Photochemical Machining & Acid Metal Etching Vector Phototool Prepress Guide

Photochemical Machining (PCM)—also referred to as chemical milling, photo-etching, or photochemical etching—is an ultra-precision subtractive manufacturing process used to produce burr-free, stress-free micro-metal components. Common products include lead frames, RF/EMI shielding cans, medical implant meshes, speaker grilles, spring contacts, optical encoder discs, and luxury decorative metal inlays.

Unlike mechanical stamping or laser cutting, PCM uses chemical etchants (primarily aqueous Ferric Chloride, $FeCl_3$) to dissolve unmasked metal areas. Because the acid spray dissolves metal laterally underneath the photoresist as well as vertically through the sheet, prepress tooling engineers must mathematically compensate the vector artwork to ensure target dimensional tolerances.

1. The Geometry of Acid Undercut and Etch Factor ($EF$)

As acid attacks exposed metal, it bites into the sidewall beneath the polymerized photoresist mask. This lateral erosion is termed undercut ($U$). The ratio of downward vertical etching depth ($d$) to lateral undercut ($U$) is defined as the Etch Factor ($EF$):

EF = Vertical Etch Depth (d) / Lateral Undercut (U)
U = d / EF

In standard double-sided etching (where acid is sprayed simultaneously onto both the top and bottom faces of the metal sheet):

• Vertical depth per side: d = T / 2 (where T is the sheet thickness).
• Undercut per edge: U = T / (2 * EF).
• Because an aperture has two opposing edges, the total aperture expansion during etching is 2 * U.

2. Double-Sided "Hourglass" Cross-Section Profile

In double-sided spray etching, the two chemical etching fronts meet at the sheet midpoint. This creates an hourglass cross-section with a knife-edge waist at the center of the sheet:

• The aperture dimension at the outer sheet surface is wider than the dimension at the central waist.
• By standard engineering convention, part dimensions are measured at the narrowest metal land (or widest through-hole aperture) unless specified otherwise on the engineering drawing.
• Inside sharp corners naturally round to a radius approximately equal to 0.75 * Depth (e.g. R_min ≈ 0.38 * T for double-sided etching).

3. Half-Etching for Bend Lines and Relief Pockets

One of PCM's most powerful capabilities is half-etching (blind etching). By exposing a feature on only one side of the dual-sided phototool while keeping the opposing side solid black/opaque, the acid dissolves 50% to 60% of the metal thickness:

• Fold & Bend Channels: Half-etched V-grooves allow precision sheet metal enclosures to be bent easily by hand without press brake tooling.
• Surface Part Numbers & Logos: Fine serial numbers, logos, and calibration scales can be half-etched into the metal surface at zero additional machining cycle cost.

4. Alloy Etch Factors and Tolerances

5. Phototool CAD & Vector Requirements

Laser photoplotters print film emulsions at resolutions exceeding 25,000 to 40,000 DPI. To achieve dimensional accuracy down to ±5 microns, vector source files must adhere to strict prepress standards:

• Strict Binary 1-Bit Graphics: Never submit anti-aliased rasters (PNG/JPG). Gray gradient pixels cause partially cross-linked photoresist that blisters and creates jagged acid attack fringes.
• Tangent Closed Polylines: All part boundaries must be closed loops with zero overlapping nodes or micro-gaps.
• Micro-Tabs (Tie Bars): Etched parts require small retention tabs connecting them to the sheet border frame so they don't get lost in the conveyor wash. Vector artwork must include 0.5x-thickness break-away tabs.