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  • Microspheres as bond line spacers in epoxies

    Posted on September 12th, 2011 Microsphere Expert

    Today’s electronics are demanding tighter and tighter tolerances, and the assembly of many items require holding precise spacing between parts during assembly.  The spherical shape and precise sizes available make microspheres the ideal candidate as a precision spacer in various liquid adhesives and epoxies.

    Important considerations:

    1) The actual bond line will correspond to the largest spheres, not the average size.  Narrow size ranges of a few micron ensure the proper gap is maintained.

    • If a 30um gap is required use a spacer grade size range of 27-30 micron.
    • If a 40um gap is required use a spacer grade size range of 37-40 micron.
    • If a 53um gap is required use a spacer grade size range of 50-53 micron.

    3) Functionality gained from using different materials.

    • Glass offers the best mechanical and chemical stability at a wide range of temperatures.
    • Plastic such as PMMA can be used where some deformation is desirable.
    • Metal coated glass spheres can be used where conductivity is desirable.
    • Hollow Glass spheres can be used where assembly pressures are low, and reduced thermal conductivity is desired, available as uncoated hollow glass or silver coated hollow glass spheres.

    4) The importance of sphere loading (% spheres by volume in the adhesive)

    • Theoretical maximum loading by volume for a monolayer is 61%
    • A mixture of about 5% by volume should work for most applications.
    • Narrow bond lines with high assembly forces will require higher loadings
    • Low crush strength spheres will require higher loadings.
    • Proper dispersion in the adhesive will help to minimize the loading needed.

    5) Adhesive / Epoxy selection

    • High viscosity epoxies will help maintain sphere dispersion.
    • For best results choose an adhesive that adheres to the spheres and the base material.
    • Long pot-life materials work best, as they allow excess adhesive to flow out of the bond line during assembly.

    6) Spacer Availability – Cospheric LLC stocks a wide variety of sizes and materials, and can custom produce spacer grade microspheres for your application.

    Applications for Bond Line Spacers

    Image courtesy of Nikkei Business PublicationsSpacer Grade Glass Microspheres are presently used in gas plasma displays, automotive mirrors, electronic displays, flip chip technology, filters, sporting goods equipment, calibration standards and transformer manufacturing.  Every day engineers are finding new and innovative uses for bond line spacers.  One area that has had the most publications is in die attachment in the semi-conductor industry, a particularly interesting area is in using spacers for building multi-die packages.

    From: US Patent Application 20070178666 “Integrated circuit system with wafer scale spacer system”

    “Stacking of chips with varying die sizes requires a spacer between the die when the top die is either the same size or larger than the bottom to avoid damage to its wires. Numerous spacer materials have been used, including silicon, adhesive paste with large spacer spheres or thick tape. Silicon is widely used because it fits the infrastructure and is cost effective, but it has more processing steps. Epoxy with spacer spheres requires fewer process steps, but has more epoxy bleed. Tape or film has no bleeding, but is more costly. “

    From: Nikkei Business Publications “Spacer Filled Pastes Control Bond Line”

    “Upon close examination, it was noticed that the die with a large amount of tilt showed a significant absence of spacers. These early experiments concluded that it was possible to have too few spacers under a given die. The next logical step was to determine the effect of too many spacers. A Ag-filled paste was filled with 1-mil (25.4micron) spacers to a volume percent loading of 3%. This experiment concluded that there was a 25% loss of die shear strength as a result of the high spacer loading percentage.

    The results of those experiments demonstrate that there are negative effects of having too many or too few spacers, but there exists a large “sweet spot” between these two extremes that allows for an ideal spacer loading to cover the largest range of die sizes.”

    From Electro-IQ ” Stacked Die packaging: Technology Toolbox:

    “Epoxy with spacer spheres is preferred for <100-µm-thick die, because it minimizes the overhanging span of the top of the die and enables wire bonding.”

    • Proper dispersion in the epoxy helps to minimize the loading needed.

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