Cu pillar bumps as a lead-free drop-in replacement for solder-bumped, flip-chip interconnects

We evaluated Cu pillar bumps with a lead-free SnAg solder cap as 1
^st
-level soldered interconnects in a standard organic BGA (ball grid array) flip chip package. Various pillar heights and solder volumes were investigated at a minimum pad pitch of 150 mum. Flip chip assembly followed the exact process as in the case of collapsible SnAg solder bumps, using an identical material set and identical equipment. Thanks to the properties of Cu pillar bumps, substrate design tolerances could be relaxed compared to solder bump interconnects of the same pitch. This, together with the fact that no solder-on-pad (SOP) is required for Cu pillar bumps, allows for lower substrate cost, which is a major factor in flip chip packaging cost. Cu pillar bumps also offer significantly higher current carrying capability and better thermal performance compared to solder bumps. We found that flip chip assembly with Cu pillar bumps is a robust process with regard to variations in assembly parameters, such as solder cap volume, flux depth, chip placement accuracy and substrate pad size. It is possible to attach the Cu pillar bumps to the metal lines of the non-SOP substrates with line spacing down to 40 mum plusmn15 mum without any solder bridging. It is shown that flip chip packages with suitable 150 mum-pitch Cu pillar bumps provide high mechanical and electrical reliability. No corrosion on the Cu pillars was found after humidity tests (MSL2 level and unbiased HAST for 192 h). Thermal cycling passed 2000 cycles, even at JEDEC JESD22-A104-B condition C (-65 degC to 150 degC). High Temperature Storage passed 2000 h at 150 degC. Cross sections reveal that after 1000 h at 150degC all Sn in the solder is transformed into Cu-Sn intermetallic compounds (IMCs). Preliminary electromigration test results at highly accelerated conditions (T
_bump
ap 177 degC, I = 0.8 A) show almost 2 orders of magnitude longer lifetime compared to SnAg solder bumps at 200 mum pitch. Cu pillar lifetimes at high current and temperature are expected to highly exceed those of solder bumps, because current crowding in the solder is avoided and solder is transformed into much more stable intermetallics.