Interfacial microstructure evolution and shear behavior of Au–20Sn/(Sn)Cu solder joints bonded at 250 °C
Introduction
The eutectic Au–20Sn (20 wt%) solder alloy is widely used for high-power optoelectronics due to its high electrical and thermal conductivity, high shear strength, excellent fatigue resistances, and the possibility of free flux soldering [1], [2], [3]. One important application of Au–20Sn in high-power optoelectronic packaging is to join with Cu heat sink materials to dissipate heat for high-power electronic systems [4], [5], which is critical for the normal operation of electronic devices. During the reflowing process, liquid Au–20Sn alloys reacts with the Cu substrate, forming the (Au,Cu)5Sn interfacial bonding layer [6], [7]. A uniform thin and continuous (Au,Cu)5Sn layer is essential for reliable bonding. Usually, the peak reflowing temperature of Au–20Sn/Cu is 20–30 °C higher than the melting point of Au–20Sn (278 °C) [6], [7], [8]. But such a high temperature does harm to the intrinsic structures and physical properties of electronic components, expecially temperature-sentitive chips adjacent to Cu substrates. Actually, the peak reflowing temperature has limited the development of electronic industry to some extent, and is an increasingly important parameter in electronic design [9]. For those reasons, it is of great significance to join Au–20Sn with Cu at low temperature.
Analogizing to the diffusion bonding technology [10], [11], an active interlayer is needed to realize the rapid bonding between Au–20Sn and Cu substrates at low temperature. To our knowledge, the Sn layer may be an ideal interlayer for such purpose. The melting point of Sn is 231.9 °C, far lower than 278 °C (the melting point of Au–20Sn). Besides, liquid Sn has high reactivity with both Au–20Sn and Cu. According to previous studies on interfacial reaction of Au/Sn [12], [13], [14], Au–20Sn/Sn [15], Au/Sn/Cu [16], [17], AuSn2 and AuSn4 layers would form at the Au–20Sn/Sn interface. (Cu,Au)6Sn5 and Cu3Sn layers form at the Sn/Cu interface. In addition, AuSn2 and AuSn4 layers would all transform into AuSn after prolonged interfacial reaction [16]. It is well known that thick intermetallic layers are sensitive to stress and trend to initiate micro cracks because of their inherent brittleness [18]. Our previously study [15], [19], [20] indicates that the hardness and Young's modulus of AuSn4 and AuSn are lower than (Au,Cu)5Sn and Cu–Sn intermetallics. Besides, the growth of hard (Cu,Au)6Sn5 layers could be suppressed by interfacial Au–Sn intermetallic layers, since Au has decreasingly diffusivities in increasing Au-rich Au–Sn intermetallics [12]. Thus AuSn4 and AuSn layers are likely to benefit the strength of solder joints.
In this study, the possibility of using Sn interlayers to join Au–20Sn with Cu substrates at 250 °C was explored. The temperature of 250 °C was selected because it is the typical peak reflowing temperature of Sn-based Pb-free solder joints. If reliable bonding between Au–20Sn and Cu is successfully realized at this temperature, the existing packaging equipment could be utilized without major modification, which would be an economical strategy for industrial application. Our present work focused on the interfacial microstructure evolution, shear strength and fracture behaviors of Au–20Sn/(Sn)Cu after multiple reflows and isothermal aging. The objective is to establish a correlation between interfacial microstucture and shear fracture of the solder joints. Also, our present study aims to gain an insight into the possible methods to realize high-temperature packaging at low temperaure with the aid of Sn interlayers.
Section snippets
Experimental procedure
Copper (99.99%) plates with the dimension of 25 mm×25 mm×1 mm were used as the substrate. Before soldering, Cu plates were grinded, and then polished with 0.3 μm alumina suspension. Square Au–20Sn slices (5 mm×5 mm×0.2 mm, with purity of 99.99%) and Sn foils (5 mm×5 mm×0.03 mm, with purity of 99.99%) were placed in sequence on the Cu plate. Sn foils with the thickness of 0.03 mm was selected because obvious gaps would form at the interface if thinner Sn foils were used. Fig. 1(a) shows the structure of
Multiple reflowed Au–20Sn/(Sn)Cu solder joints
Fig. 2 shows the interfacial microstructure of Au–20Sn/(Sn)Cu solder joints reflowed at 250 °C for various times. Different intermetallic layers formed at the Au–20Sn/Sn interface and the Sn/Cu interface. To evaluate the compositions of those IMCs, EDS point analysis was conducted and the result is shown in Table 1. Combined with the Au–Sn–Cu ternary phase diagram shown in Fig. 3 [6], A–E were indentified to be AuSn2, AuSn4, Sn, AuSn4, and (Cu,Au)6Sn5. Besides, both AuSn layer and Cu3Sn layer
Conclusions
Au–20Sn was successfully joined with Cu via Sn layer at 250 °C. Interfacial microstructure evolution and shear fracture behavior of Au–20Sn/(Sn)Cu solder joints during multiple reflowing and isothermal aging were investigated. Comparisons were made between Au–20Sn/(Sn)Cu and Au–20Sn/Cu solder joints. The following conclusions can be drawn from the present research:
- 1.
AuSn/AuSn2/AuSn4/Sn/(Cu,Au)6Sn5/Cu3Sn layers formed initially at the interface of Au–20Sn/(Sn)Cu. As the reflow times increased, Sn
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