PCB manufacturing primarily comes in two methods: chemical mode and physical mode. Nowadays, the most prevalent PCB fabrication methods high schools take advantage of are thermal transfer printing and physical engraving. The former belongs to chemical domain while the latter to physical domain.
• Thermal Transfer Printing
To carry out thermal transfer printing, first, an ordinary laser printer is used to print PCB image onto specialized thermal transfer paper. Then, PCB image that has been printed on thermal transfer paper is transferred on Copper Clad Laminate (CCL) through thermal transfer equipment. Finally, a high-precision PCB is produced after later processes including etching and PCB drilling. In high school labs, thermal transfer printing is leveraged to produce high-precision PCBs and can be achieved in one hour. When it comes to double-side PCB fabrication, the protruding issue lies in misalignment that hasn’t been successfully solved with current manufacturing method used. As a result, double-side PCBs fabricated in thermal transfer printing method fail to optimally work on electronic products.
Thermal transfer printing sticks to the following PCB fabrication process:
1. Bottom PCB image is printed on a piece of thermal transfer paper by an ordinary laser printer.
2. Top PCB image is printed on another piece of thermal transfer paper by an ordinary laser printer.
3. Two pieces of thermal transfer paper are closely stuck to a well-cut double-side CCL. Clear tape can be used to fix the position of paper and plate to avoid misalignment.
4. Thermal transfer printing is carried out by putting CCL with thermal transfer paper stuck into thermal transfer printing equipment. A CCL with PCB image printed on is then obtained after cooling.
5. Etching and PCB drilling are then implemented.
Among all the phases of thermal transfer printing for PCB fabrication, application of Step 3 can accurately lead to alignment of two pieces of thermal transfer paper. When it comes to Step 4, however, misalignment tends to be caused in thermal transfer equipment. As a result, a double-side PCBs can be hardly successfully manufactured unless a couple of trials have been made.
• Physical Engraving
Based on basic principles of physical engraving, physical engraving conforms to milling principle with extra or unnecessary parts of CCL milled off. The applied equipment is actually a type of small CNC drilling and milling machine that is also called circuit board engraving machine. In the process of double-side PCB manufacturing, after one side is finished milling, board should be flipped over for milling on the other side of the board. The flipping process possibly leads to misalignment that should be corrected through software and hardware so that rejection rate of double-side PCBs will be reduced.
• Comparison between Thermal Transfer Printing and Physical Engraving
According to the comparison between the two methods, thermal transfer printing performs better than physical engraving in terms of the following aspects:
1. CNC drilling and milling machine features relatively high cost and takes up more lab space. Thermal transfer printing machine features low cost and takes up less space than CNC drilling and milling machine.
2. A Double-side PCB can be completed within one hour when thermal transfer printing is used. In spite of possible misalignment calling for multiple times of thermal transfer printing, a double-side PCB can be fabricated within three hours at most. CNC drilling and milling machine, however, takes more time, at least four hours. Furthermore, CNC drilling and milling machine can deal with only one piece of PCB board simultaneously while thermal transfer printing is capable of producing multiple double-side PCBs within the same time period.