New die attach adhesives enable low-stress MEMS packaging

High flexibility is one of the key requirements on die attach materials for most MEMS packages as temperature changes during the assembly process and application lead to thermo-mechanical stress as a consequence of thermal mismatch, i.e. dissimilar coefficients of thermal expansion of substrate, chip and adhesive. A distortion of the signal characteristics of the extremely stress-sensitive MEMS device may be the consequence of this thermo-mechanical stress. For the first time, newly developed adhesives provide an outstanding combination of high flexibility and high die shear strength, giving them a competitive edge over the currently used MEMS die attach adhesives. This paper describes highly flexible heat-curing adhesives on the basis of acrylates and a patented mCD chemistry with Young's modulus values down to 5 MPa (0.725 ksi) at room temperature. DMTA measurements show that temperature storage at +120 °C (+248 °F) does not cause adhesive embrittlement that could negatively affect the reliability of the MEMS package. The curing temperatures of these adhesives are extremely low down to +100 °C (+212 °F), which reduces stress development during the assembly process. In addition, the adhesives have very process-friendly properties and allow processing times of one week. The option of dual curing enables preliminary light fixation of the chip within just seconds.