The Effect Of Stand-off Height For Miniature Solder Joint On The Reliability

Lead-free and miniaturization are the trends of electronic packaging. Under these trends, the patch, pad size and the volume of the solder joint will reduce greatly in the traditional BGA (Ball Grid Array) technology. The solder geometry that includes solder volume, lower/upper contact angles (θ), stand-off height (SOH) and solder joint profile is the key to the design of miniature electronic packaging, and the stand-off height of solder joint is one of the most important factor.The proper thickness of interfacial IMCs (Intermetallic Compound) layer in solder joint is the symbol of good connection, but due to IMCs intrinsic brittleness, thicker interfacial IMCs layer will degrade the reliability. The change of the SOH will affect the solder joint's microstructure heavily, such as the morphology, the component and the thickness of the interfacial IMC, and then the reliability will be challenged by those changes.In this paper, a new a clamp apparatus was designed by the author to control the solder joint's SOH exactly. The Cu/Ni/Au/solder/Au/Ni/Cu systems were used to study the effect of SOH changes on the morphology, thickness and the proportion of interfacial IMCs layer, then the shear test and impact test were applied, and the effect of the SOH changes on the reliability of miniature solder joint was analyzed too. The results showed that the thickness and the proportion of interfacial IMCs layer increased with the reduction of SOH, as well as the miniature solder joint's mechanical behavior.In additional, the chance of metal elements interaction between UBM (Under Bump Metallization) and pad in miniature solder joint will increase with the reduction of SOH. The metal elements both at the UBM and the pad may diffuse to the opposite side and form complex IMCs. The Cu/Ni/Au/Sn3.5Ag/Cu system was used in this paper to study the interaction in miniature solder joint with the reduction of SOH. The result showed there existed the interaction between UBM and pad in miniature solder joint, and the degree increased with the reduction of SOH.