When the design and fabrication of a printed circuit board is done, it is time to assemble it. Assembling printed circuit boards require another set of processes to follow.The first three processes are drilling, plating and coating, solder resist application. These are followed by legend printing, board testing, and protection and packaging.
Drilling holes is the first process in assembling.
Holes are drilled using drill bits which are very small. They should be made of solid coated tungsten carbide. It is recommended that they are coated because many board materials are abrasive. To ensure cost-effectiveness, drilling should be done at high RPM and high feed. Additionally, drill feeds have to be very sharp.Drilling is usually performed using automated machines. The placement of these drilling machines is controlled by computer-generated drill files. The drill file automatically locates the hole and knows the size of each drilled hole.
All the holes can be made conductive through electroplating and inserting metal eyelets.They are made conductive in order to electrically and thermally connect board layers. Another option is to connect only some of the copper layers rather than passing through the entire board. This can be done through controlled-depth drilling. For boards with two or more layers, they are made conductive and electroplated with copper through plated-through holes.
The next process is plating and coating.
A printed circuit board is usually plated with gold over nickel, tin and solder. This is done to resist etching away the underlying copper, which is not needed.After etching, the PCBs are rinsed with water. Then, solder mask is applied. Other exposed copper is coated to avoid corrosion.
After plating and coating, the next process is solder resist application. The surface that should not be soldered will be covered with solder mask. Legend printing is when symbols, letters and numbers are printed on either one or both sides of the printed circuit board. It has the designators, test points, switch settings, and other helpful indications for assembling, testing and servicing the board.The three methods used to print the board are silk screen printing with epoxy ink, liquid photo imaging, which is a more accurate method, and ink jet printing.
The next phase is the bare-board test.
They are tested for shorts – connections that should not be together. They are also tested for opens, which are missing connections, making them the exact opposite of shorts.
The next phase in the assembly process is when the bare boards are populated with electronic components. These are done through two known methods: through-hole technology or surface-mount technology. In both methods, component leads are mechanically fixed and electrically connected to the board through soldering. There are many soldering methods used to fasten components to a printed circuit board.
It is important to test a populated board.
Testing is also done in a variety of ways. That includes visual inspection and even an automated optical inspection. There is also power-off testing while the power is turned off. On the other hand, there is an in-circuit test as well as a functional test when the power is on. The question is what happens when boards fail the test?The boards are then soldered again and failed components are replaced. This is referred to as reworking.
The last part of the process is protection and packaging. Printed circuit boards that will be placed in extreme environments will have conformal coating. Conformal coating is applied by dipping or spraying after soldering. This coating prevents corrosion and leakage. Conformal coats are dips of silicone rubber or polyurethane solutions. Many PCBs are placed in antistatic bags during transport.