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Emerald Group Publishing Limited
Copyright © 2006, Emerald Group Publishing Limited
Institute of Circuit Technology Annual Symposium 2005
Institute of Circuit Technology Annual Symposium 2005
Keywords: Electronics industry, Laminates, Conferences
ICT Technical Director Bill Wilkie welcomed nearly fifty delegates to the Manor House Hotel near Guildford on 15 June 2005 for the 31st Annual Symposium of the Institute of Circuit Technology, this year with the very topical theme of “Design for Lead-Free Technology”.
First speaker was David Humby of Cookson Electronics, on the subject of laminate choices for lead-free assembly.
All of Cookson's laminate grades were suitable for lead-free assembly to more or less extent, but the survivability of substrates and through-hole interconnections, and the long-term reliability of assemblies, were subject to many factors which needed to be taken into account in choosing the appropriate material to suit the purpose.
Cooksons had devised a practical aid to laminate selection, using design characteristics and performance requirements of specific printed circuit examples as inputs in the preparation of a two-dimensional matrix chart with substrate thickness in one axis and number of reflow cycles in the other. For a given laminate in a specified range of reflow temperatures, the matrix was divided into zones of different colours depending on the attributes of the job: green indicating suitability for purpose, yellow a marginal condition and red indicating that the material was not a safe choice. Attributes such as high layer-counts, heavy copper weights or microvias would result in these zone boundaries being shifted downwards and to the left on the matrix, effectively reducing the recommended safe-operating window of a particular laminate.
Matrices were shown for a range of laminate grades. For example, traditional dicy-cured FR4 had a green zone up to three reflows at 1.6 mm thickness, with a yellow zone extending to 2.4 mm and red after that. The green zone was larger for filled dicy-cured FR4, with a limit of five reflows at 1.6 mm or 4 reflows at 2.4 mm. At the extreme, filled high-performance phenolic-cured FR4 gave an all-green matrix, although as a reminder to those engineers who traditionally drive up costs by over-specifying materials, the laminate-selection tool was equally available for use by purchasing people so that the most cost-effective choice could be made to suit specific cases. Various actual case-studies were described and, in answer to a question from the floor, it was commented that the next revision of the IPC 4101 laminate specification was expected to have additional classifications to recognise grades designated as lead-free compatible.
Developing the theme of lead-free choices to include solderable finishes and wave-soldering alloys, Cookson's Gerry Campbell and Frando van der Pas described current trends and developments.
It was seen as the responsibility of the supplier to educate the market as to the suitability of available finishes so that an educated and cost-effective choices could be made to suit particular application and performance requirements. It was forecast that there would be substantial growth in the use of OSP and immersion silver finishes, at the expense mainly of HASL. ENIG would remain a high cost finish for niche applications, but still with some issues regarding manufacturability and reliability. And although there would remain certain regional and OEM- specific demands for immersion tin, potential whiskering and the environmental consequences of thiourea there would be ongoing areas of concern.
The latest generation of immersion silver processes offered more controllable chemistry, longer bath life and better tarnish resistance, together with a natural compatibility with SAC solder alloys. It had been shown that thick deposits were not necessary to achieve tarnish resistance. The thinner the deposit, the less the tendency to effects like champagne voiding and crevice corrosion and good wettability could still be maintained over six reflow cycles.
Immersion silver was predicted to become the metallic finish of choice. However, if there were no metallic contact-pad considerations and the only requirement was solderability, then OSP finishes offered the most cost effective solution. Current technology OSPs, based on substituted benzimidazoles, were sufficiently thermally stable to be fully compatible with lead-free soldering processes through multiple reflow cycles. Because of natural differences in the wetting characteristics of SAC solders, some modification of design rules for stencil apertures might need to be considered. Also, careful selection of flux was important if optimum results were to be achieved.
The right choice of lead-free solder alloy for wave soldering was a compromise based on total cost of ownership. The popular SAC alloy, Tin/3 Silver/0.5 Copper, had been widely adopted but there were some concerns about micro-cracking and the need to modify design rules to avoid yield losses. Also, in cost sensitive applications, the presence of 3 per cent silver made it an expensive option. A modified SAC alloy, Tin/0.3 Silver/0.7 Copper, labelled SACX, had been shown to offer better drainage and reduced incidence of bridging, at a 30 per cent cost saving over the higher-silver material.
Surface roughness and the appearance of micro-cracks upon solidification of tin-silver-copper alloys were natural metallurgical effects related to the formation of Ag3Sn intermetallics. Revision D of IPC-A-610 recognised these effects and demonstrated acceptability criteria.
Paul Watson from Cemco-FSL began his presentation with the rhetorical question “HASL, the End?” then went on to describe the opportunities presented by lead-free legislation, and how these had been taken up and developed into a new generation of hot-air-solder-levelling technology. Lead-free HASL was already the preferred finish of several assemblers and, despite the negative forecasts, was continuing to gain ground. Cemco's lead-free programme began in 1999, with a Tin/0.7 Copper alloy in a vertical air-leveller, and was made available to the industry for evaluation from 2000. There had been considerable interest from North America and South East Asia, but little from the UK. The tin-copper alloy was not cosmetically attractive; tin-copper-nickel alloy had an appearance similar to traditional tin lead, and was more uniform in thickness after levelling. Other alloys had been evaluated in cooperation with fabricators, EMSs and OEMs, and chemical suppliers had worked closely with Cemco to develop a range of suitable oils and fluxes.
Key process differences lead-free and traditional tin-lead HASL were higher temperatures and longer contact times. In some instances the process sequence had been modified to include a pre-dip to help pre-heat the substrate. Copper control was more difficult because copper dendrites sank to the bottom of the lead-free solder rather than floated on the top, but methods were being developed to overcome the problem. New air knives had been developed for lead-free which operated at lower pressures and, in conjunction with the natural flow characteristics of lead-free solder alloys, gave improvements in thickness uniformity which were acknowledged by assemblers as a positive benefit in the paste-printing operation.
In terms of its solderability, lead-free HASL would always have the advantage over other finishes in presenting a copper surface already totally wetted with solder – “the ultimate solderable finish”.
David Woodley from Technograph Microcircuits added an alternative perspective to the proceedings. He described the diverse range of interconnection and packaging technologies undertaken within Technograph, and how the cross-feed of knowledge between market sectors including space and defence, telecommunications, security systems, optoelectronics, microelectronic mechanical systems and high temperature sensing systems for nuclear fusion reactors, offered opportunities for innovative solutions to interconnection and packaging challenges. Technograph's traditional speciality was the thick film hybrid and, although PCB-based modules had taken a proportion of the market, thick-film was still chosen where the ultimate in reliability was required. For example, the new Airbus 380 had 16 thick-film multi-chip modules within its electronics systems.
As a sub-contract assembler and EMS provider, Technograph continued to experience issues of lead-free component and material availability and identification. Although many of Technograph's customers had already adopted the lead-free concept, there were others, some of whom were subject to exemptions from RoHS legislation, who took an ultra-cautious position in respect of reliability and would not willingly move to lead-free until the technology had a 25 year proven track record.
The event concluded with a tour of the Woking facility of Cookson Electronics, to whom grateful thanks are due for their generous sponsorship of the Symposium.
Pete StarkeyICT Council