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The EMPS-7 Workshop was held at the University of Portsmouth (UoP), UK on 13–14 April 2016. This Workshop followed on from an IPC/UoP Lead-free Risk Management (PERM) meeting at the same venue—convenient for participants attending both events and arriving from continental Europe and the U.S.
The Organizing Committee of EMPS-7 consisted of Jussi Hokka, European Space Agency (ESA) and Misha Filip, UoP. The Programme Committee consisted of Barrie Dunn; Jussi Hokka; Bill Strachan, ASTA Technology; and Martin Wickham, National Physics Laboratory.
A brief welcome and introduction to the EMPS-7 Workshop was given by Jussi Hokka and Barrie Dunn. This was followed by the opening address provided by Professor Graham Galbraith, vice chancellor of the UoP. Ninety-five delegates from industry, space agencies and academia attended this two-day workshop which was held in the magnificent Portland Building auditorium (Figures 1 and 2).
The participants this year represented 16 countries and included engineers from the U.S. and China. Prof. Galbraith was proud to note that this workshop had been founded by the University in 2010 and has continued each year at a different European venue.
Figure 1: The EMPS-7 Social Dinner was held in the Still and West public house in this area of Old Portsmouth, also known as Spice Island, which is where all the spices used to enter the city. The area features part of the fortifications for the harbor entrance built in 1415.
Portsmouth is a dynamic and vibrant waterfront city and is unique being the UK’s only island city and is noted for its links to the British navy, shipbuilding and in recent years it has become a hub to Space industries such as Astrium-Airbus, Spur Electron and BAE Systems. These companies rely on advanced materials for high strength/low weight spacecraft structures—many are based on carbon fibre technology. Coincidentally, these CFRP materials are also selected by the Ben Ainslie Racing team for the fabrication, in Portsmouth, of world class racing yachts. Summer 2016 will see these yachts take part in the Americas Cup which is being hosted by the City of Portsmouth, and it is here where the Land Rover BAR team is on a mission to bring the America's Cup back to British waters!
Figure 2: Some participants to EMPS-7 (photo thanks to Leo Schöberle).
The first Workshop session concerned PCBs and circuit technologies. Maartin Cauwe from the Centre for Microsystems Technology, Belgium, presented the results of an extensive study into the embedding of passive components into PCBs for space applications. The reliability of such embedded passive components has been evaluated and a functional demonstrator manufactured. It was found the embedded technology samples performed almost as well as surface mount technology (SMT) counterparts environmentally tested in parallel. It was only the embedded 0201 resistors that had a minor impact on component reliability. At present, standarised testing of such technology is challenging as there are no design rules for space products and, as required by spacecraft contracts, there is no possibility for component replacement or repair with this new technology. Dr. Cauwe considered further embedding activities should include MOSFETs, power components and more complex modules.
The next presentation concerned the development of new PCB surface finishes using “deep eutectic solvents” (DESs) based on environmentally benign liquids with remarkably high salt content. This work was described by Karl Ryder from the University of Leicester. DESs have found many applications related to electroplating, electro-polishing, immersion coatings, metal recycling and energy storage. DES technology has enabled the plating of PCB copper tracks with silver, nickel, tin and gold. A so-called “universal surface finish” has been developed by the Leicester team. Its plating chemistry and main successes were detailed by Dr. Ryder. The main PCB finishes comprise of electro-less nickel, immersion palladium and immersion gold (ENIPIG)—these potential space PCB finishes have uniform coverage combined with excellent planarity. The DES processes are free from acids and cyanides, have little effect (dissolution) on the substrate copper and were stated to result in highly solderable surfaces.
The reliability of flexible PCBs for aerospace applications was discussed by Hans-Peter Klein of Dyconex AG, Switzerland. Interconnect mechanical stress testing and exposure to extensive temperature cycling was performed on microvias having a diameter of less than 100 microns. It was found that flex materials having a low Tg could be subject to early in-service damage and their failure mechanisms were described in detail. Materials with a high Tg were recommended as they are not subject to either the assembly stresses caused by solder-assembly or the conventional spacecraft in-service life stresses. Flexible circuits with a high Tg can undergo rapid acceptance testing and provide meaningful predictions of service life.
Other research performed at Swera IVF, Sweden focused on the cracks that may form in PCB laminate during the thermal cycling of assembled electronic components. It was noted by Per-Erik Tegehall that tin-silver-copper (SAC) alloys are far stiffer than the tin-lead solders they are replacing. One effect, during large delta T thermal cycling of area grid array (AGA) packages mounted to PCBs, is that the SAC solders stress the solder pads far more than the “compliant” SnPb solders and this can cause cracks to develop beneath the copper pad terminations. Many reliability tests have been performed at Swera, particularly related to Ball Grid Arrays assembled onto daisy chained monitoring circuits within PCBs. In general, the reliability of the SnPb solders was found to be “lower” than that of the SAC alloys, but it is only when detailed metallography is performed that defects are observed in both configurations. It is essential that any micro-sectioning of solder joints assemblies, after completion of environmental testing, includes the removal of all flux residues followed by mounting the samples in a room temperature hardening epoxy (under low vacuum). When a fluorescent agent is added to the molding resin fine cracks can be observed during optical inspection under UV light. SnPb solders degrade by thermal fatigue within the solder alloy, however it was surprising to find that the SAC soldered assemblies can also degrade in two distinct failure modes. Some SAC joints become completely recrystallized, and comprise of single grains. These are very strong in the Z direction and cause extensive pad lifting and fracture in the resin below the pad. Beneath the same AGA some SAC joints are seen to be multi-grained and these were noted to fail at the solder to pad interface. Figure 3 shows images of SAC solder joints taken with Electron Back Scatter Diffraction (EBSD) after thermal cycling—the left-hand image shows a single grain of random orientation (other single-grained balls had different orientations) and there is no crack within this grain, only a lifted pad. The right hand ball is multi-grained and here the failure is within the solder.
Dr. Tegehall concluded that cracks forming in the PCB laminate might improve the fatigue life of SAC joints, but they can lead to an overestimation of the fatigue life of such joints by at least a factor of three.
Figure 3: AGA ball joints made from SAC alloy but having various grain sizes, orientation and two distinct thermal-fatigue failure modes. Courtesy of SWERA-IVF.
Jussi Hokka outlined the roadmap identified by ESA-Estec, the Netherlands for advancing technologies related to PCBs and several assembly technologies until 2020. He detailed the activities of several ESA-Industry working groups actively engaged with SMT and PCBs and emphasized the importance of miniaturizing circuitry with high density interconnect technology. The roadmap also addresses spin-in opportunities and legislation threats posed by REACH and RoHS.
To read this entire article, which appeared in the August 2016 issue of SMT Magazine, click here.