Standard Microsection Inspection Procedures for Class Two and Class Three Annular Rings
Class 3 microsections require zero inner layer breakout and 0.050 mm minimum ring width, whereas Class 2 permits 90-degree breakout if conductor spacing holds.
Etch-Back Depth
Chemical Process
Multi-layer circuit boards require the controlled removal of resin from the walls of drilled holes to expose the copper pads of inner circuit layers. This removal is quantified by the etch-back depth, which measures the distance that the resin surface is recessed behind the copper edge. An optimal depth ensures that the subsequent chemical deposition can bond to the top, bottom and rear faces of the inner foil.
This configuration is often called a three-point connection, and it provides high mechanical and electrical reliability. Industrial standards typically specify a value between 0.2 and 1.0 mils for military and high-reliability aerospace products.
Electrical Connection
Inadequate chemical action can result in poor adhesion between the sputtered metal and the inner conductor. If the etch-back depth is too small, the plated-through hole may suffer from electrical open circuits when subjected to thermal stress. A three-point contact is achieved only when the recess is sufficient.
Processing Control
Excessive exposure to desmear chemistries can cause over-etching and create voids behind the copper plates. To control the etch-back depth, fabricators monitor the temperature, concentration and dwell time of the permanganate baths. This monitoring prevents the formation of weak points in the multilayer structure.