Issue link: https://iconnect007.uberflip.com/i/521718
40 The PCB Magazine • June 2015 applying a second layer of stretchable material at the bottom (i.e., by a second LIM step). A critical factor in the use of stretchable interconnections is the design of copper me- anders. Extensive modelling was performed to minimize stress and strain concentrations in the meander under deformation. The current me- ander shape of choice is the horseshoe shape, as shown in Figure 9. A horseshoe-shaped me- ander is a connection of circular segments, of- fering a good compromise between sufficient stress distribution along the meander on one hand, and ease of design and circuit layout on the other. For minimal stress the width (W) of the track should be as small as possible. The minimum width is determined mainly by tech- nological constraints. During the cycling elon- gation of the stretchable circuit, a permanent deformation or plastic strain is induced in the metal causing a fatigue failure. Therefore, in order to improve the fatigue reliability of the stretchable interconnect; the plastic strain has to be minimized. The ratio R/W determines the maximum plastic strain in the meander for a given de- formation. In general for deformations of 20% and less, the maximum plastic strain decreases with increasing R/W. High R/W ratios, however, also mean meanders with high overall width, and thus a low density of parallel running me- anders. A ratio of R/W = 10 normally offers a good compromise between low maximum plas- tic strain in the meander and high number of parallel running meanders per unit width of the stretchable interconnect area. A further improvement of the mechanical reliability of the meanders is achieved by sup- plying the meanders with a flexible support, i.e., a PI film. Numerical modelling showed that the PI width is the main parameter, reducing the maximum plastic strain in the Cu meander [5] . Focus of the reliability investigation for stretchable interconnections has been on the optimization of the performance under me- chanical stress like (e.g., periodic or random deformation of the circuit). No standard tests for stretchable circuits exist today. In order to quantify and assess the mechanical reliability of stretchable circuits uniaxial stretching tests are executed on samples that contain a number of parallel running meanders. The main test is cyclic stretching at moderate maximum elonga- tion until conductor rupture. Results of these cyclic stretching tests have been described extensively elsewhere [6] . The main conclusions are that supporting the Cu meanders with a flexible material like poly- imide drastically improves the lifetime of the interconnection. As an example, a double sid- ed PI-enhanced stretchable interconnect with- stands more than 90,000 stretching cycles at 5% elongation, and even for elongations up to 20%, the PI-enhanced stretchable interconnect can survive more than 400 cycles. Proper design and fabrication of the tran- sitions between component islands and the stretchable interconnects is at least as important as the stretchable interconnects themselves. There is no quantitative structural design rule or optimized design available for solving this reliability issue. A qualitative rule is that the transition from rigid over flexible to stretchable circuit parts should be as smooth as possible. Optical microscopy was used to analyse both types of samples after the endurance test. The copper interconnects break at the top of the meanders, which is the place where the highest accumulated plastic strain is present [7] . The start of the micro crack as seen in Figure 10 is not detected by the resistivity measurements due to the very low increase of resistivity. On non- supported copper tracks, the crack propagates during one or just a few cycles leading to the sudden breakdown. Delayed crack propagation is observed for the meanders with PI support, which translates in a slowly increasing track re- sistance. FLExIBLE AND STRETCHABLE CIRCUIT TECHNOLOGIES FOR SPACE APPLICATIONS continues FeAtuRe Figure 9: Horseshoe shaped meander with relevant design parameters.