The basic process of the FPC photocoating layer is the same as the photoresist film used for rigid printed boards. The materials used are also dry film type and liquid ink type. In fact, the solder mask dry film is still different from the liquid ink. Although the coating process of the dry film type and the liquid type are completely different, the same device can be basically used for the exposure and subsequent processes. Of course, the specific process conditions will be different. The dry film must be pasted first, and all the circuit diagrams are covered with dry film. The ordinary dry film method is likely to have air bubbles between the lines, so a vacuum filming machine is used.
Ink type is to use screen printing or spraying method to coat the ink on the circuit pattern. Screen printing is the use of more coating methods, the same as the rigid printed board process. However, the thickness of the ink coated by a missed printing is relatively thin, basically 10 ~ 15um, due to the squareness of the circuit orientation, the ink thickness is uneven in one printing, and even skip printing occurs. In order to improve the reliability, the missing printing direction should be changed and then the second missing printing should be performed. The spraying method is a relatively new technology in the process of printed boards. The spraying thickness can be adjusted by the nozzle, and the adjustment range is also wide. The coating is uniform, there are almost no parts that can not be coated, and it can be continuously coated, suitable for mass production.
The ink used for screen printing is epoxy resin type and polyimide type, both of which are two-component. They are mixed with curing agent before use. Solvents are added to adjust the viscosity as needed. Drying is required after printing. The double-sided circuit can be lighted. After the coated side is temporarily dried, the other side is coated on the other side and temporarily dried, and then dried and cured after exposure and development.
The pattern exposure of the photocoating layer requires a positioning mechanism with a certain accuracy. If the size of the disk is about 100um, the position accuracy of the covering layer is at least 30-40 m. As discussed during the pattern exposure, if the mechanical capability of the device is guaranteed, this precision requirement can be achieved. However, after the flexible printed board has been processed by multiple processes, it is difficult to meet higher requirements due to its own size expansion or partial deformation accuracy.
There is no big problem in the developing process. The precise pattern should pay full attention to the developing conditions. The developer and the resist pattern developer are both sodium carbonate aqueous solutions. Even in small batch production, avoid sharing the same developer with pattern development. In order to completely cure the developed photocoating layer resin, post-curing must also be performed. The curing temperature will vary depending on the resin, and it must be cured in an oven for 20-30 minutes.
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