What is the difference between gold plating and silver plating on a PCB board?
2023-12-13
The wide variety of PCB colors used in the market is dizzying. The more common PCB colors are black, green, blue, yellow, purple, red, and brown. In addition, some manufacturers have also developed PCBs in different colors such as white and pink.
In the traditional impression, black PCBs seem to be positioned as high-end, while red, yellow, etc. are dedicated to low-end, so is that so?
1. The copper layer of PCB without solder mask paint is easily oxidized when exposed to air
We know that the front and back of the PCB are copper layers, and in the production of PCBs, the copper layers will get a smooth and unprotected surface regardless of whether they are manufactured by addition or subtraction.
Although the chemical properties of copper are not as lively as aluminum, iron, magnesium, etc., under the condition of water, pure copper and oxygen are easily oxidized, because there is oxygen and water vapor in the air, so the surface of pure copper will be oxidized soon after contact with air.
Due to the thin thickness of the copper layer in the PCB, the oxidized copper will become a poor conductor of electricity, which will greatly damage the electrical properties of the entire PCB.
In order to prevent the oxidation of copper, to separate the soldered and non-soldered parts of the PCB during soldering, and to protect the surface of the PCB, engineers invented a special coating. This coating can be easily applied to the surface of the PCB, forming a protective layer with a certain thickness and blocking the contact between copper and air. This coating is called a solder mask, and the material used is solder mask.
Since it's called lacquer, it must have different colors. Yes, the original solder mask can be made colorless and transparent, but the PCB often needs to print small text on the board for repair and manufacturing convenience.
Transparent solder mask paint can only reveal the base color of the PCB, so whether it is manufacturing, maintenance or sales, the appearance is not good enough. Therefore, engineers added a variety of colors to the solder mask paint, resulting in a black, red, or blue PCB.
2. The black PCB is difficult to see the wiring, which brings difficulties to maintenance
From this point of view, the color of the PCB has nothing to do with the quality of the PCB. The difference between a black PCB and other color PCBs such as blue PCBs and yellow PCBs is the different color of the solder mask paint on the brush. The public account "Mechanical Engineering Wencui", the engineer's gas station!
If the PCB design and manufacturing process are exactly the same, the color will not have any impact on the performance and will not have any effect on heat dissipation.
As for the black PCB, it is a color that is not very convenient to manufacture and use because the surface wiring is almost completely covered, which causes great difficulty in the later maintenance.
Therefore, in recent years, people have gradually reformed and abandoned the use of black solder mask paint, and turned to the use of dark green, dark brown, dark blue and other solder mask paints, in order to facilitate manufacturing and maintenance.
Speaking of which, everyone has basically understood the problem of PCB color. The reason why there is a saying that "color represents or low-end" is because manufacturers like to use black PCBs to make high-end products, and red, blue, green, yellow, etc. to make low-end products.
To sum it up: the product gives meaning to the color, not the color that gives meaning to the product.
3. What are the benefits of precious metals such as gold and silver used in PCB?
The color is clear, let's talk about the precious metals on the PCB! Some manufacturers will specifically mention that their products use special processes such as gold plating and silver plating when promoting their products. So what exactly is this craft for?
The surface of the PCB needs to be soldered, so a part of the copper layer is required to be exposed for soldering. These exposed copper layers are called pads, which are usually rectangular or round and have a small area.
As we have seen above, we know that the copper used in PCBs is highly susceptible to oxidation, so when applied with solder mask, it is the copper on the pads that is exposed to air.
If the copper on the pad is oxidized, it is not only difficult to solder, but also has a large increase in resistivity, which seriously affects the performance of the final product. That's why engineers have come up with all sorts of ways to protect the pads. For example, it is coated with an inert metal, gold, or chemically covered with a layer of silver, or covered with a special chemical film to prevent the pad from coming into contact with the air.
The exposed pads on the PCB are directly exposed to the copper layer. This part needs to be protected to stop it from being oxidized.
From this point of view, whether it is gold or silver, the purpose of the process itself is to prevent oxidation and protect the pads so that they can ensure yield in the subsequent soldering process.
However, the use of different metals will require the storage time and storage conditions of the PCBs used in the production plant. Therefore, PCB factories generally use vacuum molding machines to package PCBs before PCB production is completed and delivered to customers to ensure that PCBs are not damaged by oxidation to the greatest extent.
Before the components are soldered on the machine, the board manufacturer also needs to test the oxidation degree of the PCB, remove the oxidized PCB, and ensure the yield rate. The board obtained by the end consumer has undergone various tests, and even after long-term use, oxidation will almost only occur in the plugging and unplugging connection parts, and has no effect on the pads and components that have been soldered.
Since silver and gold have lower resistance, will the use of special metals such as silver and gold reduce the heat generated by the PCB when used?
As we know, the factor that affects the heat generation is the resistance, which in turn is related to the material of the conductor itself, the cross-sectional area and the length of the conductor. The thickness of the metal material on the surface of the pad is even much less than 0.01 mm, and if the pad is treated with OST (organic protective film), there will be no excess thickness at all. The resistance of such a tiny thickness is almost zero, and it is impossible to calculate, and of course it does not affect the heat generation.
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