Citation
(2014), "Review of the Fourth Electronic Materials and Processes for Space Workshop (EMPS-4) held on 29 May 2013 in Aalborg, Denmark", Circuit World, Vol. 40 No. 1. https://doi.org/10.1108/CW-10-2013-0044
Publisher
:Emerald Group Publishing Limited
Review of the Fourth Electronic Materials and Processes for Space Workshop (EMPS-4) held on 29 May 2013 in Aalborg, Denmark
Article Type: Conferences and exhibitions From: Circuit World, Volume 40, Issue 1
The EMPS Workshops are a University of Portsmouth (School of Engineering) initiative which started in 2010 with EMPS-1. The workshops are held at least once per year, typically at one of the six ESA-approved skills training schools. The workshops are non-profit making and held to develop and promote the materials and manufacturing processes utilised for spacecraft applications. Materials engineers and students are encouraged to attend.
EMPS events are usually free of charge, but when participation is limited due to capacity constrains at any venue, priority is given to the workshop presenters, specialist engineers and scientists and students occupied in the fields of materials, manufacturing and the space industry.
The events are publicised on a University of Portsmouth web site from which all past EMPS presentations can be freely downloaded: http://https://sites.google.com/a/port.ac.uk/emps/
Past locations for EMPS events were at: the University of Portsmouth (2010); the Institut de Soudure, Paris (2011); and the Italian Welding Institute, Genoa (2012). EMPS-4 was held last May at the Danish Institute of Technology and Research – Hytek, Aalborg. The modern lecture hall was filled by 64 enthusiastic engineers and academics (from 12 countries) together with several local university students.
Poul Juul, the Technical Manager of Hytek, commenced the presentations with a review covering the effect of voids in soldered joints on the reliability of electronic assemblies. Failure investigations and case studies undertaken by his institute were illustrated by means of an impressive array of laboratory tools. The fine focus X-ray tomography facility was able to assess the quality of hybrid package seals, as well as the shape of microwire bonds between package and chips, together with the extent of voiding in solder alloy between chip and package. The effect of voids within the solder balls of area grid arrays was discussed and again X-ray tomography was used, after assembly to PCBs, to establish the number of balls having a well-defined void size. High-REL ball grid assemblies can be specified to contain voiding within 50 per cent of their balls. However, in order to survive extensive thermal cycling in operation, the maximum void size in any ball must be less than 20 per cent of the ball area.
Voids in solder were also shown to originate from the outgassing of component terminations during high temperature soldering as well as from the circuit boards laminated substrate. Large, well defined voids within solder joints were considered to be less of a risk than a continuous network of microvoids occurring along any solder-to-substrate interface.
Luca Moliterni, from the Istituto Italiano della Saldatura Group in Genoa, presented the very detailed failure analysis he had performed on CQFP352 component leads following both vibration and thermal cycling. It is usually the corner leads that crack and cause open circuits during qualification testing. Even when these very large-sized packages are perfectly solder-assembled, care must be taken to avoid stressing the leads by over torqueing any nearby PCB mounting screws.
Staking compounds applied to the corners of such packages will also cause premature failure of the leads unless they have a low coefficient of expansion (made possible with the addition of carboxyle powder to the adhesive). Also, care must be taken during lead bending to avoid bends too close to the lead braze, to select the correct bend radius and to avoid stressing the leads during any soldering operation (assembly or rework).
Dr Per-Erik Tegehall from the Swerea-IVF Institute in Molndal, Sweden, summarised his extensive studies related to the intermetallic layers which form between various solder alloys and several surface finishes. Specific intermetallic compounds were related to solder joint reliability.
The effect of intermetallic compounds on the reliability of BGAs was illustrated by means of SEM fractographs showing ductile (in solder) and brittle (in/at intermetallic interface) fractures.
Dr Tegehall demonstrated the effect of increasing strain rate on the failure mode of tin-lead solder – the solder strength increases with increased strain rate. Simultaneously, the intermetallics strength decreases with increased strain rate. These results highlight the fact that as strain rate is increased, the tin-lead solder will undergo a ductile-to-brittle transition. High strain rate stresses can be attributed to operations such as in-circuit testing, de-panelling by snapping end tabs, attachment of fasteners, aggressive key pad actuation and, by simply dropping a product to the ground. Interestingly, the impact of lead-free soldering is shown to cause a reverse effect: as strain rate increases, the ductile to brittle transition is decreased because, in general with these solders, resultant intermetallic compounds have a lower strength than the bulk lead-free solder. The paper continued, relating solder composition and termination finish to intermetallic growth. Per-Erik ended his presentation by reviewing one cause of micro-void formation (due to contaminated plated layers) and the effect on reliability when soldering with SnAgCu (SAC) alloys to modern electrolytic and electroless finishes.
On a similar subject, Mr Jussi Hokka, previously from the Aalto University in Finland and now employed at ESA-ESTEC in The Netherlands, presented extensive data on the reliability of SAC solder interconnections. Jussi described the various parameters selected by industry for thermal shock testing and temperature cycling testing of tin-lead solders. He discussed how they might be adapted when an electronic assembly has been constructed from a SAC solder alloy.
Dr Tegehall, in his previous presentation, had also shown the properties of SnPb alloys to be vastly different to those possessed by SAC solder alloys. Testing at Aalto University was performed using samples of BGA144 packages mounted to eight-layer (FR4) MLBs. A multitude of thermal profiles were evaluated, each having different lower and upper dwell temperatures, different dwell times and ramp-rates. Failures were monitored (on about 30 packages per test) by recording the resistance values of the daisy chained networks – a 20 per cent increase from the initial resistance was selected as the criterion for failure. Failure sites were later established by metallography, cross-sections being made in a diagonal plane through the BGA packages.
Jussi indicated in his conclusions that standardised accelerated thermal cycling tests create failure mechanisms that are not seen in conditions representing real-use operation.
Activities related to material investigations and component failure analyses at the RUAG (previously Saab Space) laboratory in Gothenburg, Sweden, were described by Dr Lars Ryen. RUAG is one of Europes largest product suppliers to the space industry and is involved with launch vehicles, satellite structures and their mechanical equipment and electrical systems. Lars demonstrated how focused ion beam (FIB) cross-sectioning was used to determine the failure mode of MMICs after temperature step stress testing. An open circuit at a contact window was cross-sectioned to reveal extensive Kirkendall voiding within the device, together with voluminous aluminium palladium intermetallic phases. Several other examples demonstrated how this laboratory has enhanced the quality of RUAG products in orbit.
An interesting presentation was given by Mr Dale Walkden concerning the awareness of UK Ministry of Defence contractors to certain lead-free issues. Information was gathered by means of questionnaires and company visits. It appeared to be clear that most companies had attempted to have mechanisms in place to manage lead-free issues. Most were aware of potential problems resulting from the growth of tin whiskers, mixing SnPb with lead-free alloys and the need for incoming inspection for pure tin. Actual issues relating to lead-free had been experienced by the majority of responders, some related to tin whiskers and others to manufacturing, repair and solder joint reliability. This is a difficult and sensitive subject to monitor and Dale was keen to emphasise company and product anonymity. Defence industries and similar organisations in other countries will probably have the same status regarding lead-free as determined by Mr Walkdens statistics. An important minority of responders were considered to be of "high risk", as they either had insufficient understanding of lead-free issues and/or, they had limited mitigation strategies in place to manage the issues.
All topics related to the growth of tin whiskers on electronic materials are of great interest and Martin Wickham of the National Physics Laboratory in the UK was sure to highlight this during his presentation entitled "The attraction of Sn whiskers". He began by outlining the many "whisker works" presently being undertaken at NPL. One, the NPL tin whisker databank, involves partners forwarding components for storage and annual inspection – several of the submitted lead-free components have developed whiskers during the past 12 months, however for the DIL devices, no whiskers greater than 50 per cent of the lead gap spacing has been observed. The results of whisker resistance and, voltage versus current plots, were detailed by Martin. His results, not surprisingly, confirmed the measurements and findings published by the writer, based on his tin whisker studies performed in the ESA-Estec laboratory 26 years ago!
A video was then shown and the audience were able to witness the electrostatic attraction of whiskers. The whiskers were seen to bend and become attracted to the pointed end of a microprobe situated at a distance of 50 microns and at a potential difference of 10 V. Eventually the whiskers made physical and electrical contact with the probe, so creating a short circuit. The effect of electric field was also demonstrated by means of whiskers growing on the surfaces of metal plates separated by a distance of 600 microns. The likelihood of electrical short circuits will increase due to this electrostatic attraction between whiskers and conducting surfaces when they are at different electrical potentials. Failure sites were later established by metallography, cross-sections being made in a diagonal plane through the BGA packages.
"Industrial failures due to deviations from design rules", was the following presentation by Mr Karl Ring, a veteran electronics failure analyst and Head of the ESA skills training school at the ZVE-Fraunhofer Institute in Oberfaffenhofen on the outskirts of Munich. Examples of mechanical overloading and fracture of soldered joints were shown and the lessons learnt included:
* ensure proper mechanical support when manually attaching press fit connectors to PCB assemblies; and
* small PCB assemblies should be detached from industrially assembled larger boards by milling because shear cutting bends the smaller board and cracks chip terminations.
Other examples:
* cracked metallised chip capacitors were shown to be defective before assembly;
* gold embrittlement and fracture occurred when soldering to 3 microns thick gold on metallised ceramic;
* thermal cycling fatigue failures of joints in plated through holes;
* absence of conformal coating caused dendritic growth and short circuits between adjacent tracks on a PCB exposed to some humidity; and
* high voltage melting of tracks due to poor requirements in international standards.
Karls final case study involved examining the effect of fretting corrosion that had occurred between spring connectors and the mating surfaces of connector pads located on component assembled. These circuits had been mechanically attached to a flexible metal sheet within the front door of a gambling machine. Here, the electronic units design possessed no rigidity. Continuous movements and shocks, caused by the players of the machine, produced fretting and the build-up of oxides and corrosion products at the connection interface. The machines malfunctioned once the debris was thick enough to cause an open circuit.
Dr Pavel Shashkov introduced the EMPS audience to the novel "aluminium nanoceramic substrates for thermal management in electronics". The process for producing this material has been patented by Cambridge Nanotherm, Haverhill, UK. Its high thermal conductivity and dielectric strength make it suitable for heat pipes and chip-on-heatsink applications.
The nanoceramic surface could be built up to a given thickness, possibly 40 microns, on thin aluminium sheet and used in spacecraft design as a printed circuit board substrate. Conducting tracks based on gold, copper or platinum, can be easily coated onto the dielectrics surface.
The novel uses of automatic optical inspection and X-ray inspection for high reliability PCB assemblies were described by Kim Plauborg, Director of the Danish company Terma A/S, based in Aarhus. Numerous examples were shown where automated inspection proved to be more capable and more reproducible than human optical inspection.
Another presentation, given by Thomas Mueller of Zestron, Germany underlined the importance of "cleaning before the subsequent processing" during electronic assembly.
Professor Jens Nielsen described the successful design, construction (by his students at the Aalborg University) and launch of a small earth observation satellite. Students at this local university were again active with Lars Alminde, Director of GomSpace in Aalborg, during the completion and successful flight of a nano-satellite mission to demonstrate improved situational awareness for air traffic control. The resulting spacecraft images are extremely interesting and will be of great use to both ship and aircraft operators.
The Chairmans long term colleague and friend, Mr Claes Berlin, Former Product Assurance and Quality Manager at Saab Space, and now owner of QUBE, Gothenburg, presented the post-lunch talk. Light-hearted and philosophical, "from what to how" process management, a competitive tool, was much appreciated by the workshop participants as evidenced by the lengthy applause.
Barrie Dunn
European Space Agency, Noordwijk, The Netherlands