Characterisation of chip-on-board (COB) encapsulate epoxies used in the printed circuit board assembly (PCBA) industry using variable frequency microwave (VFM) facilities

Abstract

[Abstract]: In the printed circuit board assembly (PCBA) industry, integrated circuit (IC) dies are attached onto the printed circuit board (PCB) with epoxy. The die pads are first connected to the PCB tracks with gold or aluminium wires, utilising an ultrasonic wire-bonding machine. The dies are then coated with thermally cured epoxy. The chip-on-board (COB) is finally placed in an electrical oven for baking at certain temperature for a certain period of time, depending upon the type of epoxy used, until the adhesive is completely cured. The baking process using an electrical oven is time-consuming and costly. An alternative way of curing the epoxy is desirable searched. A variable frequency microwave (VFM) source is identified as the possible solution. The first step in curing the epoxy using VFM heating is to find out the best frequency to process the materials in microwaves and this is termed as ‘characterisation’ of the epoxy by the VFM oven manufacturers. Two VFM facilities were employed to perform the characterisation. One is in the frequency range of 2 – 8 GHz and the other is in the frequency range of 6.5 – 18 GHz. By using these two facilities, the best frequency range to process an epoxy by VFM can be identified. Two high quality and commonly epoxy resins, Uniset adhesive and Hysol encapsulant were chosen for consideration. The two resins were then characterised by VFM facilities. From the graphs of the characterisation, a better resin of the two can be identified.