corrosion. Even if they did

The gold fingers of several years’ worth of memory sticks still glint as they did when they were made of copper, aluminum and iron, which now rust into piles of junk.

The gold plated layer is widely used in the parts of the circuit board pads, gold fingers, connector shrapnel and other positions. If you find that the circuit board is actually silver, that need not say, directly call the consumer rights hotline, certainly is the manufacturer jerry-built, did not make good use of materials, with other metals deceive customers. We use the most extensive mobile phone circuit board motherboard is mostly gold plate, sunken gold plate, computer motherboard, audio and small digital circuit board are generally not gold plate.

The advantages and disadvantages of the gold sinking process are not difficult to come to:

Advantages: not easy to oxidize, can be stored for a long time, the surface is smooth, suitable for welding small gap pins and components with small solder joints. Preferred PCB board with key (e.g. mobile phone board). Reflow soldering can be repeated many times without much loss of solderability. It can be used as the base material for COB(Chip On Board) cabling.

Disadvantages: high cost, poor welding strength, because of the use of nickel plating process, easy to have black plate problems. The nickel layer oxidizes over time, and long-term reliability is an issue.

Now do we know that gold is gold and silver is silver? Of course not. Tin.

Three, spray tin circuit board

The silver plates are called tinjet plates. Spraying a layer of tin over the copper wire can also aid in welding. But it doesn’t offer the same long-term contact reliability as gold. There is no impact on soldered components, but reliability is not sufficient for pads exposed to air for a long time, such as ground pads, spring pin sockets, etc. Long-term use easy oxidation corrosion, resulting in poor contact. Basically used as a small digital product circuit board, without exception is tinjet board, the reason is cheap.

Its advantages and disadvantages are summarized as follows:

Advantages: low price, good welding performance.

Disadvantages: Not suitable for soldering thin gap pins and too small components, due to poor surface flatness of tinjet plate. In PCB processing, it is easy to produce solder bead and cause short circuit for fine pitch components. When used in the double-sided SMT process, because the second surface has been a high temperature reflow welding, it is very easy to re-melt tin spraying and produce spherical tin beads or similar water beads dripping under the influence of gravity, resulting in more uneven surface and affecting welding problems.

Earlier, we talked about the cheapest light red circuit board, the miner’s lamp thermoelectric separation copper substrate

Four, OSP process board

Organic welding aid film. Because it’s organic, not metal, it’s cheaper than tin-spraying.

Advantages: With all the advantages of bare copper welding, expired boards can also be refinished.

Disadvantages: Susceptible to acid and humidity. In the case of secondary reflow welding, the time required to complete the second reflow welding is usually poor. If stored for more than three months, it must be resurfaced. Use up within 24 hours after opening the package. The OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer to contact the needle point for electrical testing.

The sole function of this organic film is to ensure that the inner copper foil does not oxidize before welding. The film evaporates as soon as it is heated during welding. Solder can be used to weld copper wires to components.

But it is not corrosion-resistant. An OSP circuit board, exposed to air for more than ten days, cannot weld components.

Many computer motherboards use OSP technology. Because the circuit board is too big to afford gold plating.