Non Local Strain Gradient Cohesive Zone Formulation for Microvia Corner Step Debonding
Non-local strain gradient cohesive zone modeling eliminates mesh-dependent stress singularities at microvia corner steps to accurately predict reflow debonding.
Microvia Delamination
Meaning
Structural separation occurs between the base of an electroplated microvia and the underlying copper target pad. Multilayer printed circuit boards experience microvia delamination when the adhesive bond at the metallization interface fails under thermomechanical stress. This separation creates an open circuit or a high resistance connection that compromises the reliability of the electronic assembly.
Quality engineers use thermal shock tests to detect this hidden defect before boards are shipped.
Adhesion Failure
Inadequate chemical cleaning leaves organic residues or oxide films on the target pad before the plating process begins. These residues prevent the formation of a metallic bond, which directly leads to microvia delamination during subsequent heating. Tight control over the desmear and micro-etch processes is necessary to ensure clean metal-to-metal contact.
Thermal Expansion
Mismatch in the coefficient of thermal expansion between the epoxy resin and the copper via creates vertical tension. During lead-free soldering processes, the temperature rises to two hundred and sixty degrees Celsius and causes the polymer matrix to expand rapidly. This expansion pulls the microvia upward while the copper via tries to resist, resulting in microvia delamination if the bond strength is low.
The separation often closes when the board cools down, making the defect difficult to detect with standard electrical testing.
Reliability Test
Highly accelerated thermal cycling tests evaluate the durability of the plated interfaces under extreme temperatures. If microvia delamination occurs, the electrical resistance of the circuit increases permanently or fluctuates during the test. Continuous resistance monitoring during thermal testing ensures the detection of transient failures.