Solid-state intermetallic compound layer growth between copper and 95.5Sn-3.9Ag-0.6Cu solder

Abstract

Long-term, solid-state intermetallic compound (IMC) layer growth was examined in 95.5Sn-3.9Ag-0.6Cu (wt.%)/copper (Cu) couples. Aging temperatures and times ranged from 70°C to 205°C and from 1 day to 400 days, respectively. The IMC layer thicknesses and compositions were compared to those investigated in 96.5Sn-3.5Ag/Cu, 95.5Sn-0.5Ag-4.0Cu/Cu, and 100Sn/Cu couples. The nominal Cu 3 Sn and Cu 6 Sn 5 stoichiometries were observed. The Cu 3 Sn layer accounted for 0.4-0.6 of the total IMC layer thickness. The 95.5Sn-3.9Ag-0.6Cu/Cu couples exhibited porosity development at the Cu 3 Sn/Cu interface and in the Cu 3 Sn layer as well as localized "plumes" of accelerated Cu 3 Sn growth into the Cu substrate when aged at 205°C and t Ͼ 150 days. An excess of 3-5at.%Cu in the near-interface solder field likely contributed to IMC layer growth. The growth kinetics of the IMC layer in 95.5Sn-3.9Ag-0.6Cu/Cu couples were described by the equation x ϭ x o ϩ At n exp [Ϫ∆H/RT]. The time exponents, n, were 0.56 Ϯ 0.06, 0.54 Ϯ 0.07, and 0.58 Ϯ 0.07 for the Cu 3 Sn layer, the Cu 6 Sn 5 , and the total layer, respectively, indicating a diffusion-based mechanism. The apparent-activation energies (∆H) were Cu 3 Sn layer: 50 Ϯ 6 kJ/mol; Cu 6 Sn 5 layer: 44 Ϯ 4 kJ/mol; and total layer: 50 Ϯ 4 kJ/mol, which suggested a fast-diffusion path along grain boundaries. The kinetics of Cu 3 Sn growth were sensitive to the Pb-free solder composition while those of Cu 6 Sn 5 layer growth were not so.

The Influence of Current Direction on the Cu-Ni Cross-Interaction in Cu/Sn/Ni Diffusion Couples

“…The increase in Cu 6 Sn 5 thickness could deteriorate the mechanical properties of the interface due to the brittle nature of this intermetallic; however, the decrease in Cu 3 Sn would substantially improve the drop reliability due to the reduction in the growth of Kirkendall voids as a function of the Cu 3 Sn thickness. 7,[14][15][16][17][18] The Cu-Ni interaction continued but at a relatively low rate during subsequent solid-state annealing. 2,[7][8][9][10][11] It is clear that cross-interaction refers to an up-hill diffusion process wherein both Cu and Ni diffuse from regions of low concentration to those of high concentration.…”

Intermetallic Compound Formation and Evolution in Solid-State Sn/Immersion-Ag/Cu Trilayer Interfacial Reactions on a Flexible Polymer Board

“…These voids were so-called Kirkendall voids, being a result of unequal interdiffusion between Sn and Cu in the Cu 3 Sn phase. [18][19][20] Voids were also found in Fig. 1d and h, but the quantity and size were smaller than that in Fig.…”

Optimized Cu-Sn Wafer-Level Bonding Using Intermetallic Phase Characterization

“…However, several studies have presented values of n that vary with temperature. 11,23-25 Vianco et al 11 also obtained n = 0.4 for an annealing temperature range below the melting point of Sn (70°C to 205°C). Our present work shows an exponent n < 1/2 for T < T m , implying that the IMC growth rate is slower than can be explained by diffusion rates, showing that the chemical reaction kinetics is slow in this temperature regime.…”