Critical Characteristics: Ensuring Product Safety and Compliance

In modern manufacturing, ensuring that every individual unit produced in series production remains fully compliant with regulatory requirements is essential. Conformity is not only a design or validation outcome, but must be consistently maintained across all stages of production and across all manufactured units.

Every product and process has features, or characteristics, that need to be achieved and consistently maintained. Failing to do so can have serious consequences. In industry, these are often called special characteristics (ISO/TS 16949, sect. 3.1.12).

Critical characteristics are those product specifications — like dimensions, functional performance requirements, or material properties — or process parameters — like rates, temperatures, or pressures — where any deviation could seriously affect compliance with regulations, product safety, or even product functionality.

Note: ISO 9001 requires a quality management system to monitor and measure product and process characteristics to ensure requirements are met. IATF 16949:2016, sect. 8.3.3.3 (an automotive standard widely applicable across industries) clarifies that organizations must establish, document, and implement processes to identify special characteristics.

Risk Filtering: Identifying What Truly Matters

Failure-related risk assessment is used to determine which product or process characteristics could negatively impact safety or regulatory compliance if they deviate. This assessment should be done at every stage of product development.

Common methods include:

• Failure Mode and Effects Analysis (FMEA)

• Fault Tree Analysis (FTA)

• Event Tree Analysis / Value Engineering

• List comparisons

• Expert teams

Not all characteristics that could impact safety need to be controlled in production. If a characteristic has already been secured during the concept phase, through robust design, or via a robust production process, it does not need further tracking. This is known as risk filtering.

Concept Filter: Designing Out Failures Early

The concept filter evaluates whether the product concept itself prevents potential failures during operation. It checks whether deviations from product specifications are sufficiently mitigated by the design concept, for example through:

• Error tolerances

• Redundancies

• Secure fallback levels (“failsafe” mechanisms)

Specifications validated at the concept stage are no longer tracked. Non-validated specifications that cannot be secured by the concept are carried forward to the design stage as potential critical characteristics.

Design Filter: Robust Design Practices

The design filter focuses on the non-validated potential critical characteristics from the concept phase. It combines this with detailed specifications, experience from previous projects, simulations, tests, and verifications.

The first step is to check whether the design ensures robustness. Robust characteristics, once validated, do not need further tracking.

If robustness cannot be achieved through design, the characteristic is forwarded to the production planning filter as a potential critical characteristic.

Production Planning Filter: Securing Critical Characteristics Before Production

The production planning filter ensures that potential critical characteristics can be reliably manufactured. This involves evaluating:

• Manufacturing feasibility

• Testability

• Risk analysis and inspection concepts

• Overall production planning

Characteristics validated at this stage are no longer tracked. If a potential critical characteristic cannot be secured through the production concept, adjustments to the production concept are considered. If that fails, the characteristic moves to the production process filter.

New potential critical characteristics may also be identified at this stage and should be communicated back to development as needed.

Production Process Filter: Ensuring Robust Manufacturing

The production process filter verifies that potential critical characteristics are controlled during actual manufacturing. Robustness can be ensured, for example, through:

• Statistical process control (SPC) with cpk ≥ 1.33

• Demonstrating insensitivity to disturbances and process fluctuations

Robust processes must be documented.

If a characteristic cannot be secured through the production process, process changes are considered. If these fail, the characteristic is confirmed as critical.

Confirmed critical characteristics, along with the methods used to check and monitor them, are included in the production control plan for full-scale production.

Key Takeaways

• Critical characteristics are a subset of product and process specifications that directly affect safety or compliance.

• They are filtered and validated step by step: Concept → Design → Production Planning → Production Process.

• Only those characteristics whose criticality cannot be mitigated are carried forward as confirmed critical characteristics.

• Proper documentation and control ensure safe, compliant, and robust production.