Design & Engineering Considerations
Undercut, the lateral removal of material beneath the photoresist mask that occurs during chemical etching, represents one of the most important phenomena designers must understand when creating parts for photochemical etching. This sideways material removal happens because chemical etchants dissolve metal isotropically, meaning they attack exposed metal surfaces equally in all directions rather than exclusively in the vertical direction through the material thickness. As the etchant penetrates downward through the sheet, it simultaneously etches horizontally underneath the edges of the protective photoresist, progressively removing material from beneath the mask and effectively enlarging openings while shrinking solid features.
Understanding and properly accounting for undercut is essential for achieving accurate final part dimensions. However, in most cases, designers do not need to manually compensate for undercut in their CAD files. Instead, experienced photochemical etching manufacturers handle this compensation during the phototool creation process, adjusting artwork dimensions based on material thickness, etchant chemistry, and established process parameters to ensure the final etched parts match the specified dimensions. Designers should provide drawings showing the desired final dimensions, and the manufacturer will apply appropriate compensation to produce those results.
To understand how undercut compensation works, it helps to visualize the etching process in cross-section. When a sheet of metal coated with patterned photoresist enters the etching chamber, etchant sprays contact all exposed metal surfaces. Where the photoresist has been removed during development, the underlying metal is vulnerable to chemical attack. The etchant immediately begins dissolving metal atoms from these exposed areas, creating a depression that grows progressively deeper as etching continues.
However, the etchant does not attack only the surface directly beneath the opening in the photoresist. The chemical solution also makes contact with metal at the interface where the photoresist edge meets the metal surface. At this boundary, the etchant begins dissolving metal laterally, undermining the photoresist edge and creating a shelf or overhang where resist remains but underlying metal has been removed. This lateral etching continues throughout the entire etching cycle, advancing inward beneath the photoresist mask as the vertical etch front advances downward through the thickness.
In double-sided etching, the standard approach where etchant attacks from both top and bottom surfaces simultaneously, undercut occurs from both sides. Each surface develops its own undercut extending inward from the photoresist edge, and the cross-sectional profile through an etched opening resembles an hourglass shape. The opening is widest at the top and bottom surfaces where undercut has had full opportunity to advance beneath the photoresist, and narrowest at the midpoint of the material thickness where the two etch fronts meet.
The amount of undercut that develops for a given etch depth is quantified by the etch factor, expressed as the ratio of vertical depth etched to horizontal undercut distance. A 1:1 etch factor means that for every unit of depth etched, one unit of lateral undercut occurs. A 2:1 etch factor indicates that for every two units of vertical penetration, one unit of horizontal undercut develops. A 3:1 etch factor shows three units of depth for each unit of lateral etching, representing more directional, less undercutting behavior.
Typical etch factors for photochemical etching range from 1:1 to 3:1 depending on the specific material being etched, the etchant chemistry used, etchant temperature and concentration, spray pressure and flow rate, and other process parameters. Stainless steel with ferric chloride might achieve 2:1 to 2.5:1 etch factors under optimized conditions. Copper with cupric chloride might reach 2:1 to 3:1. Aluminum may show different etch factor characteristics depending on alloy and process conditions.
For practical compensation calculations, consider a specific example. Etching 0.020 inch thick stainless steel from both sides with a 2:1 etch factor means each surface must etch 0.010 inch deep to achieve breakthrough. With the 2:1 ratio, this 0.010 inch depth produces 0.005 inch of undercut from each surface. Since both top and bottom surfaces contribute undercut, the total lateral material removal is 0.010 inch, with 0.005 inch coming from each side.
When a designer submits a drawing specifying a 0.100 inch diameter hole in 0.020 inch material, the manufacturer understands this represents the desired final dimension after etching. Knowing the material thickness, etchant system, and typical etch factor from established process data, the manufacturer calculates the expected undercut and adjusts the phototool artwork accordingly.
If the expected total undercut is 0.010 inch as calculated above, the manufacturer creates phototools showing a 0.090 inch hole rather than 0.100 inch. During etching, as the process removes metal vertically through the 0.020 inch thickness, it simultaneously removes 0.010 inch laterally. The 0.090 inch opening in the phototool grows to 0.100 inch in the final part, matching the design specification.
This compensation applies to all feature types. Slots specified as 0.050 inch wide might appear as 0.040 inch in the phototool, growing to 0.050 inch after undercut. Solid features like webs or lands experience the opposite effect. A web specified as 0.030 inch wide must appear as 0.040 inch in the phototool, as undercut will remove 0.005 inch from each edge, reducing the final width to the specified 0.030 inch.
External part dimensions also require compensation. If the part outline should measure 2.000 inches wide in the final etched state, the phototool must show 1.990 inches, allowing 0.005 inch undercut from each edge to reach the specified 2.000 inch dimension.
The division of responsibility for undercut compensation typically follows a straightforward principle. Designers provide drawings showing final desired dimensions after etching, clearly specifying material type and thickness. All dimensions should represent the final part as it should measure after all processing is complete. Designers should not attempt to pre-compensate dimensions in their CAD files based on estimated undercut, as this creates confusion and may result in double compensation where both the designer and manufacturer adjust for the same phenomenon.
Manufacturers review the provided drawings, determine appropriate compensation based on material, thickness, and process parameters, and apply this compensation during phototool creation. The phototools incorporate all necessary dimensional adjustments to account for predictable undercut. First article inspection verifies that the compensation has been correctly applied and that parts meet dimensional specifications.
While manufacturers control undercut through established process parameters, several variables can influence the amount of undercut that develops. Material composition affects etch rate and directionality. Etchant concentration impacts how aggressively the solution attacks metal. Temperature influences reaction rates, with higher temperatures generally increasing etch rate and potentially affecting etch factor. Spray pressure and etchant flow rate determine how effectively fresh etchant reaches the etching surface and how quickly depleted solution is removed.
Time represents perhaps the most critical variable. Longer etching times allow more lateral material removal. Over-etching, where parts remain in contact with etchant after breakthrough has occurred, results in excessive undercut and enlarged features. Process monitoring and control systems track etching progress and trigger advancement to the next process stage at the optimal moment to achieve target dimensions without over-etching.
While manufacturers handle compensation, designers can make choices that minimize sensitivity to undercut and process variation. Using thinner materials when functionally acceptable reduces the total undercut that develops. Designing features larger than the minimum size makes dimensional control easier, as undercut represents a smaller percentage of the total dimension. Providing adequate tolerances on non-critical dimensions allows normal process variation without rejecting parts. Identifying truly critical dimensions that require tight control helps manufacturers focus attention where it matters most.
Understanding undercut and trusting the manufacturer to apply appropriate compensation creates a collaborative relationship where designers focus on functional requirements while manufacturers ensure those requirements are met through proper process execution and dimensional control.
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