March 13, 2014

Adhesive Bonding of Sub-miniature Electronic Devices to Dissimilar Materials: A Case Study

Our CECON Group consultants, leaders in their fields, are always pushing boundaries. In this blog, one of our chemistry consultants , a Coatings, Adhesives, and Laminates Expert, writes about a recent case study. 

Could an alternative adhesive be found which would bond two dissimilar materials for a longer service life?

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 were made of stainless steel and the mounting fixtures were glass-filled nylon. These devices are used in actual service for many years and are continuously exposed to high humidity and a temperature of ~95 °F. The goal of the project was to identify alternative adhesives that would significantly enhance bond strengths and be less susceptible to humidity. The result would be longer service life in current applications and development of new applications.

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

During routine quality control testing to measure adhesive bond strengths, bonded assemblies were exposed to elevated temperature and humidity for a pre-determined period. This was followed by mechanical testing to determine the force required to de-bond the assembly. In many applications, the minimum allowable strength is 10 newtons force.

Baseline evaluations of alternative adhesives with potential for enhanced bond strengths were an essential component of this project in its initial phase. Literature reviews and technical discussions with suppliers who specialize in adhesives for joining dissimilar materials and for applications involving severe environments were used to select candidates for the study. Special attention was given to materials used in the very demanding applications found in micro-electronics, medical devices, and dental procedures.

In its first phase, the scope of the project purposefully allowed for “added risk” in making selections of materials for bond strength enhancements that could be implemented in the very near term. The strategy was to evaluate numerous good candidates—knowing that many would be disqualified—so that near-term solutions could be identified.

Since 1985, The CECON Group has been placing experts in over 200 scientific disciplines. CECON consultants include pharmaceutical consultants, clinical trials experts, and chemistry experts.

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