The glued polyimide film can be chemically etched with chemical etchants, such as NaOH, KOH and other strong alkaline solutions, to form various through holes. This is typically used for two-layer labels with only polyphthalimide and metal. The vias are formed by etching a polyphthalimide film after covering the metal layer. If the polyphthalimide film cannot pass through the caustic soda.
Since polyimide film is a thermosetting polymer and has no typical softening point or melting point, flexible circuit boards made of polyimide film will not degrade and decompose the film during hot-melting and soldering.
Thermal shrinkage is one of the important performance considerations for high-density flexible packaging substrates. Low shrinkage rate is not only the guarantee of the precision of the fine pattern produced by multiple exposures in the watchmaking process, but also the guarantee of the mutual alignment of the through holes of different layers in the multi-layer substrate process; also, the low shrinkage rate is particularly important when making large-area fine patterns .
A low coefficient of thermal expansion is also an important indicator to consider. The thermal expansion coefficient of the polyimide film is required to be as close as possible to the copper signal line to reduce the internal stress due to the large difference in the thermal expansion coefficient between them. It is estimated that if the thermal expansion coefficient of the polyimide film is less than 18 ppm/°C, the above-mentioned internal stress accumulation can be effectively avoided.
For high-density flexible packaging substrate applications, the lower the hygroscopicity of the polyphthalimide film, the better. In fact, due to the presence of phthalimide groups, the water absorption of polyimide films is less than 1.5%. It is reported that when the moisture absorption rate is lower than 2%, the flexible packaging substrate will not generate bubbles or peel off when subjected to a high temperature of 250-300 °C during the patterning process.
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