Form, Fit, Function (FFF)
Definition
Form, fit, and function (FFF) are the three criteria for judging whether one component can replace another. Form is the physical size and shape, fit is how the part interfaces and mounts, and function is its performance and electrical or mechanical behavior. A replacement that matches all three is a true drop-in.
Why it matters
FFF is the test behind every cross-reference and second source. When a part reaches End of Life, an FFF-equivalent alternate is the cleanest fix, because it drops into the existing design without a redesign or requalification. A candidate that matches function but differs in form or fit may still be usable with modifications or a formal qualification, which is why FFF comparison, datasheet against datasheet, is the first step in evaluating any alternate.
Related terms
- Cross-Reference (Alternate Part): judged by form, fit, and function.
- Second Source: qualified against FFF criteria.
3E Technology and the guide on qualifying a second-source supplier help you find and evaluate FFF-equivalent parts. Start a search.
Frequently Asked Questions
What does form, fit, function mean?+
Form, fit, and function (FFF) are the three criteria used to judge whether one part can replace another. Form is the physical size and shape, fit is how it interfaces and mounts, and function is its performance and behavior. A replacement that matches all three is a drop-in and generally needs no redesign.
Why does form, fit, function matter for obsolescence?+
When a part goes obsolete, an FFF-equivalent replacement is the ideal fix because it slots into the existing design without changes. If a candidate matches function but differs in form or fit, it may still work with modifications or requalification, which is where a formal second-source qualification comes in.
Related Resources
Cross-Reference (Alternate Part)
A cross-reference is a functionally equivalent replacement for a component. Here's why cross-referencing is often the fastest fix for an obsolete part.
Second Source
A second source is an alternative qualified supplier or manufacturer for a part. Here's why second sourcing protects against shortages and single-supplier risk.
EOL (End of Life)
EOL, or End of Life, is a manufacturer's notice that a component will stop being produced. Here's what it means for sourcing and how to respond.
How to Qualify a Second-Source Supplier
A practical workflow for identifying, evaluating, and qualifying an alternate supplier — from candidate identification through vendor audit and approval.
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