What is quality function deployment?
Quality function deployment (QFD) makes sure that what customers care about is still driving decisions months later, when engineers choose tolerances and process engineers choose controls. It does this with a chain of matrices: the HOWs of one matrix become the WHATs of the next, and the importance weights flow along. A requirement that matters a lot to customers stays visible all the way down, and a control that cannot be traced to any customer need is easy to spot.
The four phases of QFD
| Phase | WHATs (rows) | HOWs (columns) |
|---|---|---|
| 1 Product planning | Customer needs | Design requirements |
| 2 Part deployment | Design requirements | Part characteristics |
| 3 Process planning | Part characteristics | Process parameters |
| 4 Production planning | Process parameters | Production controls |
Phase 1 is the house of quality. This tool keeps each phase to the core relationship matrix so the whole cascade fits on one page; use the house of quality tool when you also need the roof, competitor ratings and targets.
QFD example
A blood pressure monitor team deploys four customer needs. In phase 1, peak cuff pressure and measurement time (27.9% each) and measurement error (24.6%) are the critical design requirements. Phase 2 shows that the pump flow rate drives two of them and becomes the most important part characteristic (36.8%), ahead of valve deflation linearity and pressure sensor accuracy. In phase 3, pump diaphragm fit, valve orifice diameter and sensor calibration trim are the critical process parameters, and phase 4 puts the pump flow test at assembly (30.7%) and the 100% end-of-line accuracy test (25.3%) at the top of the control plan.
The golden thread runs from cuff is comfortable through peak cuff pressure, pump flow rate and pump diaphragm fit to the pump flow test. The tool also flags that LCD bonding pressure has no production control yet.
Tips
- Deploy only what is critical. Carrying every item through four phases is a lot of work; focus on the top items of each level.
- Involve the right people per phase: marketing and design for phase 1, design and suppliers for phase 2, manufacturing for 3 and 4.
- Make phase 4 your control plan input, and link it to the FMEA.
- In pharmaceutical development the same idea appears as quality by design.
How to use this Quality Function Deployment tool
- List the customer needs. Add the needs in phase 1 and rate their importance from 1 to 5.
- Add the HOWs of each phase. Click + next to the column headers of each phase to add design requirements, part characteristics, process parameters and production controls.
- Rate the relationships. Click the cells of each matrix to set ◎ 9, ○ 3 or △ 1.
- Follow the cascade. The importance of each HOW becomes its weight as a WHAT in the next phase; the top three items of each level are highlighted.
- Check the gaps. The tool flags items not deployed to the next phase or not linked to anything before, and shows the golden thread.
- Export. Export all four matrices and the priorities to PDF or Excel.
Frequently asked questions
What is quality function deployment (QFD)?
Quality function deployment is a method for carrying the voice of the customer through every stage of product development. Customer needs are translated into design requirements, then into part characteristics, process parameters and finally production controls, using a chain of matrices. Each step keeps the customer's priorities, so the controls on the shop floor protect what customers care about most.
What are the four phases of QFD?
The common four-phase model (from the American Supplier Institute) is: 1 product planning (customer needs → design requirements, the house of quality), 2 part deployment (design requirements → part characteristics), 3 process planning (part characteristics → process parameters) and 4 production planning (process parameters → production controls).
How do the weights cascade between phases?
In each phase, the importance of a HOW is the sum of (weight of each WHAT × relationship strength 9, 3 or 1), normalised to 100%. That percentage becomes the weight of the same item when it is a WHAT in the next phase. So a customer need with high importance pulls its design requirements, parts, processes and controls up the priority list.
Who invented QFD?
QFD was developed in Japan by Yoji Akao and Shigeru Mizuno in the late 1960s and first used at Mitsubishi Heavy Industries' Kobe shipyard in 1972. Toyota and its suppliers adopted it in the 1970s, and it spread to the US and Europe in the 1980s.
Do I need all four phases?
Not always. Many teams do only the house of quality. The full cascade pays off for complex products, regulated industries and when design and manufacturing are in different organisations, because it documents why each part characteristic and process control exists.
What is the golden thread?
The golden thread is the chain of strongest links from a production control back to a customer need. It shows the customer reason for a control, which is useful in design reviews, process validation and when someone proposes to remove a control.