EVT is a stage in the NPI process. EVT units are intended to test the functionality of your product against its requirements. In some sense, EVT is a “feasibility study” of the design.
EVT builds will often be the first time that a proper Form Factor Engineering Prototype device is built – one that both works like and looks like the intended product. However, it is still common to produce Non-Enclosed Devices. Some materials, such as housings, may be in short supply. NEDs are also useful for software development and EE teams, as they provide easier access to the components for debugging and measurement purposes.
True yields at the EVT stage are quite low. There will be manufacturing process errors, out-of-spec components, and other problems. This is useful, however, as the engineering team will investigate failures and improve the design and manufacturing processes to improve yield (though significant improvements may not be seen until future builds).
EVT units should meet the requirements outlined in the Product Requirements Specification before proceeding DVT. There will often be significant design changes that need to be made at this stage, requiring at least another EVT event (“EVT-2”). We have rarely worked on products that made it past EVT with a single build event.
Qualities of EVT
- EVT builds produce a small quantity of units. Of course, what “small” means varies according to company resources and product cost.
- Small may be “5-25 units”, built in 1-5 unit batch sizes
- Small may be “50-100 units”, built in 5-10 unit batch sizes
- Small may be “500-1000 units”, built in 50-100 unit batch sizes
- EVT builds often involve several configurations (e.g., distinguished by using different component vendors)
- Production-intent materials are used, however:
- Cosmetics are almost always ignored at this stage
- Production-intent materials may not be available (e.g., tooling has not been kicked off), so 3D printing, soft tooled parts, or milled parts may be used in place of dye cast or molded parts
- Testing happens, but is often a secondary concern
- Often, you’re bringing up test stations for the first time at EVT.
- Software may not yet be stable, requiring frequent software updates to address issues.
- At a minimum, test stations should be collecting data. Limits may be wide open, or selected to catch only egregious failures.
- If parametric test limits exist, you will often still pass every unit through the line even when out-of-spec.
- Units that fail in some particular way can still be useful for other teams, e.g., for software development.
- Manufacturing process steps are refined as the team works through the assembly, testing, and repair processes
Uses of EVT Units
EVT units are used for:
- Internal development
- Validation of the design
- Identification of issues that need to be fixed
- Comparing alternative configurations (e.g., different component vendors)
NPI Process Flow
- EVT follows the Proto Stage, when the team has a path forward on a design that is worth the manufacturing effort.
- The EVT stage is completed when there is at least one production-worthy product configuration that meets the requirements outlined in the Product Requirements Specification. If this has not happened, another EVT event will be scheduled, incorporating improvements from the previous design.
- Some companies will set yield targets for exiting EVT, but these will usually be quite low (e.g. 60% yield).
- After EVT is completed, the DVT stage begins.
Related Concepts
- Manufacturing Test and Manufacturing Firmware often make their debut at EVT
- Engineering Prototype
- Non-Enclosed Device
- Form Factor
References
- Hardware engineers speak in code: EVT, DVT, PVT decoded by Anna-Katrina Shedletsky
The EVT build is the first time you combine looks-like and works-like into one form factor, with production intent materials and manufacturing processes.
Purpose:
- To select the production intent design, sometimes from a build matrix of options
- To identify all of the issues that need to be fixed with that design
Typical Quantities: 100 to 1000
- Units must be fully functional and testable, made from the intended materials and with the intended manufacturing process, but may be from soft-tools (if you’re using 3D printed parts, it’s not EVT!)
- All functional test stations must be present and collecting data
Things that Go Wrong:
- A new revision of an intended design does not work after reliability testing
- Tighter than expected (or capable) tolerances are needed to meet the intended performance specifications — such as with an antenna element
- Depending on product complexity, up to ~40% of the units built may fail for a variety of functional or performance reasons and need to be analyzed
- Engineering has started the battle to get glue processes, hand-soldering, environmental seals, and other tricky steps under control
Exit Criteria: one production-worthy configuration that meets all of the product requirements for functionality, performance, and reliability
- The different engineering validation stages in a nutshell | EVT, DVT, PVT | by Chris Boucher | Medium
- Overview of the hardware product development stages: POC – EVT – DVT – PVT explained
- The objective of the EVT is to combine look-alike and work-like subsystem prototypes made of intended components to meet the functional requirements in the form factor as per your PRD (product requirements Document).
- EVT prototype quantities: 3-50 units, depending on the design complexity and BOM cost. On average, 5-12 prototypes are required to complete the EVT.
- Technologies: 3D printing, laser cut/milled PCBs, soft tooling (silicon molds), professional hardware development kits (HDK), rapidly cut/milled parts;
- Outputs / Deliverables: fully-functional prototype with key components performing as intended.
- Limitations: Prototypes delivered throughout the EVT phase may look somewhat ugly, raw and have a lack of beautiful cosmetic finish. The EVT prototype can also miss some non-key mechanical features such as handles, curves in enclosure, painting, etc.
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