Sandia Labs FY22 Laboratory Directed Research & Development Annual Report

NEW TECHNOLOGY FOR HETEROGENEOUSLY INTEGRATED DEVICES ALLOWS FOR UNDERFILLING OF COMPLEX GEOMETRIES USING DRIVEN FLUIDS.

Underfilling is a widely used manufacturing process that helps stabilize and reinforce solder joints in electronic parts. However, heterogeneously integrated (HI) devices are challenging to underfill using standard capillary flow methods, due to large areas with narrow gaps—trenches that act as strong flow barriers, and high bump density that cause filler particle segregation. Through this LDRD project, the team developed a magnetic underfill that can be driven with multiaxial magnetic fields having key symmetries to create vorticity in the fluid, enabling it to fill narrow gaps, cross barriers in any orientation, and uniformly disperse the filler

particles. This approach was demonstrated using driven fluids consisting of epoxy and magnetic nanoparticles. A new Sandia capability was developed to create vorticity vector orbits with low frequency triaxial magnetic fields. It was discovered that significant torque density can be generated in viscous particle suspensions, enabling driven underfilling of complex geometries—a viable new technology for HI devices. Discoveries during this two-year project led to new concepts for low moisture permeation coatings now under study and generated one Technical Advance. (PI: Lauren Shea Rohwer)

(Left) Without a magnetic field, there are voids in the HI devices. (Middle) Using the phase-modulated 1+dc:2:3 magnetic field, the HI device is void-free, completely gap filled, with no filler particle segregation. (Right) A set of vorticity vector orbits computed for a phase-modulated 1+dc:2:3 field.

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LABORATORY DIRECTED RESEARCH & DEVELOPMENT