Editorial

Editorial

Article Type: Editorial From: Circuit World, Volume 39, Issue 3

As electronic products become more sophisticated, with the need to include additional functionality and greater interconnect density, the demands on printed circuit board manufacturers continue to increase. This is particularly true in terms of ensuring product reliability, where the provision of reliable, assembled circuit boards is one of the greatest challenges for manufacturers. The more that interconnects and components reduce in size, the greater the difficulty in ensuring reliability. There are a number of factors that cause concerns, but most of them ultimately relate to the formation of solder joints between components and the printed circuit board. The key influences are well known and require careful optimisation in order to give the yields and robust performance demanded by end-users. These are too numerous to list here, but mention must be made of the need to use lead-free solders, with their higher processing temperatures, different mechanical properties and related reliability challenges. In this issue of Circuit World each of the papers relates to reliability in some way and they provide good examples of the work that is continually being undertaken around the world to address this important issue, be it via new approaches to drilling, through the better understanding of the problems associated with solderable finishes, or via any of the other critical areas that have an influence.

The first paper reports on practical studies of failed circuit boards undertaken by Ramanauskas et al. from the Center for Physical Sciences and Technology in Vilnius., Lithuania. This paper details work to confirm the results of earlier research carried out as part of a European Commission funded project called Aspis, which aimed to develop more reliable nickel-gold (ENIG) solderable finishes and to gain a better understanding of the well-known problems associated with these coatings. The Aspis work enabled the mechanisms of black pad failures to be elucidated and the work reported here records the results of practical investigations of ENIG-related failures on real problematic PCB samples from different supplies. The results of this investigation helped to confirm that the failure mechanisms identified in the earlier work really did apply to production boards.

Voiding during reflow soldering has been a well-known problem for the assembly industry for many years and its occurrence has been exacerbated by the adoption of BGA and CSP packaging, especially when combined with the use of microvias. The conversion of assembly to use lead-free solders has also made the problem worse. In the second paper of this issue, Chen et al. from the University of Electronic Science and Technology of China, report on the failure mechanisms related to solder bubbles in PCB vias during high-temperature assembly. The cause and mechanisms inducing solder bubbles are considered in the context of drilling effects on the microstructure of drill bits and the blowing of gas in via holes. It was found that a basic cause of the problem was moisture absorbed inside the holes of plated via layers that evolved during lead-free soldering to induce the formation of solder bubbles. However, the use of less than optimum drill bits was also found to have an influence and the authors recommend that dull bits should be avoided in order to prevent drilling related effects.

As the number of holes required in many circuit boards continues to grow, and hole sizes reduce, the demands on the placement accuracy and subsequent performance of the holes drilled presents major challenges for PCB fabricators. The next two papers detail the results of work to improve the performance of drilling operations and to improve the quality of drilled holes and vias.

The use of entry materials when drilling circuit boards has long been a requisite but, as hole sizes have diminished and the number to be drilled per unit area has grown, increasing attention has been paid to the influence these entry materials can have on both drilling efficiency and subsequent hole quality and reliability. This has been particularly true when seeking to overcome the difficult challenges of drilling high-aspect-ratio boards and the processing of exotic substrates. In the paper by Hu Zhou et al. from Hunan University of Science and Technology, the authors report research on the preparation and performance of coated aluminium entry boards. The coatings provided endothermic and lubricant properties to enhance the performance of PCB drilling. The aim of the work was to develop a new entry board material that was superior in terms of heat removal, lubrication and hole location capability. It could thus provide an enhanced process capable of forming high quality holes. The results from drilling tests showed that, due to the endothermic and lubricant resins used, the entry boards offered superior performance compared to conventional aluminium foils, not only in terms of hole location accuracy, but also in hole wall quality and the protection of drill bits.

Finally for this issue, the paper by Qinglong An et al. from Shanghai Jiao Tong University reports the results of work to develop a solution for micro drill condition monitoring during PCB drilling that utilised data obtained from vibration signals generated during the process. The quality of drilled holes is heavily influenced by variations in the geometric profile of the drills used and these profiles change as the bits are used and wear. Worn bits also have a greater propensity to break, causing damage to valuable boards and loss of production. There is thus a clear need to establish a detection method for tool condition monitoring, which can also be used as an effective way to prevent micro-drilling tool breakage. In this study, vibration signals were used to distinguish between the progressive stages of wear in micro drill bits and the paper provides a method and a system for PCB drill wear monitoring. The method and system described can achieve on-line monitoring of the condition of PCB drill bits.

I do hope that you enjoy reading these papers and find the information they provide useful to your own work. As always, I welcome your comments and feedback and would be pleased to hear from you if you have comments, suggestions or recommendations. I can be contacted at: m.goosey@lboro.ac.uk.

Martin Goosey

May 2013