Adhesive Bonding of Sub-miniature Electronic Devices to Dissimilar Materials

At issue in the 26-week project summarized here were adhesive bonds used to attach sub-miniature electronic microphones to plastic fixtures that allow the assembly to be incorporated into application-specific packaging. Specifically, the microphone cases are made of stainless steel and the mounting fixtures are glass-filled Nylon. These devices are in actual service for many years and are continuously exposed to high humidity and a temperature of ~95 °F. The specific goal of the project was to identify alternative adhesives that would significantly enhance bond strengths and would be less susceptible to warm, high humidity environments. The result would be longer service life in current applications and development of new applications.

Selections of candidate alternative adhesives were based on three technical issues: (1) the bonded assembly comprises one adhesive layer and two dissimilar interfaces that must be more resistant to temperature and humidity; (2) due to the heat sensitivity of the electronic device itself, thermal curing of an adhesive would be limited to about 150 °F; and (3) application of adhesives during the bonding process would be done manually. The two interfaces noted in item #1 are metal-to-adhesive and the Nylon-to-adhesive.

An initial task in this project was to identify technologies that have solutions to similar adhesive bonding problems. They are :

• Industrial adhesives for high strength metal-to-metal and metal-to-plastic bonding
• Dental adhesives for long-term bonding of small ceramic and metal parts to moist, porous, complex surfaces
• Medical device adhesives for bonding hypodermic needles to various types of plastic hubs
• Micro-electronics applications requiring bonding dissimilar materials with small bond areas

Physical and / or chemical treatments of substrate surfaces are methods for enhancing bond strengths. Modifications of the stainless steel and Nylon surfaces to enhance bond strengths that were considered for the project include the following :

• Grit blasting to increase surface area
• Chemical etches to increase surface area
• Physical vapor deposition (PVD) of metallic coatings to act as compliant layers
• Inorganic and organic primers to act as compliant layers
• Oxygen plasma processing to chemically modify plastic surfaces and clean metallic surfaces
• Silane coupling agents to chemically bond substrate to adhesives

Expert Analysis

Twenty-two candidate adhesives were selected. For screening, a standardized test procedure was defined that involved bonding groups of five electronic device cases to a one-inch diameter Nylon mount. The forces required to fail the bonds in shear were determined on as-fabricated and humidity-exposed samples. The photograph shows the shear test in progress.

Using the bond strength measurements coupled with microscopic examinations of fracture surfaces, ten “good” adhesives were identified of which five were identified as “superior.” The superior adhesives exhibited shear values as high as 200 newtons-force (45 pounds-force) for bond areas of only 0.016 square inch. Some of these were put into small-scale manufacturing trials as the first step in adopting them for production.

To see the resume of the expert associated with this case study, see the link below.