Flexible Hybrid Electronics (FHE) brings together the best of printed and flexible electronics with rigid Si-based electronics. A critical and often limiting bottleneck is the interconnect between printed (often wide) and Si ICs (often narrow pitches).
Normal solder can not easily be used because (1) substrate such as TPU (stretchable electronics) and PET (flexible electrodes) impose severe temperature limitation, often even below bismuth-based low-T solders, and (2) some inks, specially Ag inks, dissolve in solder. Furthermore, these interconnects need not only support the pitch sizes of the ICs, but also survive flexibling as well as stretching, and be compatible with standard industry processes.
One option is to deploy particle filled (often Ag particle) epoxies to form the interconnects. Here, unless it is anisotrophic, then the pitch sizes will be limited. Furthermore, particle loadings are often high to achieve high conductivity, adding to cost.
Sunray has developed a novel solution: they disperse ferromagnetic particles within a two-part epoxy system. Under an external magnetic field, the particle align vertically, forming z-axis conductive paths. Here, the pitch can be down to 100um. The curing temperature can be as low as 80C, making compatible with TPU and PET. The material can sustain extreme repeated stretching.
Furthermore, the process is, as shown below, compatible with standard SMT process. The material can be stencil printed or dispensed. Once the component is pick and placed, a magnetic pallet is used to align the particles before sending the film through a curing step (batch over, reflow, vertical oven)
This is an interesting process. It of course lacks the self alignment properties of solder. The ptich is also currenly limited to 100um, which is too wide for many ICs.
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