Measurement of Coating Thickness via Micro‐resistance

Measurement of through‐hole copper in double‐sided and multilayer boards has been possible for a number of years by a micro‐resistivity method. Recent developments of this method, together with the introduction of the use of an eddy current system, have opened up new possibilities. The eddy current system enables on‐line pre‐etch measurements to be made where early reject detection greatly reduces costs. The micro‐resistance method is now coupled with automatic hole dimension sensing for simple measurement of mixed hole boards while a unique multi‐measurement technique enables a quality assessment to be given to each hole measured. Copper discontinuities, porosity, cracking, breakout and local high/low thickness areas are thus more readily detected.