Microstructure induced galvanic corrosion evolution of SAC305 solder alloys in simulated marine atmosphere

doi:10.1016/j.jmst.2020.03.024

Abstract

Abstract:

Motivated by the increasing use of Sn-3.0Ag-0.5Cu (SAC305) solder in electronics worked in marine atmospheric environment and the uneven distribution of Ag₃Sn and Cu₆Sn₅ intermetallic compounds (IMCs) in β-Sn matrix, comb-like electrodes have been designed for in-situ EIS measurements to study the microstructure induced galvanic corrosion evolution of SAC305 solder in simulated marine atmosphere with high-temperature and high-humidity. Results indicate that in-situ EIS measurement by comb-like electrodes is an effective method for corrosion evolution behavior study of SAC305 solder. Besides, the galvanic effect between Ag₃Sn IMCs and β-Sn matrix can aggravate the corrosion of both as-received and furnace-cooled SAC305 solder as the exposure time proceeds in spite of the presence of corrosion product layer. Pitting corrosion can be preferentially found on furnace-cooled SAC305 with larger Ag₃Sn grain size. Moreover, the generated inner stress during phases transformation process with Sn₃O(OH)₂Cl₂ as an intermediate and the possible hydrogen evolution at local acidified sites are supposed to be responsible for the loose, porous, cracked, and non-adherent corrosion product layer. These findings clearly demonstrate the corrosion acceleration behavior and mechanism of SAC305 solder, and provide potential guidelines on maintenance of microelectronic devices for safe operation and longer in-service duration.

Key words: SAC305 solder, Marine atmosphere, Galvanic corrosion, In-situ EIS, Comb-like electrode

Mingna Wang, Chuang Qiao, Xiaolin Jiang, Long Hao, Xiahe Liu. Microstructure induced galvanic corrosion evolution of SAC305 solder alloys in simulated marine atmosphere[J]. J. Mater. Sci. Technol., 2020, 51: 40-53.

Figures/Tables 15

Table 1 Composition of the as-received commercial SAC305 solder (wt.%).

Ag	Cu	Pb	Sb	In	Fe	As	Ni	Sn
2.95	0.466	0.019	0.019	0.004	0.001	0.001	0.005	Bal.

Fig. 1. Schematic diagram in illustrating two-electrode system used for in-situ EIS measurement. (a) Top view of the comb-like electrodes. (b) Arrangement in illustrating functions of the electrodes.

Fig. 2. Microstructures of as-received commercial SAC305 solder (a) and furnace-cooled SAC305 solder (b).

Fig. 3. EPMA mapping results of distributions of Sn, Ag, and Cu elements on furnace-cooled SAC305 solder substrate.

Fig. 4. Potentiodynamic polarization curves of furnace-cooled (a-d) and as-received (a'-d') SAC305 samples as a function of CCT cycle in simulated high-temperature marine atmosphere with various RH and NaCl concentration.

Table 2 Obtained corrosion current density icorr (μA cm-2) from each polarization curve in Fig. 4.

Sample	70% RH and 2.0 wt.% NaCl		80% RH and 2.0 wt.% NaCl		70% RH and 3.5 wt.% NaCl		80% RH and 3.5 wt.% NaCl
Sample	Furnace-cooled	As-received	Furnace-cooled	As-received	Furnace-cooled	As-received	Furnace-cooled	As-received
1 CCT	5.00	2.33	4.18	3.38	0.93	2.28	10.0	1.35
2 CCT	2.88	6.61	22.0	5.78	1.96	2.22	3.79	1.54
3 CCT	6.20	6.41	77.7	16.3	4.67	1.52	5.71	8.59

Fig. 5. Evolution in open circuit potential (OCP) of paralleled SAC305 samples in 3.5 wt.% NaCl solution as function of immersion time.

Fig. 6. Evolution in obtained Rp within each CCT cycle from 1 CCT to 5 CCT for as-received and furnace-cooled SAC305 samples in simulated marine atmosphere of 80% RH and 3.5 wt.% NaCl at 45 °C.

Fig. 7. Bode plots of in-situ EIS results for as-received SAC305 and furnace-cooled SAC305 as a function of exposure time in simulated marine atmosphere with 80% RH and 3.5 wt.% NaCl at 45 °C: (a) and (c) modulus plot; (b) and (d) phase angle plot.

Fig. 8. Equivalent electrical circuit for selected in-situ EIS data of as-received and furnace-cooled SAC305 solder samples as a function of exposure time: (a) 0 h and 48 h; (b) 96 h, 144 h and 192 h. (Rs: solution resistance; Ro and CPEo: resistance and CPE impedance of corrosion product layer; Rct and CPEdl: resistance and CPE impedance of electrical double layer at the interface of corrosion product layer and solder substrate; W: Warburg diffusion impedance at the interface of corrosion product layer and solder substrate).

Table 3 Fitting results of the EIS data of as-received and furnace-cooled SAC305 as a function of exposure time in simulated marine atmosphere of 80% RH and 3.5 wt.% NaCl at 45 °C.

Solder	Corrosion time (h)	R_s (Ω cm²)	R_o (Ω cm²)	Q_o×10^-6 (F cm^-2 sⁿ^-1)	n_o	R_ct (kΩ cm²)	Q_dl×10^-6 (F cm^-2 sⁿ^-1)	n_dl	R_w × 10^-4 (Ω cm² s^0.5)	χ²×10^-4
As-received SAC305	0	5.44	650.1	7.296	0.877	45.01	9.565	0.836	1.444	6.18
	48	4.44	245.4	19.40	0.607	39.43	21.99	0.562	1.532	8.53
	96	7.52	586.7	19.06	0.615	29.96	25.19	0.623	/	9.95
	144	10.6	1185	47.83	0.587	23.93	53.97	0.791	/	9.89
	192	4.24	548.3	53.58	0.525	18.19	14.08	0.670	/	8.95
Furnace-cooled SAC305	0	5.98	1039	7.992	0.858	72.26	5.667	0.818	0.841	9.12
	48	9.16	895.0	10.86	0.704	14.44	51.05	0.573	8.298	4.60
	96	13.2	500.1	54.81	0.571	11.77	55.88	0.634	/	9.76
	144	12.3	795.4	62.10	0.579	8.142	86.35	0.580	/	8.36
	192	12.6	469.2	92.36	0.540	7.401	96.28	0.712	/	8.72

Fig. 9. XRD results of surface corrosion product on as-corroded samples of as-received and furnace-cooled SAC305 solder after exposure to marine atmosphere of 80% RH and 3.5 wt.% NaCl for 192 h.

Fig. 10. Surface and cross-sectional morphologies of as-corroded SAC305 solder samples after exposure to simulated marine atmosphere with 80% RH and 3.5 wt.% NaCl at 45 °C for 192 h: (a) and (b) as-received sample; (c) and (d) furnace-cooled sample.

Fig. 11. EPMA mapping on cross-sectional profile of corrosion product layer on as-received SAC305 sample after exposure to the simulated marine atmosphere of 80% RH and 3.5 wt.% NaCl at 45 °C for 192 h.

Fig. 12. EPMA mapping of cross-sectional profile of the corrosion product layer on furnace-cooled SAC305 sample after exposure to the simulated marine atmosphere of 80% RH and 3.5 wt.% NaCl at 45 °C for 192 h.

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