Effects of bump height and UBM structure on the reliability performance of 60µm-pitch solder micro bump interconections

Abstract

Recently, three dimensional integration circuits technology has received much attention since the demands of functionality and performance in microelectronic packaging for electronic products are rapidly increasing. For high-performance 3D chip stacking, high density interconnections are required. In the current types of interconnects, solder micro bumps have been widely adopted. For fine pitch solder micro bump joints, selections of bump height and UBM structure are the important issues that would show the significant effects on the reliability performances of solder micro bump interconnection. In this study, effects of bump height and UBM structure on the reliability properties of lead-free solder micro interconnections with a pitch of 60μm were discussed. The chip-to-chip test vehicle having more than 4290 solder micro bump interconnections with a bump pitch of 60μm was used in this study. To evaluate the effects of bump height and UBM structure on the reliability performance of micro joints, two groups of solder joint were made. The first group of micro joints had a total bump height of 29μm. In this group, Cu/Sn/Cu joint with a thickness of 7μm/15μm/7um, Cu/Sn/Ni/Cu joint having a thickness of 7μm/15μm/2μm/5μm and Cu/Ni/Sn/Ni/Cu joint with a thickness of 5μm/2μm/15μm/2um/5μm were selected. The second group of micro joints had a total bump height of 24μm. In this group, Cu/Sn/Cu joint having a thickness of 7μm/10μm/7um, Cu/Sn/Ni/Cu joint with a thickness of 7μm/10μm/2μm/5μm and Cu/Ni/Sn/Ni/Cu joint having a thickness of 5μm/2μm/10μm/2um/5μm were chosen. We used the fluxless thermocompression bonding process to form these two groups of micro joints. After bonding process, the chip stack was assembled by capillary-type underfill. Reliability tests of temperature cycling test (TCT), high temperature storage (HTS) and electromigration test (EM) were selected to assess the effect of bump height and UBM structure on the reliability properties of those two groups of solder micro bump interconnections.