In addition to better throughput, foil construction eliminates the need for covering what will become the outer layer surface with a film etch resist. Resist would be required on an all-core construction on the outer layers to keep the copper from being etched away. In this same vein, handling scratches on this protective etch resist would cause lower yields. This is all avoided by adding foil to top and bottom after the inner layer etch process, but before pressing. Tom Coyle Field Services Engineer HADCO Corporation [log in to unmask]