Effect of intermetallic compound thickness on shear strength of 25 μm diameter Cu-pillars

• The shear strength of a single Cu-pillar solder bump 25 μm in diameter is studied.
• Different thermal ageing have been applied to the Cu-pillars.
• The distance from the shear tip to the initial Cu/solder interface varies in the range 1–5 μm.
• The thicknesses of intermetallics have no significant influence on mechanical strength.
• Cu3Sn/Cu interface seems to be as strong as Cu3Sn/Cu6Sn5 interface.

The chip-to-chip bonding technique using Cu-pillars bump is widely applied in 3D chip stacking technology. The excessive growth of intermetallic compounds (IMC) is expected to increase the propensity to brittle failure. Typically, the thickness of the IMC layer is used to indicate the risk of failure of solder joints. This study investigates the effects of Cu6Sn5 and Cu3Sn compounds on the single-joint shear strength of Cu-pillar bumps 25 μm in diameter joined with Sn–3.5 wt%Ag–0.7 wt%Cu (SAC 357) alloy. The influence of heat treatment on the shear strength of the Cu-pillar structures is studied by applying two types of heat treatment: (i) a standard conveyor furnace process and (ii) isothermal holding at 240 °C for holding times up to 30 min. The change in mechanical strength was then established as a function of total IMC thickness through shear test experiments. Shear strength was measured with different displacement rates of 70, 130 and 500 μm s−1. The shear height, from the tip of the shear tool to Cu pad substrate, varied from 11 μm to 16 μm in 1 μm steps. The variation in shear force values through the interfacial system, from pure Cu-pillar to solder ball, are discussed in relation to the failure shape observations. For low shearing heights (11–12 μm), mainly Cu material is probed and high shear force values are measured. For high shearing heights (15–16 μm), the probed materials are Cu6Sn5 and Sn and low shear force values are measured. For intermediate shearing heights (13–14 μm) the probed materials are Cu3Sn and Cu6Sn5 and the Cu/Cu3Sn interface seems to be as strong as the Cu3Sn/Cu6Sn5 interface, despite the fact that almost all the Kirkendall voids are located in Cu/Cu3Sn interface.

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