PCB Technical - PCB factory: PTF printed circuit on PET matters needing attention

PCB factory: PTF printed circuit on PET matters needing attention

PTF: Polymer Thick Film (Polymer Thick Film)

PET: Polyethylene terephthalate (polyethylene terephthalate)

As everyone knows, compared with FPC, PTF soft board does have many advantages such as low price and short delivery time, but PTF has its inherent capacity limitation. Due to the use of the screen printing process, PFT cannot produce electronic circuit spacing smaller than FPC. Although it is claimed that the minimum electronic circuit spacing of 5mils (0.12mm) can be achieved, in fact, 10mils (0.25mm) is already reaching the limit. In addition, PTF can't withstand double folding, because its electronic circuit is made of silver paste and conductive carbon. It does not have the ability to stretch like FPC's copper circuit, and can withstand a small number of bending.

The following will be based on the personal experience of using PTF, and put forward the process capacity limitations and solutions of PTF in production.

Foreign matter contamination: If non-conductive foreign matter (particle) adheres to the electronic circuit, its smallest effect is to increase the resistance of the circuit, because the resistance is inversely proportional to the area through which the electrons pass, and it is likely to cause electrical properties Open; if conductive foreign matter is contaminated between the circuit and the circuit, it is likely to cause an electrical short (short). This shortcoming is the most annoying, because it is in the circuit board manufacturer During production, there is no way to screen it out completely through testing methods. Usually, problems will only occur after being installed in the product for a period of time, especially when it is used in high-temperature and high-humidity areas. If you have the opportunity in the future, you can write an article to discuss this topic.

Foreign matter may come from the air in the environment, including dust, fibers, paper scraps, etc. Generally speaking, they are man-made foreign objects, such as clothes that have a lot of fibers, especially sweaters or cotton clothes. Corrugated cardboard boxes are also a major source of pollution, with dander, paper scraps and fibers. The best prevention and control is to set up a 100K clean room (Clean room), personnel wear dust-proof clothing, and strictly control various possible sources of pollution. The importance of environmental control, especially when producing products with small pitch lines.

Another possible source of foreign matter is ink. In the production of ink, some metal additives are added. If these additives are not properly controlled, they may form lumps. Larger lumps cannot flow through the screen printing mesh. Block the screen of the silk screen, resulting in defects in printing, or even broken electronic circuits; smaller hard blocks can flow through the mesh, but will form bumps on the electronic circuit, if there are overlapping upper and lower layers at this time When wiring, the bumps are likely to penetrate the dielectric layer and cause short circuits between the upper and lower circuits. The pollution of the ink may also come from the bad ambient air during the production of the ink, or the environment where the ink is stored. The best way to control foreign matter in ink is to filter ink contamination with an appropriate size filter; if the contamination is serious, you can also use a coarse filter first and then a fine filter.

Staining of the conductive layer: when screen printing, the viscosity of the conductive ink is very important. When too much diluent is added, the ink will become thinner and easily penetrate into the places where the ink should not be printed. This disadvantage It is called printing stain, and what is worse is that this stain will continue until the next printing, or even the next time, until the stained ink dries up. Severe printing contamination will directly cause a short circuit between adjacent conductive lines; slight printing contamination may also affect the flow of electrons between lines. If voltage is applied as energy and humidity is a catalyst, electrons are likely to be generated. The phenomenon of eletromigration is between the circuit and the circuit. Over time, electrical conductivity will occur and cause a short circuit.

The method to solve this kind of conductive layer printing contamination is to select appropriate ink and control the addition of thinner; the second is to clean the screen screen regularly; the third is to strengthen the inspection, and it is best to check every time a sheet is made. Zhang, if there is an AOI (Auto Optical Inspector), it would be even better.

Printing holes and holes in the insulating layer: The screen printing screen is composed of warp and weft threads. Since it is composed of threads, there will be places where the lines cannot be printed during printing, especially where the warp and weft intersect. Will form holes that cannot be printed. These holes are extremely small and difficult to observe with the eyes, but electrons may still pass through. At this time, if the holes just fall on the conductive lines of the adjacent layer, it will cause layers and layers. Short circuit between.

This method of printing holes and holes in the insulating layer can reduce the viscosity of the insulating layer ink, so that the insulating ink can penetrate under the warp and weft lines. ). Another method is to increase the number of printing of the insulating layer by 2 to 3 layers, and stipulate that each insulating layer uses a screen printing screen with different meshes and warp and weft directions. The purpose is to allow different screen printings to fill the holes with each other. (PCB)