Effect of Cobalt Doping on the Microstructure and Tensile Properties of Lead Free Solder Joint Subjected to Electromigration

doi:10.1016/j.jmst.2016.09.007

Abstract

Abstract:

Rapid Cu diffusion is one of the main causes of electromigration (EM) failure in lead-free solder joints. In this study, an effort has been made to investigate the detrimental effects of EM on microstructure and mechanical performance of solder joint by introducing Co nanoparticles (NP) doped flux at the interface between SAC305 solder and copper substrate. EM tests were conducted on un-doped SAC305 and Co-doped SAC305 solder joints for different time intervals, with the maximum duration of 1128 h. A DC current was applied to the both types of solder joints to achieve a current density of 1 × 10⁴ A/cm². EM tests were performed in an oil bath maintained at a constant temperature of 80 °C. It is found that Co-doped flux significantly reduced the formation of cracks and voids at the cathode interface. Co atoms entered into the lattice of Cu₆Sn₅ leading to the formation of (Cu, Co)₆Sn₅. This thermodynamically stabilized the interfacial intermetallic (IMC) layers both at the anode and cathode sides and suppressed the change in their thickness. The average anodic growth rate of (Cu, Co)₆Sn₅ interfacial IMC in the doped sample was about one order of magnitude lower compared with that of Cu₆Sn₅ in the un-doped samples. Co-NP also improved the tensile strength considerably before and after EM. The report suggests that the reliability of solder joint during EM can be improved by using Co-NP doped flux.

Key words: Electromigration damages, Tensile strength, Intermetallic compounds, Nanoparticles, Reliability

Nasir Bashir M.,Haseeb A.S.M.A.,Zayed Mohammad Saliqur Rahman Abu,Fazal M.A.. Effect of Cobalt Doping on the Microstructure and Tensile Properties of Lead Free Solder Joint Subjected to Electromigration[J]. J. Mater. Sci. Technol., 2016, 32(11): 1129-1136.

Figures/Tables 14

Fig. 1. (a) Photograph of a prepared line type Cu/solder/Cu solder joint, (b) mounted and ground sample in epoxy resin used for EM, (c) schematic diagram of the sample with all dimensions, and (d) description of central angle used for measurement of cross-sectional area[33].

Fig. 2. SEM back-scattered electron images of (a), (c) SAC305 and (d), (f) SAC305+2 wt% Co NP solder joints. Each micrograph corresponds to one end of the joint and (d), (e) correspond to the solder bulk of SAC305 and SAC305+2 wt% Co NP solder joints.

Table 1. EDX elemental compositions of IMC phases in SAC305+ 2 wt% Co NP sample

Location	Element composition (wt%) -------------------------
Location	Cu	Ag	Sn	Co
Interfacial IMC layer near to solder	54.24	1.64	42.62	1.5
Interfacial IMC near to Cu substrate	55.01	1.12	43.01	0.86
IMCs in solder matrix	50.355	1.435	47.31	0.9

Fig. 3. FESEM backscattered electron images of SAC305 solder joint after EM test for 192, 384, 768 and 1128 h. (a), (b), (c) and (d) show cathode side, while (e), (f), (g) and (h) show anode side.

Fig. 4. FESEM backscattered electron images of SAC305+2 wt% Co NP solder joint after EM test for 192, 384, 768 and 1128 h. (a), (b), (c) and (d) show cathode side, while (e), (f), (g) and (h) show anode side.

Fig. 5. IMC thinness variation as a function of EM time in cathode and anode of SAC305 solder with 0 and 2 wt% Co-doped flux.

Table 2. Anodic growth rate of IMCs in SAC305 solder with 0 and 2 wt% Co doped flux at various EM time interval

EM time (h)	Cu₆Sn₅ (×10^-6 µm/s)	(Cu, Co)₆Sn₅(×10^-7 µm/s)
192	1.591	1.44
384	1.880	2.170
768	1.591	2.531
1128	1.280	2.201
Average	1.585 ± 0.245	2.231 ± 0.460

Fig. 6. EDX elemental maps of SAC305 + 2 wt% Co NP solder joint after EM test for 1128 h. Elemental maps for (a) SEM image, (b) Co, (c) Sn, and (d) Ag.

Fig. 7. Stress-strain curves of (a, b) SAC305 solder joint at 0 h and 192 h EM, (c, d) SAC305 + 2 wt% Co NP solder joint at 0 h and 192 h EM test.

Fig. 8. Tensile strength of SAC305 and SAC305 + 2 wt% Co NP solder joints at 0 h and 192 h.

Fig. 9. Description of fracture path in (a) SAC305 at 0 h (b) SAC305 + Co NP at 0 h (c) SAC305 at 192 h and (d) SAC305 +Co + NP at 192 h of EM test.

Fig. 10. Schematic diagram of structural changes at interfaces of SAC305 and SAC305+2wt% Co NPs solder joints before and after EM.

Fig. 11. Estimated value of constant n of Eq. (7) by using curve fitting technique.

Table 3. Comparison of estimated value of n and k with literature

Composition	n	k (m²/s)	Current density (A/cm²)	Temperature T (k)	Maximum EM time t (h)	References
Cu/SAC/Cu	0.88	8.85 × 10^-12	1.0 × 10⁴	353	1128	This study
Cu/SAC+Co/Cu	1.40	6.31 × 10^-16	1.0 × 10⁴	353	1128	This study
Cu/SAC/Cu	0.65	5.72 × 10^-13	1.0 × 10³	343	360	^[50]
Cu/SAC/Cu	0.63	1.41 × 10^-12	2.0 × 10³	343	360	^[50]

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