Dynamic Four Wire Testing Essentials for Microvia Reliability Assessment
Dynamic four-wire testing isolates ambient and transient microvia resistance shifts during thermal stress to catch latent target pad defects before shipment.
Z Axis Strain
Mechanical Force
Dimensional change in the thickness direction of a substrate occurs as the material reacts to thermal or mechanical loading. This z axis strain is particularly high in organic laminates because their coefficient of thermal expansion is greater than that of copper. The force acts perpendicular to the surface of the board.
Displacement Effect
Plated through holes and microvias act as anchors that resist the expansion of the resin. The z axis strain is the primary driver for failures in high-density multi-layer boards. As the temperature rises, the resin pulls on the copper barrels and the internal pad connections.
This tension can lead to the fracture of the plating or the separation of the via from the inner layer copper. High-temperature assemblies are more susceptible to this type of failure during the reflow process.
Reliability Threshold
Material selection is the primary way to manage the impact of these forces. Designing for low z axis strain involves selecting materials with a low expansion coefficient. Laminates with a high glass transition temperature exhibit less expansion and therefore exert less pressure on the copper structures.
If the strain exceeds the elongation limit of the electrodeposited copper, the circuit will fail. Inspection after thermal cycling confirms whether the design can survive the expected life of the product.