Design & Engineering Considerations
The minimum hole size and line width achievable through photochemical etching depend fundamentally on the thickness of the material being processed, with the finest features possible in ultra-thin foils measuring as small as 0.001 to 0.002 inches (25 to 50 micrometers). These extraordinarily fine dimensions represent some of the smallest features producible in any metal fabrication process, enabling applications at the boundary between conventional manufacturing and microfabrication. However, achieving such fine features requires working with correspondingly thin materials, as the practical rule of thumb states that minimum hole diameter and line width should approximately equal the material thickness, typically sizing holes at 110% of thickness or greater to ensure reliable, repeatable production.
Understanding the relationship between material thickness and minimum feature size is essential for designers seeking to optimize parts for photochemical etching. This relationship is not an arbitrary process limitation but rather a fundamental consequence of how chemical etching removes material, reflecting the isotropic nature of the etch process where material dissolves equally in all directions, creating lateral undercut beneath the photoresist mask as the etchant penetrates vertically through the material thickness.
At the extreme lower end of the material thickness spectrum, photochemical etching demonstrates remarkable capability to produce incredibly fine holes and narrow lines. In 0.001 inch (25 micrometer) foils, holes as small as 0.001 inch diameter can be reliably etched, creating openings barely visible to the naked eye. At 0.002 inches (50 micrometers), holes of 0.002 inch diameter represent the practical minimum. These dimensions are comparable to fine human hairs, yet photochemical etching produces them with clean edges, precise positioning, and excellent repeatability.
Applications requiring such ultra-fine holes include precision filter screens where pore size in the 25 to 100 micrometer range provides specific particle retention characteristics for pharmaceutical manufacturing, water purification, or industrial filtration. Microelectronic components including lead frames, contact arrays, and interconnect structures benefit from the ability to create fine conductors and small via holes with precise spacing. Scientific instruments incorporate precision apertures and collimating screens with hole sizes that control beam characteristics for optical, x-ray, or electron beam applications. Medical devices including drug delivery systems may require microfluidic passages with dimensions measured in tens of micrometers.
Producing holes at these scales requires exceptional process control. The photoresist coating must be perfectly uniform, as any thickness variation affects exposure and development. Registration between top and bottom phototools must be maintained within micrometers. Etching parameters including temperature, concentration, spray pressure, and timing require precise control to balance adequate material removal against excessive undercutting that could enlarge holes beyond specification or close them entirely if undercut from the perimeter progressively consumes the opening.
The guideline that holes should be sized at approximately 110% of material thickness or greater provides a practical, conservative design rule that ensures reliable manufacturing with good dimensional control and acceptable yield. This recommendation accounts for the undercut that occurs during etching and provides margin for process variation while maintaining predictable results.
The reasoning behind this guideline relates directly to the mechanics of chemical etching. When etching through material from both sides simultaneously, each surface etches approximately halfway through before the two etch fronts meet. For 0.010 inch material, each side etches roughly 0.005 inches deep. With a typical etch factor of 2:1, this produces approximately 0.0025 inches of undercut from each surface. For a circular hole, undercut occurs around the entire perimeter, progressively consuming the opening from all directions as etching proceeds.
If attempting to etch a hole smaller than the material thickness, the cumulative undercut from both sides may consume much or all of the intended opening. A 0.008 inch hole in 0.010 inch material faces undercut totaling 0.005 inches (0.0025 from each side), leaving only 0.003 inch of the original 0.008 inch diameter. Process variations in etchant temperature, concentration, or timing could easily cause such marginal features to close up entirely, become oval rather than round, or vary significantly from part to part.
Sizing holes at 110% of thickness or larger provides sufficient margin that undercut consumes a manageable percentage of the feature, leaving adequate dimensional control. A 0.011 inch hole in 0.010 inch material (110% of thickness) allows the predictable 0.005 inch undercut while still producing a reliable 0.006 inch final opening. Larger holes provide even better dimensional control, as undercut represents a smaller percentage of the total diameter.
Minimum line width, meaning the width of a narrow conductor, web between openings, or thin bridge of material, follows similar principles to hole size. The line experiences undercutting from both edges as the adjacent areas are etched away. A line significantly narrower than the material thickness may etch away partially or completely, or may survive etching but emerge too thin and fragile for handling.
The 110% guideline applies to line widths as well, recommending that the narrowest webs, conductors, or bridges measure at least 110% of material thickness. For 0.005 inch material, this suggests minimum line widths of 0.0055 inch. For 0.020 inch material, lines should be at least 0.022 inch wide for reliable results.
Lines can be much longer than they are wide without particular difficulty. A 0.010 inch wide line can extend for many inches in length, creating narrow conductors, slim support structures, or elongated features. The aspect ratio of length to width can be very high, with lines 100 or even 1000 times longer than wide presenting no special challenges beyond the requirement that the width meet the minimum size guideline.
Designers seeking to minimize hole size or line width while ensuring manufacturability should consider several strategies. First, use the thinnest material that provides adequate strength, rigidity, and functionality. If the application can work with 0.005 inch material instead of 0.010 inch, the minimum hole size and line width immediately halves. Second, design holes and lines larger than the absolute minimum when possible. Features sized at 150% or 200% of material thickness are far easier to produce consistently than features at exactly 100% or 110%, and the additional size may not compromise functionality.
Third, discuss requirements with the photochemical etching manufacturer early in design. If the application genuinely requires features smaller than the standard guidelines suggest, the manufacturer can advise on feasibility, recommend process optimizations, and may suggest prototyping to validate that the desired features can be achieved reliably before committing to production. Fourth, recognize that tighter tolerances typically apply to larger features. A hole sized at 110% of thickness may have dimensional variation of ±15%, while a hole at 200% of thickness might hold ±10% or tighter.
When parts include arrays of many small holes or dense patterns of fine lines, additional considerations apply. The spacing between holes or lines becomes critical, as closely spaced features create thin webs that must survive processing. Dense patterns with minimal spacing may require thicker webs between openings to maintain structural integrity, effectively limiting how closely features can be packed. Very high open area percentages, where holes consume 70% or 80% of the material, create delicate structures that require careful handling.
The practical minimum hole sizes of 0.001 to 0.002 inches in appropriate thin materials, guided by the 110% of thickness rule, enable extraordinary design flexibility and precision. Understanding these relationships and designing accordingly ensures that parts leverage photochemical etching’s remarkable fine feature capabilities while maintaining reliable, cost-effective manufacturing with predictable dimensional control and excellent part-to-part consistency.
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