Epoxy Cracking in High-Temperature Electronics

Summary

A structural epoxy bond between a ceramic substrate and aluminum heat sink developed cracks after 500 thermal cycles (-40°C to 125°C). The cracks originated within the adhesive layer, characteristic of cohesive failure under cyclic stress.

Root Cause

The CTE mismatch between aluminum (23 ppm/°C) and ceramic (6-8 ppm/°C) created significant cyclic shear stress at the bond line. The rigid epoxy formulation (Tg 150°C) had insufficient elongation to accommodate the differential expansion, leading to fatigue cracking.

Solution

Replaced the rigid epoxy with a flexible filled epoxy (Henkel EA 9394) that provides 5% elongation at break versus 1% for the original. The flexible formulation maintains adequate shear strength (2,200 PSI) while accommodating thermal cycling. Passed 2,000 thermal cycles without failure.

Lessons Learned

• •Always calculate expected shear strain from CTE mismatch before selecting adhesive.
• •Flexible or toughened epoxies are essential when CTE mismatch exceeds 10 ppm/°C.
• •Thermal cycling qualification should exceed expected service life by 2x minimum.