I. List of Special Characteristics: The "Panoramic Map" of Quality Risks
The special characteristics list is the core document for an enterprise to identify the key quality points of products/processes. Its value lies in systematizing the scattered risk points to ensure that all characteristics affecting product safety, compliance, and customer satisfaction are "covered without omission". The list needs to include three types of characteristics:
Special characteristics specified by the customer: Derived from the customer's clear technical requirements (such as contract annexes, product specifications, or quality agreements), they are the "mandatory agreements" of the customer regarding the product's functions/performance (for example, the customer requires that "the waterproof rating of the new energy battery PACK must reach IP67").
Special characteristics required by laws and regulations: Specified by national/industry mandatory standards (for example, the breaking strength of automobile seat belts shall comply with GB 14166 - 2013), which are directly related to product compliance and public safety and belong to the "untouchable bottom line".
Special characteristics identified by the enterprise: "High-risk characteristics" discovered through FMEA, historical defect data, or process capability analysis (for example, the shrinkage rate of a certain injection-molded part, which once caused 15% assembly defects due to process fluctuations) are the enterprise's "active management" based on risk prevention.
II. Identification rules for special characteristics: Use symbols to convey the "control priority"
The core of identification is to enable all links to quickly understand the importance of features, and two major principles need to be followed:
Keep the customer's identification unchanged: The characteristics specified by the customer must use their original symbols (for example, if the customer uses "SC" to represent safety characteristics, the enterprise shall not change it to "★") to avoid information deviation across organizations.
Documented correspondence of enterprise - self - defined identifiers: For the characteristics identified by the enterprise on its own, self - defined symbols (such as ★ ▲) can be used. However, the corresponding relationship with customer identifiers must be clearly defined through the Comparison Table of Special Characteristic Identifiers (for example, the enterprise's ★ corresponds to the customer's SC) to ensure that internal implementation is consistent with external requirements.
III. Control methods for special characteristics: Tools serve for "risk reduction", not for form
A common misconception is that "control charts must be used for special characteristics." Control charts are tools for monitoring process variation, but they are not the only option. The method should be selected based on the "risk type" and "data attributes" of the characteristics:
Error-proofing substitution: If the failure of a characteristic can be eliminated through "100% error-proofing" (for example, an electronic ruler is used to detect the shaft diameter, and the machine stops when the deviation exceeds the tolerance), there is no need for a control chart.
Piece-by-piece inspection: If the characteristics are related to safety (such as the propellant content of airbags), 100% inspection is required to replace statistical monitoring.
Applicable scenarios of control charts: When the characteristic is measurement data (such as shaft diameter size) and the process stability needs to be monitored, control charts are effective tools.
Premise of alternative methods: If control charts are not used, two conditions need to be met - either quantitative data cannot be obtained (e.g., appearance defects are of the attribute type), or the customer gives written consent (e.g., attribute characteristics need to meet the "zero defect" requirement).
IV. APQP and Special Characteristics: The "Core Binding" of Preventive Quality
The essence of APQP (Advanced Product Quality Planning) is "risk prevention from design to production", and special characteristics are the "anchor points" of this process. If special characteristics are not identified, it's equivalent to "not knowing what to prevent"; if they are identified but not controlled, it's equivalent to "knowing the risks but taking no action". For example, a certain automotive parts factory failed to identify "brake oil pipe torque" as a special characteristic, resulting in a large - scale leakage recall. In essence, APQP failed to complete the closed - loop of "risk identification → control measures". The core signs of APQP failure are the "absence of identification" or "failure of control" of special characteristics.
V. Product characteristics and process characteristics: Correspondence between "results" and "causes"
It is necessary to clarify the boundaries of two basic concepts:
Product characteristics: The technical requirements of the final product (such as the hardness of the mobile phone screen ≥ 6H and the battery capacity ≥ 4500mAh) directly determine whether the product "meets the customer's needs".
Process characteristics: Parameters that affect product characteristics during the production process (such as injection molding temperature and welding current), which are the "prerequisites for product characteristics to meet the standards" (for example, too low injection molding temperature will cause product shrinkage, directly affecting the dimensional characteristics).
VI. Key Characteristics and Important Characteristics: The "Clear Standard" for Hierarchical Management
Special characteristics shall be divided into two levels according to the degree of risk impact and clearly marked with symbols:
Key characteristics (safety category): Characteristics that affect product safety or regulatory compliance (such as the hardness of automobile brake discs), are marked with ★. The failure of such characteristics may result in personal injury or regulatory penalties, and 100% control is required.
Important characteristics (performance/fit category): Characteristics that affect product functions or customer satisfaction (such as the clearance between the engine piston and the cylinder block) are marked with ▲. The failure of such characteristics will lead to a decline in product performance (such as insufficient power), and the control requirements need to be clearly defined in the quality plan (such as the process capability Cpk ≥ 1.33).
The essence of special characteristic management
Special characteristics are not "extra work", but rather use "risk thinking" to focus on key links - cover all risk points through checklists, convey priorities with markings, and control risks with matching methods, ultimately achieving the goal of "getting the product right the first time". The essence of the success of APQP is to "solidly manage special characteristics".