Elsevier

Materials & Design

Volume 108, 15 October 2016, Pages 418-428
Materials & Design

Suppression of Cu6Sn5 in TiO2 reinforced solder joints after multiple reflow cycles

https://doi.org/10.1016/j.matdes.2016.06.121Get rights and content

Highlights

  • TiO2 additions suppressed the Cu6Sn5 phase, both as primary crystals and as the interfacial layer.

  • TiO2 free Sn-0.7Cu alloys had a higher undercooling resulting in longer contacts times between the IMC and liquid solder.

  • With the suppression of primary and interfacial Cu6Sn5 in TiO2 containing solders, the solder joint strength was improved.

  • The strength of Sn-0.7Cu solder joints decreased with increasing reflows.

  • The solder joint strength of TiO2 containing solder was relatively insensitive to the number of reflow cycles.

  • TiO2 containing solder had a higher solder joint strength across all reflow cycles compared to Sn-0.7Cu.

Abstract

In the current generation of 3D electronic packaging, multiple reflows are often required during soldering. In addition, electronic packages may be subjected to additional solder rework or other heating processes. This paper investigates the effects of multiple reflow cycles on TiO2 reinforced Sn–0.7Cu solder fabricated by a powder metallurgy microwave sintering technique. Compared to TiO2-free equivalents, a relative suppression of the Cu6Sn5 phase, both as primary crystals and as an interfacial layer was observed. The likely mechanism relates to the TiO2 nanoparticles promoting nucleation and decreasing the amount of time that liquid is in contact with the interfacial layer. The TiO2 particles appear to stabilise the interfacial Cu6Sn5 layer and result in a more planar morphology. The suppression of Cu6Sn5 results in TiO2 reinforced solder joints having a higher shear strength after multiple reflow cycles compared to Sn–0.7Cu solder joints.

Introduction

Solder alloys play a crucial role in determining performance and reliability in the assembly and interconnection of electronic products and have electrical, thermal and mechanical functions [1], [2]. The relative importance of solder alloy properties has increased due to continued miniaturization of microelectronic circuitry and the use of finer pitch interconnects. Higher functional densities in printed circuit boards (PCBs) have been made possible by surface mount technology (SMT) using reflow soldering, often with multiple reflow cycles. Other heating cycles can be present in manufacturing such as additional solder rework [3]. One challenge associated with current generation Pb-free solder alloys is that during multiple thermal cycles, the joint strength may degrade due to the rapid growth of the interfacial layer of intermetallic compounds [4], [5], [6], [7], [8], [9], [10], [11], [12]. In a typical Pb-free solder joint, Cu6Sn5, which may form either as primary crystals or an interfacial layer during soldering can play a determining role in solder joint strength. There is evidence that by suppressing the Cu6Sn5 interfacial layer, solder joint properties could be improved [13], [14], [15], [16], [17] and as such there are benefits associated with controlling the growth of this layer during multiple reflows.

It has recently been reported that additions of reinforcement to a variety of solder matrices, with compounds including silicon carbide (SiC) [18], [19], [20], nickel oxide (NiO) [21], alumina (Al2O3) [22], [23], [24], zirconia (ZrO2) [25], [26], [27], [28], titanium oxide (TiO2) [29], [30], [31], [32], [33], [34] and silicon nitride (Si3N4) [35], [36] result in suppression of the growth of the interfacial layer during soldering [37]. In our recent study [38], we developed a method of fabricating a reinforced solder using a powder metallurgy microwave sintering method that results in a homogeneous distribution of TiO2 in the solder material and an improvement in the bulk solder material thermal and mechanical properties. However, properties related to the solder joint strength after multiple reflows of this reinforced solder are yet to be explored.

This paper investigates the effects of multiple reflow cycles on the TiO2 reinforced Sn–0.7Cu solder joint by comparing it with a base Sn0.7 wt.%Cu (unreinforced) solder joint. This includes investigating the evolution of Cu6Sn5 intermetallics both as a primary phase in the bulk solder and as an interfacial compound layer during multiple reflow and its effect on the solder joint strength. Since it is impossible to investigate the evolution of primary Cu6Sn5 in real time using conventional methods, advanced real time experimental techniques including synchrotron X-ray imaging were used.

Section snippets

Sample fabrication

In this study, Sn–0.7Cu solder powders of spherical morphology with an average particle size of 45 μm were supplied by Nihon Superior Co. Ltd. and used for the base matrix material, along with 99.7% purity TiO2 anatase powder supplied by Sigma Aldrich, which had an average particle size of < 50 nm. To fabricate the Sn–Cu containing TiO2 nano-composite solder, 1 wt.% of TiO2 particles was incorporated into the Sn–0.7Cu solder matrix using a powder metallurgy route similar to previous research [38].

Thermal reactions in solder joints during multiple reflow

Thermal reactions during heating and cooling of solder joints are important in understanding the reactions of the solid-liquid-solid transition during soldering. Fig. 2 shows a differential scanning calorimetry curve of multiple reflow cycles of Sn–0.7Cu with TiO2 (Fig. 1a and b) and Sn–0.7Cu during cooling and heating (Fig. 1c and d). During heating, the endothermic melting peaks were consistent throughout the multiple cycles for both types of solder joints. However, during cooling the

Conclusions

In conclusion, the effect of multiple reflow cycles on the formation of Cu6Sn5 primary crystals and the interfacial layer in Sn–0.7Cu and Sn–0.7Cu + TiO2 solders on copper substrates was investigated. In addition, the effects of the growth of Cu6Sn5 after multiple reflows on the solder joint strength were evaluated. The following conclusion can be made:

  • a)

    Multiple reflow and TiO2 additions to Sn–0.7Cu affect the Cu6Sn5 primary and interfacial layer growth mechanisms.

  • b)

    Sn–0.7Cu solder joints displayed

Acknowledgements

Real-time observation experiments were performed at the SPring-8 BL20XU beamline (2014B1620 and 2015A1675) while SEM and DSC were performed at the Centre for Microscopy and Microanalysis (CMM) and Australian National Fabrication facility (ANFF). High speed shear tests were conducted at Nihon Superior Japan. This work was financially supported by the University of Queensland (UQ)-Nihon Superior (NS) (NSCMEM 2012000864) collaboration research project, the Australian Synchrotron International

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