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64 The PCB Magazine • March 2017 thermosetting resins increases temperature re- sistance and durability issues, other challenges remain. These PTFs must be compatible with the substrate they are applied to. The PTF needs to adhere well to film and the film needs to be capable of withstanding the curing regime of the thermosetting resin (Figure 3). Circuits made with polymers (regardless of thermoplas- tic or thermoset) are orders of magnitude less conductive than those made of solid metals like copper. Additionally, the volume fraction of the resin required for a conductive paste to achieve cohesive and adhesive integrity generally pre- vents soldering, which means that these circuits remain limited to interconnection via mechani- cal connectors or conductive adhesives. One option for creating higher conductiv- ity circuits using additive processes like screen printing or inkjet printing is the use of sinter- able metal technology. Sintering is the process of using heat to fuse discrete metal particles (like powders and flakes) into solid mass with- out melting it to the point of liquefaction. De- pending on the formulation, silver, copper and other metal powders may be sintered with heat or high energy light sources. In terms of bulk conductivity, the sintered metal pastes tend to fall between their bulk metal counterparts and PTFs made with particles of the same metal. Nanoparticle technology is being incorporated into some sintered paste formulas. Nanoparti- cles behave very differently than larger particles of the same materials. For example, nano-scale metal particles can significantly reduce the amount of heat required for sintering to occur. In addition to improved conductivity, circuits formed by sintering can often withstand elevat- ed assembly and end use temperatures, and in some cases, they may be soldered. The thermo- setting stretchable film from Panasonic noted above has been successfully metalized with sin- tered conductive pastes. While these conduc- tors formed in this manner are not stretchable, they can be used to selectively create islands of rigidity and solderable areas for attaching com- ponents via conventional reflow solder paste processing. Creating these types of features on stretchable films may open the door for high volume assembly with existing PCBA equip- ment and materials. Some clever work has been done creating stretchable conductor structures with bulk met- als. One approach involves creating metal struc- tures on the surface of pre-stretched elastomers. When the substrate is relaxed, the metal buckles into wave structures as the substrate contracts. This type of structure can then be repeatedly stretched and relaxed in the pre-stretch direc- STRETCHING BEYOND FLEX Figure 3: Panasonic developmental thermosetting materials: stretchable PTF circuits created on stretchable film.