New MEMS die attach adhesives combining maximum flexibility and outstanding dispensability including screen printing

摘要

High flexibility is one of the key requirements imposed on die attach materials for most MEMS packages as temperature changes during assembly and application lead to thermo-mechanical stress resulting from 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. In addition to maximum flexibility, easy processing is becoming increasingly important for MEMS die attach adhesives due to increasing volume and progressing miniaturization. Thin, accurate bondlines and fillets are essential for ever smaller packages. Screen printing is one promising application process which combines high UPH and accurate dispensing patterns. However, the use of standard MEMS die attach adhesives based on silicones for new screen printing processes is challenging. Tailing and spreading of the adhesives is difficult to prevent. For the first time, newly developed adhesives provide an outstanding combination of high silicone-like flexibility and outstanding processability similar to adhesives based on epoxies, giving them a competitive edge over currently used MEMS die attach adhesives. In particular, the performance in screen printing is unique for adhesives with high flexibility. This paper describes highly flexible, heat-curing adhesives with Young's modulus values down to 5 MPa (0.73 ksi) at room temperature on the basis of mCDs which is a chemistry patented by DELO. DMTA measurements show that storage at temperatures up to +120 °C (+248 °F) does not cause adhesive embrittlement that could affect the MEMS package's reliability. 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 based on mCDs have very process-friendly properties, allowing bondline- up to 50 μm in one needle dispensing step, do not show any tailing in showerhead dispensing, and are suitable for new dispensing methods such as screen printing. Here, structure widths down to 100 μm are possible with hardly any spreading.