Thermo-electromigration phenomenon of solder bump, leading to flip-chip devices with 5,000 bumps

摘要

High performance logic devices have rapidly advanced in network system. In order to reply the demand of high pin count and high speed, Flip-chip BGA (FC-BGA) package applied high-density organic substrate has been developed. This package has the superior possibility of flexible bump locations by virtue of high via densities and fine line capabilities of the substrate. The feature of substrate is adopting the stacked method of finer via pitch layers. Utilizing the density, it is possible to either minimize the LSI die size or maximize the number of bumps on the die. Also at the high performance devices, the high current density through the bump is strongly demanded. In order to satisfy the demand and realize the high pin counts devices, thermo-electromigration phenomenon of solder bump is one of the key reliability items. The thermo-electromigration phenomenon of solder bump was investigated to be consisting of three steps as below. At 1st step, the lead (Pb) migrates as electron flow under high-density current. And at 2nd step, the Under Bump Metals(UBM) migrates and disappears. Finally at 3rd step, Aluminum (Al) routing metal migrates and it results in open failure. And from the High Temperature Operating Life (HTOL)results, the life time of solder bump on current density has been estimated theoretically based on Black's equation. The lifetime was predicted more than 20 years with the current being 160 mA/bump in 220μm pitch cases.