Why do design reviews?
Electronics design services can be a complex undertaking over several weeks or months. Mistakes, oversights and omissions can delay project releases. They can add costs through additional prototype stages. Additionally, if not spotted until after product release, they can even cause product recalls or warranty claims. This is why an in-depth design review is highly recommended.
Familiarity with the schematic, printed circuit board or source code can quickly make the design engineer blind to these errors. They have been seen so regularly that it is very difficult to spot errors, which in some cases are obvious to a fresh pair of eyes.
Design reviews, especially those performed by a different engineer to the designer, can be invaluable in identifying issues before sending designs out for manufacture. If you are the sole engineer in a company, it is possible to review your own work to some degree. There are dangers in doing so, as you will review the design with the same assumption you used to design the electronics in the first place.
The process
Most complex tasks are made easier and more repeatable with a process. The process helps to avoid missing out review steps and, in a multi-engineer environment, helps keep review consistency. Typically for a design review, we want to check that all aspects of the design requirements have been addressed. While ensuring that the designed circuits are free from errors.
If you are new to design reviews, or if your company hasn’t previously embraced them, it’s useful to create a design review checklist. At Ignys we’ve split these reviews into:
- design & schematic
- printed circuit board, and
- manufacturing pack reviews.
This is to ensure that the schematic is correct prior to putting effort into a printed circuit board (PCB) layout. We then review the PCB layout. Then, finally, we check the manufacturing pack outputs include a correctly formatted bill of materials (BOM) to the correct design variant. The Gerber files need to include all of the copper, silkscreen, solder mask, paste mask and layer stack information for manufacturing. For a guide to Gerber files and general electronics see our brief guide to CAD for electronic PCBs.
The checklists should include everything you need to check during the design reviews. They can include part of your quality standards. Such as ensuring each sheet of the schematic has a company drawing box, page numbers, change control and is legible. More importantly, is to capture the collective prior experience of the team. This is likely to be the things they have previously got wrong which has caused a design to be reworked.
Some areas to consider adding for schematic reviews are:
- Power supply voltages
- Signal voltage level compatibility and buffering
- Connector pinouts and orientation
- New component pin numbering and PCB footprint review
- Device power consumption
- Decoupling capacitors
- ESD protection for exposed ports
- Impact of component tolerancing
- Unconnected nets and mismatched net names
Understanding the design
In order to review a design, it’s essential to first know what the design needs to do. This should be documented in a requirements specification, although it’s common for this document to not exist at all. As a starter, an overview of what should be included in the requirements specification can be seen at the bottom of this page on Specification and Feasibility. If there isn’t a requirements specification already in place now is a great time to draft one. It doesn’t need to be a lengthy time, it just needs to accurately capture the product requirements.
Print out a copy of the requirements, schematic and checklist and work through together with any available design notes. The aim is to verify that each of the required functions has been addressed. For each circuit work through the previously created checklist, and any design simulations or notes. This is to check that best practise has been followed and that the circuitry will perform as required. It is also worth checking at this point that the design is as simple as it can be, complexity adds component cost and engineering time.
There is little value in the formal write up of design reviews. It is often enough to mark up the schematic, checklist and requirements and then scan them in as a design review pack. This should then be worked through with the designer to ensure the identified issues are understood and worked through. The updates are then captured in an up issued schematic, prior to moving to PCB layout.
Similar processes with their own checklists can be worked through for printed circuit board design and for manufacturing pack reviews as required.
The benefits
The most obvious benefit of design reviews is capturing mistakes and omissions early on in the design process before prototypes are manufactured. Additional benefits come from the spreading of knowledge within the design team. This can be from best practice observations, which should be added to an up-issued version of the design review checklists. A second benefit comes from another engineer having some familiarity with the design. This allows them to assist more readily if issues occur during bring up and debug. They can more quickly to pick up the project if there are resourcing or illness issues.
The challenges
Design engineers should know it is expected that their work is completed to a high standard, which could go straight to prototyping. Having a review process is not an excuse for sloppy work to be picked up by someone else later.
1. Should never be a tick-box exercise
Design reviews need to be taken seriously and resourced properly. In a pressured engineering environment, where time is precious, there is a temptation for design reviews to be a quick tick box exercise. This approach can be worse than having no review at all. Design reviews should take enough time for each aspect of the review checklist to be verified. There should be no assumptions made, especially where the designing engineer is more senior than the reviewing engineer. Each check should be made fully. A thorough design review takes between 2 hours for a fairly simple design. Whereas a more complex one may take up to a few days.
Following the review, there is also additional time required to address the issues identified. Contrast this with the time and cost for a prototype to be manufactured and then tested. There it can be readily seen that design reviews can bring a significant return on the time taken to complete.
2. Don’t fear missed errors
There is a danger of allocating too much time and effort to design reviews through fear of having to have designs right first time especially after spending extra time on reviews. The unfortunate reality is that, even with design reviews in place, most designs need a second spin of printed circuit boards to address unforeseen issues.
These can be:
- unexpected behaviour from the integrated circuits used
- changes in functionality from user testing
- design mistakes missed by both the design and reviewing engineer
- unforeseen interactions between circuitry
- mechanical enclosure clash issues, or
- other areas where change is required.
There comes a point where prototypes simply must be manufactured and tested to verify their operation and performance. For first off builds, only manufacture a small number of prototypes as the more you build the lower their quality, see Fletcher’s Law for more information on this.
Reviews must be an integral part of any design process. We hope we’ve shown you why this is the case here and, of course, if you would like to talk about how we can help you to design or implement a design review process, get in touch.
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