Low-Cycle Fatigue of Sintered Silver in Polymeric-Substrate Printed Hybrid Electronic Assemblies Subject to Extreme Strain Induced by Mechanical Shock

The advantages of Printed Hybrid Electronic assemblies are of considerable interest to designers of electro-mechanical systems, especially for applications in extreme environments. This study considered the reliability of electrical components embedded into polymeric substrates and interconnected with printed conductors as elements of PHE assemblies subject to extreme acceleration levels experienced during mechanical shock. Passive components were recessed into cavities in injection-molded polysulfone beams by way of a unique ‘mill-and-fill’ method combining traditional subtractive milling with extrusion-based paste printing. The components were interconnected to printed silver traces using printed solder, with circuits then formed from the silver traces. The populated beam specimens were subjected to drop testing in a clamped-clamped configuration without secondary impact using an accelerated-fall drop tower with shock amplifier. Excitation acceleration levels ranged from 25,000 to 100,000 g, resulting in substrate strain magnitudes of up to 50,000 µm/m at rates up to ~1000 /s. Repeated shocks induced cracking within the sintered silver traces adjacent to the components, eventually causing component separation from the substrate. The dependence of rates of degradation and failure on acceleration level was quantified based on strain levels expected within the silver trace. Experimentally-calibrated global and local modeling was used to assess substrate response and stress levels within the traces, with plastic strain magnitudes as high as 18,000 µm/m at rates of up to ~1500 /s. Based on experiments and modeling, a low-cycle fatigue curve for sintered silver was generated.