Lead-free solder bumping by the electroplating process for electronic packaging

Replacement of tin-lead solder bumps by a suitable lead-free material for flip chip attachment has become an urgent necessity due to the impending ban on the use of lead in electronics. Alloy compositions containing Sn and Ag. with some additions of Cu and Bi are the most promising candidates for the replacement of Sn-Pb. These materials have shown better mechanical strength and wetting, while its fatigue resistance surpasses even that of Sn-Pb. Amongst various competing deposition techniques, electroplating is the most attractive technique due to low cost and capability of preparing small dimension bumps. However, the co-deposition of elements is seriously complicated by the large difference in the electrode potentials of each element. Though these problems are not intractable, suitable chemicals need to be selected as additives by way of chelating agents, surfactants, buffers etc, so as to deposit multiple elements simultaneously. In this work, we report preliminary results of a co-deposition bath for Sn-Bi-Cu. This bath was used to obtain Sn-Ag-Bi-Cu alloy after reflow, with a targeted composition of Sn-3.1Ag-3.1Bi-0.5Cu. For this, Ag was deposited separately. The plating bath chosen for the co-deposition of Sn-Bi-Cu contains sulphate salts of these metals dissolved in sulphuric acid in the targeted proportion, along with chelating agents and buffers for a pH of ∼4.5. Titanium was chosen as substrate in view of its importance as barrier layer in under bump metallurgy (UBM). The bath was found to be stable for more than a week and produced compact films having composition close to the target. The bath was characterized for the dependence of film composition on the storage time up to 72 hours and current density between 5 and 25 mA/cm². In both experiments, it was found that the film composition is more or less invariant. To obtain solder bumps, Cu and Ni were deposited on patterned Ti substrates, followed by Ag and Sn-Bi-Cu. The deposited films show slight deviation from the targeted composition, which could be due to higher duration deposition done to obtain large thickness. The reflown bumps show variation in bump size and composition close to that targeted. Further study and optimization seems necessary in order to obtain improved bumps.