Emerald Group Publishing Limited
Copyright © 2004, Emerald Group Publishing Limited
Surface mount technology (SMT) has become the main technique for the board level assembly of microelectronic devices since 1980s. For surface mounted components (SMCs), the solder joints not only provide for the passage of electrical signal and power, but also the mechanical support to hold the package in position on the printed circuit board (PCB). Since the dimensions of solder joints are relatively small and the electronic devices may experience thermal-mechanical loadings during operation, the solder joint reliability of SMCs is a major concern in the electronics manufacturing industry.
This special issue of SSMT is a collection of seven papers on the subject of solder joint reliability. All of the papers were subject to a process of peer review and revision.
The first paper gives a comprehensive discussion on the long-term reliability of lead-free solders. This paper considers the transition in terms of performance, with particular emphasis on long-term, high reliability applications. Comparison of key mechanical properties indicated generally beneficial outcomes of the transition to lead-free alloys, although there might be a lack of understanding surrounding anomalous observations. Sole reliance upon empirical testing has become non-viable, and there is an urgent need for a greater understanding of solder behaviour to act as a basis for phenomenological methods of life prediction and design.
The second paper presents an experimental study to assess the reliability of solder ball attachment to the bond pads of a plastic ball grid array (PBGA) substrate for various plating schemes. Solder ball shear tests were conducted to characterise the ball attachment strength for comparison. Three different kinds of electroless plating solutions were used to deposit the Ni layer. Also, conventional electrolytic Ni/Au plating was performed to provide a benchmark. In addition to the ball shear tests, scanning electron microscopy (SEM) was performed to inspect the cross-section and the fracture surface of the tested specimens. From the failure analysis, certain characteristics were identified for the various Ni plating schemes.
The third paper presents some strategies for improving the reliability of solder joints on power electronics. Schemes for improving solder joint reliability by adjusting solder joint geometry, underfilling and utilisation of flexible substrates are discussed. Thermal cycling data and finite element analysis indicated significant improvements in reliability when the proposed configurations were used.
The fourth paper introduces a simple method for the estimation of thermal fatigue life of ceramic ball grid array (CBGA) solder joints. A closed-form solution, based on calculation of the equilibrium of the displacements within the electronic package assembly, was first derived to calculate the solder joint strains during temperature cycling. In the calculation, an iteration technique was developed to obtain a convergent solution for the solder strains, and the elastic material properties were used for all the electronic package assembly components except for the solder materials, which had elastic-plastic properties. A fatigue life prediction model, evolved from an empirically derived formula based upon a modified Coffin-Manson fatigue model, was established and then validated against previous experimental results. It was concluded that the proposed model could be used as an effective tool to estimate the thermal fatigue life of CBGA solder joints.
The fifth paper is an experimental study of solder joint reliability, together with the relevant data analysis. This study investigated temperature cycling tests and statistical analysis of various high-density packages on PCBs with SnCu HASL, NiAu, and OSP finishes. The emphasis was placed on the determination of the life distribution and reliability of the lead-free solder joints under thermal cycling conditions. Much useful first-hand experimental data are presented in this paper.
The sixth paper is on the failure analysis of solder joints, which is a sequel to the fifth paper. A series of failure analyses on the solder joints of assemblies of high density packages with various PCB surface finishes was performed. The emphasis was placed on identifying the failure location and failure mechanism of the solder joints after large numbers of temperature cycles. The presented results will be very helpful to researchers who are working on the computational modelling of solder joint reliability.
The seventh paper investigates the thermal cycling reliability of chip resistors with lead-free solder joints. In this study, the reliability of 2512 chip resistor lead-free solder joints under thermal cycling was investigated. Two temperature ranges ( 40 to 125°C and 40 to 150°C) and five different solder alloys were examined. In addition to two-parameter Weibull analysis of the experimental data, a second set of thermally cycled samples was used for microscopy studies to examine crack propagation, changes in the microstructure of the solders, and intermetallic growth at the solder to PCB pad interfaces.
This special issue covers a wide range of topics on solder joint reliability, which should bring benefits to the readers of SSMT from various points of view. The Guest Editor would like to thank authors of all papers for their contribution and co-operation. Special thanks are due to the Chief Editor of SSMT, David Whalley. Without his strong support, this special issue would not have materialised in time for publication. Furthermore, due to his effective proof-reading, the quality of this special issue has been substantially improved.
Ricky LeeGuest Editor