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.
Ammonium Hydroxide Etch
Alkaline Chemistry
An alkaline chemical subtractive process removes exposed copper foil from printed circuit board substrates after pattern plating. Fabricators apply an ammonium hydroxide etch solution containing ammonium chloride and copper ammonium complexes to dissolve unwanted copper without attacking metallic tin or solder resists that shield active trace pathways. The solution operates in continuous spray chambers at controlled temperature and specific gravity ranges.
Continuous replenishment maintains chemical equilibrium while preventing copper precipitation on circuit features. This process stops at the interface of the organic substrate or underlying barrier plating.
Subtractive Mechanism
Spray pressure and chemical concentration drive the dissolution rate across outer circuit layers. Differential fluid flow between isolated conductors and dense trace arrays can alter lateral undercut rates, which narrows line widths on fine-pitch features. Cupric ions act as active oxidizers while dissolved oxygen continuously regenerates the complexing agents within the chemical bath.
Automated dosing pumps maintain chemical balance through density monitoring and pH adjustment.
Process Boundary
Etchants operating outside target specific gravity thresholds produce non-uniform conductor geometries or incomplete copper removal. Excessively high copper concentrations slow dissolution rates and promote sludge formation inside processing tanks. Lower fluid temperatures reduce reaction speed, increasing dwell time and promoting lateral copper undercut beneath the etch resist.
Controlling spray nozzle pressure prevents physical masking from spent fluid accumulation.