Evaluating Step Stencil Keep out Zones for Mixed Pitch Assemblies
Step stencil keep out zones require a 10 to 1 distance ratio per mil of step height difference to prevent squeegee levitation and solder paste bridging.
Photochemical Etching
Chemical Milling
Chemical reactions remove unwanted metal from a foil or sheet based on a photographic mask during the creation of circuit traces. Photochemical etching allows for the production of complex patterns in copper, stainless steel, aluminum or nickel with high precision. This technique avoids the mechanical stresses and burrs associated with stamping or laser cutting.
It is the primary method for creating fine line circuitry on rigid and flexible printed boards.
Material Removal
Production begins with the application of a light sensitive resist over the metal surface. During photochemical etching, the exposed areas of the resist harden while the unexposed areas are washed away to reveal the underlying metal. An acid or alkaline solution then dissolves the unprotected metal to form the desired traces.
The speed of the conveyor and the concentration of the etchant determine the final conductor width.
Dimensional Control
Quality control focuses on the etch factor which describes the ratio of downward etching to lateral undercut. Successful photochemical etching results in clean edges and consistent spacing between conductors. If the chemistry becomes unbalanced, the traces may suffer from over etching which reduces the cross sectional area.
Operators monitor the pH and specific gravity of the bath to maintain the target dimensions.