Compared with FPC(Flex Printed Circuit) and PTF(Polymer Thick Film) soft boards, we know that PTF has many advantages, namely low price and short delivery time, but PTF has its innate ability limitation.
Due to its silkscreen use, the PFT was not able to produce a smaller electronic circuit pitch than the FPC. Although the PTF claimed to achieve a minimum of 5mils (0.12mm) electronic circuit pitch, in fact 10mils(0.25mm) was close to the limit. In addition, PTF can not bear the dead fold, because the material of its electronic circuit is silver paste and conductive carbon ink, they are not as good as FPC copper foil (cooper) line has excellent extension ability, can withstand the limited number of bending.
Foreign body contamination:
The non-conductive foreign body (particle), if stuck on the electronic line, its minimum impact is to increase the resistance value of the line, because the resistance value is inversely proportional to the area through the electron, and is likely to cause electrical open (open); If the conductive foreign matter contaminates the circuit and between the lines, it is very likely to cause electrical short circuit (short circuit). This shortcoming is the most annoying, because it cannot be screened out completely through testing when it is produced in the circuit board manufacturing factory. Usually, the problem will not occur until it is installed in the product for a period of time. Especially in the high temperature and humidity environment. I will write again on this subject at a later time if I have the opportunity.
Foreign bodies can come from air pollution in the environment, such as dust, fiber, and paper scraps. Generally speaking, it is mostly man-made foreign bodies, such as clothes, there are a lot of fibers, especially sweaters or cotton clothes. Wallen carton is also a big source of pollution, there are dandruff, paper and fibers, the human body will always drop cuticle. The best prevention method can be set up a 100K grade Clean room/clean room, personnel wear dust jackets, and strict control of various possible pollution sources. Environmental control is really important, especially in the production of products with small spacing lines, because you never know when there will be something dirty on the line, resulting in irregular quality shortcomings.
Another possible source of foreign body is ink, ink in the production of some metal additives, if the additives are not properly controlled, it is likely to form a hard block, large hard block can not flow through the screen printing mesh, will block the screen screen, resulting in printing holes, or even PCB board circuit break; A smaller block can flow through the mesh but will form a dielectric dot on the electronic circuit and there is a good chance that if there are overlapping upper and lower circuits, the dielectric dot will Pierce the insulation layer and cause a short circuit on the upper and lower circuits. Ink pollution may also come from the poor environment of the ink production air, or ink storage environment. The best way to control ink foreign body is to filter ink stains with the appropriate size of the screen; If the contamination is serious, you can also use coarse filter screen after fine filter screen.
Printing stain of conductive layer:
When screen printing, the viscosity of conductive ink is very important. When the thinner is added too much, the ink will become thin and easy to penetrate into the original place where the ink should not be printed. This shortcoming is called printing stain. Serious printing contamination will directly cause short circuit between adjacent conductive lines; Slight printing stains may also affect the flow of electrons between lines. If voltage is used as an energy source and humidity is used as a catalyst, it is likely to produce electron migration between lines. Over time, electrical conductivity will be generated and short circuit will be caused.
One of the ways to solve this conductive layer printing stain is to select appropriate ink and control the addition of diluent; The second is to clean the screen screen regularly; The third is to strengthen the inspection, it is best to inspect every circuit board, if there is AOI(Auto Optical Inspector), it is better.