Editorial
Article Type: Editorial From: Soldering & Surface Mount Technology, Volume 21, Issue 3
Welcome to the third issue of Volume 21 of Soldering and Surface Mount Technology (SSMT). Following on from our previous issue, which was dedicated to selected papers from a specific conference, the journal has returned to its more usual format with an eclectic mix of papers covering a range of topics that are both topical and highly interesting. The six papers in this issue have been submitted by authors in North America, Europe and the Far East and clearly reflect the level of global interest in the research topics covered by the journal.
For many companies, the move to lead-free assembly was successfully completed some years ago and the materials and processes used have been subjected to a huge amount of study with a large accompanying body of published literature. Consequently, many people now regard the use of lead-containing solders as history and lead-free has become the established norm for most electronics. However, and despite all the work that has already been undertaken, lead-free reliability continues to be an important topic that still causes concerns in some areas. In a paper entitled the “Effect of multiple reflow cycles on ball impact responses of Sn-Ag-Cu solder joints”, Yi-Shao Lai, C.R. Kao,Hsiao-Chuan Chang and Chin-Li Kao, detail work they have carried out using ball impact testing to determine the influence of reflow processing on tin-silver-copper alloys soldered to a tin finished printed circuit board. They confirmed that the thickness and grain size of the interfacial Cu6Sn5intermetallic increased with the number of reflow cycles and that thicker Cu6Sn5degraded the interfacial strength.
Thermal management is another issue that continues to be prominent and the study of reliability in assemblies where there may large operational temperature ranges is critical to the successful use of power electronics in many applications. A paper with a focus on this type of reliability issue has been provided by Guangcheng Dong, Guangyin (Thomas) Lei, Xu Chen, Khai Ngo and Guo-Quan Lu, who have published the results of their work on the influence of edge tail length on the reliability of direct-bond-copper (DBC) substrates. These substrates were found to crack after thermally cycling over 15 cycles from−55 to 250°C. The phenomenology of this cracking was studied to determine the suitability of DBC for high-temperature packaging. Interface cracking was observed to initiate at the short edge of the bonded copper and to propagate into the ceramic layer. They found that an edge tail could decrease the thermal strain along the short edge of the DBC substrate and thermal cycling performance was found to be greatly improved for DBC substrates with a 0.5 mm edge tail length compared to those without an edge tail.
In addition to covering the more conventional solder-based flip chip joints,this issue also has two papers detailing work relating to assemblies using adhesively bonded flip chip devices. In the first paper, entitled “Effect of substrate material and thickness on reliability of (ACA) bonded flip chip joints”, Laura Frisk and Anne Cumini report on their work to investigate the effect of substrate material type and thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives(ACA). The substrates studied were conventional an FR4 laminate and a liquid crystal polymer based material. The results indicated that the thermal cycling reliability of this type of joint was markedly influenced by both the thickness and the type of material from which the substrate was made. The second paper on this topic is by Kati Kokko, Hanna Harjunpää, Pekka Heino and Minna Kellomäki and it details work they have undertaken to assess the impact of composite conformal coatings on adhesive flip chip joints used in assemblies destined for use in medical applications, where the reliability performance is often of critical significance.
The first wafer level packages were commercialized over ten years ago and since then their small form factors, where the packages are of a similar size to the die, have seen them increasingly employed in a wide range of applications such as mobile telephones. However, and despite this popularity, there are currently at least four approaches to WSP that have been adopted by a number of major organizations and probably many more less popular ones as well. In the paper, by Ming-Chih Yew, Mars Tsai, Dyi-Chung Hu, Wen-Kun Yang and Kuo-Ning Chiang work to assess the reliability of a novel fan-out type of wafer level package is reported. The work proposes a low-cost packaging technology that could rapidly be brought to market and which is especially suitable for high-density IC devices. The so-called Panel Base Package technology is claimed to have the ability to meet the requirements of key reliability testing conditions and to have a high potential for use in real applications in the near future.
Considering the huge numbers of surface mount devices that have been assembled since the technology was introduced, it is perhaps surprising that relatively little attention has been paid to the influence of component pickup conditions and parameters on subsequent placement accuracy. It is therefore interesting that, in their paper entitled “Six sigma analysis of SMD feeding parameters and board assembly quality”, Pekka Kytösaho and Timo Liukkonen report the results of a study to investigate the possible link between the pickup conditions and placement accuracy of typical discrete chip components. Owing to the ever decreasing spacings between chips, pickup is becoming more important and the indications from this work are that components will simply have to be picked up more and more on their centres in order to avoid collision with components that have already been placed. The paper also highlights, the future need for placement machine manufacturers to develop more accurate pickup tools.
For future issues of SSMT, I hope to include more review papers and I am pleased that we have the first of these currently being prepared. However,I would welcome the submission of additional review papers on any of the key areas covered by SSMT. The journal now has an increasing number of papers being submitted for publication and I am pleased that it has also been possible to implement improvements to the review process. This will enable authors to receive faster feedback on their submissions, so that their can be published sooner than has previously been possible.
As always, I welcome your comments, feedback and suggestions about the journal and its content. I can be contacted at: m.goosey@lboro.ac.uk
Martin Goosey
