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.
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