Photo chemical machining (PCM), also known as photo etching or chemical milling, is a precision metal fabrication process widely used in industries ranging from aerospace to electronics. It enables manufacturers to produce complex, burr-free parts from thin metals with high repeatability and tight tolerances—without introducing mechanical stresses or heat-affected zones. Despite its use in highly regulated industries, PCM is not considered a “special process” under Nadcap, the global cooperative accreditation program administered by the Performance Review Institute (PRI). Understanding why requires a closer look at how Nadcap defines special processes and what sets PCM apart.

Nadcap (National Aerospace and Defense Contractors Accreditation Program) is an industry-managed approach to conformity assessment that focuses on processes that cannot be verified entirely through final inspection or testing. Examples include heat treating, welding, non-destructive testing, chemical processing, and coating—processes in which the results depend heavily on the method, environment, and operator skill, and where defects may be hidden or impossible to correct after the fact.

In these cases, product quality must be assured through rigorous process control, traceability, and operator qualification. Nadcap accreditation ensures that suppliers meet these expectations, minimizing risk in critical aerospace and defense components.

Fields of Nadcap activities

The Nadcap program provides accreditation for special processes in the aerospace and defense industry.

These include:

• Aerospace Quality Systems (AQS)
• Aero Structure Assembly (ASA)
• Chemical Processing (CP)
• Coatings(CT)
• Composites(COMP)
• Conventional Machiningas a Special Process (CMSP)
• Elastomer Seals (SEAL)
• Electronics(ETG)
• Fluids Distribution (FLU)
• Heat Treating(HT)
• Materials Testing Laboratories (MTL)
• Measurement & Inspection (M&I)
• Metallic Materials Manufacturing (MMM)
• Nonconventional Machining and Surface Enhancement (NMSE)
• Nondestructive Testing(NDT)
• Non Metallic Materials Manufacturing (NMMM)
• Non Metallic Materials Testing (NMMT)
• Sealants(SLT)
• Welding(WLD)

PCM does not meet Nadcap’s criteria for classification as a special process for several reasons:

One of the core reasons PCM is excluded from Nadcap’s special processes list is that the results of photo chemical machining are fully inspectable using standard measurement and quality control tools. Features such as dimensions, cut quality, surface finish, and etch depth can be measured and validated after processing. This contrasts with processes like heat treating, where internal microstructural changes are not directly observable and must be inferred through destructive testing or coupons.

PCM is a highly repeatable process when properly controlled. The key variables—photoresist application, UV exposure, and etching chemistry—are managed through automated systems and standard operating procedures that reduce variability. Because the process is largely photographic and chemical in nature, it does not involve the kinds of manual intervention or operator skill that characterize Nadcap special processes like welding or brazing.

Unlike heat treating or surface enhancement processes that alter the metallurgical or mechanical properties of a part, PCM is subtractive—it removes unwanted metal without introducing stress, distortion, or hardness changes. The base material remains unchanged in its properties outside the etched areas, reducing the likelihood of hidden defects that require special oversight.

While PCM is used in aerospace applications—for example, to produce EMI/RFI shielding, fuel system components, or precision screens—its role is typically in producing lightweight or ancillary structures rather than load-bearing or mission-critical components. As such, the risk of catastrophic failure from an undetected flaw in a PCM part is generally much lower than in welded joints, thermally treated parts, or bonded structures, which Nadcap does regulate.

Although PCM does not fall under Nadcap’s special process umbrella, the process is still subject to rigorous quality assurance practices. Suppliers must meet ISO 9001 or AS9100 quality system standards and may also comply with customer-specific requirements, such as First Article Inspection (FAI), PPAP documentation, or statistical process control (SPC). Traceability of materials, documentation of process parameters, and regular equipment calibration are standard in any reputable PCM facility.

Photo chemical machining is a valuable fabrication technology in high-performance industries, offering precise, clean, and repeatable results without the need for post-processing or mechanical finishing. However, because the quality and conformance of etched parts can be fully verified through conventional inspection methods, PCM does not meet Nadcap’s definition of a special process. Still, aerospace and defense OEMs rely on experienced PCM providers with robust quality systems to ensure their components meet demanding application standards.