General Process & Basics
Photochemical etching is primarily optimized for thin to medium gauge sheet metals, with the standard production range spanning from 0.001 inches to 0.040 inches (25 micrometers to 1.0 millimeter). This range encompasses the vast majority of applications where photochemical etching provides the most significant advantages over alternative manufacturing methods. In specialized cases, the process can accommodate thicker materials up to approximately 0.080 inches (2.0 millimeters), and partial etching techniques can be applied to even thicker substrates when through-cutting is not required.
Understanding the relationship between material thickness and the photochemical etching process is essential for designers seeking to optimize their components for this manufacturing method. The thickness capabilities are not arbitrary limitations but rather reflect the fundamental physics and chemistry of how the etching process works.
The lower end of the thickness spectrum, starting at 0.001 inches (25 micrometers), represents some of the thinnest metal foils commercially available. At this extreme, photochemical etching demonstrates its remarkable capability to handle delicate materials that would be nearly impossible to process through mechanical means. These ultra-thin foils are used in applications such as precision filters, microelectronic components, gaskets, and specialized shielding where minimal thickness and weight are critical.
As thickness increases through the 0.002 to 0.010 inch range (50 to 250 micrometers), photochemical etching reaches what many consider its “sweet spot” where the process delivers optimal precision, speed, and economy. This range includes the most common applications: contact springs, flex circuits, lead frames, encoder discs, precision shims, filtration screens, and decorative grilles. The etching process removes material quickly and uniformly, tolerances remain tight, and feature resolution is excellent. Parts in this thickness range often etch in minutes rather than hours, making production efficient and cost-effective.
Moving into the 0.015 to 0.040 inch range (approximately 0.4 to 1.0 millimeters) represents the upper limit of standard production photochemical etching. Materials in this thickness range are commonly used for structural components, mounting brackets, heat sinks, stencils, and parts requiring greater rigidity than thinner gauges provide. While photochemical etching remains highly effective at these thicknesses, the etching time increases proportionally with material thickness.
The relationship between material thickness and the etching process is governed by fundamental chemical and physical principles. Chemical etchants work by dissolving metal through direct contact. In through-hole etching where material is completely removed to create openings, the etchant sprays simultaneously from both sides of the sheet, progressively dissolving metal until the two etch fronts meet in the middle.
As material thickness increases, several factors come into play. First, more metal must be dissolved to achieve breakthrough, requiring longer exposure to the etchant. Second, and more significantly, the nature of chemical etching means material is removed not only vertically through the thickness but also laterally beneath the photoresist mask. This isotropic etching creates what is known as undercut.
The etch factor, expressed as the ratio of depth etched to lateral etch, typically ranges from 1:1 to 3:1 depending on the material, etchant chemistry, and process parameters. For thicker materials, greater undercut becomes inevitable. If you are etching through 0.040 inch thick material with a 2:1 etch factor, you can expect approximately 0.020 inch of undercut from each side. This means a feature designed as 0.100 inch wide might measure 0.140 inch after etching. Experienced photochemical etching companies compensate for this by adjusting the phototool artwork, essentially “pre-shrinking” features so the final etched dimension matches your design intent.
Feature resolution is also directly related to thickness. A practical rule of thumb is that the minimum reliably achievable feature size is approximately equal to the material thickness. In 0.010 inch material, features as small as 0.010 inch are readily achievable. In 0.040 inch material, attempting features smaller than 0.040 inch becomes challenging due to undercut considerations and the difficulty of maintaining dimensional control through the full thickness.
While 0.040 inches represents the standard upper limit for routine production, photochemical etching can extend to approximately 0.080 inches (2.0 millimeters) in certain circumstances. These thicker materials require longer etching cycles, specialized process parameters, and typically result in increased undercut. However, for applications where the unique benefits of photochemical etching (no mechanical stress, no heat-affected zone, no hard tooling required, burr-free edges) outweigh the reduced precision, processing thicker materials remains viable.
Design considerations become more critical at these extended thicknesses. Features must be sized appropriately to accommodate increased undercut. Sharp internal corners are not possible; corners naturally radius to approximately the material thickness. Minimum web widths and feature spacing must increase proportionally.
An important variation involves partial or half-etching, where material is selectively removed from one or both surfaces without etching completely through the thickness. This technique dramatically expands the range of materials and applications photochemical etching can address. Partial etching can be applied to materials well beyond the 0.080 inch limit of through-etching, potentially accommodating substrates several millimeters thick.
Partial etching applications include creating stepped thickness variations, forming shallow recesses, engraving identification marks, reducing weight in specific areas, creating controlled flex zones, and forming channels or grooves. One particularly valuable application involves combining through-etching and partial etching on the same part to create complex functionality that would be extremely difficult to achieve through mechanical means.
The depth control in partial etching depends on precise timing and process monitoring. Depths can typically be controlled to within ±10 to 15% of the target depth, with tighter control possible for shallow etches in thin materials.
When designing parts for photochemical etching, selecting the optimal material thickness involves balancing multiple factors. Thinner materials offer faster etching, tighter tolerances, finer feature resolution, and lower material costs. Thicker materials provide greater structural rigidity, better load-bearing capacity, and increased durability.
The versatility of photochemical etching across this wide range of thicknesses makes the process remarkably adaptable to diverse applications across industries from microelectronics to aerospace, from medical devices to automotive systems.
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