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Are You Using a Requirements-Based PCB Enclosure Selection Process?

Are You Using a Requirements-Based PCB Enclosure Selection Process?
Image by Clker-Free-Vector-Images from Pixabay 
Avoid wasting time trying to find the optimal PCB enclosure.

Printed circuit board (PCB) enclosures are very common in medical devices, and although they are not overly complicated, a requirements-based approach can save a lot of time and effort during development. Enclosures typically protect a PCB from impacts and take the strain off the PCB’s onboard connectors through the use of bulkhead connectors. They can also provide ingress protection (IP) from dust, debris, or liquids; isolate the patient/operator or technician from electrical hazards; or deliver heat isolation or electromagnetic interference (EMI) shielding as required.

The pitfalls of a wrong enclosure choice can be steep. Speaking generally, issues with enclosure design often stem from forcing a design to fit a modified off-the-shelf enclosure. All requirements may not be understood when making the design adjustments. This can lead to serious performance issues that roadblock regulatory certification. For example, regulatory requirements might include fire resistance, very tight EMI sealing, or a higher level of ingress protection than specified. Realizing this later on in development with a custom enclosure would mean a tweak to the design, but for a modified off-the-shelf enclosure there is no tweaking option. You have to start again. There is no worse time to learn that your enclosure needs a complete design change than when you fail IEC 60601 testing. Enclosures may seem trivial, but they often have a lot of safety and efficacy implications that are key to medical device certification.

There are three main approaches to enclosure design: fully custom, modified off the shelf, and off the shelf. The following table and descriptions of each type are a tool to help you decide the best (and cheapest) option for your needs. The table’s rows list the options in order of cost (off the shelf being the cheapest). The columns contain the benefits of each type so that you can identify the most cost-effective way to meet all your requirements.

Fully off-the-shelf enclosures are a bit of a unicorn in medical devices. They seem ideal, but they are really quite useless unless they have the exact holes for I/Os or fans pre-drilled into them in exactly the right spot. Otherwise, the only real use for them is off-the-shelf PCBs with standard layouts (such as common microcontrollers) or custom PCBs, which you can design with the same layout as a common microcontroller. I recommend fully off-the-shelf enclosures only for extremely simple PCBs that you can customize to match existing I/Os and for applications that do not need EMI sealing.

Modified off-the-shelf enclosures are useful when you have just a few specific requirements. They generally fit into one of two types: an off-the-shelf enclosure you buy and then modify (typically by machining holes into it), or one that you have a vendor customize one or more of the sides so your enclosure has the desired holes. The former is best done by a secondary machining vendor but can sometimes be done simply with a power drill (for one-offs). For the latter, many vendors offer design services so that you don’t even need an in-house mechanical designer or even computer-aided design (CAD) software, which can be a huge asset for smaller organizations. Whether you or the vendor make the modifications, lead times tend to be quick, and the prices very reasonable in small volumes—about 40-50% cheaper than fully custom options. The downsides to this approach are fairly clear: you have to pick the closest shape or size to what you want, options for EMI sealing are often limited, and sometimes you have no control over the positioning of the PCB retention features. This can be doubly annoying when the baseline enclosure layout requires you to install standoffs, which come unthreaded all the time during removal of the PCB. Another downside is that baseline designs don’t always have features for attaching them to your medical device. In this case you would need to strap down the enclosure. That is not the end of the world, but it typically adds more effort. I recommend modified off-the-shelf enclosures when size is not essential, a fairly suitable option exists, volumes are moderate to low, and all the I/Os are on the same side.

Another modified off-shelf-enclosure option involves designing a custom lid but using an unmodified off-the-shelf base, sourced independently from each other. This approach is advantageous because the base is the most expensive part to make yourself. In addition, more options are available to suit more complex requirements (such as EMI sealing features and space for larger I/Os and fans) if the lid is fully custom. Unfortunately, this approach results in a bunch of waste as the lids that come with the base will be scrapped. Double check to confirm they are recyclable, or ask your vendor not to send them at all if possible.

Fully custom enclosures can be made to spec by a variety of processes, including injection molding, forming, extruding, sheet metal, 3D printing, and machining. They can be made as small as possible and shaped to fit in odd places; their PCB retention features are also positioned in places suitable for the PCB size and layout. Custom enclosures can incorporate custom shapes and finishes to fit brand aesthetics, ergonomics, or even unique placements when the enclosure is outward-facing. Custom enclosures can be designed with complex EMI gasketing or sealing. Another benefit is there is no risk of a custom enclosure being discontinued. The main downside to a fully custom enclosure is up-front time and money. They take the longest to design and typically require CAD software and a mechanical designer to avoid numerous iterations. They also cost an order of magnitude more in small volumes than the other options. However, in larger volumes custom enclosures might actually be cheaper than off-the-shelf options. I recommend a fully custom enclosure when space is an issue, your PCB layout or I/Os are highly complex, or you have other unique requirements.

Overall, the ability to quickly choose and customize a PCB enclosure will often reap rewards in medical devices. Fully off-the-shelf enclosures may seem like the best option, but they typically are not as they usually don’t have holes in the right spots. Modifying off-the-shelf enclosures is typically the best choice when the requirements allow. I suggest designing an enclosure from scratch when the requirements are complex or when volumes are projected to be very high (as they may be cheaper in the long run). I hope this blog has helped you choose the option that works best for your requirements!

TAGS: Automation
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