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Stretchbale, flexible substrate compatible with SMT processes & enabling high temperature ink curing

Current substrate technologies impose severe limits on potential of stretchable or flexible hybrid electronics. This is because (a) they often limit curing temperature of conductive inks which limits conductivity levels far below bulk metal and (b) they rule out compatibility with standard SMT processes and materials such as solder reflow.

The table below is a comparison of common flexible and stretchable substrates. The most common ‘flexible’ substrate is PET, which is low cost, resistant to chemicals, and offers a good surface energy for printing of inks. It however has poor heat resistance, generally making it incompatible with SMT processes and imposing temperature constraints on the curing of the ink, which can limit achieved conductivity levels.

The most common ‘stretchable’ substrate is TPU which offers excellent stretching as well as a good surface for printing, but has very intolerant of heat and humidity, and imposes even more severe constrains on ink and solder/conductive adhesive processing temperatures than PET.

Therefore, there is a need for a substrate that it flexible and stretchable and offers compatibility with SMD and higher temperature processes. Panasonic is developing such a product based on a novel patented fully cross-linked thermoset polymer system.

Below you can film stretch comparison, showing how the new thermoset substrate survives 100% stretch cycle without deformation, unlike even TPU. In the next slide, it can be seen how this substrate survives a solder float operation (1m@260C) whilst PET and TPU are fully damaged. This clearly demonstrates more compatible with standard SMT processes. Next you can see the thermal stability of the film- it maintains its elongation and tensile properties even after 1000 thermal cycles (-55 C to 125C).

To demonstrate some applications, they sintered Cu inks at 230C to form highly conducting copper inks. They also demonstrated a stretchable LED foil together with stretchable Ag inks.

It is of course relatively early stage. Cost and volume questions will need to be addressed, paste makers may need to adjust paste formulations for good printing on this substrate, printers will need to learn how to process on this substrate.

Nonetheless, this substrate is promising because it can enable more conductive pastes and SMT processes. It is not a solution looking for a problem, and clearly address a market need

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