A miniature 3D stress measurement module for in-situ stress analysis of heterogeneous system in package devices

This paper presents an ultra-miniature diagnostic module capable of resolving all nine components of 3-D strain in plastic encapsulated heterogeneous system in package devices (HSIP) and details the correlation between 3-D stress simulations and measurements from the module. HSIP's present a multimaterial structure with complex three-dimensional geometry that defies easy multi-physics simulation and prognostic analysis. Accurate measurement of reliability stresses would help in analyzing HSIP reliability and provide data that can be used to verify and calibrate simulations. However, collecting 3D reliability stress data is very difficult without significantly changing the HSIP structure to incorporate measurement devices defeating the objective of characterizing the HSIP itself. This paper demonstrates the use of a miniature, drop-in, 3D stress measurement module for monitoring in-situ 3D stress in HSIP devices over time. The system comprises three miniature, off-the-shelf, three-gauge, rectangular strain rosettes that are positioned so all components of stress can be measured. The rosettes are placed at right angles to each other and are connected to a miniature time-stress measurement device (TSMD) with gauge conditioning circuitry, a 24-bit sigma-delta ADC gauge interface to a microcontroller which sends the data directly to a PC or to an onboard eeprom and on-board temperature sensor for thermal compensation of the gauges. The total size of the module is only 5 ? 5 ? 5 mm. To verify module design and to calibrate it, modules were encapsulated in "dog-bone" test samples using a widely-used epoxy encapsulant. The encapsulated modules were subjected to mechanical loading tests using an Instron mechanical tester and the results of the tests were compared with ANSYS simulations. Since the material properties of the encapsulant are known, the experimentally collected strain data could be converted into stress readings. All 9 components of stress were derived from the rosettes and the experimental data was found to match Ansys simulations closely. The module is of sufficiently small size that it can be assembled as a component in a HSIP and used for diagnostic and prognostic purposes without compromising the HSIP structure.