General Process & Basics
The minimum feature size achievable through photochemical etching is intrinsically linked to the thickness of the material being processed, with the fundamental rule of thumb stating that minimum feature dimensions should approximately equal the material thickness. In practical terms, openings, slots, holes, and web widths can be produced as small as 0.004 inches (100 micrometers) in very thin foils, while thicker materials require proportionally larger minimum features.
This relationship is not an arbitrary limitation imposed by equipment capabilities, but rather a fundamental consequence of how chemical etching removes material. Unlike mechanical cutting that can theoretically produce features much smaller than material thickness, or laser cutting that cuts with a beam diameter independent of material thickness, chemical etching operates according to principles that create this direct correlation.
Chemical etching is an isotropic process, meaning it removes material equally in all directions. When etchant contacts exposed metal, it dissolves that metal not only downward through the thickness but also laterally underneath the edges of the photoresist mask. This lateral etching, called undercut, occurs simultaneously with vertical penetration through the material thickness.
The etch factor, defined as the ratio of vertical etch depth to horizontal undercut, typically ranges from 1:1 to 3:1 depending on the material, etchant chemistry, temperature, and spray pressure. A 1:1 etch factor means that for every unit of depth etched, one unit of lateral undercut occurs. A 3:1 etch factor indicates one unit of undercut for every three units of depth, representing more directional etching.
When etching through the full thickness from both sides simultaneously (the standard approach), each surface etches approximately halfway through before the two etch fronts meet. If you are etching 0.010 inch thick material, each side etches roughly 0.005 inches deep. With a 2:1 etch factor, this creates approximately 0.0025 inches of undercut from each edge. For narrow features like slots or webs between openings, undercut occurs from both edges, consuming 0.005 inches total of the original feature width.
Features significantly smaller than material thickness become difficult to control because the undercut from opposing edges begins to interact unpredictably, potentially causing the feature to etch away entirely or produce inconsistent results. The guideline that minimum feature size should approximately equal material thickness provides a conservative, reliable design rule that maintains good process control and part-to-part consistency.
At the extreme lower end of material thickness, photochemical etching demonstrates remarkable capability. In 0.001 inch (25 micrometer) foils, features as small as 0.001 inch can be reliably produced. In 0.002 inch material, 0.002 inch features are achievable. At 0.004 inches (100 micrometers), this represents incredibly fine detail by any manufacturing standard.
These ultra-fine features enable applications at the boundary between conventional manufacturing and microfabrication. Precision filter screens with pore sizes in the 100 to 200 micrometer range provide highly specific particle retention. Microelectronic components with conductor widths measured in fractions of a millimeter enable dense circuit layouts. Precision apertures for optical, laser, and x-ray applications require exact opening dimensions that photochemical etching delivers.
Working at these scales requires exceptional process control. Material handling becomes critical as foils become more delicate. Photoresist coating must be perfectly uniform. Registration between phototools must be maintained within micrometers. Etching parameters must be precisely controlled to balance adequate material removal against excessive undercutting.
As material thickness increases, minimum feature sizes scale proportionally. For 0.005 inch material, commonly used in contact springs and lead frames, minimum feature sizes of 0.005 inches are standard. At 0.010 inches, a very common thickness for encoder discs and electronic components, the 0.010 inch minimum feature size still represents quite fine detail. Moving to 0.020 inches for brackets and structural components, the 0.020 inch minimum feature size allows substantial design flexibility. At 0.040 inches, the upper end of standard photochemical etching, features of approximately 0.040 inches remain quite small at just over one millimeter.
The minimum feature size guideline applies to different geometric elements with slight variations. Holes and circular openings with diameter equal to material thickness are generally reliable. Holes smaller than material thickness become increasingly challenging as undercut progressively consumes the opening.
Slots follow similar principles, with width being the critical parameter. Slot width should meet or exceed material thickness for reliable production, though length can be much greater without difficulty.
Web widths, the distance between adjacent openings, must be wide enough to withstand undercutting from both sides. Webs significantly narrower than material thickness may etch away partially or completely. This consideration is particularly important in mesh patterns where maintaining adequate web thickness preserves structural integrity.
Internal corners naturally radius during etching due to the isotropic nature of the process. The corner radius typically approximates the material thickness. True sharp internal corners cannot be produced through photochemical etching.
Isolated features, such as small islands of material surrounded by etched area, experience undercutting around their entire perimeter. Islands should generally be at least twice the material thickness in diameter to ensure adequate retention.
While the guideline provides a conservative design rule, several strategies can help achieve smaller features. Optimizing etch chemistry and process parameters can improve the etch factor, reducing lateral undercut. Using thinner material when functionally acceptable represents the most straightforward approach. Partial etching techniques can create surface features smaller than through-thickness limitations would allow. Hybrid approaches combining photochemical etching with other processes can extend capabilities, though they add complexity and cost.
Always specify minimum feature sizes equal to or greater than material thickness when possible. Maintain adequate spacing between features, ideally equal to or greater than material thickness. Consider feature aspect ratios, as extremely high ratios may experience etch uniformity variations. Provide clear documentation of critical dimensions to help manufacturers focus inspection efforts appropriately.
The relationship between material thickness and minimum feature size represents a fundamental characteristic of photochemical etching. Within this framework, the process offers remarkable capability to produce extraordinarily fine features, intricate patterns, and complex geometries across diverse applications and industries.
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