Design & Engineering Considerations
Designing parts for photochemical etching requires understanding several fundamental principles that ensure manufacturability, optimize quality, and maximize the unique advantages the process offers. While photochemical etching provides remarkable design freedom compared to stamping or machining, following established design guidelines helps achieve the best results in terms of dimensional accuracy, production efficiency, and cost effectiveness. These rules are not arbitrary restrictions but rather reflect the physics and chemistry of how the etching process works, enabling designers to create parts that leverage the process’s strengths while avoiding potential pitfalls.
The most successful designs come from engineers who understand photochemical etching’s capabilities early in the development process, designing specifically for the process rather than adapting designs originally conceived for other manufacturing methods. This design-for-manufacturing approach produces parts that are easier to produce, more cost effective, and often perform better than designs created without consideration of the manufacturing process.
The single most important design rule for photochemical etching is that minimum feature dimensions should equal or exceed the material thickness. This guideline applies to holes, slots, web widths between openings, and any other dimensional feature. The relationship exists because chemical etching removes material isotropically, meaning it etches laterally beneath the photoresist mask as it etches vertically through the thickness. When etching through the full material thickness from both sides, undercut occurs from each surface, and features smaller than the material thickness become difficult to control reliably.
For a practical example, if designing a part in 0.010 inch thick material, specify holes no smaller than 0.010 inch diameter, slots no narrower than 0.010 inch, and webs between openings no thinner than 0.010 inch. Following this rule ensures predictable, repeatable results with good dimensional control. Features can certainly be larger than material thickness without any issue, and in fact, larger features often hold tighter tolerances because undercut consumes a smaller percentage of the total dimension.
While features slightly smaller than material thickness are sometimes achievable with optimized processing, they require special attention, may have reduced yield, and should be attempted only after consultation with your photochemical etching supplier. When pushing limits, prototyping becomes essential to validate that the desired features can be produced reliably before committing to production quantities.
Adequate spacing between features is crucial for maintaining structural integrity during processing and ensuring parts remain handleable throughout manufacturing. The metal between adjacent openings forms webs or bridges that must be strong enough to survive the etching process, resist stripping, and withstand subsequent handling without tearing or distorting.
As a general guideline, maintain spacing between features equal to or greater than the material thickness. For 0.020 inch material, leave at least 0.020 inch of solid metal between adjacent holes or slots. This spacing ensures adequate structural support and helps maintain dimensional control, as closely spaced features can interact during etching with overlapping undercut zones affecting final dimensions.
For parts with very high open area percentages where many openings consume most of the material, even thinner webs may be feasible, but these delicate structures require careful handling and may need to remain connected to supporting frames during processing. Dense mesh patterns with thousands of small openings separated by minimal webs represent some of the most challenging etching applications, requiring specialized expertise and handling protocols.
Understanding and compensating for undercut represents a critical aspect of designing for photochemical etching. Undercut occurs because the isotropic nature of chemical etching removes material laterally beneath the photoresist mask as it etches vertically through the thickness. The etch factor, typically ranging from 1:1 to 3:1 depending on material and process parameters, quantifies this relationship as the ratio of vertical depth etched to horizontal undercut.
For through-thickness etching from both sides, material is removed from both the top and bottom surfaces, with undercut occurring from each side. If etching 0.020 inch material with a 2:1 etch factor, each side etches approximately 0.010 inch deep, producing roughly 0.005 inch of undercut from each surface. This consumes 0.010 inch total from any feature dimension, as undercut occurs from both edges of a slot, both sides of a web, or the entire perimeter of a hole.
Experienced photochemical etching manufacturers compensate for this predictable undercut by adjusting the phototool artwork. If the design calls for a 0.100 inch wide slot in 0.020 inch material, the phototool might show a 0.090 inch slot, knowing that 0.010 inch of undercut will occur during etching to produce the desired 0.100 inch final dimension. Designers generally do not need to make these adjustments themselves but should be aware that the compensation occurs and should provide dimensions representing the desired final etched size rather than attempting to pre-compensate in the CAD file.
Internal corners, where two edges meet at an inside angle, naturally develop radii during chemical etching due to the omnidirectional nature of the etch process. The corner radius typically approximates the material thickness, as this represents the extent of undercutting. True sharp internal corners with zero radius cannot be produced through photochemical etching. Designs requiring sharp internal corners must either accept the natural radius that develops, be modified to specify acceptable radii, or use alternative manufacturing processes.
External corners on the outside perimeter of parts or features remain relatively sharp because there is no competing undercut from another direction. While some rounding occurs, external corners maintain much sharper definition than internal corners.
For slotted features, the ends of slots naturally become semicircular with radius approximately equal to half the slot width. A 0.020 inch wide slot will have rounded ends with roughly 0.010 inch radius. Designers can specify whether slot ends should be rounded or if additional material should be removed to create more rectangular slot terminations, though truly square slot ends are not achievable.
Chemical etching produces edges that taper slightly due to undercut. When etching from both sides, the edge profile becomes roughly hourglass-shaped, with the narrowest dimension at the midpoint of the material thickness and the widest dimensions at the top and bottom surfaces. This taper is gradual and predictable, typically not problematic for most applications, but designers should be aware that edges are not perfectly vertical.
The amount of taper depends on material thickness and etch factor. Thicker materials develop more pronounced taper than thin foils. For applications where edge profile is critical, designers should discuss requirements with the manufacturer to understand the expected profile and determine if the natural taper is acceptable or if additional specification or post-processing is needed.
Providing clean, accurate CAD data greatly facilitates the phototools creation process and ensures the final parts match design intent. Supply files in standard vector formats such as DXF, DWG, or STEP. Ensure all dimensions are to scale at 1:1 ratio. Clearly identify which features appear on the top surface versus the bottom surface if patterns differ. Specify material type, thickness, and quantity. Include tolerance callouts for critical dimensions and note any special requirements for finish, flatness, or other characteristics.
Avoid supplying raster images or low-resolution PDFs for manufacturing, as these lack the precision needed for accurate phototool generation. Vector files maintain precise dimensions regardless of scale and translate directly to photographic masters without loss of fidelity.
Select material thickness based on strength and rigidity requirements rather than defaulting to thicker-than-necessary gauges. Thinner materials etch faster, hold tighter tolerances, enable finer features, and cost less per part. Choose material grade and temper appropriate for the application, remembering that photochemical etching preserves whatever properties the starting material possesses. Specify surface finish requirements if appearance matters or if specific finishes improve subsequent plating or coating adhesion.
Following these design rules and working collaboratively with your photochemical etching supplier during the design phase produces parts that maximize quality, minimize cost, and fully leverage the unique capabilities of the photochemical etching process.
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