Controlling Whisker Formation in Tin-Based Solders Using Electrically Mediated Electrodeposition

Abstract

The global trend to eliminate lead from electronic devices is driven by legislative and marketing pressures based on environmental, health and social concerns on the cumulative effects of lead in waste streams. Tin and tin-rich alloys are the forerunning replacements for tin-lead solders.However, pure tin electrodeposits have a tendency to form tin whiskers that may cause short-circuiting, metal vapor arcing and failure of the component. The major cause of whisker growth is thought to be compressive stress in pure tin deposits after DC plating. Faraday Technology, Inc. has developed an electrically mediated electrochemical process for depositing lead-free solders, based on non-steady state electric fields to control the deposition process and consequently the physical properties of the deposit. Results of electrically mediated tin deposits are presented and compared to DC deposits in terms of grain size, internal stress type/magnitude and surface roughness.

Analysis of the Particle Size and Morphology of Tin Powder Synthesized by the Electrolytic Method

“…The experimental results demonstrated that the increase in tin concentration in the electrolyte leads to an increase in the current efficiency of the electrolysis. The increase in the initial concentration of SnSO 4 ensures the availability of Sn 2+ ions on the interface of the cathode and electrolyte, thereby reducing the fraction of the electrical current used for hydrogen gas formation from the H + ions . The current efficiency is lower at the lower tin concentration because more electric current is consumed for the hydrogen gas evolution (i.e., 2H + + 2e = H 2 ).…”

“…During the electrodeposition process to produce tin powder, thiourea will be adsorbed on the Sn nuclei and form a soluble complex species with Sn 2+ ions; thus, it will hinder the growth process and reduce the diffusion rate of an adatom on the cathode surface . The low diffusion rate of the adatom on the cathode surface leads to the formation of new nuclei, and the nucleation process is more dominant than the grain growth process . The limitation of the existing grain growth resulted in the tin powder having a smaller grain size.…”

“…The formation of a compact plate deposit at the electrolysis time of 120 min is due to a decrease in the concentration of adatoms on the cathode surface that leads to the growth of the existing grains rather than the process of forming new nuclei. In addition, the electrolysis time is inversely proportional to the rate of electrocrystallization . As a result, when the electrolysis time reached 120 min, a deposit was in the form of a compact plate with a larger grain size than the shorter electrolysis duration.…”

“… 17 The low diffusion rate of the adatom on the cathode surface leads to the formation of new nuclei, and the nucleation process is more dominant than the grain growth process. 29 The limitation of the existing grain growth resulted in the tin powder having a smaller grain size.…”

“…In addition, the electrolysis time is inversely proportional to the rate of electrocrystallization. 29 As a result, when the electrolysis time reached 120 min, a deposit was in the form of a compact plate with a larger grain size than the shorter electrolysis duration.…”