Effects of Thermal Lids Gold Plating Thickness on Thermal Interface Reliability for Flip Chip Packaging

This paper summarizes the intermetallic compounds (IMC) formation at the interface between thermal interface material (TIM) and nickel/gold plated integrated heat spreader (IHS) at varying Au thickness, and its impact on thermal reliability. Indium solders due to their high thermal conductivity are commonly used as the TIM to dissipate heat from silicon die to the thermal lids for new generation microprocessors with higher operating die temperatures. Indium solders readily wet the Au plating on thermal lids to form IMC during soldering. Optimal Au thickness is essential; Au thickness should be thick enough for reliable soldering, but must also be thin enough to offset the high cost and to prevent formation of a brittle Au-rich IMC layer in the solder joint. AuIn₂ is the preferred IMC for indium-gold soldering and does not embrittle the solder joint. Resulting IMC type depends on the Au:In ratio which can be predicted by a In-Au binary phase diagram. On this basis, critical Au plating thickness to form AuIn₂ IMC can be estimated using the known density values for electroplated gold and indium. In this study, Au thicknesses ranging from 0.035 to 0.2μm with a fixed gold pad size were electrolytically plated on a nickel plated copper lid. Assembled units were then subjected to Temperature Cycling-B (TCB). An in-house developed metrology for measuring junction-to-case thermal impedance (Rjc) is described. In this study, varying the thermal lids Au-plating thickness between 0.035 to 0.2 μm only lead to slight increase in center and corner Rjc values through 115 cycles TCB. The maximum center Rjc degradation post thermal cycling observed was only ∼ 1.7% on the lids with Au pad thickness between 0.035 – 0.04 μm. There were also no clear indications of impact of Au pad thickness on center and corner Rjc performance at EOL or post 115 cycles TCB. Thermal lids/TIM interface integrity remains unchanged for the range of Au pad thickness considered. However, detailed scanning electron microscopy and energy dispersive spectroscopy showed thicker Au plating results in greater incidence of AuIn₂ IMC nodules beneath In-Ni-Au ternary IMC layer at end of line (EOL) ie post packaging and test. AuIn₂ IMC is formed right after assembly and is what that holds the solder to the lid. As such, it follows that the presence of a more continuous and possibly greater number of AuIn₂ IMC nodules can be expected to provide a better lid-solder joint at EOL.