If you are sourcing precision-machined components for liquid cooling systems, you will encounter three acronyms: CPK, FAI, and PPAP.
Understanding these terms helps you evaluate suppliers and ensure consistent quality.
CPK: Process Capability Index
CPK measures how consistently a manufacturing process produces parts within tolerance.
What It Tells You
| CPK Value | Meaning |
|---|---|
| <1.00 | Process is not capable |
| 1.00–1.32 | Process is acceptable |
| ≥1.33 | Process is capable and stable |
A CPK of 1.33 means approximately 1 in 1,500 parts may fall outside tolerance. For critical liquid cooling components, this is the industry standard.
What to Ask
"What is your CPK on critical dimensions?"
"Do you monitor CPK during production runs?"
FAI: First Article Inspection
FAI is a full dimensional inspection performed on the first part from each production run. It verifies that the manufacturing process produces parts that meet all drawing requirements.
What It Includes
Full dimensional measurement
Material verification
Surface finish inspection
Documentation of all results
What to Ask
"Do you provide FAI reports?"
"How many parts do you inspect for FAI?"
PPAP: Production Part Approval Process
PPAP is a standardized documentation package used in automotive and high-reliability industries. It provides evidence that a supplier can produce parts that meet all requirements consistently.
What It Typically Includes
Design documentation
Process flow
FAI results
CPK data
Material certifications
Test results
What to Ask
"Do you offer PPAP-level documentation?"
"What is your standard inspection package?"
Quick Comparison
| Aspect | CPK | FAI | PPAP |
|---|---|---|---|
| What it measures | Process stability | First part compliance | Full process capability |
| When used | Throughout production | Start of production | During qualification |
| Output | Statistical value | Inspection report | Full documentation package |
| Industry standard | ≥1.33 | Per drawing | Varies |
What to Ask Your Supplier
For liquid cooling components, ask these four questions:
1. "Do you provide FAI reports with first shipments?"
FAI confirms the first parts meet your drawing. Always request it.
2. "What is your CPK on critical dimensions?"
The answer should be 1.33 or higher.
3. "Do you provide material certifications?"
Confirm material grade, sulfur content, and heat lot traceability.
4. "Can you provide PPAP-level documentation if required?"
Some applications require full traceability and process documentation.
Final Thought
CPK, FAI, and PPAP are not just paperwork. They are evidence that your supplier can deliver consistent quality.
If a supplier cannot provide CPK data or FAI reports, consider that a red flag.
Need CPK-controlled components for your cooling system? Send your drawing for a quote.
