Spalling of Cu6Sn5 spheroids in the soldering reaction of eutectic SnPb on Cr/Cu/Au thin films

Abstract

The growth and morphology of intermetallic compounds between the solder and substrate play an important role in the solderability and reliability of electronic solder joints. Solder on thin films, as in chip joint, acts as an electrical and mechanical/physical interconnection between the chip and the substrate. We have studied the interfacial reactions between eutectic SnPb (63Sn37Pb, wt%) and Cr/Cu/Au thin films. Our results found here have been compared to the solder reaction on bulk Cu. The eutectic solder has 7° of wetting angle on Cr/Cu/Au thin films rather than 11° on Cu substrate. Sideband around the solder cap was found in both the thin film case and the Cu case. Spalling of Cu6Sn5 compound grains occurred in the thin-film case when the Cu film was consumed but not in the case of bulk Cu. We observed a shape change from hemispherical ‘‘scallops’’ to spheroids before spalling took place. The shape change is assisted by ripening a reaction among the scallops. We have calculated a critical size of the scallop, depending on the Cu film thickness, when the shape change or spalling starts.

Cross interactions on interfacial compound formation of solder bumps and metallization layers during reflow

“…Kao reported that the addition of Ni particles may inhibit the redistribution of AuSn 4 IMCs during solid state aging. 23 Tu et al also found that Au-Sn IMC deposited on the surface of Cu 6 Sn 5 IMCs after reflow, 10 and they reported that there was no AuSn 4 redistribution observed in Pb-free solder. 9 Moreover, the solubility of Ni in the eutectic SnPb solder is estimated to be 0.052 at.% at 220°C, but it is 0.28 at.% in the eutectic SnAg solder at 250°C.…”

“…Tu et al also observed similar tiny particles coated on the surface of Cu 6 Sn 5 IMCs when eutectic SnPb solder was reflowed on a Cr/Cu/Au UBM. 10 Furthermore, Au atoms dissolve very fast in SnPb solder. 16 Although the Au concentration in our SnPb case after the second reflow was less than the solubility (0.3 wt.%), the Au could be depleted from the solid solution when the AuSn 4 particles on Cu 6 Sn 5 IMCs has a lower chemical potential than that of Au or AuSn 4 dissolved in the eutectic SnPb.…”

“…Since the thickness of the Cu UBM was about 0.7 m, spalling of IMCs might occur after the first reflow. 10 Figure 4(b) depicts the SEM plan-view image for the chip side, in which Cr-Cu-Sn layer was detected. It is speculated that the spalled or partially spalled Cu 6 Sn 5 IMCs were removed during the selective etching of the SnAg solder.…”

“…[6][7][8] Spalling of IMCs were reported when thin film Cu UBM reacted with eutectic SnPb and Pb-free solders. 9,10 A method has been developed to prevent the spalling of IMCs by the opposite interfacial reaction on the substrate side. 11 Furthermore, Tu et al reported that Au layer on the substrate-side affected the spalling of IMCs on the chip-side across a solder joint.…”

Flip Chip Bump Formation of Sn–1.8Bi–0.8Cu–0.6In Solder by Stencil Printing

“…Berry and Ames 22) have reported that the IMCs spalled into the solder when Cu was consumed. Liu et al 23) reported similar results and suggested that the IMC changed to circletype in shape when spalling began. The spalling in this study started before 500 h aging.…”