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June 2014 • The PCB Magazine 27 case is much smaller than that in the single- disk case (Figures 2b and 2d). We further illustrate our concept with experi- mental recording of the strain across the surface of the substrate (Figures 2b and 2d). We introduce an ordered matrix of PDMS microposts distrib- uted all over the top PDMS substrate. The PDMS substrate was moulded against a silicon wafer pat- terned with cylindrical holes, 2 µm deep and 2 µm in diameter, arranged in a square array with a pitch of 12 µm. These microposts are small enough compared to the thick PDMS substrate not to affect the mechanical properties at its surface. They are used as visual markers to mea- sure the local surface strain of the PDMS. While stretching, the distance variation between the mi- croposts on the PDMS surface was measured with an optical microscope, and the local strain profile along the stretch direction was calculated. Each strain value was calculated by measuring the dis- tance between ten adjacent pillars. We assumed the error in determining the position of the edge of each micropost should be approximately the same as the micropost diameter (2 µm), giving ±1 µm for each micropost and a total of ±2 µm for the row. The error was then calculated assuming a ±2 µm uncertainty for each strain value. Figures 2b and 2d display the recorded strain across the single and double disk sample when a 20% strain is applied along the X-axis. The double disk sample is made of a 5 mm diameter PI disk embedded 100 µm below the top surface and a second PI disk (8 mm diameter) embedded at H2 = 0.5 mm. The single disk sample has the 5 mm diameter disk embedded 100 µm below the top surface. The experimental data concur with the finite element model. At 20% applied strain, a large peak reaching ~40% is observed at the edge of the single disk while it is suppressed in the case of the double disk design. Furthermore, the strain is locked at 0% across the complete surface of the top disk and the strain increases from 0% to 20% in the region above the second larger disk is very gradual. Vertical lines mark the position of the edge of the 5 mm and 8 mm diameter disks. Disks of this size were studied as they provide a rigid re - gion large enough to hold a standard rigid electri- cal component. The strains determined from the finite element simulation remarkably agree with the experimental data. III. Electrical Interconnects Thin-metal film interconnects are patterned on the top surface of the hybrid stretchable substrate and define a stretchable electrical net- work between the non-deformable areas of the substrate. Their electromechanical performance is presented Figure 3a as a function of applied strain and for both substrate designs. Gold thin- film stripes were evaporated on top of the single and double disk substrate, and their electrical resistance was recorded as a function of applied strain and number of cycles. The interconnect is a 2 mm wide and 1.5 cm long stripe of Cr(5 nm)/Au(50 nm) bi-layer evaporated on the PDMS substrate through a shadow mask. Both substrates can carry highly stretch- able interconnects but interestingly the double disk design allows for a much smaller change in resistance with cycling. After 1000 cycles, the interconnect resistance of the stripe run- ning above the double disk design increases by a factor of seven (at 20% applied strain), com- pared to a factor 30 for the single disk design. This suggests reduced mechanical damage in the gold thin film on the double disk design. Note that the evolution of the R (strain) pro- file with cycling from a triangular to a box-like profile is typical of evaporated thin gold film on PDMS13. We further observed the surface of the gold interconnect patterned on a "double disk" sub- strate using scanning electron microscopy. We monitored its topography as a function of ap- plied strain in a FEI/Philips XL30 Environmen- tal Scanning Electron Microscope (ESEM). For ease of surface tracking, the top surface of the PDMS is covered with the same PDMS micro- posts as described above. Figure 3b shows a se- ries of micrographs taken above the rigid plat- form, at the edge of the 5 mm diameter disk and on the stretchable PDMS region after 100 0 stretch cycles to 20% strain. Row (a) displays the gold surface at the edge of the 5 mm disk at 0%, 5%, 10%, 15%, and 20% strain. The dotted line indicates the approximate position of the edge of 5 mm diameter disk. Row (b) shows the gold surface above the rigid region. There is no sign of strain or cracking in the gold film, even after 1000 stretch cycles. Row (c) shows the highest strain regions around the edge of the smaller HyBRID STRETCHABLE CIRCUITS oN SILICoNE SUBSTRATE continues