Electromigration enhanced intermetallic growth and void formation in Pb-free solder joints

Abstract

A kinetic analysis was formulated for electromigration enhanced intermetallic evolution of a Cu-Sn diffusion couple in the Sn-based Pb-free solder joints with Cu under bump metallurgy. The simulated diffusion couple comprised the two terminal phases, Cu and Sn, as well as the two intermetallic phases, Cu 3 Sn and Cu 6 Sn 5 , formed between them. The diffusion and electromigration parameters were obtained by solving the inverse problem of the electromigration enhanced intermetallic growth, and they were compatible with the literature values. Finite difference method was applied to the whole simulated domain to solve for the mass transport kinetics within the intermetallic phases and across each interface of interest. Simulation showed that, when electromigration effect was absent ͑zero current͒, intermetallic growth followed a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing ͑4 ϫ 10 4 A/cm 2 ͒, the growth of the dominant intermetallic Cu 6 Sn 5 clearly followed a linear law, suggesting a reaction controlled mechanism for electromigration. Simulation results were consistent with the experimental observations. The analysis of vacancy transport was also incorporated with the model, and the results showed substantial increase in vacancy concentration at the Cu 6 Sn 5 phase near the Cu 3 Sn/ Cu 6 Sn 5 interface. The peaking of the vacancy concentration explains the substantial Kirkendall void formation under electromigration at this region.

Influence of Electromigration on Kinetics of Reaction Diffusion in the Cu—Sn System

“…Що стосується кінетики росту ІМС, то в цьому питанні не має повної ясности: у частині робіт отримано лінійну залежність ширини прошарку ІМС від часу x  t [10][11][12][13], в іншій -параболічну x 2  t [6,8,13,14]. Аналіза кінетики і морфології ІМС у припайних з'єднаннях є до-сить непростою, оскільки різні ефекти можуть бути домінантними: електроміґрація, стиснення струму, термоміґрація.…”

“…Потім він одержав розвиток шляхом враху-вання існування нерівноважних вакансій у системі та скінченної швидкости реакції на міжфазних межах [23,24]. Згідно з цим мо-дельом реакційної дифузії при електроміґрації рівняння швидкос-ти росту нової фази буде наступним: [11,12]. Залежність ширини фази (Cu 3 Sn  Cu 6 Sn 5 ) від часу проведення експерименту є складною.…”

Intermetallic Layer Growth Kinetics in Sn‐Ag‐Cu System using Diffusion Multiple and Reflow Techniques

“…The growth rate of the Sn/Cu IMCs follows a parabolic kinetic law as it is reported in the literature. [15,26,27,[44][45][46]48,52,54,56,57] The corresponding activation energies are reported in Table 3. The growth rate of the IMC layers follows the parabolic rate law (dX=dt ¼ k=X), the observed correlation was in the range from [31] Cu|Sn thin films 101.3 83.9 25-220 [64] Cu thin film|Sn 6 Cu|Sn-0.6Cu-0.05Ni -66.3 80-150 [47] 0.95 to 0.98 (R 2 linear least squares fitting).…”

A framework for studying dynamics and stability of diffusive–reactive interfaces with application to Cu 6 Sn 5 intermetallic compound growth

“…where Z * is a parameter that relates the diffusive flux to the applied electrical current determined experimentally or through ab initio models [29,30]. Taking the gradient of the chemical potential in equation (2.6) and adding to the electromigration driving force of equation (2.7), the total driving force for diffusion is obtained as…”