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Microspheres as Bond Line Spacers in Epoxies

Microspheres as Bond Line Spacers
Image Courtesy of Nikkei Business Publications

Microspheres are widely used as bond line spacers because today’s electronics are demanding tighter and tighter tolerances.  Assembly of many items requires holding precise spacing between parts.

The spherical shape and consistency of dimensions that does not require aligning the particles in a specific orientation, as well as the precision of the dimensions currently available commercially, makes microspheres the ideal candidates for us as precision bondline spacers in various liquid adhesives and epoxies.

Important Considerations for Bond Line Spacers

1) Controlling the bondline thickness

Actual bondline thickness will correspond to the diameter of the largest spheres, not the average diameter of the population of all particles. The narrower particle size distribution is always preferred, but the largest diameter particles are the ones that will actually hold the dimensions of the bond line.

When sourcing microsphere product for a specific bondline thickness:

First, select the size range with the maximum diameter of the spheres matching the desired bond line thickness.

  • 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.

Second, select the narrowest size range of particles that you can afford. In most cases it is not necessary to have a very narrow size range, as long as there is enough large particles to hold the bond.

2) Selecting bondline spacer materials

  • Glass offers the best mechanical and chemical stability at a wide range of temperatures.
  • Plastic such as poly(methyl methacrylate), also called acrylic or 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.

3) Bondline spacer 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.

4) Selection of Adhesive / Epoxy

  • 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.

5) 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

Spacer 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-um-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.