Authored By:
Michael J. Renn, Ph.D., and Kurt K. Christenson, Ph.D.
Optomec, Inc.
St. Paul, MN, USA
Donald Giroux
Resin Designs, LLC
Woburn, MA, USA
Daniel Blazej, Ph.D.
Assembly Answers, LLC
Dracut, MA, USA
Summary
Traditional dispensing technologies are typically limited to feature sizes above 100 um and have limited 3D capability due to near- or direct-surface contact with the substrate. Conductive adhesives for these systems typically contain large metallic flakes which lead to inconsistent conductivity in small features such as dots. In this paper we combine a new, nanoparticle conductive adhesive with Aerosol Jet dispensing and demonstrate non-contact printing over 3D surfaces and into recesses such as through silicon vias (TSV).
The conductive adhesive is a high-solids, nanoparticle system with resistivity as low at 5 -cm at 150 degrees C cure temperatures. The adhesive has superior thermal conductivity, adhesion, and elasticity, which are desirable for fabricating heterogeneous connections, such as between IC chips and circuit boards. The material can be dispensed with dot sizes down to 25 um, which enables high density die attach. Similarly, TSV filling has been demonstrated with via diameters down to 50 um and depths of 300 um. The high-solids formulation reduces the material shrinkage during curing resulting in a dense conductive plug.
Conclusions
A new nanoflake silver epoxy has been developed for small feature, die attach applications. Using the Aerosol Jet tool, uniform dot sizes as small as 1 mil (25 um) can be dispensed. The epoxy is curable at 150 degrees C and at that temperature the die shear strength exceeds MIL-STD-883, method 2019. The electrical resistivity is 100 times greater than bulk silver with a 1 hour cure, but can go down to 2 times greater with extended curing. Laser assisted curing results in similar resistance values but with dramatically shorter processing time.
The Aerosol Jet dispensing system is capable of non-contact printing at high standoff heights. Consequently, the silver epoxy can be printed into recesses and over steps. This capability should enable various TSV and chip-on-chip configurations and support high density I/O packages.
Initially Published in the SMTA Proceedings
|
