Effect of heat treatment temperature and surface roughness to the PDMS-FR4 adhesive bonding

Abstract

The current work investigates the use of liquid polydimethylsiloxane (PDMS), with a 10:1 ratio of prepolymer:curing agent = 10:1 as the intermediate layer for adhesive bonding with Flame Retardant 4 (FR4). The spin coating of liquid PDMS on FR4 allows irreversible adhesive bonding with the solid PDMS. The strength of the proposed adhesive bonding technique has been investigated under different treatment temperatures: oven-heated at 60 °C, 70 °C, and 80 °C, cooled down in the room temperature of 25 °C, and exposed to direct sunlight at 35 °C. All the samples were left for 6 h. Investigations were conducted to analyze the effect of FR4 surface roughness on the strength and quality of the adhesive bonding. The standard procedure of American Standard Test Measurement (ASTM) D1002 was followed to verify the strength of PDMS-FR4 adhesive bonding. Strongest adhesive bonding, free from air bubbles, was obtained from a sample with smooth surface of FR4 that has undergone a cooling down treatment in the room temperature of 25 °C. FR4 was coated by solder mask to obtain smooth surface. The techniques reported in this paper are simple, straightforward, and effective to be implemented with FR4 as a base material for adhesive bonding with PDMS, thus eliminating expensive and complicated operating equipment as widely used in the oxygen plasma-assisted bonding treatment. Another benefit from this adhesive bonding technique is the fact that it avoids the use of expensive oven or hot plate.