Search results

This paper aims to identify a variety of binary system solders by alloying, and relevantly derive multiple system Pb-free solders from the former, attempting to replace the high temperature Sn-Pb solder.

The basis of the paper is the synthesis of previous studies. In terms of some binary high temperature solder alloys, such as Au-20Sn, Bi-2.5Ag, Sn-5Sb, Au-12.5Ge, Zn-6Al and Zn-Sn, taking the alloy phase diagram as the starting point, the melting characteristics, microstructure, mechanical properties, wetting ability and reliability of solder joint are analysed and the prospect is consequently indicated.

Based on the analysis of the six groups of Pb-free solders, the present binary system solder alloys, from the perspective of melting properties, mechanical properties, soldering or reliability of solder joint, rarely meet the comprehensive requirements of replacing the high-temperature Sn-Pb solder. It is assumed to be a solution that multiple-system Pb-free solders derive from a variety of binary system solders by means of alloying. The future development of high temperature Pb-free solder may focus on some factors such as physical properties, mechanical properties, processing, reliability of solder joint, environmental performance and expense.

The paper concentrates on the issue of Pb-free solders at high temperature. From a specific perspective of binary system solders, the presently available Pb-free solders are suggested from the starting point of the alloy phase diagram and the prospect of alternatives of Sn-Pb solders at high temperature are indicated.

The research aims to explore the high‐cycle fatigue performance of Pb‐free alloys and compare them to Sn‐Pb. In doing this, it also aims to demonstrate the viability of a new testing method.

The method introduced uses existing test equipment in a novel way to combine the speed and applicability of general vibration testing with the control of single, model specimen testing. Model solder joints are constructed in a repeatable manner and repeated tensile stress cycles are applied until failure.

It is found that in the regime studied, all of the Pb‐free alloys tested show significantly decreased performance compared to Sn‐Pb, at ambient temperatures. No obvious mechanical or microstructural features have been identified as the cause of this discrepancy. The test method employed demonstrates good correlation with existing fatigue test methods despite the known variance of solder mechanical test results.

It is recognised that results pertaining to essentially only a one‐dimensional stress state are obtained, and that practical stresses will vary. The performance difference between Pb and Pb‐free alloys warrants further investigation.

The results obtained are of interest to high‐reliability electronics sectors such as aerospace, defence and automotive, where vibrations in service are encountered. Very little work exists on the subject of solder high‐cycle fatigue performance and to the author's knowledge none comparing Pb to Pb‐free alloys in an objective manner.

The study investigates the sub process behaviour in stencil printing of type‐6 and type‐7 particle size distribution (PSD) Pb‐free solder pastes to assess their printing limits.

Two solder pastes were used in a design of experiments approach to find optimal printing parameters

Solder paste printing has been achieved to ultimately produce 30 μm deposits at 60 μm pitch for full area array patterns using a type‐7 Pb‐free solder paste. For a type‐6 PSD solder paste, full area array printing was limited to 50 μm deposits at 110 μm pitch. However, for peripheral printing patterns, 50 μm deposits at 90 μm pitch were obtained. The disparities in the behaviour of the two paste types at different geometries can be attributed to differences in the sub‐processes of the stencil printing. The paste release of the type‐6 paste from the stencil apertures at fine pitch was superior to the type‐7 paste, which may be attributed to the finer particle paste producing an increased drag force along the stencil aperture walls. However, the type‐7 paste was able to fill the smallest aperture openings, ultimately to 30 μm, thus producing full array printing patterns at uniquely small pitches.

This advancement in the stencil printing process has been made possible by refinements to both solder paste design and stencil manufacturing technology. Adjustments in the solder paste rheology have enabled successful printing at ultra fine pitch geometries. This, together with selecting appropriate printing parameters such as printing speed, pressure, print gap and separation speed, allows a practical printing process window. Moreover, advancements in stencil fabrication methods have produced “state‐of‐the‐art” stencils exhibiting very precisely defined aperture shapes, with smooth walls at very fine pitch, thus allowing for improved solder paste release at very small dimensions.

The results can be used to present a low cost solution for Pb‐free flip chip wafer bumping. Furthermore, the results indicate that type‐6 and type‐7 solder pastes should be applied to/selected for specific application geometries.

A numerical study on the reliability of soldered interconnects of c-Si solar photovoltaic cells has been conducted.

A three-year data (2012–2014) from outdoor weathering of PV modules was used to generate temperature cycle profiles to serve as thermal loads and boundary conditions for the investigation of the thermo-mechanical response of the soldered interconnects when subjected to real outdoor conditions using finite element analysis (FEA) Software (Ansys. 18.2). Two types of soldered interconnections, namely, Sn60Pb40 and Sn3.8Ag0.7Cu (Pb-free), were modelled in this study.

Life prediction results from accumulated creep energy density damage show that the solder interconnects will achieve maximum life under the 2014 thermal cycle loading. In particular, the Sn60Pb40 solder interconnection is expected to achieve 14,153 cycles (25.85 years) whilst the Pb-free solder interconnection is expected to achieve 9,249 cycles (16.89 years). Additionally, under the test region average (TRA) thermal cycle, the Pb-free and Pb-Sn solder interconnections are expected to achieve 7,944 cycles (13.69 years) and 12,814 cycles (23.4 years), respectively. The study shows that Sn60Pb40 solder interconnections are likely to exhibit superior reliability over the Pb-free solder interconnections at the test site.

This study would be useful to electronics manufacturing industry in the search for a suitable alternative to SnPb solders and also the thermo-mechanical reliability research community and manufacturers in the design of robust PV modules.

The study has provided TRA data/results which could be used to represent the test region instead of a particular year. The study also indicates that more than six thermal cycles are required before any meaningful conclusions can be drawn. Finally, the life of the two types of solders (SnPb and Pb-free) as interconnecting materials for c-Si PV have been predicted for the test region (Kumasi in sub-Saharan Africa).

Many theories regarding whisker growth exist. It has been demonstrated in a variety of reference sources that tin whiskers can form in both pure tin and tin alloy deposits. Conversely, an equal number of claims exist in the literature demonstrating no whisker growth in the same types of deposits. The lack of an industry standard whisker test is a significant limitation in addressing tin whiskers. Historically in the electronics industry, addition of lead (Pb) was found to be an effective method of minimizing tin whisker formation and so for many years electronic components have been electroplated with tin‐lead (Sn‐Pb). With the advent of Pb‐free electronics finishing, the risk of tin whiskers is again a significant concern. This paper will review the theories behind whisker formation, identify the common characteristics of same, and demonstrate how Pb‐free electroplating processes can be formulated to minimize the risk of whisker formation.

To investigate effects of the thermal history on intermetallic thickness and morphology and on the resulting shear strength of the ball attachment for a variety of BGA components.

In this study, a variety of BGA components with balls made of Pb‐free Sn‐Ag‐Cu (SAC) 305, Sn‐Pb eutectic and high‐temperature 90Pb‐10Sn alloys, were subjected to different thermal histories, including up to ten reflow cycles, and aged at 125°C from 24 to 336 h. The intermetallic thickness and morphology after these thermal events were then examined under optical and scanning electronic microscopes. Ball shearing tests were conducted to investigate effects of the thermal history and intermetallic thickness and morphology on shearing strength of these solder balls.

The results show that effects directly from intermetallic layers may or may not be detectable; and the shear strength of solder balls is largely dependent on the solder alloy and its microstructure. Shear strength increases are observed after multiple reflow cycles and ageing at elevated temperature for the two Pb‐bearing alloys, while the SAC305 lead‐free alloy shows slight reductions in both strength and ductility after thermal exposure.

Presented results can be used for estimation of reliability for electronic assemblies subjected to multiple rework and repair operations, which expose sensitive components, such as BGAs, to elevated temperatures.

It is believed that a sound understanding of the effects of intermetallic morphology and thickness on reliability of BGA solder balls can lead to more intelligent choice of soldering processes, as well as to rework/repair process optimisation and to establishing their operational limits.

The impact strength of solder joint under high strain rate was evaluated by board level test method. However, the impact shear test of single solder bump was more convenient and economical than the board level test method. With the miniaturization of solder joints, solder joints were more prone to failure under thermal shock and more attention has been paid to the impact reliability of solder joint. But Pb-free solder joints may be paid too much attention and Sn-Pb solder joints may be ignored.

In this study, thermal shock test between −55°C and 125°C was conducted on Sn-37Pb solder bumps in the BGA package to investigate microstructural evolution and growth mechanism of interfacial intermetallic compounds (IMCs) layer. The effects of thermal shock and ball diameter on the mechanical property and fracture behavior of Sn-37Pb solder bumps were discussed.

With the increase of ball size, the same change tendency of shear strength with thermal shock cycles. The shear strength of the solder bumps was the highest after reflow; with the increase of the number of thermal shocks, the shear strength of the solder bumps was decreased. But at the time of 2,000 cycles, the shear strength was increased to the initial strength. Minimum shear strength almost took place at 1,500 cycles in all solder bumps. The differences between maximum shear strength and minimum shear strength were 9.11 MPa and 16.83 MPa, 17.07 MPa and 15.59 MPa in φ0.3 mm and φ0.4 mm, φ0.5 mm and φ0.6 mm, respectively, differences were increased with increasing of ball size. With similar reflow profile, the thickness of IMC decreased as the diameter of the ball increased. The thickness of IMC was 2.42 µm and 2.17 µm, 1.63 µm and 1.77 µm with increasing of the ball size, respectively.

Pb-free solder was gradually used to replace traditional Sn-Pb solder and has been widely used in industry. Nevertheless, some products inevitably used a mixture of Sn-Pb and Pb-free solder to make the transition from Sn-Pb to Pb-free solder. Therefore, it was very important to understand the reliability of Sn-Pb solder joint and more further research works were also needed.

Using two different conceptual approaches to environmental life‐cycle assessment, attributional and consequential, the purpose was to test the hypothesis that a typical lead free solder paste Sn95.5Ag3.8Cu0.7 is worse than Sn63Pb37 as far as global environmental impacts are concerned.

Single index weighting indices within the impact methodology Life cycle Impact Assessment Method based on Endpoint Modelling (LIME) impact methodology, were applied to the flows of three life cycle inventory models and their globally related flows. The LIME results based on three environmental impact categories, i.e. resource consumption, global warming and ozonelayer depletion are presented and discussed.

The attributional LCA (ALCA) results point towards a larger impact for Sn95.5Ag3.8Cu0.7 than Sn63Pb37 mostly due to the higher Sn and Ag content. This study confirms earlier similar ALCAs. The system expansion for the Consequential LCA (CLCA) did not change this conclusion.

The present study has not included the affected microelectronics packaging parts of electronic products, nor has it included toxic effects as they are local. ALCA was considered to be equal to CLCA for Sn95.5Ag3.8Cu0.7, where no Ag nor Sn recycling was included.

For the first time a global environmental impact assessment of the shift to Pb‐free solder paste using the LIME weighting method applied to a CLCA is reported. Environmental life‐cycle investigations intended to support decisions of an ecological nature in the microelectronics packaging industry should benefit from the consequential approach.