Photochemical machining

E-Fab's screening rule uses the larger of 10% of sheet thickness and 0.0005 in (0.0127 mm). On 0.200 mm sheet, a 1.000 mm aperture screens at ±0.0200 mm. A tighter ±0.010 mm requirement needs agreed capability and inspection. Establish phototool compensation separately from etch trials. E-Fab tolerance guidance

Photochemical machining uses an imaged resist and etchant to pattern thin metal sheet. Specify the intended feature and material condition for the selected etch route. Fotofab, Design Guide to Photochemical Machining

From design to stripped part

A typical negative-working resist route is:

1. Clean the sheet: remove contamination that could prevent resist adhesion.
2. Apply photoresist: coat or laminate the selected resist onto the required surfaces.
3. Image and develop: expose selected resist regions, then remove soluble regions to reveal metal that will be attacked.
4. Etch: apply the qualified chemistry until the required through-features or partial depths are obtained.
5. Strip, rinse, and dry: remove remaining resist and prepare the part for inspection or secondary operations.

Tecan's process description uses UV-hardened resist, laser direct imaging, and spray etching on both faces. Its imaging laser defines the resist pattern; it does not cut through the sheet as a laser-cutting process would. Its page has no explicit technical issue date, so the site's current copyright year is not treated as an edition. Tecan, Chemical etching process

PEI describes top/bottom tooling registration, compensation for lateral etching, and controls including conveyor speed, chemistry, temperature, and oxidation-reduction potential. Its stripping stage is followed by deionized-water rinsing and drying. These describe its manufacturing route, not a transferable bath formulation or operating recipe. PEI, Precision Photochemical Machining Process, undated

Material and condition are part of the process definition

Specify alloy, temper, thickness and thickness tolerance, surface finish, coatings, and any required traceability. Confirm compatibility with the supplier's actual etching route. A supplier's ability to etch stainless steel does not establish that its same chemistry and settings apply to titanium, aluminium, or every nickel alloy.

Fotofab lists a broad set of etchable metals and explicitly connects material selection with thickness and feature design. Its claim that etching preserves temper should be interpreted as avoiding the cutting heat and mechanical working of alternative processes, not as proof that every delivered surface is acceptable without inspection. Fotofab design guide, material selection

Do not equate “no cutting stress” with a guarantee of final flatness. Precision Micro notes that material with high residual stress may relax depending on the design. Define free-state or restrained measurement conditions for delicate parts and resolve subsequent forming, plating, or heat treatment separately. Precision Micro, Technical Guidelines, flatness

Undercut and two-sided geometry

Etchant attacks laterally beneath resist edges as well as through the thickness. Consequently, the finished edge can be tapered or cusped; an aperture may have different dimensions at the face and within the thickness. Specify the functional opening and measurement location rather than assuming a perfectly vertical wall.

Hirai's technical page relates dimensions and tolerance to material, thickness, and side etching. It illustrates greater taper for single-sided etching and a reduced edge effect with both-sided etching. It also distinguishes pitch tolerance from etched feature-size tolerance. These are different contributors: precise image placement does not eliminate uncertainty in the dissolved edge. Hirai Seimitsu Kogyo, Photo chemical Etching Process, undated

Two-sided artwork can combine through-openings and surface recesses. “Half etch” is a process description, not an automatic promise of exactly half the stock thickness with a flat, tightly toleranced floor. State the required depth, residual thickness, side, alignment, and transition shape. Precision Micro illustrates single-sided depth etching, double-sided depth etching, shaped profiles, and multiple-depth routes requiring additional imaging. Precision Micro, profile geometries

Supplier rules are screening guidance

Published rules can help screen a design, but use the chosen supplier's confirmed limits for the actual material and geometry:

These are not equivalent specifications. E-Fab's retrieved sheet has no explicit issue date; its upload path is not evidence of publication date. The Photofab and micrometal pages are also treated as undated technical guidance. E-Fab design guide, Photofab FAQ, micrometal technology comparison

Worked example: tolerance screening is not artwork compensation

For an original hypothetical enquiry, assume 0.200 mm sheet and a 1.000 mm nominal aperture. Applying E-Fab's published general rule:

• Ten percent of thickness = 0.0200 mm.
• The absolute floor is 0.0005 × 25.4 = 0.0127 mm.
• The larger value gives a screening tolerance of ±0.0200 mm, or a nominal interval 0.980–1.020 mm.

If the functional requirement is 1.000 ±0.010 mm, that screening rule does not establish capability. Ask for a specifically agreed tolerance and inspection method; do not silently widen the drawing requirement. This calculation says nothing about the correct phototool aperture size. Compensation for undercut requires the supplier's process knowledge and trials. E-Fab, General Tolerances

Keep the product drawing separate from production artwork

Supply nominal finished geometry and identify critical features. The etcher should control the compensated manufacturing artwork, its revision, sidedness, registration, and any process allowances. Do not apply your own undocumented compensation and then ask the supplier to interpret the result as nominal part geometry. Precision Micro specifically requests scaled, continuous CAD geometry with nominal dimensions and a drawing identifying critical inspection features and surface-etched areas. Its retrieved PDF has no explicit issue date; no June 2024 edition is inferred from its upload directory. Precision Micro, Technical Guidelines, CAD data supply

Resolve whether parts are delivered loose or retained in a sheet. Tags can aid handling, registration, plating, or assembly, but their removal leaves a separate edge condition to control. Precision Micro describes protruding, recessed, and half-etched tag variants. A chemically etched edge's burr advantage does not establish the condition of an edge later cut or broken at a tag. Precision Micro, component tags

Acceptance and useful handoff

Agree on feature size and position, edge profile, minimum remaining webs, partial-etch depth, surface defects, flatness, and tag-removal limits. For screens or fluid passages, state whether acceptance concerns the smallest opening, a face opening, open area, or functional flow. Measurement must resolve the relevant edge and account for part support and optical interpretation.

Stripping and rinsing are manufacturing steps, not evidence of a particular ionic, particulate, medical, or bonding cleanliness level. Define the relevant cleanliness test, handling, and packaging when required. PEI describes inspection and application-specific packaging after processing; those capabilities do not automatically certify every etched part for every downstream use. PEI, stripping, inspection, and packaging

A useful enquiry includes controlled drawing/CAD revision, alloy and condition, thickness, quantity, critical edges and features, required secondary operations, inspection conditions, and delivery form. Confirm exceptions before accepting a supplier's general design table as the order's acceptance criteria.

Related reference

Manufacturing processes