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June 2015 • The PCB Magazine 39 contact pad finish is applied and components are mounted using a conventional reflow solder process. At this point, the circuit can be tested and repaired, if required. Figure 7 shows an SMI substrate after component assembly. The second part of the SMI process flow in- volves the application of the stretchable carrier material, giving the circuit its resilience (Figure 8). In the first step, the top of the circuit is cov- ered by a layer of stretchable material by liquid injection moulding (LIM) of a two-component polydimethylsiloxane (PDMS) mixture. Alterna- tives for the application of the stretchable car- rier material by LIM include cover lamination and spray coating. Besides PDMS, also polyure- thane (PU) or any polymer for which a liquid precursor or a thermoplastic variant is available can be applied. In a second step, the assembly is removed from the temporary carrier. After cleaning, the circuit is completely embedded by Figure 7: Example of a stretchable system for respiratory monitoring before encapsulation. Figure 8: Second part of the SMI process flow. FLExIBLE AND STRETCHABLE CIRCUIT TECHNOLOGIES FOR SPACE APPLICATIONS continues FeAtuRe