Connection Fracture
An electrical open circuit occurs when the copper plating within a high-density interconnect separates from its target landing pad. This separation, known as microvia failure, occurs primarily during thermal stress from assembly reflow or operational temperature swings. Because the separation can close again when the board cools, it often produces intermittent faults that escape standard testing.
The issue is highly critical in aerospace and automotive circuit boards where reliability is mandatory.
Stress Concentrator
The geometry of these ultra-small holes creates natural focal points for mechanical and thermal stress. During the manufacturing process, poor desmear steps leave a thin layer of resin debris at the bottom of the drilled hole, weakening the bond between the plated copper and the target pad. When the board undergoes assembly, the immense heat causes the laminate to expand much faster than the copper via.
The mismatch in expansion coefficients creates immense stress that pulls the via away from the pad. Even a tiny defect in the copper plating thickness can act as a starting point for a crack to spread rapidly through the metal.
Thermal Cycle
Detecting these structural flaws requires subjecting the manufactured boards to repetitive temperature fluctuations during reliability testing. Testing units measure the resistance of daisy-chained vias continuously while cycling the boards between extreme cold and heat. If the resistance spikes during the heating phase, it proves the existence of a hidden fracture.
The test must be conducted before boards are approved for harsh environment use.
