Covering film is the earliest and most commonly used technology for the application of flexible printed board covering layer. It is to apply the same adhesive as copper foil on the same film as the base film of copper clad laminate, making it a semi cured adhesive film. It is sold and supplied by copper clad laminate manufacturers. When supplying, the adhesive film is coated with a release film (or paper), and the semi cured epoxy resin adhesive gradually solidifies at room temperature. Therefore, it should be stored under low temperature refrigeration. Printed circuit manufacturers should keep it in a refrigerated warehouse at around 5 ℃ before use or send it by the manufacturer before use. Generally, material manufacturers guarantee a usage period of 3-4 months, and can use it for 6 months under refrigeration conditions. Acrylic adhesives hardly solidify at room temperature, and can still be used even if stored for more than six months without refrigeration. Of course, the lamination temperature of this adhesive must be very high.

One of the most important issues in the processing of covering films is the flowability management of adhesives. Before the coating film leaves the factory, the material manufacturing factory adjusts the fluidity of the adhesive to a specific range. Under appropriate temperature and refrigerated storage conditions, it can ensure a service life of 3-4 months. However, during the validity period, the fluidity of the adhesive is not fixed, but gradually decreases over time. Due to the high fluidity of the adhesive, the newly produced covering film is prone to flow out and contaminate the terminal area and connecting plate during lamination. At the end of its service life, the adhesive has very little or no fluidity. If the lamination temperature and pressure are not high, it cannot obtain a covering film that fills the pattern gaps and has high bonding strength.