Preparation and properties of soluble thermoplastic polyimides with phenol side-groups for flexible copper clad laminates

Polyimide (PI), which is derived from bisphenol A-type diether dianhydride (BPADA) and 4,4-diamino-diphenyl ether (ODA), and known as the typical commercial UltemTM thermoplastic PI, is widely used a heat-resistant engineering plastic. However, its film material still has some practical problems such as insufficient adhesion and poor high temperature resistance when it is used as the supporting film or bonding layer of FCCL. A novel diamine monomer 4-(p-hydroxyphenyl)-2,6-bis(p-aminophenyl) pyridine (p,p-HAPP) containing pyridine heterocyclic structure and bulky phenol side-group at its para position was firstly synthesized, and subsequently employed as the second diamine monomer to prepare a series of PI resins by co-polycondensation with ODA and BPADA via the conventional two-step method. Then, the corresponding PI film was prepared by casting the purified PI solution on glass plate, and finally used to prepare flexible copper clad laminate (FCCL) by hot pressing. The chemical structure, composition and purity of p,p-HAPP, as well as the chemical and phase structures of the resulting PIs are confirmed by relevant means. It is found from the following tests that by introducing a certain amount of p,p-HAPP as a second diamine monomer for ternary copolycondensation, it not only ensures that the resulting PIs (PI-1 similar to PI-3) have good solubilities and relatively low dielectric properties, but also improves their heat resistance, thermal stabilities, thermal dimensional stabilities, mechanical properties and hydrophobicities. In addition, from the tests of FCCL resulted by the preferred PI films (PI-0 similar to PI-3), which have good comprehensive performance, it is also found that the average peel strength (PS) value of the FCCL, prepared by PI-1 with a 25% molar content of p,p-HAPP in mixed diamines, is up to 1.23 N mm-1, and the solder float resistance tests at 288 degrees C and 310 degrees C are all passed. Therefore, PI-1 is a good candidate material for interlayer dielectric application of FCCL without requiring any alloplastic adhesion promoter in this study, which provides an effective strategy for the preparation of thermoplastic PI film with excellent comprehensive properties such as excellent solubility, good mechanical property, low water absorption rate (WAR), high adhesion and heat resistance.