Editorial
Article Type: Editorial From: Soldering & Surface Mount Technology, Volume 25, Issue 2
Welcome to Volume 25 Issue 2 of Soldering & Surface Mount Technology. In this issue there are six papers covering several areas of importance to those involved in electronics assembly and reliability. Lead-free solders and their processing continue to be major topics of interest and ongoing research activity around the world and the first three papers of this issue address various key aspects ranging from lead-free materials to the influence of reflow on specific alloy formulations.
Power device packages often require high melting point die attach solders and bismuth silver has been proposed as one suitable candidate. The first paper details the results of a study of a bismuth silver solder alloy at high application temperatures and the aim of the research work was to determine the soldering, thermal and mechanical properties of Bi-11Ag solder. As with many alloys, this alloy’s properties represent something of a compromise, but the fact that it is lead-free, bismuth is non-toxic and that the shear modulus is similar to high lead solders, make this an interesting material for high temperature soldering applications. In this work, joint formation was studied on a range of substrates in a shielding atmosphere and the mechanisms of joint formation analysed.
In the past, there has been significant interest in the electrolytic deposition of solder alloys from metal solutions and the approach was often used with tin-lead solders. More recently there has been work to use the same approach for lead-free solders and the second paper covers the electrodeposition of lead-free alloys. Electrodeposition of lead-free solder may become an increasingly important deposition technique due to its ability to produce fine geometry bumps. In this paper the authors review the literature on the electrodeposition of lead-free solder systems and identify the challenges in this area, so that new studies can be undertaken to solve key processing problems. Controlling the composition of electrodeposited alloys has always been a key challenge and it seems that the issue continues with new solder alloys. However, there are a wide range of chemical additives that can be utilised to assist in maintaining the compositions of deposited alloys and, through on-going research in this area, it seems likely that the “Electrodeposition of lead-free solder alloys”may become more established as another route for the mass production of solder deposits.
Regular readers of this journal will be aware of the interest in using nanoparticle additives to tailor the properties of solders and solder joints. The third paper continues with this theme, detailing the effects of multiple reflow cycles on the interfacial characteristics between zinc nanoparticle modified SAC solder and copper substrates. The wetting behaviour of the solders was characterized by analysing the contact angles and spreading rates and the interfacial microstructure of the solder joints was investigated using field emission scanning electron microscope and energy dispersive X-ray spectroscopy. It was found that the zinc nanoparticles retarded growth of the interfacial intermetallic compounds, although excessive amounts induced the formation of an additional IMC layer. It was concluded that the zinc nanoparticles underwent melting/reaction during reflow and imparted their influence on the interfacial intermetallic compounds through alloying effects.
Vapour phase soldering is a technique that has been promoted over many years for specific applications, but which has never really managed to achieve the same widespread use as the mainstream soldering processes with which most electronic products are produced. However, it is a valuable process that can offer a number of key benefits in some assembly applications. The fourth paper presents the results of a process characterization study using direct feedback on the state of the vapour, to give better monitoring, control and understanding of the overall vapour phase soldering process. The results revealed the dynamics of the vapour blanket generation and provided a novel approach offering the benefits of more precise saturation detection and better control of heat transfer. The proposed approach could enable more efficient production using this technique with reductions in idle time, improved soldering quality and reduced power consumption.
Ensuring the integrity of assemblies comprising disparate materials is a key challenge for those involved in the production of reliable electronics,especially those used in power devices and applications. The fifth paper details the results of a study into the thermal cycling induced evolution of curvature in a sandwich assembly bonded by sintered nano-silver paste and the effect of bond line thickness on residual stress. It was found that the curvature significantly decreased with increasing bond line thickness and the severity of the residual curvature was mitigated with increasing number of cycles, due to stress relaxation or initiation of micro-cracking in the sintered silver layer.
The final paper of this issue addresses another important reliability related aspect of electronics manufacturing and, in this case, the authors cover the subject of void formation in QFN assemblies. In this paper, the application of an algorithm-based approach for investigating critical attributes of void formation is discussed. Data mining was utilised as a new diagnostic scheme for investigating void formation at the thermal pad in QFN assembly. Occurrences of voiding with various component designs, materials and manufacturing processes were analysed. It was found that stocking PCBs for beyond ten days could lead to increased levels of voiding. Also, it was found that the use of via in pad designs caused a concave geometry on the surface of thermal pads which contributed to void formation. By using this algorithm-based approach to derive an optimal process, voiding could be appreciably reduced.
As always, I do hope that you find these papers both interesting and useful. I appreciate feedback, suggestions and comments from readers and I can be contacted at: m.goosey@lboro.ac.uk
January 2013
Martin Goosey
