Detailed Explanation of the 24 Fields in a Control Plan

The Day the CPK Report Came Out, the Entire Room Fell Silent for Three Seconds

I still remember a while ago, the yield for a batch of new products at the factory just couldn't improve, stuck in an awkward, middling range. Quality Assurance was hounding me daily, asking what the problem was. Then, a week later, at the monthly meeting, when we presented the process CPK report, wow, the CPK value for one critical dimension was a mere 1.08! The entire room went quiet for three seconds, the air instantly freezing. The boss's face was paler than the lights in the cleanroom. At that moment, I thought, "I'm definitely going to be crucified."

Where's the Problem? Stop Relying on Intuition!

Frankly, many times we adjust parameters based on feeling, tweaking a bit here and a bit there, only to find the problem unresolved and the process even more complicated. In such situations, the Control Plan is your savior. A Control Plan might sound like some rigid Quality Assurance document, but honestly, it's essentially a "Process Operation Manual." It lists all variables in the process that "affect product quality" and then tells you "how to control them." If you're an engineer responsible for products or processes, this document is your ultimate guide. It has a total of 24 fields, each hiding clues to solving problems.

How to Actually Do It? Here's How to Use the 24 Fields

Alright, let's cut straight to the chase: how do you interpret and use these 24 fields?

Document Basic Information Section (Fields 1-5):

1. Prototype/Production/Other: Which stage is this Control Plan for? Typically, we start with "Prototype" during new product development and switch to "Production" for mass manufacturing.
2. Control Plan Number: Simply a serial number for easy file management.
3. Product/Process Name: Clearly state which product and process it is, e.g., "A100 Wafer Dicing Process."
4. Part Number: The part number of the product; the more detailed, the better.
5. Supplier/Plant: Which supplier or plant is responsible for this process segment.

Process Step and Characteristics Section (Fields 6-10):

1. Process Step Number: This is the step number from your process flow diagram, e.g., "10. Wafer Grinding."
2. Process Name/Operation Description: A detailed description of the step, e.g., "Grinding performed using a CMP machine."
3. Machine/Jigs/Tools: What equipment is used, e.g., "KLA Tencor Grinder."
4. Product Characteristics (CTQ): These are the "points most critical to the customer" in the final product. For example: wafer thickness, surface roughness.
5. Process Characteristics (CP): These are the "parameters you can adjust or monitor" during the process, e.g., grinding time, grinding pressure, flow rate. Typically, one CTQ corresponds to multiple CPs.

Control Methods and Reaction Plan Section (Fields 11-24):

1. Specification Limits (Upper/Lower): The "acceptable range" for your product or process characteristics. This is very important and directly impacts CPK.
2. Measurement Technique: What method do you use for measurement? E.g., optical microscope, AOI.
3. Sample Size/Frequency: How often is it measured? How many pieces per measurement? E.g., sample 5 pieces hourly.
4. Control Method: What method do you use for monitoring? E.g., SPC Control Chart (X-bar R Chart), patrol inspection.
5. Reaction Plan: What should you do if measurement results exceed specifications or control limits? E.g., stop production, quarantine product, notify engineer.
6. Responsible Person: Who is responsible for executing this control point? E.g., Operator, Engineer.
7. Control Plan Approval (Date): Signature, date.
8. Customer Approval (Date): Some critical processes require customer signature.
9. Revision Date: When was this Control Plan last updated?
10. Revision Number: What revision number is this?
11. Reason for Revision: Why was it changed? E.g., process improvement, customer complaint.
12. Notes: Other matters requiring explanation.
13. Customer Special Characteristics Symbol: A symbol specifically designated by the customer, indicating a critical characteristic.
14. Inspection Plan: A more detailed description of the inspection process.

So the key point is, when your CPK drops to 1.08, you need to go back and look at the "Process Characteristics (CP)" field. Are grinding time, pressure, and other parameters being effectively monitored? Is the measurement frequency sufficient? Is the reaction plan truly being executed?

The Most Common Pitfalls

The most common mistake I've seen is having a beautifully written Control Plan that is completely different from actual execution. For example, the Control Plan might state that 5 pieces are sampled hourly, but on-site, to rush production, it becomes once every two hours, or even skipped altogether. Such a "paper-only" Control Plan is utterly useless. Another pitfall is setting specifications too loosely, which might result in passing yield targets but endless customer complaints because you haven't grasped the customer's true "pain points." Frankly, a Control Plan isn't for merely fulfilling requirements; it's for "problem prevention."

One Thing You Can Do Today

Take out your process Control Plan, and for a process characteristic with a CPK below 1.33, check the "Control Method" and "Reaction Plan" fields to see if actual execution is in place.