A brief outline of the stages in making a plated Though Hole(PTH) Printed circuit board
PCB’s have been made for over 80 years and their complexity has increased with the constant march of the Electronics Industry. Materials have also changed due to applications and regulations, so when designing boards the end use will have as much bearing on how the board is made and its overall cost, as will choice of any specific component.
All Printed Circuit Boards (commonly referred to as PCBs) are unique and bespoke. They are a solution to having to hard wire components in a circuit layout. They form a platform on which to arrange components for a specific circuit design .
PCB Manufacturing involves many different techniques and disciplines, whose interaction needs to be understood and controlled to produce reliable and consistent results. It only needs one fault in as many as 100 of the processes used to make a PCB, to result in a failure.
All PCBs start with a cad file or gerber file which will have to be “Tooled” as each manufacturer works with different machines and a different set of tooling holes to position the panels for alignment from one process to the next. Photographic tools will also have to be produced for several stages of production. This is the one off “Tooling charge” often seen on quotations.
The Raw material used is a double sided piece of copper clad glass fiber and epoxy mix. There are various grades of other materials also available dependent on final application requirements. Some of which require special processing, which is not commonly stocked or readily available.
The most common is FR4 1.6mm thick with 1oz copper. (FR4 refers to the fire retardancy grade, 1oz copper is 36microns thick)
This raw material comes precut into sheets of differing sizes, that are easily handled by the processing machines. Pre- production will determine best yield to cut down on waste and therefore best sheet size for each bespoke build.
The panel is then drilled with the required holes for the circuit and any tooling holes or slots for production.
All the previously drilled holes are non conductive so in order to connect them (allowing for interconnection between top and bottom Copper layers on the base substrate panel) the whole panel goes through a series of wet chemical processes to “metalize” the entire surface which adds 1-5 microns of copper to all surfaces.
The panel is then coated both sides with a film of photosensitive resist which with the aid of the photographic tool, will give a plating resist. For the areas where copper tracks are required and to reinforce the drilled plated holes, are now electroplated with copper and a thin coating of Tin. The Tin is there to form an etch resist for the next stage.
The plating Resist is then removed and the panel is then passed through an etching solution. This will etch copper, but not in the areas where the tin acts as an etching resist, thereby leaving behind ‘conductive tracks’.
The Tin is removed and the copper etched and cleaned before a coating of electrically insulating and protective photo-imagable soldermask ink is applied. With a photo tool an image is generated leaving a hard coating over the entire circuit just leaving copper pads exposed to allow component interconnection.
At this stage a solderable surface finish is applied which can be either Hot Air Solder Leveled(HASL) or Electroless nickel immersion Gold (ENIG) depending on application and complexity of the board design.
There are other surface finishes available but these two cover the majority of demand and are most commonly used.
A Legend can be applied at this stage either by photo-imaging or silk screen printing. This allows for easier component placement and acts as an aid in the field should repairs/upgrades be required.
The circuits are now ready to rout out as single circuits or left in a panel prior to Bare board testing.
Once tested for breaks and shorts, they are visually inspected prior to dispatch.