Microspheres as bond line spacers in epoxies

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.

• Spacer Grade Solid Glass Spheres, narrow 3um ranges, >95% in range, 22-53 micron
• Silver Coated Solid Glass Spheres, >90% in range, 22-110 micron sizes
• Soda Lime Glass Spheres, >90% in range, 27 micron to 3.4 mm sizes
• Solid PMMA Spheres, >90% in range, 20-150um sizes

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. Continue reading “Microspheres as bond line spacers in epoxies” »

High Density Glass Microspheres

Barium Titanate Glass microspheres are high density solid glass spheres.  At a Density of 4.5g/cc these solid glass spheres can be used for many scientific applications where high density and optical clarity is needed.  In addition to high density they also offer an index of refraction of more than 1.9.

Bulk quantities are available in 30-100um and 0.3-1.0mm size ranges.

For experiments requiring the highest precision the beads can be purchased in classified grades offering >90% of the spheres within a specific size range.  Standard Sizes include the narrow ranges below.

38-45um, 45-53um, 53-63um, 63-75um, 75-90um, 90-106um, 0.25mm-0.3mm, 0.3mm-0.35mm, 0.36mm-0.42mm, 0.43mm-0.5mm, 0.5mm-0.6mm, 0.6mm-0.7mm, 0.71mm to 0.85mm, and 0.85mm to 1.0mm.

Product pricing and availability can be found under Cospheric’s BTGMS (barium titanate glass microspheres)

BioCompatability of Metal Coated Spheres

For those scientists who are looking to use silver coated materials such as silver coated microspheres in biomedical applications, it is important to understand whether they are bio-compatable.  A selection of abstracts and article references related to the biocompatability of silver follow:

The Biocompatibility of Silver²

The experiments reported have referred to some of the characteristics of the biocompatibility of Ag. Silver has been shown to display interactions with albumin, as an example of a plasma protein, quite different from those of most metals. Such studies shed further light on the complex issue of protein adsorption on biomaterials. It has also been demonstrated that Ag at concentrations < 1 ppm exerts a considerable influence on the activity of lactate dehydrogenase, this effect being reversed in the presence of albumin. A significant but transient increase in blood levels of Ag following intramuscular implantation of the metal has been observed. This is not reflected in any raised urine level. It is proposed that the richly vascular tissue immediately surrounding the implant in the acute phase of the response gives rise to the transient increase, but a subsequent decrease in vascularity reduces this possibility. It appears that Ag released from implants following this initial period substantially remains in the local area.²

Lack of toxicologocial side-effects in silver-coated megaprostheses in humans¹

Deep infection of megaprostheses remains a serious complication in orthopedic tumor surgery. Furthermore, reinfection gets a raising problem in revision surgery of patients suffering from infections associated with primary endoprosthetic replacement of the knee and hip joint. These patients will need many revision surgeries and in some cases even an amputation is inevitable. Silver-coated medical devices proved their effectiveness on reducing infections, but toxic side-effects concerning some silver applications have been described as well. Our study reports about a silver-coated megaprosthesis for the first time and can exclude side-effects of silver-coated orthopedic implants in humans. The silver-levels in the blood did not exceed 56.4 parts per billion (ppb) and can be considered as non-toxic. Additionally we could exclude significant changes in liver and kidney functions measured by laboratory values. Histopathologic examination of the periprosthetic environment in two patients showed no signs of foreign body granulomas or chronic inflammation, despite distant effective silver concentrations up to 1626 ppb directly related to the prosthetic surface. In conclusion the silver-coated megaprosthesis allowed a release of silver without showing any local or systemic side-effects.¹

Specific Article References for the biocompatability of silver are below: See the References

Patent Review: Process for forming hollow glass spheres up to 5mm

Hollow glass microspheres are currently commercially available in sizes of up to 0.2mm, but not larger. William Mathews et. al offer a potential solution for producing larger hollow glass microspheres in patent 3,838,998.   They present a method which would enable the production of hollow glass spheres in sizes up to 5mm in diameter.   As can be seen in the details of the patent, the process is quite complex, and seems to only offer pilot production capability, which would reason why we do not currently see hollow glass spheres in the 0.2-5mm range.

US Patent: 3,838,998

PROCESS FOR FORMING HOLLOW GLASS MICRO-SPHERES FROM ADMIXED HIGH AND LOW TEMPERATURE GLASS FORMERS

Abstract:

A process for forming hollow glass micro-spheres with walls of controllably variable thickness in a size range of 50 to 5,000 microns, embodying (1) preparation of a water slurry of finely particulated, high temperature and low temperature glass formers; (2) prilling the slurry in a vertical spray drying tower; (3) separating and supporting the individual prilled feed material; (4) heating the feed material to glassification of the high temperature glass former while maintaining appropriate geometry and shell thickness and (5) cooling the finished product. The high temperature glass former is preferably a naturally occurring soda feldspar. The process is particularly adapted to form thicker walled micro-spheres of larger size and high quality.

Advantages of Borosilicate Glass Microspheres in 0.03mm to 0.2mm sizing

BSGMS 63-75 micron at 40x magnification

High quality glass microspheres for research and development are always in high demand.  In an effort to better serve scientists Cospheric recently added a complete line of high quality borosilicate microspheres, and microbeads.

Borosilicate glass offers  advantages over standard soda lime glass microbeads.

• Roundness is >90%   (Soda lime >85%)
• Density is the lowest of most glasses at 2.2g/cc  (Soda Lime 2.5g/cc)
• Very low thermal expansion coefficient  32×10^-7/°C   (for 30-300°C)   (Soda Lime:  90×10^-7/°C)

The high roundness, and low thermal expansion make borosilicate spheres an excellent candidate for use as spacers in epoxy bond lines, or other applications which require stability over a wide temperature range.

Borosilicate glass spheres are now offered in narrow size ranges from 0.03mm to 0.2mm with greater than 90% of the particles in range.

Continue reading “Advantages of Borosilicate Glass Microspheres in 0.03mm to 0.2mm sizing” »