Avoiding Common Failure Modes in Bonding Applications

Understanding common failure mechanisms allows engineers to implement preventive measures during the design and qualification phases.

Delamination and Bond Failure

Delamination—the separation of the adhesive from one or both substrates—is the most common failure mode in bonding applications. Root causes include but are not limited to inadequate surface preparation, insufficient adhesive coverage, improper measuring/mixing, incomplete curing, and long term operating temperatures that exceed the adhesive's glass transition temperature (Tg).

Prevention strategies:

• Roughen smooth or polished metal surfaces before bonding—highly polished substrates are particularly challenging
• Clean substrates to remove oils, oxidation, and contaminants
• Select adhesives with Tg values well above the long term operating temperature
• Verify adhesive compatibility with all substrate materials in the assembly

Stress Cracking and Thermal Fatigue

Repeated thermal cycling can induce stress cracking in rigid adhesives, particularly when bonding substrates with mismatched coefficients of thermal expansion (CTE). Over time, micro-cracks propagate and compromise the mechanical integrity and overall performance.

Prevention strategies:

• For applications with aggressive thermal cycling, select flexible or toughened adhesive formulations such as silicones or toughened epoxies.
• Consider the CTE values of all materials in the assembly—mismatches create stress during temperature excursions
• Conduct accelerated thermal cycling tests during qualification, to identify potential fatigue issues

The Importance of Process Control

Once a qualified adhesive and process have been established, maintaining consistency is critical. Variations in dispensed quantity, cure conditions, or surface preparation can reintroduce failure modes that were eliminated during qualification. Document the qualified process thoroughly and implement controls to ensure adhesive quantities, cure times, and temperatures remain within specification throughout production.

Key Takeaway: Most bonding failures trace back to surface preparation, process control, or severe application conditions. Invest time in qualification testing that replicates actual service conditions and establish tight process controls to maintain consistent performance in production.

Disclaimer: This article is for informational purposes only and is not meant to be used for specification purposes.