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Printed Circuit Board Solder resist

Solder resist has become an integral part of Printed Circuit Board (PCB) technology today. It is hard to find any PCBs that have no solder masks, although there may be some private individuals who have developed printed circuit boards without solder masks. Even PCBs ordered in very small quantities for prototyping have solder resist. Using solder resist on PCBs manufactured commercially is now the standard worldwide.

Why use PCB solder resist?
Quite evident from its name, solder resist is used on a PCB to resist solder from becoming attached to specific areas of a PCB’s surface. Only the specified areas where components are to be connected will receive solder while the rest of PCB is covered with solder resist. This ensures that only the required areas will be soldered. The key benefit of solder resist is that solder is applied only to the specified region, which prevents solder from entering the rest of the circuit and therefore reduces the chances of solder bridges, which can cause short circuits. Modern day PCBs have very thin and closely spaced tracks and component pins that can cause short circuits after the soldering process. By utilizing solder resist, this problem is prevented.

Solder resist not only prevents short circuits and small bridges, it can also be considered as an additional PCB protective layer. The solder resist layer functions as PCB insulation layer which stops corrosion, oxidation or damage from chemical exposure. Since these issues damage a PCB over a relatively long period of time, solder resist can be seen as improving the reliability of a PCB.

What is PCB solder resist?
During the manufacturing process, a coating of resins is permanently applied on the bare surface of a PCB, which is called solder resist. It is a resin-based formulation that sticks permanently and is normally green coloured. It safeguards the PCB surface except in the areas that will be soldered.

While green is the typical colour of solder resist, any colour can be used. On PCBs it’s difficult to maintain the exact colour but it is possible to obtain similar colours. When green isn’t used, red and blue are normally the colours of choice.

PCB Solder Resist Application
At the present time, Surface Mount Technology (SMT) has very fine pitch calculations, which sometimes do not work well with the conventional solder resist process. In these situations, Liquid Photo Imageable (LPI) solder resist is utilized. When first used, PCB solder resist was applied using a silkscreen and stencil printing. The newer LPI technique varies from the old stencil pencil technique quite a bit. In LPI, a very precise solder resist process is ensured by separating both imaging and coating into two different operations. PCB solder resist is actually a liquid photopolymer used by PCB manufacturer on bare PCB surfaces. Epoxy-acrylate or epoxy resin technology is used and the whole PCB surface is coated with it. In general, the material thickness is kept to 20 microns over copper tracks and 30 microns on the bare PCB surface. After applying the epoxy resin material to the PCB, it is dried and then exposed to the required image, which develops the desired solder resist pattern on the PCB. The PCB is then heat-cured, which makes sure that the solder resist has a firm and long lasting finish.

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