Advantages, disadvantages and application scene of different PCB surface treatment technics

PCB technology has undergone tremendous changes along with the continuous development of electronic science and technology, and the manufacturing technics needs to be improved. At the same time, the technics requirements of PCB boards have gradually improved in each industry. For example , in the circuit boards of mobile phones and computers, both gold and copper are used, which makes the board's advantages and disadvantages gradually become easier to distinguish.

Let us learn about the surface technics of the PCB board, compare the advantages and disadvantages of different PCB board surface treatment technics and application scene.

Simply from the appearance, the external layer of the pcb 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,it is easy to judge whether the hardware manufacturers have the action of cutting material and technics from the color . However, the internal circuit of the board is mainly pure copper, which is bare copper.

First, pcb bare copper board

The advantages and disadvantages are obvious:

Advantages: low cost, smooth surface and good weldability (Under no oxidation condition).

Disadvantages: It is easy to be affected by acid and humidity. It can't be placed for a long time. It needs to be used up within 2 hours after unpacking, because copper is easily oxidized when exposed to air; it can't be used in double sided board because the second side is oxidized after the first reflow. If there is a test point, solder paste must be applied to prevent oxidation, otherwise the subsequent contact with the probe will not be good.

Pure copper is easily oxidized if exposed to air, and the external layer must have the above protective layer. And someone think that the golden color part is copper, which is absolutely wrong idea, 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 technics that you have learned before.

Second, gold plating board

Gold color 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. In Shenzhen, there are many merchants who specialize in buying used circuit boards. It is a good income to wash out gold through certain means.

Gold is used as the coating layer, firstly for the convenient welding and secondly for anti-corrosion. Even the golden fingers that have been used for several years are still flashing. If copper, aluminum, and iron were used, they are now rusted into a pile of waste.

The gold plating layer is widely used in board places of component pads, gold fingers, connector shrapnel, etc. If you find that the circuit board is actually silver, then needless to say, directly call the consumer rights hotline, it is certainly the manufacturers cut down material and technics, do not use the materials well but use other metal to fool customers. The motherboards of mobile phone circuit boards are mostly gold plating boards, and the immersion gold boards, computer motherboards, audio and small digital boards are generally not gold plating board.

It’s not difficult to conclude the advantage and disadvantage of immersion gold.

Advantages: It is not easy to oxidize, can be stored for a long time, and the surface is flat. It is suitable for soldering fine gap pins and components with small solder joints.Which is the preference for a button PCB board (such as a mobile phone board, that can be repeated multiple times over reflow soldering and it is less likely to reduce solderability. Can be used as substrate for COB (Chip On Board) wire bonding.

Disadvantages: high cost, poor soldering strength, because the usage of electroless nickel process, easy to have black pad problems. The nickel layer oxidizes over time and long-term reliability is a problem.

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

Third, spray tin circuit board

The silver board is called spray tin board. Spraying a layer of tin on the external layer of the copper circuit can also aid in welding. But it can’t provide long-lasting contact reliability like gold. It has nearly no effect on the components that have been welded, but for pads that are exposed to the air for a long time, reliability is not enough, such as ground pads, pin sockets, etc. Long-term using is prone to oxidative corrosion, resulting in poor contact. Basically circuit board used for small digital products, without exception, are spray tin board due to cheap cost.

Its advantages and disadvantages are summarized as below:

Advantages: low price and good welding performance.

Disadvantages: It is not suitable for soldering fine gap pins and small volume components because the poor fatness of spray tin board. Solder bead is easy to be produced in PCB processing, and it is easy to cause short circuit to fine pitch components. When used in the double-sided SMT technics, because the second side has been through once high-temperature reflow soldering, it is very prone to re-melting the tin to produce solder bead or ball-shape tin spots similar to water droplets that are dripped by gravity, resulting in serious unfairness surface and thus affecting the welding.

Previously we said the cheapest light red circuit board, namely the miner's lamp thermoelectric separation copper substrate

Fourth, OSP technic board

Organic solder mask. Because it is organic, not metal, it is cheaper than the tin spraying technics.

Advantages: with all the advantages of bare copper board welding, the expired board can also be re-surface treatment.

Disadvantages: susceptible to acid and humidity. When used in secondary reflow soldering, it needs to be completed within a certain period of time. Usually, the effect of the second reflow soldering will be poor. If the storage time exceeds three months, it must be re-surface treatment.it needs to be used up within 24 hours once unpacking. The OSP is an insulating layer, so the solder paste must be applied to the test point to remove the original OSP layer in order to contact the pin point for electrical testing.

The only effect of this organic film is to ensure that the inner copper foil is not oxidized before welding. This film evaporates as soon as it is heated. Solder can weld copper wires and components together.

But it is not resistant to corrosion. An OSP board can't be soldered if it is exposed to the air for ten days.

There are OSP technics in many computer motherboards. Because the board area is too large, it’s too expensive to use gold plating.