Effects of Sn-3Ag-0.5Cu solder ball size and content on the solder anisotropic conductive film (ACF) joint properties for flex-on-board assembly using ultrasonic bonding method

Abstract

In this paper, the electrical and reliability properties of various solder 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. And the exact value of current handling capability of all the ACF joints could not be evaluated, because metal lines of PCB were burnt before ACF joint failure. After 120 hours of PCT (Pressure Cooker Test), the open circuit failures were mainly observed at Ni ACFs, and 5-15 μm solder ball size from 10 wt% 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 hours of PCT in the solder ACF joints who has lower wetted solder areas. As a result of this study, the solder ball size and content of solder ACFs joints were optimized at 25-32 μm sizes and 30 wt% content for 300-μm-pitch FOB assembly.