Metal chemical etching (photochemical machining) relies on a series of specialized industrial chemicals throughout the whole production workflow, covering surface cleaning, photoresist processing, core metal dissolution, resist stripping and posttreatment. Each chemical category performs a unique function. Improper chemical selection, concentration, temperature or mixing will directly lead to defects such as incomplete etching, undercutting, pinholes, poor pattern adhesion, surface roughness and part corrosion. Different base metals including stainless steel, copper alloys, titanium and special nickelbased alloys require matched etchant formulations. Understanding the function and characteristics of these chemicals is critical for stable quality control, yield improvement and wasteliquid environmental management in precision metal etching manufacturing.

1. Main Etchants (Core Dissolving Chemicals)

Etchant is the most critical chemical in the whole process, whose function is to chemically dissolve exposed unprotected metal surfaces under controlled working conditions. Etchant formula varies significantly according to workpiece metal material.

Ferric Chloride (FeCl₃)

Ferric chloride solution is the most widelyapplied generalpurpose etchant for metal chemical etching, dominating stainless steel, copper, brass, nickel and alloy etching. In industrial continuous spray etching lines, ferric chloride offers balanced etching rate, controllable isotropic sideetch performance and relatively stable chemical properties. Its etching mechanism is based on oxidationreduction reaction: ferric ions oxidize metallic elements into soluble metal ions dissolved inside the liquid system.

Concentration, temperature and specific gravity are three key control indicators. Typical working concentration ranges from 38 °Bé to 48 °Bé, working temperature commonly 4555 °C. As etching proceeds, ferric ions are continuously consumed and converted into ferrous ions, which gradually slow down etching speed. Manufacturers continuously monitor solution specific gravity and metal ion accumulation, supplement fresh liquor or perform partial solution renewal to maintain consistent etching performance.

Limitations: ferric chloride is not suitable for titanium, tantalum and certain highperformance superalloys; these metals form stable passive oxide films against standard ferric chloride etchant.

Acidbased Mixed Etchants for Special Metals

For metals that are hard to etch by ferric chloride alone, mixed acid systems are adopted, usually composed of mineral acids together with oxidizing agents.

Titanium & titanium alloy etchant: Mixed formula containing hydrofluoric acid and nitric acid. Hydrofluoric acid destroys the dense inert titaniumoxide passivation layer, while nitric acid acts as oxidizer to control uniform dissolution and prevent overcorrosion. Strict parameter control is required due to high chemical hazard.

Kovar, Invar, highnickel alloy etchant: Modified ferric chloride with hydrochloric acid additives to break stable surface passivation film and guarantee uniform etching rate.

Aluminum etching solution: Mixture of strong alkaline caustic soda together with oxidizing agents. Aluminum reacts violently with acid, so alkaline etching system is preferred for aluminum chemical etching.

2. PhotoresistRelated Chemicals

Dryfilm photoresist itself is a polymer film material, while supporting liquid chemicals complete pattern development and resist removal after etching.

Alkaline Developer Solution

The mainstream developer for dryfilm photoresist is dilute aqueous sodium carbonate solution. Function: selectively wash away unpolymerized unexposed photoresist, while leaving UVcured crosslinked photoresist firmly adhered as antietch mask. Working concentration is normally 0.8%1.2% sodium carbonate, working temperature controlled around 2832 °C.

Overhigh concentration or excessive temperature causes overdeveloping, eroding edges of cured resist and aggravating sideetch defects. Insufficient concentration results in incomplete development with residual photoresist scum covering metal surface, leading to missing etching features. During mass production, developer becomes contaminated with dissolved photoresist polymer debris, requiring regular filtration and solution replacement.

Resist Stripping Solution (Stripper)

After etching completion, stripping chemical removes fully cured photoresist mask from metal panels. Industrial hot dilute sodium hydroxide (caustic soda) aqueous solution is the most common stripper. Under elevated temperature (5060 °C), strong alkali hydrolyses crosslinked photoresist polymer structure, making it peel off from metal substrate in flake form.

Stripping bath needs filtration to filter resist fragments. Incomplete stripping leaves tiny organic residues, which may cause spot discoloration or poor performance for subsequent plating or passivation processes. Some special highresistance photoresist types need dedicated organic aminebased stripping agents.

3. Pretreatment Cleaning Chemicals

Cleaning quality determines photoresist bonding strength; oil, grease, oxide and fingerprint contamination are major sources of etching rejects.

Alkaline Degreaser

Alkaline degreasing agents are composite formulations built upon sodium hydroxide, sodium silicate, phosphate and surfaceactive surfactants. Applied via immersion or spray, they saponify cutting oil, pressing grease and antirust oil on raw metal sheets. Degreasing must be thorough; residual oil will cause photoresist delamination and pattern lifting.

Acid Pickling & Activating Agents

Pickling removes surface oxide scale, thermal oxidation film and passive layers. For stainlesssteel workpieces, dilute hydrochloric acid or sulfuric acid mixed formulations are widely used. For titanium and highalloy materials, activating agents break compact passive oxide films before lamination. After pickling, thorough water rinsing is mandatory to avoid acid carryover contaminating subsequent working tanks.

Rinsing Water

Multistage tapwater and deionizedwater rinsing stations separate each chemical process step, preventing crosscontamination when different liquors are carried from one tank into another. Deionized water is used for final rinsing to avoid mineral salt deposits on metal surface.

4. Etching Additives

Additives are auxiliary trace components added into main etchant bath, not performing primary metaldissolving function, yet greatly improving finishedpart quality.

Surface wetting agents (surfactants): Reduce surface tension of etchant liquor, assist etchant to fully wet microhole and narrowslot inner walls, eliminate gasbubble stagnation on metal surface. Bubbles adhering to workpiece surface will generate unetched pinhole defects.

Inhibitors / leveling agents: Moderately suppress excessive lateral undercut reaction, improve sidewall smoothness, reduce surface roughness, optimize uniformity of etching depth across large panels.

Dispersing agents: Disperse insoluble metalsalt precipitates and solid particles, prevent sediment from reattaching onto workpiece surface and generating blemishes.

Additive dosage must be strictly controlled. Excess addition may conversely slow etching rate or cause poor adhesion of photoresist.

5. PostTreatment Chemicals

After stripping, posttreatment chemicals improve finishedpart surface performance.

Passivation solution: For stainlesssteel components, nitricacidbased passivation liquid forms a compact chromiumoxide protective film on metal surface, enhancing corrosionresistance performance.

Bright dipping / polishing chemicals: Used when low surface roughness or bright metallic appearance is required.

Antirust agents / corrosion inhibitors: Applied for intermediate storage to prevent thin metal parts from flash rusting before plating or final assembly.

Chemical Selection Principles for Different Metals

Stainless steel (304 / 316): Ferric chloride etchant + alkaline degreaser + carbonate developer + caustic stripper, the most mature chemical system.

Copper, brass: Ferric chloride works well; lower etching temperature is adopted to restrain excessive sideetch.

Nickel and nickelcobalt alloy: Modified ferric chloride with hydrochloric acid additive to counter passivation tendency.

Titanium alloy: Hydrofluoricnitric mixed acid etchant; special pretreatment activating agents. High operational safety requirements.

Aluminum: Alkaline etching solution; acidbased etchant is generally avoided due to violent reaction.

Safety & Environmental Considerations

Almost all chemicals used in metal chemical etching belong to hazardous industrial chemicals. Etchants, strong alkalis and mixed acids are corrosive to human skin, eyes and respiratory tract. Factory operators must wear protective garments, goggles and anticorrosion gloves; ventilation systems are essential for workshop environment.

Spent etching waste liquid contains highconcentration dissolved heavymetal ions, residual acid / alkali and organic contaminants. Direct discharge is prohibited. Factories must install professional wasteliquid treatment facilities for neutralization, precipitation, heavymetal removal and filtration before compliant discharge or outsourced hazardouswaste disposal. Chemical concentration monitoring, regular liquor renewal and sludge collection are indispensable parts of stable production. Improper wasteliquid handling brings severe environmental risks.

Conclusion

Metal chemical etching is a multistep manufacturing process driven by systematic matching of industrial chemicals. The whole chemical chain includes degreasing & pickling pretreatment agents, core metaldissolving etchants, photoresist developer and stripping agents, functional etching additives and posttreatment passivation & antirust reagents. Different metal substrates demand customized etchant formula and process parameter tuning. Mastering chemical characteristics, realtime concentration monitoring and standardized safety & wasteliquid management are prerequisites for producing highquality burrfree precision etched metal components.