Microstructure and Shear Properties Evolution of Minor Fe-Doped SAC/Cu Substrate Solder Joint under Isothermal Aging

doi:10.1007/s40195-024-01691-3

Abstract

Abstract:

Different amounts of Fe (0.005, 0.01, 0.03, 0.05, and 0.07 wt%) were added to SAC305 to study the shear behavior damage of Fe-doped SAC solder joints under thermal loading (170 °C, holding time of 0, 250, 500, and 750 h). The results show that during isothermal aging at 170 °C, the average shear force of all solder joints decreases with increasing aging time, while the average fracture energy first increases and then decreases, reaching a maximum at 500 h. Minor Fe doping could both increase shear forces and related fracture energy, with the optimum Fe doping amount being 0.03 wt% within the entire aging range. This is because the doping Fe reduces the undercooling of the SAC305 alloy, resulting in the microstructure refining of solder joints. This in turn causes the microstructure changing from network structure (SAC305 joint: eutectic network + β-Sn) to a single matrix structure (0.03Fe-doped SAC305 joint: β-Sn matrix + small compound particles). Specifically, Fe atoms can replace some Cu in Cu₆Sn₅ (both inside the solder joint and at the interface), and then form (Cu,Fe)₆Sn₅ compounds, resulting in an increase in the elastic modulus and nanohardness of the compounds. Moreover, the growth of Cu₆Sn₅ and Cu₃Sn intermetallic compounds (IMC) layer are inhibited by Fe doping even after the aging time prolonging, and Fe aggregates near the interface compound to form FeSn₂. This study is of great significance for controlling the growth of interfacial compounds, stabilizing the microstructures, and providing strengthening strategy for solder joint alloy design.

Key words: SAC305 solder, Fe doping, Shear mechanical behavior, Isothermal aging, Interfacial intermetallic compounds (IMC)

Quanzhen Li, Chengming Li, Xiaojing Wang, Shanshan Cai, Jubo Peng, Shujin Chen, Jiajun Wang, Xiaohong Yuan. Microstructure and Shear Properties Evolution of Minor Fe-Doped SAC/Cu Substrate Solder Joint under Isothermal Aging[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(7): 1279-1290.

Figures/Tables 16

Fig. 1 Solder ball preparation and DSC testing

Fig. 2 Preparation and shear testing of shear samples

Fig. 3 Shear force and absorbed energy of xFe/Cu joints at 170 °C for 0-750 h aging

Fig. 4 Mechanical properties improvement percentages of Fe-doped SAC305/Cu joints compared to SAC305/Cu BGA joint

Fig. 5 Shear force-displacement curves of Fe (0-0.07 wt%) doped SAC305 solder/Cu BGA joints at 170 °C for a 0 h, b 500 h, c 750 h

Fig. 6 Interfacial IMC growth for a-d SAC305/Cu and e-h 0.03Fe/Cu aged for 0, 250, 500, and 750 h, respectively [13]

Table 1 EDX results of the tested position in Fig. 6

Elements (at.%)	EDX result
Elements (at.%)	1	2	3	4
Fe	0.24	0.21	0.14	0
Cu	37.99	60.22	39.15	34.34
Ag	0.09	0.2	0.01	6.06
Sn	61.68	39.37	60.7	59.59
Total	100	100	100	100

Fig. 7 Thickness of interface IMC growing with aging time from 0 to 750 h at 170 °C before and after 0.03Fe-doped

Fig. 8 0.03Fe/Cu solder joint 0/750 h EPMA, a SEM 0 h, a1 Fe 0 h, a2 Cu 0 h, a3 Ag 0 h, a4 Sn 0 h; b SEM 750 h, b1 Fe 750 h, b2 Cu 750 h, b3 Ag 750 h, b4 Sn 750 h; c IMC 750 h; d IMC 750 h EPMA

Fig. 9 Microstructures of a-d SAC305/Cu and e-h 0.03Fe/Cu aged for 0, 250, 500, and 750 h, respectively [13]

Fig. 10 Typical DSC curves of SAC305 and 0.03Fe alloys

Fig. 11 Intermetallic compound (IMC) statistical analysis of SAC305 and 0.03Fe joints: a, b particle radii, c percentages, and d the interparticle space [13]

Fig. 12 a Load-displacement curve and b nanoindentation mechanical properties of solder joints

Table 2 Young's modulus of solder composition

Solder composition	E (GPa)			Refs.
Solder composition	β-Sn	Cu₆Sn₅	Cu₃Sn	Refs.
SAC305	61.05	68.15	119.7	This paper
SAC305-0.03Fe	56.8	117.45	115.1	This paper
SAC105	-	59	-	[41]
SAC105-1Bi-0.05Fe	-	85.5	-	[41]
SAC105-2Bi-0.05Fe	-	92	-	[41]
SAC387	-	96.73	-	[52]
SAC387-Ni	-	164.9	-	[52]
Cu/Sn/Cu	-	114	-	[53]
Cu-Sn/Mo-100 nm Co	-	136	-	[53]
Cu-Sn/Mo-500 nm Co	-	151	-	[53]
Cu-Sn-Cu	-	113.6	-	[54]
Cu-Sn-Co	-	136	-	[54]
Cu-Sn-In-Cu	-	132.6	-	[54]

Table 3 Hardness (GPa) of solder composition

Solder composition	E (GPa)			Refs.
Solder composition	β-Sn	Cu₆Sn₅	Cu₃Sn	Refs.
SAC305	0.39	4.59	6.63	This paper
SAC305-0.03Fe	0.28	4.72	6.54	This paper
SAC105	-	0.21	-	[41]
SAC105-1Bi-0.05Fe	-	0.28	-	[41]
SAC105-2Bi-0.05Fe	-	0.40	-	[41]
Cu/Sn/Cu	-	6.7	-	[53]
Cu-Sn/Mo-100 nm Co	-	6.9	-	[53]
Cu-Sn/Mo-500 nm Co	-	8.9	-	[53]
Cu-Sn-Cu	-	6.7	-	[54]
Cu-Sn-Co	-	6.9	-	[54]
Cu-Sn-In-Cu	-	9.1	-	[54]

Fig.13 SAC305 and 0.03Fe joints: a shear force and absorbed energy, b, d $\Delta \tau /a$, c $(\Delta \tau /a)\times {\text{Elongation}}$

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