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.
Desmear Weight Loss
Material Removal Rate
Gravimetric analysis of a laminate sample before and after exposure to chemical cleaning agents provides a quantitative measure of the amount of resin extracted from the hole walls. Calculating the desmear weight loss allows a process engineer to verify that the permanganate or plasma cycle effectively removes drill debris and resin smear from the internal copper interconnects. This measurement confirms that the chemical process is aggressive enough to facilitate a reliable bond for subsequent electroless copper plating.
Process Control
Standardized coupons undergo a specific sequence of baking, weighing, chemical treatment and reweighing. The process control of the desmear weight loss requires a balance between temperature, chemical concentration and dwell time. Over-processing leads to excessive resin removal.
This creates deep voids or wedge defects at the copper interfaces.
Surface Topography
Physical changes to the resin surface create the necessary anchor points for the metallic layers. The surface topography of the desmear weight loss relates directly to the micro-roughening achieved during the etching phase. A target weight loss between 0.7 and 1.0 milligrams per square inch often indicates an ideal surface for copper adhesion.
If the value falls below this range, the electroless copper may peel away from the hole wall during soldering. If the value exceeds this range, the resulting topography becomes too rugged and may trap plating chemicals or moisture.