Circuit Board Processing - Etching Overview
- Iva Leung
- Jan 14, 2024
- 2 min read
What is Etching?
In the production of printed circuit boards (PCBs), etching is a crucial step used to remove unwanted copper from the outer layer where the circuit pattern is pre-plated with a layer of tin-lead corrosion-resistant material. Through a chemical process, the unprotected non-conductive portions of copper are etched away, forming the circuit.
Depending on the process, etching is categorized into inner layer etching and outer layer etching. Inner layer etching employs acidic etching with dry or wet films serving as corrosion-resistant agents, while outer layer etching uses alkaline etching with tin-lead as the corrosion-resistant agent.
Basic Principles of Etching Reaction
Acidic Copper Chloride Etching
Developing: Use a weak alkaline solution like sodium carbonate to dissolve the unexposed portion of the dry film that hasn't undergone ultraviolet radiation, leaving the radiated portion intact.
Etching: Dissolve and corrode the exposed copper surface using an acidic copper chloride etchant solution.
Stripping: Dissolve the protective film on the circuit traces using a specific solution at a set temperature and speed.
Alkaline Etching
Stripping: Use a stripping solution to remove the photoresist film from the circuit board surface, exposing the unprocessed copper.
Etching: Utilize an etching solution to etch away the unwanted base copper, leaving behind the thickened circuit traces. Additives like accelerators promote oxidation reactions, prevent the deposition of subcopper ions, shore up against side etching, and suppress the dispersion of ammonia and copper deposition while accelerating copper etching.
New Rinse: Use an ammonia solution without copper ions to eliminate any residual chemicals on the board surface.
Through-Hole Plating: Exclusive to the gold-plating process, this step removes remaining palladium ions from non-plated through-holes, preventing gold ion deposition during gold plating.
Desoldering: Use a nitric acid solution to remove the tin-lead layer.
Four Etching Effects
Ponding Effect
During etching, a layer of liquid forms on the board's surface due to gravity, preventing new etchant from contacting the copper surface.
Channeling Effect
The adhesive nature of the etchant causes it to adhere to the circuit traces and gaps between the lines, resulting in varying etching quantities in dense and open areas.
Over-Hole Effect
Etchant flowing through holes accelerates around the holes, increasing etching speed in the vicinity of the holes.
Nozzle Oscillation Effect
For lines parallel to the nozzle oscillation direction, the etchant is easily dispersed by new etchant, resulting in faster etching. For lines perpendicular to the nozzle oscillation direction, the etchant disperses less easily, resulting in slower etching.
Common Issues and Improvements in Etching Process
Incomplete Stripping
Causes: Low solution concentration, excessive line speed, nozzle blockage.
Solutions: Adjust solution concentration, readjust speed and parameters, clear nozzle blockages promptly.
Surface Oxidation
Causes: High solution concentration, elevated temperature.
Solutions: Timely adjust solution concentration and temperature.
Incomplete Copper Etching
Causes: Excessive etching speed, composition discrepancies in the etchant, copper surface contamination, nozzle blockage, low temperature.
Solutions: Adjust etching speed, review etchant composition, handle copper surface contamination with care, clear nozzle blockages, and regulate temperature.
Excessive Copper Etching
Causes: Machine operating at a slow speed, high temperature.
Solutions: Adjust machine speed and temperature.
In conclusion, the etching process is a critical aspect of circuit board manufacturing, and understanding the principles, effects, and troubleshooting methods ensures a smoother and more efficient production process.
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