The Residual Stress In The Evolution Of Mems Package And Its Associated Reliability

The thesis focuses on the stress issue induced by die attachment in MEMS (namely, Micro-Electronic-Mechanical System) packaging. With silicon piezoresistive sensor chip as the media of in-situ measurement, a series of investigation have been carried out including the studying of residual stress affected by different substrates, different adhesives, different positions of the chip attached on the substrate as well as the thermal treatment. The main results are summarized as follows:1. While the chips are attached to different kinds of substrates, the evolution trends of surface residual stress are also different. When the substrate is FR4, the surface shear stress and normal stress difference is at a low level and near zero during curing process and build up during cooling down because of the mismatch of CTEs of silicon and substrate. When the substrate is AliOs ceramic substrate, the shear stress hovers over a relatively high negative level during curing process and steps to a relatively low level during cooling down. If low residual stress and better residual stress distribution map are expected, A12O3 ceramic substrate is a better choice compared to FR4. The mean residual stress of A^Os ceramic substrate is lower than the mean residual stress of FR4 with the difference around 40%.2. While the chips are attached to substrates with different organic adhesives, the evolution trends of surface residual stress are alike. It is found during the cooling down in some local region that the normal stress difference varies from positive to negative and then turns positive again. The stress accumulates rapidly in the followed thermal treatment in this region, where the chip-on-board package tends to fail from. If low residual stress and betterresidual stress distribution map are expected, adhesives with low CTE is preferred, which could be a criteria to select organic adhesives.3. While the chips are attached to near center or near a edge of subatrate, the mean, deviation and maximum are at the almost same level; when the chip is attached near a corner, the deviation and the maximum is far larger than those of the above two positions; During thermal treatment followed ,the residual stress demonstrates a regular cycle trend when the chips are attached near center or edge of the substrate and fluctuates in a wide range when the chip is attached near a corner. Sudden increase or decrease can be found in the trend of residual stress of the latter case.4. After 20 days' storage in air at room temperature after curing, the residual stresses accumulate significantly in re-curing process and after additional curing, the residual stress stabilizes at a relatively low level. Thermal analysis of the adhesive was performed to identify the incomplete cure of the adhesive after first curing process and the penetration of moisture into the package is the main reason for sharp stress increase. This result could be utilized to lower and stabilize the residual stress induced by the adhesive attachment.5. In the simulated distribution residual stress map, the results of FE simulation is close to those measured by silicon piezoresistive sensor chip while the distribution of negative region and positive region matches with the measured map. However, the FE method has its limit, experimental measurement is the essential way to investigate the packaging related issues. Only by experimental measurement, can results accords with reality be obtained.