Flexible hybrid electronics lends itself to a broad range of current and future applications, some for long term use but many only intended for use for a few days or weeks.  In either case, however, reliable performance is always a concern.  Attempts to generalize test results, from common test procedures, to cover all possible variations in use conditions may overestimate the actual life under realistic conditions by one or more orders of magnitude.  Generalization of systematics to a variety of potential applications, as well as optimization of reliability and identification of the ‘worst case’ use scenario to test for, will eventually require simulation based on a credible mechanistic picture of damage evolution under combinations of different and often continuously varying loading parameters.  The present work aims at the establishment of a general understanding of the evolution of damage but does not address the relatively few cases where thermal mismatch induced fatigue is a serious concern.  Our focus is based on combinations of mechanical loading and aging.  This is usually only a concern for so-called ‘dynamic flex’ applications in which the flex is subject to either overstress or fatigue under repeated loading. The measurement of how much a sample can be deformed once without failing is straightforward, but the assessment of a fatigue life of practical relevance is not.  Realistic use conditions for a ‘dynamic flex’ product most often involve combinations of loading in tension, bending, twisting, and perhaps compression with different amplitudes and most likely different strain rates.  Almost all use conditions are certain to involve significant dwells, either under load or unloaded.  Strong and far from trivial interactions between all these factors are commonly overlooked in simple testing.  

The present work focuses on aerosol jet printed (AJP) Ag and Cu nano-particle traces on Kapton and Upilex substrates. The traces were oven sintered at 200°C or by (830 nm CW IR laser with an 80 μm spot size and 1 W power) laser irradiation.  Printed AJP substrates were cycled in tension and bending, or adhesively bonded to surfaces and peeled off again, while monitoring the electrical resistance.  A fundamental mechanistic understanding requires the characterization of damage evolution from a relatively early stage, but high-resolution electron microscopy did not allow the identification of damage induced in cycling even after doubling or tripling of the resistance.  At that stage local cracks must thus still be distributed widely, albeit with a very high density, but the only quantitative measure of the damage remained the electrical resistance.  Measurements of resistance versus temperature offered insight into the evolution of damage.  The development of a mechanistic picture had to rely on a comprehensive study of effects of combinations of loading modes, amplitudes, dwells, and thermal treatments.  Based on experimental work, a mechanistic picture is proposed to explain these trends to serve as a basis for future modelling studies. The picture leads to recommendations of reliability test protocols and interpretations of results.  Finally, examples are also provided to understand variations in rigidity when the printed trace transitions to the top of a component or passes over a location where a chip is mounted on the opposite side of the flex. 


Assessing the Reliability of Aerosol Jet Printed Nano-Particle Traces on Flex Under Realistic Use Conditions

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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