A Study on the Solder Ball Size and Content Effects of Solder ACFs for Flex-on-Board Assembly Applications Using Ultrasonic Bonding

In this paper, the electrical and reliability properties of various solder anisotropic conductive film (ACF) joints for flex-on-board (FOB) assembly using ultrasonic bonding were assessed in terms of short circuits, contact resistance, current handling capability, wetted solder areas, and reliability evaluation. It was found that the wetted solder areas increased and reliability was enhanced as the size and content of the solder ball increased. Regardless of solder ball size and content, the contact resistances of solder ACF joints were almost the same, because the current went through the electrode/electrode contact, which is like a short circuit to the solder joint. In addition, the exact value of current handling capability of all the ACF joints could not be evaluated, because metal lines of printed circuit board were burnt before ACF joint failure. After 120 h of pressure cooker test (PCT), the open-circuit failures were mainly observed at Ni ACFs, and 5-15-μm solder ball size from 10 to 30 wt% solder content and 25-32-μm size and 10 wt% solder content of solder ACFs, presumably due to lower wetted solder areas on metal electrodes. In addition, the crack was noted after 120 h of PCT in the solder ACF joints who has lower wetted solder areas. As a result of this paper, the solder ball size and content of solder ACFs joints were optimized at 25-32-μm sizes and above 30 wt% content for 300-μm pitch FOB assembly.