Planarity-tolerant fine-pitch reworkable interconnections with sharp protrusions and microbumps

Abstract

Fine-pitch inter-strata connections in 3-D stacking lack rework solutions which can aide in early learning for 3-D integration. Reduced solder volume in the microbump or micropillar is also less tolerant to non-planarity and non-uniformity of the strata and interconnection array, especially as die size and number of interconnections increase. At fine pitch, heavily-loaded thermal underfills can have high viscosity and potential risk of filler entrapment. To address these issues, reworkable interconnections have been fabricated in our facility. In these structures, sharp metal tips were formed in a silicon mold and transferred to a silicon substrate to form one side of the reworkable interconnect. The other side of the reworkable interconnect is a silicon substrate with lead-free microbumps coated with underfill. The two silicon substrates were brought into contact at room temperature without reflowing the solder to perform electrical test.. The temporary stack can be easily separated (reworked) if test results indicate failure. Non-planarity is tolerated up to an amount which is the lesser of the height of the tips and solder thickness. A test vehicle with approximately 45,400 pairs of reworkable interconnects at 50 μm pitch was fabricated. Contact resistance as low as 115 mΩ was obtained at a contact force of 0.2 g per interconnect after insertion and partial cure. This same structure was also used to make initial permanent interconnects, the contact resistance measured 34 mΩ after reflow. Typical yield over the initial 10 mm × 12 mm test vehicles built was found to be > 99.9% out of 22,000 interconnects that can be electrically measured.