High Aspect Ratio Blind via Reliability Limits under Sequential Thermal Cycling and Z-Axis Stress
High aspect ratio blind vias require low Z-CTE filled laminates and pulse plating to prevent target pad separation during lead-free thermal cycling.
IST Coupon
Reliability Test Vehicle
Specialized test patterns designed to evaluate the integrity of plated through holes and internal interconnects under accelerated thermal stress provide data for process qualification. An ist coupon utilizes direct current to heat the internal traces and vias of a board to a specific temperature. This method replaces traditional oil bath testing by allowing for rapid cycling and real time resistance monitoring.
It identifies latent manufacturing defects that might not appear during standard electrical continuity tests.
Circuit Configuration
Alternating layers of copper tracks and vias form a continuous electrical path. The circuit configuration of the ist coupon includes separate circuits for the via barrels and the internal junctions. This separation allows the test equipment to distinguish between a failure in the plated hole wall and a failure at the connection to an inner layer pad.
Failure Detection
Continuous monitoring of electrical resistance during each heating and cooling cycle reveals the onset of fatigue. The failure detection of the ist coupon triggers when the resistance increases by more than 10 percent from the initial baseline value. This change indicates the formation of micro-cracks in the copper plating.
The system records the number of cycles to failure. This data allows engineers to compare different laminate materials or plating parameters. Because the heating is localized, the test proceeds much faster than oven based thermal cycling.