Bonding SMT components via inkjet printing
They describe their recently patented approach in the AIP Journal of Applied Physics in a paper titled “Flexible hybrid circuit fully inkjet-printed: Surface mount devices assembled by silver nanoparticles-based inkjet ink”. With a 40% silver weight concentration, the AgNP ink was first used to print conductive patterns based on tracks and pads (with an average thickness of about 250nm) on different substrates, before being thermally cured in a convection oven at 150°C to evaporate the organic ink solvents and to sinter the silver nanoparticles.
Then, surface mount devices of various sizes were placed on their corresponding pads of the circuit and their SMD metallic pads were bonded with the printed pads through the non-contact deposition of an appropriate AgNP ink pattern using a high resolution drop jetting printer.
The printing technology allows selective deposition, in a non-contact process, of a few picoliters of ink, doing without masks and vacuum systems. That selective deposition exploits the capillarity action (the ink wets in between the SMD and the pad) and prevents material waste.
According to the researchers, this approach uses much less material than alternative screen-printed solder-paste or epoxies and so is more cost-effective even though the ink is based on silver. After a low-temperature thermal treatment at 150°C, the coalesced silver nanoparticles yield a strong bond with 98% of interconnecting silver.
The paper reveals that the inkjet-printed AgNP bond exhibits comparable electrical and mechanical performances of benchmark assembling materials onto several inkjet-printed flexible substrates, including paper, polyimide Kapton, and rigid glass, forming a homogeneous and indiscernible contact between the SMD and the printed pads.
Such a bonding approach is roll-to-roll compatible and would be particularly suited to the manufacture of flexible hybrid electronic circuits with a high component density and requiring fast assembling at up to 6m/s.
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