Why Do People Always Fill in "Product Characteristics" and "Process Characteristics" Incorrectly in Control Plans? —— A Five-Step Method to Align the Two Columns

During a customer audit at an automotive parts factory, the auditor pointed to a control plan for a welding process and asked: Is "welding current 80±5A" a product characteristic or a process characteristic? The quality engineer couldn't answer. Upon further review, the actual product characteristic required by the drawing, "weld penetration ≥1.2mm," was incorrectly listed under the process characteristics column. Although this mistake might seem like a simple typo, it can actually disrupt the entire control chain—parameters go unmonitored, and defects can only be caught during final inspection.

1. What Exactly Do These Two Columns Distinguish?

The most easily overlooked part of a control plan is the two columns under the "Characteristics" section: product characteristics and process characteristics. They are not just two different names for the same thing but answer two distinct questions.

• Product Characteristics: The measurable attributes that the product must possess after this process is completed. This answers "what needs to be made." It follows the part and is derived from product drawings, design specifications, and DFMEA. The specifications and tolerances are those from the drawings. Dimensions, form and position, appearance, material, and performance all fall into this category.
• Process Characteristics: The process parameters that ensure the product characteristics are achieved. This answers "what ensures it." It follows the equipment, tooling, and process, and is derived from process specifications and the failure causes in PFMEA. The specifications are often the process window rather than the drawing tolerances. Current, voltage, furnace temperature, pressure, time, speed, torque, concentration, and flow rate all belong to this category.

A simple criterion: If it still exists after the part leaves the equipment, it is a product characteristic; if it only exists during the process operation, it is a process characteristic.

The reason so many people fill these columns incorrectly is partly due to the table itself. In the standard control plan, both the "Characteristics" and "Methods" columns contain the terms "product" and "process": the left column asks "what to control," and the right column asks "how to control and to what extent." Both columns share titles like "specifications and tolerances," which can easily lead to confusion when glancing over. Additionally, many companies create their control plans by copying and pasting from previous versions, changing the process but not the characteristics, thus passing down errors from generation to generation.

2. Four Common Mistakes

1. Filling Only Product Characteristics, Leaving the Process Characteristics Column Blank. The entire control plan focuses on acceptance and lacks control, leaving parameter fluctuations unmonitored and defects to be caught only during final inspection.
2. Writing Process Parameters in the Product Characteristics Column. Treating "current 80A" and "mold temperature 80℃" as product specifications, the inspector checks the product based on equipment parameters, effectively using process values to replace product quality requirements.
3. Writing Drawing Tolerances in the Process Characteristics Column. Process parameters lack their own specification window, making it impossible to determine what level of variation is abnormal, and thus impossible to write control methods.
4. Misalignment Between the Two Columns. A product characteristic lacks a corresponding process characteristic, or a process characteristic lacks a corresponding product characteristic. This breaks the link between the control plan and the PFMEA.

3. Five-Step Alignment Method

Step One: Align the Three Source Documents. Process flow diagram, PFMEA, product drawings, and specifications (including the list of special characteristics). Every row in the control plan must have a source in these three documents; any row without a source is a potential hazard.

Step Two: List Product Characteristics by Process. Only take from the drawings, design specifications, and DFMEA, copying the specifications and tolerances and noting the corresponding drawing line number. If a characteristic is identified as a special characteristic, mark it as SC or CC in the classification column, ensuring the symbols match those in the drawings and DFMEA.

Step Three: Derive Process Characteristics from PFMEA Failure Causes. For each failure cause, ask: Which process parameter determines it, and can this parameter be set and monitored? Parameters that can be set and monitored (temperature, pressure, time, current) should be written into the process characteristics column, with specifications taken from process specifications or process validation results. For parameters that cannot be directly monitored, find a substitute parameter, such as using a furnace temperature curve to represent the actual temperature of the parts inside the furnace.

Step Four: Bidirectional Pairing Check. Check forward to ensure each product characteristic is supported by at least one process characteristic, or specify why it is not needed (e.g., incoming characteristics are guaranteed by the drawing and supplier). Check backward to ensure each process characteristic points to a specific product characteristic. Any row that does not match should be either completed or removed as an unnecessary item.

Step Five: Document and On-Site Consistency Verification. The parameters in the process characteristics column must be visible, adjustable, and recorded on the equipment. The control methods (spot checks, patrols, automatic monitoring, poka-yoke alarms) and frequencies must be detailed in the work instructions and inspection forms. Before the audit, compare the drawings, PFMEA, and control plan line by line. Any inconsistencies are often the nonconformities identified in the next customer audit.

4. An Example: Welding Process

For a welding process, the drawing requires a weld penetration of ≥1.2mm, and one of the failure causes in the PFMEA is "low welding current leading to incomplete penetration."

• Product Characteristics Column: Weld penetration ≥1.2mm (source: drawing, marked CC), with inspection method "metallographic sampling inspection, 1 piece per shift."
• Process Characteristics Column: Welding current 80±5A (source: process specification, corresponding to the failure cause "low current"), with control method "welding machine parameter lock + first piece inspection per shift, alarm and stop machine if outside the window."

The two columns, placed side by side, one explains what needs to be achieved, and the other explains how to ensure it is achieved. If only the weld penetration is filled in and the current column is left blank, the control plan will only focus on acceptance—once the parameters drift, no one will know until a nonconforming product appears on the inspection table.

5. Three Questions to Ask Yourself

• Does this characteristic still exist after the part leaves the equipment? —— Determine the category.
• Does it come from the drawing specifications or from the process specifications and PFMEA failure causes? —— Determine the source.
• Is the control method product inspection or parameter monitoring? —— Determine the control method and frequency.

By asking these three questions, you won't fill the two columns incorrectly. The true value of a control plan lies not in filling out the table but in ensuring that each product characteristic corresponds to a process characteristic and that both can be traced back to their source documents.

Product characteristics answer "what is made," and process characteristics answer "what ensures it." —— Only when both columns are aligned with their sources does the control plan truly control.

Knowledge code: 8.3.2

Version: v20261004

Author: QTank QTank is dedicated to providing systematic professional knowledge, methodologies, and practical tools for quality management practitioners, helping companies continuously improve their quality capabilities.