After the covering film of FPC circuit board is positioned, it is necessary to heat and pressurize to make the adhesive completely solidified and integrated with the circuit. The heating temperature of this process is 160 ~ 200 ℃, and the time is 1.5 ~ 2H (one cycle time). In order to improve production efficiency, there are several different schemes, the most commonly used is to use hot press. Place the printed board temporarily fixed with the covering film between the hot plates of the press, overlap in sections, and heat and pressurize at the same time. Heating methods include steam, thermal medium (oil), electric heating, etc. The cost of steam heating is low, but the temperature is basically 160 ℃. Electric heating can be heated to more than 300 ℃, but the temperature distribution is uneven. The external heat source heats the silicone oil. The heating with silicone oil as the medium can reach 200 ℃, and the temperature distribution is uniform. Recently, this heating method is gradually used more and more. Considering that the adhesive can fully fill the gap of line graphics, it is ideal to use vacuum press, which has high equipment price and slightly longer pressing cycle. However, it is cost-effective in terms of qualification rate and production efficiency. The examples of introducing vacuum press are also increasing.
The way of lamination has a great influence on the state of adhesive filling into the circuit room and the bending resistance of the finished flexible printed board. Lamination materials are commercially available general products. Considering the cost of mass production, each flexible plate factory makes lamination materials by itself. According to the structure of flexible printed board and the materials used, the materials and structures for lamination are also different.
Screen printing of FPC circuit board covering layer
The mechanical properties of the missing printing coating are worse than that of the laminated coating, but the material cost and processing cost are lower. The most widely used are civil products that do not need repeated bending and flexible printed boards on automobiles. The process and equipment used are basically the same as that of the solder resist film of rigid printed board, but the ink materials used are completely different. The ink suitable for flexible printed boards should be selected. The commercially available ink includes UV curing type and heat curing type. The former has short curing time and convenience, but the general mechanical properties and chemical resistance are poor. If it is used under bending or harsh chemical conditions, it will sometimes be inappropriate. In particular, it should be avoided for electroless gold plating, because the plating solution will penetrate under the covering layer from the end of the window, which will seriously cause the stripping of the covering layer. The curing of thermosetting ink takes 20 ~ 30min, so the drying path of continuous curing is also relatively long. Generally, intermittent oven is used