The mechanical properties of the leak printing cover layer are worse than that of the laminated cover film, but the material and processing costs are lower. The most commonly used are civilian products and flexible printed boards on automobiles that do not require repeated bending. The process and equipment used are basically the same as the printing of rigid printed board solder mask, but the ink materials used are completely different. To choose inks suitable for flexible printed boards, there are UV cured and heat cured inks available on the market. The former has a short and convenient curing time, but generally has poor mechanical properties and chemical resistance. If used for bending or harsh chemical conditions, it may sometimes be inappropriate, and it is particularly important to avoid using it for chemical gold plating, as the plating solution can seep into the cover layer from the end of the window, seriously causing the cover layer to peel off. Due to the 20-30 minute curing time required for thermosetting inks, the continuous curing oven is also relatively long, and intermittent ovens are generally used.

The covering film needs to be processed by opening a window, but it cannot be processed immediately after being taken out of the cold storage. Especially when the environmental temperature is high and the temperature difference is large, water droplets will condense on the surface. When the base film is polyimide, it will also absorb moisture in a short period of time, which will have an impact on the subsequent process. So generally, the roll shaped covering film is sealed in a polyethylene plastic bag. After being taken out of the refrigerated warehouse, the sealing bag should not be opened immediately, but should be left in the bag for several hours. When the temperature reaches room temperature, the covering film can be taken out of the sealed bag for processing.

The use of CNC drilling and milling machines or punch machines for opening the window of the covering film, and the rotation speed of the CNC drilling and milling machine should not be too high, which incurs high operating costs. This method is generally not used in mass production. Overlap 10-20 sheets of covering film with release paper together, fix them with upper and lower cover pads, and then proceed with processing. Semi cured adhesive is easy to adhere to the drill bit, resulting in poor quality. So it should be inspected more frequently than when drilling copper foil, and the debris generated during drilling should be removed. When using the punching method to process the window covering the film, a simple punching die can be used, and for the processing of batch holes with a diameter of less than 3mm, a punching die can be used for punching. When the hole size of the window is large, a punch is used, and for small and medium-sized batches of holes, CNC drilling and stamping are used for processing