Understanding PCB Manufacturing and Assembly Process
Prior to the actual PCB manufacturing and assembly process, the fabrication data is generated through CAD in which Gerber layer images, netlist, component details and Execellon and Gerber drill files are included. PCB manufacturing process and connection of assemblies at industrial level is done in a spotless environment. Most of the PCB manufacturers have proprietary manufacturing procedures; however, the following steps in PCB manufacturing and assembly are common in general.
- The Substrate
Fiberglass (FR4) is commonly used to make the substrate. It is sprayed with epoxy resin or soaked in it and then they are flattened to achieve the desired thickness. Then, the material is semicured in an oven after which the material is cut in panels. These pieces are stacked together in layers –copper foil is inserted in between each layer. All of these are bonded together by lamination, in which the application of heat, pressure and vacuum is done.
- Primary Drilling and Plating of Holes
Holes are drilled and deburred. In multilayer PCBs, VIAS have to be drilled to provide connections to the hidden layers as well. The location of holes for primary drilling is provided by the designer. For double sided and multilayer PCB, the inner side of the VIAS provides a conductive route by plating with copper while the non-conductive holes are drilled afterwards or plugged during copper plating.
- Printing the Pattern
Typically, in PCB manufacturing and assembly process, there are two processes for printing the circuit on the circuit board.
- Subtractive Process – entire board is plated with copper and later on undesired copper is etched away
- Additive Process – copper is plated in the form of desired pattern only
In homemade custom PCB manufacturing, subtractive methods are preferred and printing is done with the help of a laser printer or an inkjet printer. In industrial process, photolithography (patterning is done by optical or UV radiation) is used for printing on the substrate. In the additive process, a layer of photoresist is pressed on top of the material. The circuit pattern mask is positioned over the photoresist and then the panels are exposed to UV rays. The exposure to intense radiation irradiates the patterned portion beneath the mask, making the photoresist soluble. The mask is then removed. Alkaline solution is used to dissolve the photoresist on the printed circuit pattern. This exposes the copper foil beneath and copper plating is done on the exposed areas. The areas on which the photo resist is still present are not plated. Additionally, tin or gold plating is also done over copper plating so that the copper does not oxidize during etching. After this, the existing photoresist is also stripped which exposed bare copper.
- Etching Process or Milling
Unnecessary copper is removed by etching or milling process. PCB milling is done with the help of CNC machine. In etching, chemicals are used to remove unwanted copper. Custom PCB manufacturing by the electronic hobbyists, make use of ferric chloride or a solution of hydrochloric acid and hydrogen peroxide as an etchant. The protective tin or gold plating prevents copper traces from oxidizing during the corrosive etching process.
- Solder Resist PCB
The areas that are not supposed to be soldered are protected with solder resist which is a coating of polymer. This stops the solder from getting in contact with the traces ore other nearby component leads.
- Silk screening
With the help of a silk screen printer, legends, part numbers and symbols are printed on the PCBs.
The last step in the process of PCB manufacturing and assembling is the assembling of the electronic components. The PCB assemblies are of the following types:
- Surface Mounted Assemblies
- Through hole Assemblies
- Mixed
In through hole assembles, holes are drilled and the components are inserted through them and the leads are soldered. In surface mounted technology, the PCB assembles are mounted on top of the board.
- Inspection and Testing
In industrial PCB manufacturing processes, the PCBs are tested and inspected for defects. This is because the PCBs are used in high reliability applications and the PCB assembly and electrical connections need to be perfect.