A Soluble Polyimide Improves the Performance of UV-curable Solder Resist

UV-curable solder resist (SR) is widely used in advanced packaging substrate. However, due to the network structure generated by the photothermal double curing reaction, the elongation at break and heat resistance of UV-curable SR film cannot meet the requirement of advanced packaging substrate with high reliability. Although polyimide has the advantages of high elongation at break and high heat resistance, its application in UV-curable SR is limited due to its difficulty in dissolving in organic solvents. In this paper, we try to apply soluble polyimide (SPI) to the field of UV-curable SR by regulating the solubility of SPI and the formula of UV-curable SR. The results show that the solubility of SPI can be improved by adjusting the flexibility of the chemical structure and reducing the molecular weight of the polymer. The excellent solubility of SPI can greatly improve the elongation at break of UV-curable SR (elongation at break of 8.4%). In addition, the introduction of SPI has also improved the thermal stability of UV-curable SR (glass transition temperature (Tg) of 185.07 degrees C, 5% thermal weight loss (T-5%) of 335.1 degrees C). Moreover, the introduced SPI can also reduce the dielectric constant (D-k) and dielectric loss (D-f) of the film (D-k = 2.86*5 GHz, D-f = 0.0151*5 GHz). However, due to the limited solubility of SPI and epoxy acrylate resin systems, excessive SPI content of the UV-curable SR formula will cause internal phase separation, resulting in performance degradation of the UV-curable SR film In summary, the SPI developed in this work has great potential application for improving the performance of UV-curable SR for advanced packaging in the future.