PCB manufacturing process
- an overview, tutorial or information about the basics of the PCB
manufacturing process.
The PCB manufacturing process is very important for anyone
involved in the electronics industry. Printed circuit boards, PCBs, are very
widely used as the basis for electronic circuits. Printed circuit boards are
used to provide the mechanical basis on which the circuit can be built.
Accordingly virtually all circuits use printed circuit boards and they are
designed and used in quantities of millions.
Although PCBs form the basis of virtually all electronic
circuits today, they tend to be taken for granted. Nevertheless technology in
this area of electronics is moving forward. Track sizes are decreasing, the
numbers of layers in the boards is increasing to accommodate for the increased
connectivity required, and the design rules are being improved to ensure that
smaller SMT devices can be handled and the soldering processes used in
production can be accommodated.
The PCB manufacturing process can be achieved in a variety of
ways and there are a number of variants. Despite the many small variations, the
main stages in the PCB manufacturing process are the same.
PCB constituents
Printed circuit boards, PCBs, can be made from a variety of
substances. The most widely used in a form of glass fibre based board known as
FR4. This provides a reasonable degree of stability under temperature variation
and is does not breakdown badly, while not being excessively expensive. Other
cheaper materials are available for the PCBs in low cost commercial products.
For high performance radio frequency designs where the dielectric constant of
the substrate is important, and low levels of loss are needed, then PTFE based
printed circuit boards can be used, although they are far more difficult to work
with.
In order to make a PCB with tracks for the components, copper
clad board is first obtained. This consists of the substrate material, typically
FR4, with copper cladding normally on both sides. This copper cladding consists
of a thin layer of copper sheet bonded to the board. This bonding is normally
very good for FR4, but the very nature of PTFE makes this more difficult, and
this adds difficulty to the processing of PTFE PCBs.
Basic PCB manufacturing process
With the bare PCB boards chosen and available the next step
is to create the required tracks on the board and remove the unwanted copper.
The manufacture of the PCBs is normally achieved using a chemical etching
process. The most common form of etch used with PCBs is ferric chloride.
In order to gain the correct pattern of tracks, a
photographic process is used. Typically the copper on the bare printed circuit
boards is covered with a thin layer of photo-resist. It is then exposed to light
through a photographic film or photo-mask detailing the tracks required. In this
way the image of the tracks is passed onto the photo-resist. With this complete,
the photo-resist is placed in a developer so that only those areas of the board
where tracks are needed are covered in the resist.
The next stage in the process is to place the printed circuit
boards into the ferric chloride to etch the areas where no track or copper is
required. Knowing the concentration of the ferric chloride and the thickness of
the copper on the board, it is placed into the etch froth e required amount of
time. If the printed circuit boards are placed in the etch for too long, then
some definition is lost as the ferric chloride will tend to undercut the
photo-resist.
Although most PCB boards are manufacturing using photographic
processing, other methods are also available. One is to use a specialised highly
accurate milling machine. The machine is then controlled to mill away the copper
in those areas where the copper is not required. The control is obviously
automated and driven from files generated by the PCB design software. This form
of PCB manufacture is not suitable for large quantity but it is an ideal option
in many instances where very small quantities of a PCB prototype quantities are
needed.
Another method that is sometimes used for a PCB prototype is
to print etch resistant inks onto the PCB using a silk screening process.
Multi-layer printed circuit boards
With the complexity of electronic circuits increasing, it is
not always possible to provide all the connectivity that is required using just
the two sides of the PCB. This occurs quite commonly when dense microprocessor
and other similar boards are being designed. When this is the case multilayer
boards are required.
The manufacture of multi-layer printed circuit boards,
although it uses the same processes as for single layer boards, requires a
considerably greater degree of accuracy and manufacturing process control.
The boards are made by using much thinner individual boards,
one for each layer, and these are then bonded together to produce the overall
PCB. As the number of layers increases, so the individual boards must become
thinner to prevent the finished PCB from becoming too thick. Additionally the
registration between the layers must be very accurate to ensure that any holes
line up.
To bond the different layers together the board is heated to
cure the bonding material. This can lead to some problems of warp. Large
multi-layer boards can have a distinct warp on them if they are not designed
correctly. This can occur particularly if, for example one of the inner layers
is a power plane or a ground plane. While this in itself is fine, if some
reasonably significant areas have to be left free of copper. This can set up
strains within the PCB that can lead to warping.
PCB holes and vias
Holes, often called via holes or vias are needed within a PCB
to connect the different layers together at different points. Holes may also be
needed to enable leaded components to be mounted on the PCB. Additionally some
fixing holes may be needed.
Normally the inner surfaces of the holes have copper layer so
that they electrically connect the layers of the board. These "plated through
holes" are produced using a plating process. In this way the layers of the board
can be connected.
Drilling is then accomplished using numerically controlled
drilling machines, the data being supplied from the PCB CAD design software. It
is worth noting that reducing the number of different sizes of holes can help
reduce the cost of the PCB manufacture.
It may be necessary for some holes to only exist within the
centre of the board, for example when inner layers of the board need to be
connected. These "blind vias" are drilled in the relevant layers prior to the
PCB layers being bonded together.
PCB solder plating and solder resist
When a PCB is soldered it is necessary to keep the areas that
are not to be soldered protected by a layer of what is termed solder resist. The
addition of this layer helps prevent unwanted short circuits on the PCB boards
caused by the solder. The solder resist normally consists of a polymer layer and
protects the board from solder and other contaminants. The colour of the solder
resist is normally deep green or red.
In order to enable the components added to the board, either
leaded or SMT to solder to the board easily, exposed areas of the board are
normally "tinned" or plated with solder. Occasionally boards, or areas of boards
may be gold plated. This may be applicable if some copper fingers are to be used
for edge connections. As the gold will not tarnish, and it offers good
conductivity it provides a good connection at a low cost.
PCB silk screen
It is often necessary to print text and place other small
printed idents onto a PCB. This can help in identifying the board, and also in
marking component locations to aid in fault finding, etc. A silk screen
generated by the PCB design software is sued to add the markings to the board,
after the other manufacturing processes for the bare board have been completed.
PCB prototype
As part of any development process it is normally advisable
to make a prototype before committing to full production. The same is true of
printed circuit boards where a PCB prototype is normally manufactured and tested
before full production. Typically a PCB prototype will need to be manufactured
quickly as there is always pressure to complete the hardware design phase of the
product development. As the main purpose of the PCB prototype is to test the
actual layout, it is often acceptable to use a slightly different PCB
manufacturing process as only a small quantity of the PCB prototype boards will
be needed. However it is always wise to keep as close as possible to the final
PCB manufacturing process to ensure that few changes are made and few new
elements are introduced into the final printed circuit board.
Summary
The PCB manufacturing process is an essential element of the
electronics production lifecycle. PCB manufacturing employs many new areas of
technology and this has enabled significant improvements to be made both in the
reduction of sizes of components and tracks used, and in the reliability of the
boards.
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