PCB Solder resist

- overview or tutorial about solder resist for printed circuit boards, and how PCB solder resist can be used to ensure that solder remains within the desired areas on a printed circuit board.

Solder resist is an essential part of today's printed circuit board technology. The use of PCB solder resist has become so widespread, that it is most unusual to see any printed circuit boards without any solder resist covering, except for some home constructed circuits Today even many prototype boards have solder resist and as such its use on commercially manufactured printed circuit boards can be said to be universal.

Purpose of PCB solder resist

As its name indicates, a solder resist covering on a board is used to protect areas of a printed circuit board, PCB, from taking solder. In this way only areas that actually need to have a solder covering, i.e. areas where components are to be soldered, are free of the solder resist and able to be soldered. This provides many advantages. The main one is that only having solder whereit is needed, and prevented from reaching some areas by the solder resist, small short circuits caused by solder bridges can be significantly reduced. This is increasingly important because the very fine pitch of many printed circuit boards today means that small solder tracks caused in the soldering process could easily cause bridges and short circuits. The use of solder resist confines this problem to areas where components are to be soldered, and these areas can be designed accordingly.

In addition to its function in preventing solder from causing small bridges, PCB solder resist also acts as a protective layer to a board. The solder resist provides electrical insulation and protection against oxidation and corrosion. Over periods of time this can improve the overall reliability of a printed circuit board, especially if it is exposed to harmful agents.

What is PCB solder resist?

PCB solder resist is a permanent resin based coating applied to printed circuit boards during the manufacture of the bare board. The solder resist is a permanent coating of a resin formulation, generally green in colour, which encapsulates and protects all of the surface features of a printed circuit board except the specific areas where it is required to form solder joints.

Although green is the most widely used colour for solder resist, almost any colour can be used. While it can be difficult to maintain exact colours, it is possible to make them almost any colour. However apart from green, other popular colours are red and blue.

Applying PCB solder resist

In order that PCB solder resist layers are able to meet the very precise requirements of today's surface mount technology, SMT printed circuit boards, Liquid PhotoImageable (LPI) solder resist is used. Previously PCB solder resist application used stencil printing using a silk screen.

The LPI process for solder resist is very different to the previously used stencil printing. LPI separates the coating and imaging operations, thereby gaining the highest level of precision. The PCB solder resist material used by the bare PCB manufacturer is in the form of a liquid photo-polymer and it uses epoxy or epoxy-acrylate resin technology and the whole board is coated with the material. The material thickness is typically around 30 microns over the bare board and 20 microns over the copper. Once dry after application of the solder resist material, it is exposed to the image pattern required and then developed to give the required solder resist pattern. Then after developing the solder resist is heat cured to ensure it provides a tough durable finish.

By Ian Poole

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