Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

Abstract

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated $62 lm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm 2 at 150°C, 10 kA/cm 2 at 160°C, and 15 kA/cm 2 at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thinfilm-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni) 6 Sn 5 or Cu 6 Sn 5 intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the $52-lm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of $80% Cu-Ni-Sn IMC and $20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.

Dominant effects of Sn orientation on serrated cathode dissolution and resulting failure in actual solder joints under electromigration

“…Two approaches to inhibit the formation of a pancake-type void have been implemented successfully. One is to utilize a new joint configuration with thick Cu under bump metallurgy to effectively alleviate serious current crowding in conventional flip chip solder joints [15,16]. The other is to reduce the operating temperature of the device.…”

Toward a Better Understanding of the Effect of Cu Electroplating Process Parameters on Cu3Sn Voiding

“…[1][2][3][4][5][6][7][8] During soldering, two IMCs, Cu 6 Sn 5 and Cu 3 Sn, form at the solder/Cu interface, and continue to grow upon subsequent thermal aging or current stressing by solid-state diffusion. As the term ''Kirkendall voiding'' implies, the voiding arises from the agglomeration of excess vacancies, as a result of asymmetry between the fluxes of two interdiffusing species, Cu and Sn, in the Cu 3 Sn IMC.…”

Electromigration-induced accelerated consumption of Cu pad in flip chip Sn2.6Ag solder joints

“…[11] For Sn3Ag0.5Cu solder bump combined with 25 µm thick Cu substrate pad metallization, after current stressing with a current density of 1.0×10 4 A/cm 2 , wavelike morphology with large radiant of Cu pad was observed. [12] The wavelike consumption behavior was also observed for the Cu UBM in Sn3.5Ag solder bump combined with 18 µm thick Cu UBM, during current stressing with a current density of 5.67×10 4 A/cm 2 . [13] The present study investigated the consumption behavior of 20 µm Cu pad under electromigration for a Sn2.6Ag solder bump with Ti/Cu/Ni UBM.…”