Influence of External Interface Normal Stress on the Growth of Cu-Sn IMC During Aging

doi:10.1007/s40195-020-01059-3

Abstract

Abstract:

A U-shape clamp was designed to apply stress perpendicular to the interface of Cu/Sn/Cu solder joints, and its influence on the growth behavior of Cu-Sn intermetallic compound (IMC) during thermal aging at 150 °C was investigated. The results show that compared with the sample at general stress-free state, the growth rate of IMC under compression is faster, while that under tension is slower. Moreover, the interface between IMC and Sn is smoother under compressive stress, and the corresponding IMC grains are smaller and more uniform than that under tensile stress. According to the growth kinetic analysis, the growth of IMC under general, compressive and tensile states is all controlled by the combination of grain boundary diffusion and volume diffusion with a similar growth exponent (n ≈ 0.4). However, external stress can affect the Ostwald ripening process of grain growth, causing a change of grain size and grain boundary density in the IMC layer. As a result, the IMC growth behavior at the interface of the solder joint will be affected by the applied external normal stress.

Key words: Cu-Sn IMC, Growth behavior, External stress effect, Isothermal aging

Changchang Wang, Yinbo Chen, Zhi-Quan Liu. Influence of External Interface Normal Stress on the Growth of Cu-Sn IMC During Aging[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(10): 1388-1396.

Figures/Tables 10

Fig. 1 Photographs of a the configuration of the spring steel clamp and as-reflowed general sample, b clamps with compression and tension samples, c schematic illustration of the experiment process

Table 1 Value of variations used in estimating the stress in the sample and results obtained from each equation

Variables	Values	Results
y_max	2.5 mm	P ≈ 16.6 N
l	4 mm
E	1.97 × 10⁵ MPa
b	5 mm	I = 0.72 mm⁴
h	1.2 mm	I = 0.72 mm⁴
A_c	4 mm²	σ ≈ 4.16 MPa

Fig. 2 SEM images of a the cross section of as-reflowed general sample and b the top view of IMC phases after etching away the excessive tin

Fig. 3 Interfacial morphology evolution of samples under a compression, b stress free and c tension after (1) 100-h, (2) 250-h, (3) 500-h and (4) 1000-h aging

Fig. 4 a IMC thickness as a function of aging time and b logx versus logt, where x is the IMC thickness and t is the aging time

Table 2 Average IMC thickness of samples under different stress conditions for different aging time

Aging time (h)	Average IMC thickness (μm)
Aging time (h)	Compression	Stress free	Tension
100	2.70	2.74	2.40
250	4.75	4.19	3.43
500	5.70	5.53	4.86
1000	7.51	6.92	5.56

Fig. 5 Top view of the IMC grains under a compression, b stress free and c tension after 1000-h aging, d-f are the opposites interface of a-c, respectively

Fig. 6 Histogram of grain size distribution with normal distribution fitting in the samples under a compression, b stress free and c tension after 1000-h aging, d the comparison of the normal distribution curves

Table 3 Statistical parameters of IMC grain size under different stress states after aging for 1000 h, where rmax represents maximum radius, raver represents average IMC grain size, xc and w are the expectations and variance of the normal distribution fitting results, respectively

Parameters	Compression	Stress free	Tension
r_max (μm)	12.22	13.42	14.04
r_aver (μm)	2.74	3.37	3.91
x_c	2.42	2.82	3.29
w	1.69	1.83	1.89

Fig. 7 Illustration of different growth behaviors of IMC grains under stress states of a compression, b stress free, c tension during thermal aging

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