China PCB maker SysPCB will take you to understand the surface process of PCB boards, compare the advantages and disadvantages of different PCB board surface treatment process and applicable scenarios.
First, bare copper board
Advantages and disadvantages are obvious.
Advantages: low cost, flat surface, good solderability (in the absence of being oxidized).
Disadvantages: easy to be affected by acid and humidity, can not be stored for a long time, after unpacking need to be used up within 2 hours, because copper exposed to the air is easy to oxidation; can not be used for double-sided PCB, because after the first reflow soldering the second side has been oxidized. If there is a test point, must add printed solder paste to prevent oxidation, otherwise will not be able to make good contact with the probe in the subsequent testing.
Pure copper can be easily oxidized if exposed to air, and the outer layer must have the protective layer. And some people think that the golden yellow is copper, that is not the right idea, because that is the copper above the protective layer. So it needs to be a large area of gold plating on the board, that is, I have previously brought you to understand the immersion gold process.
Second, gold-plated board
Gold is the real gold. Even if only a very thin layer of plating, it has accounted for nearly 10% of the cost of the board. In Shenzhen there are many families specializing in acquiring scrap circuit board businessmen, through certain means to wash out the gold, is a good income.
The use of gold as a plating layer, one is to facilitate the welding, the other is to prevent corrosion. Even after several years, gold fingers on memory bank still shining as before, if the initial surface treatment is copper, aluminum, iron, now has rusted into a pile of scrap.
Gold plating layer is heavily used in the circuit board component pads, gold fingers, connector clips and other locations. the most widely used cell phone circuit board is mostly gold-plated board, immerion gold board, computer motherboards, audio and small digital circuit boards are generally not gold-plated board.
The advantages and disadvantages of gold plating process is not difficult to draw.
Advantages: not easy to oxidation, can be stored for a long time, the surface is flat, suitable for welding fine pitch pins and solder joints of smaller components. Preferred for PCB boards with keys (such as cell phone boards). Can be repeated many times reflow soldering, and is not likely to reduce its solderability. It can be used as a substrate for COB (Chip On Board) making.
Disadvantages: Higher cost, poor solder strength, easy to have the problem of black PAD because of the use of electroless nickel process. The nickel layer will oxidize over time, and long-term reliability is a problem.
Now we know that the gold color is gold, silver color is silver? Of course not, is tin.
Third, HASL circuit board
Silver-colored board is called HASL board. Spraying a layer of tin at the outer layer of copper can also help soldering. But can not provide long-lasting contact reliability as gold. For the components that have been soldered has little effect, but for long-term exposure to air pads, reliability is not enough, such as grounding pads, chips, pin sockets, etc.. Long-term use is prone to oxidation and rusting, resulting in poor contact. Basically used as small digital products circuit board, without exception, the reason is cheap.
Its advantages and disadvantages are summarized as follows.
Advantages: lower price, good soldering performance.
Disadvantages: not suitable for soldering fine gap pins and too small components, because the surface flatness of the HASL board is poor. In the PCB processing is easy to produce tin beads (solder bead), the fine pitch pins (fine pitch) components easier to cause a short circuit. When used in double-sided SMT process, because the second side has been a high-temperature reflow, it is very easy to melt the spray tin and produce tin beads or similar water droplets by gravity into drops of spherical tin spots, resulting in a more uneven surface and thus affect the soldering problem.
Previously mentioned the cheapest light red circuit board, that is, the mine lamp thermoelectric separation copper base PCB
Fourth, OSP process board
Organic flux film. Because it is organic, not metal, so it is cheaper than the HASL process.
Advantages: has all the advantages of bare copper board soldering, expired boards can also be redone once the surface treatment.
Disadvantages: Easily affected by acid and humidity. When used in secondary reflow, it needs to be done within a certain time, and usually the second reflow will be less effective. If the storage time exceeds three months, it must be resurfaced. OSP is an insulating layer, so the test point must be stamped with solder paste to remove the original OSP layer in order to contact the needle point for electrical testing.
The only purpose of this organic film is to ensure that the inner copper foil is not oxidized before soldering. Once heated during soldering, this film evaporates away. Solder is then able to solder the copper wire and components together.
But it isn’t very resistant to corrosion, an OSP circuit board, exposed to the air for ten days or so, you can not solder components.
Computer motherboards have a lot of OSP process. That is because the board area is too large to use gold plating.