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.
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.
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.
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.
By Ian Poole
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