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56 SMT Magazine • June 2015 eVenT ReVIeW: (eMPS-6) continues tions and recommended alternative processing methods were provided. An inhomogeneous stripline interface transition was detailed as a novel approach for designs incorporating a transition from alternate impedance circuits to standard 50 ohm impedance interfaces. This transition included a good degree of stress re- lief in the axial direction and this novel ISIT in- terface technology has been qualified for space use. The following two presenta- tions were made by Dr. Jan Van- fleteren from the IMEC Centre for Microsystems Technology, Gent University, Belgium. He reported on the manufactur- ing processes developed at IMEC for embedding ultra- thin chip packages into the thickness of both rigid and flexible printed circuit boards. Here, bare commercial semi- conductor dies are thinned down to thicknesses of be- tween 20 and 30 microns with- out losing their functionality. These chips are then embedded in spin-on polyimide material and, using thin-film technology and lithography, the fine pitch chip contact pads are fanned out to a more coarse pitch which is compatible for alignment and contacting with conventional PCBs. These IMAC packages have a thickness below 100 microns and they are mechanically flexible. The full description of the processes was described and physical samples were dem- onstrated to be completely bendable as they were passed around for assessment by the work- shop participants. Dr. Vanfleteren's second presentation was equally fascinating; it covered elastic and rigid 2.5D free-form circuits. The copper conductors on these flexible circuits were designed rather like springs so that they could elongate between 60% and 100% without failure. These substrates are made from elastic polymers (e.g., silicone rubbers or polyurethanes). Again, remarkable samples of (wearable) "blue light therapy" de- vices were passed around the audience. These stretchable electrical connections are thought to have many interesting features for space ap- plications. Due to their flexibility, such circuits could be incorporated as interconnections be- tween highly bendable parts such as on solar arrays and even astronauts' spacesuits. The next session concerned Processes and Re-processing and was started by Dr. Klas Brink- feldt of Swera/IVF, Sweden, entitled, "Nanosil- ver Die Attach and other Emerging Technolo- gies for Power Electronics." Dr. Brink- feldt explained his sintered silver particle material and demon- strated its use in a manufactur- ing process for low-volume, high-performance applica- tions—particularly for the attachment of dies in power electronic units. Present com- mon failure mechanisms in power electronic assemblies were illustrated using scan- ning electron fractographs (wire bond failures, delami- nation of substrate materials, and the cracking and fatigue of solder alloys, as well as burn- out failure mechanisms). The use of sintered silver nanoparticles in an organic matrix was seen for a die attach application to be of similar, or better, reliabil- ity than conventional solder-attach processing. Thermal cycling of these die-attach configura- tions between -40 o C and +125 o C gave excellent results and, for power cycles between +45 o C and +175 o C, for the same silver sintered die attach- ments, up to 160 million power cycles can be envisaged from Coffin-Manson modeling. By using the same environmental testing cycles, present-day solder interconnections would fail at about 40,000 cycles. For real-life applications it was acknowledged that some precautions must be taken in order to avoid silver migration and resulting short circuiting; suitable coatings do exist and would need to be applied to these devices. Also, Dr. Brinkfeld considered that the silver sintering process steps could be opti- mized, and should include a physical pressure cycle that would further enhance the adhesion between mating surfaces. The thermal manage- ment of compact, small scale power electronic artICle the copper conductors on these flexible circuits were designed rather like springs so that they could elongate between 60% and 100% without failure. " "