In the flip chip interconnection using solder bump, the Under Bump Metallurgy(UBM) is required to perform multiple functions in its conversion of an aluminium bond pad to a solderable surface. In this study, two kinds of the UBM system such as electro-plated Cu 5㎛/Ti 2000Å/Al 1㎛ and Au 1000 Å/electroless-plated Ni 5㎛/ Al 1㎛ were investigated for Pb-free solders (Sn-Ag-X) flip chip packaging in terms of their metallurgical and mechanica properties. In recent years, Pb-free solders become very important because of lead toxicity. Therefore, the study about the application of the Pb-free solders for flip chip solder bump is also important.
In this study, the flip chip solder bump is formed using a stencil printing method which has a the highest potential for a low cost flip chip interconnection process, and electroless-plated Ni UBM is formed using a selective autocatalytic metal deposition directly on the aluminum pads of wafers. The reaction of the electro-plated Cu and Sn-Ag solder formed $Cu_6Sn_5$(η-phase) and $Cu_3Sn$(ε-phase) phases and the electroless-plated Ni and Sn-Ag solder formed stable $Ni_3Sn_4$ phase and metastable $NiSn_3$ phase at the interface during solder reflow process. And electroless-plated Ni is amorphous with supersaturated P at the Ni surface in contact with the $Ni_3Sn_4$ phase. The solder reflow process is performed on a hot plate at 250°C temperature.
Kinetics of phase formation during interdiffusion in solid UBM-liquid solder diffusion couples are influenced by the morphology of the intermetallic compound layer. The $Cu_6Sn_5$(η-phase) has a scallop-like shape and $Cu_3Sn$(ε-phase) has a thin layer shape. Most of the Cu-Sn intermetallic compound layers is the $Cu_6Sn_5$(η-phase) phase which form a channel between scallop-like grains by grain boundary grooving effect. The channel provides a rapid diffusion path. Therefore the growth kinetics of the Cu-Sn intermetallic compound depends on interdiffusion of the Cu and Sn though the channel. The growth exponent, n, of the Cu-Sn intermetallic compound is measured as 0.38 and apparent activation energy is 24.2KJ/mole. The $Ni_3Sn_4$ phase growth as a polygonal shape and needle-like shape such as a $NiSn_3$ phase. When the $Ni_3Sn_4$ phase growth, it crumbles into molten Sn by thermal stress between amorphous Ni and $Ni_3Sn_4$ phase and saturated P layer. The Growth Kinetics of the $Ni_3Sn_4$ phase depends on crumbling effect and saturated P layer at the Ni surface. The growth exponent, n, of the Ni-Sn intermetallic compound is measured as 0.47 and apparent activation energy is 26.4KJ/mole.
The effects of intermetallic compounds at the UBM/Pb-free solder interface on mechanical solder joint strength were investigated using solder ball shear test. The flip chip solder bumps formed with 170㎛ diameter and 120㎛ height on 150×150 ㎛ bond pad size were prepared for ball shear test. According to those results, the shear strength of the Sn-Ag and Sn-Ag-Cu solder bump on electro-plated Cu UBM increased with increasing of the solder reflow time and the fractured interface occurred at solder/$Cu_6Sn_5$, $Cu_6Sn_5$ inside and $Cu_6Sn_5/Cu_3Sn$. However, the shear strength of the Sn-Ag-Bi solder decreased due to the addition of brittle Bi. In general, the Cu UBM system has a good reliability at reaction with Pb-free solders.
The ball shear strength of the Sn-Ag solder with 120㎛ and 60㎛ height on the Ni UBM decreased with increasing of the solder reflow time. The fractured interface in the former 120㎛ height case occurred at $Ni_3Sn_4$/saturated P layer due to crumbling of the $Ni_3Sn_4$ and in the later 60㎛ height case was at Ni UBM/Al bond pad due to thermal mismatch between amorphous Ni and Al.
