Synthesis of phosphite-type trifunctional cycloaliphatic epoxide and the decrosslinking behavior of its cured network

The motivation of the present work is to design and synthesize reworkable epoxy resin for electronic packaging which are required to be sufficiently stable before 200 °C and can rapidly decompose in the temperature range of 200-300 °C. For this purpose, a new trifunctional cycloaliphatic epoxide (Epo-A) with a phosphite center linked by three epoxycyclohexyl groups was prepared. The chemical structure was confirmed by FTIR, 1H NMR and 31P NMR spectra. Compared to the phosphate-type analog (Epo-B) and commercial epoxide ERL-4221, Epo-A exhibits the apparently higher curing reactivity. The shearing strength of the cured Epo-A at room temperature is 5.67 MPa, much superior to that of ERL-4221 (3.29 MPa). More importantly, the cured Epo-A can maintain the high shearing strength up to 210 °C. Upon further increasing temperature, the network rapidly decomposes, and the strength almost completely losses at around 255 °C, just lying in the desirable temperature range for reworking operation. As a result, the dismantlement of the integrated circuit or the replacement of the faulty chip can be realized without damaging the circuit board. In addition, the incorporation of phosphorus in the network results in a significantly increased limiting oxygen index (LOI) from 18.2 (ERL-4221) to 23.2 (Epo-A). The mechanism for the degradation behavior was studied by isothermal and temperature-variable FT-IR spectra in detail.