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Undulating Coils Humanize PCBAs
December 31, 1969 |Estimated reading time: 3 minutes
Collaborative development work between Belgium's University of Ghent and IMEC produced a molded interconnect device (MID) that can stretch up to 100%, while still conducting signals between surface mount devices (SMDs) and other components. The traces in this MID follow a periodically undulating "horseshoe" shape, which, combined with narrow metallization and a compliant polymer substrate, dissipates the stress created by stretching and bending electronics. SMT spoke with Jan Vanfleteren, researcher, University of Ghent, about the consumer and medical end-products enabled by stretchy PCB assemblies (PCBAs), the assembly process, and the design and materials engineering that led to developing a circuit board that endures complex mechanical loading without trace or component damage.
The design for elastic circuits relies on maintaining continuity throughout the trace. IMEC and University of Ghent researchers applied three design concepts to this end. The team discovered that a narrow trace improved mechanical performance under stresses, reducing cracks in the gold or copper. Therefore, they used narrow metallization schemes, where, for example, a 60-µm trace is split into four parallel lines of 15-µm traces and 15-µm spaces. Calculated stress was reduced by up to 10× without affecting electrical performance. Researchers also modeled various trace designs to determine the shape that suffered least from uneven loading. The undulating horseshoe shape proved to dissipate stretch and flex pressures without concentration on one curve or angle in the design. This increases elasticity while strengthening the trace.
The third aspect to consider for trace reliability rests in the compliant polymer chosen as a substrate. Polydimethylsiloxanes (PMDS), a type of silicone polymer, while not a good conductor, can transmit a signal very short distances when shunted. Should a trace be over-stretched, resulting in a micro-crack, the surrounding polymer will conduct the signal, bridging the gap created by the crack. Vanfleteren refers to this capability as "self-healing" because failures within the trace are compensated for by the substrate, to a degree.
The polymer's highly resistive qualities keep trace signals isolated beyond micro-distances. Shunting the polymer adds costs, but ensures high reliability, particularly for medical end products. The polymer's material properties also contribute to maintaining trace integrity and providing a functional circuit board. Polymer thickness limits stretch and flexibility; controlling that thickness moderates stress loads. The substrate is thicker where the horseshoe traces connect to component leads, Vanfleteren explains, creating a less stretchy, more stable region around solder joints.
These solder joints are created using normal electronics assembly processes before the silicone polymer is molded around the traces. To assemble the MID, researchers prefabricate interconnect traces on a supporting board with plating steps. This sacrificial board, usually copper foil, then moves through solder-mask deposition, printing, component placement, and reflow, which generates a functional PCBA. The PCBA is overmolded with the silicone polymer, following a precise volume design to allow predetermined deformability. The sacrificial board is then removed by wet etch, and a second overmold is performed to completely embed the assembly.
Early assembly, prior to overmolding with the compliant polymer, closely mirrors flex-circuit assembly, Vanfleteren notes. Due to accuracy issues it creates and problems with the heat of reflow, components are difficult to mount when the elastic polymer is already in place. Vanfleteren adds that PCB manufacturers or EMS providers could produce elastic PCBAs using this method, with additional equipment. Partnerships between PCB manufacturing and electronics assembly companies would also benefit the technology.
Other developments that would provide a boon to flexible, twistable, stretchable electronics include determination of tests and standards to qualify production. "With no standards for stretchable electronics, we had to buy test equipment and compile our own set of relevant test data," Vanfletern explains. Tests ran the gamut from functional to destructive. Some involved repeatedly stretching the assemblies. Breaks, predictably, occurred at the thinnest points in the polymer, where stretch was highest.
Within three years, researchers at the University of Ghent and IMEC plan to develop a viable commercial product based on this technology. Necessary stretch capacity could be about 20% for most devices, and some would require stretch only once in a product life cycle. More challenging devices such as advanced medical prototypes, would demand 100% stretchability and/or repeated stretching.
Options cover smart textiles, where a certain degree of stretch in a sensor array, transmitter, or display embedded in clothing would greatly increase user comfort and practicality of application; medical devices, everything from an implantable regulators to a simple thermometer headband; or consumer devices, a sector with possibilities for gaming, cellular phone designs, displays, and other next-generation products that could integrate well into a user's physical surroundings that they seem to be ambient technologies.
To learn more about the stretch-PCB technology and University of Ghent/IMEC research, visit www.IMEC.be and www.UGent.be.