Optimizing Modified Semi-Additive Process Parameters for Sub-30 Micron Feature Generation
Sub-30 micron mSAP feature generation requires controlling seed layer thickness, LDI resist profiles, plating additive dynamics, and flash etch undercut.
Electroless Copper Bath
Chemical Deposition
An aqueous chemical solution deposits a thin, uniform layer of metallic copper onto the non-conductive surfaces of printed circuit board holes and vias. The use of an electroless copper bath provides the initial conductive seed layer necessary for subsequent electrolytic plating. This autocatalytic reaction relies on a delicate balance of formaldehyde, copper sulfate and sodium hydroxide.
Solution Management
Stabilizers and complexing agents are added to the mixture to prevent spontaneous copper precipitation, which would ruin the working solution. Operators must constantly monitor the temperature and chemical concentrations to maintain a steady deposition rate. Air agitation and continuous filtration prevent particulate contamination from settling on the board surface.
Automated dosing systems replenish the depleted chemistry based on the square footage of boards processed.
Quality Verification
Incomplete coverage or thin deposition in the holes can lead to void defects during the assembly soldering stage. Backlight testing of sample coupons provides a quick visual check of the deposition quality within the plated holes. If the chemical ratio is incorrect, the copper layer may peel from the laminate walls and cause electrical failures under thermal stress.
Trace analysis and periodic strip down of the plating tanks ensure consistent coating thickness. By maintaining the bath in its optimum chemistry, fabricators ensure long term reliability of high aspect ratio multilayers.