Mechanical Assembly Design for Electronic Devices

Every product you assemble has to follow an assembly process, including mechanical enclosures that will contain an assembled circuit board. Mechanical enclosures are part of any real product that will be taken to market, and they have their own set of design for manufacturing rules to follow.

The next time your team plans a product for volume manufacturing, don’t forget the enclosure’s assembly process. Your contract manufacturer or your internal assembly team will need to follow a specific process that integrates the PCB assembly with the mechanical assembly. If the PCB and the enclosure are not designed in concert, it’s possible that the PCB itself interferes with a volume assembly process for the enclosure.

MEs Plan Enclosure Assembly Steps

The role of planning for assembly of a custom enclosure is the responsibility of the mechanical engineering team members. In the development of real products to be produced at volume, you will rarely see an off-the-shelf enclosure being used to house the PCB assembly. Instead, custom enclosures are often fabricated, either by a metal/plastic contract manufacturer or your EMS provider.

Build the Enclosure for an Assembly Process

The most important part of building a mechanical enclosure for a PCB is to plan out the assembly steps for the enclosure. The PCB, its components, and the enclosure mounting elements can interfere with each other if not designed to assemble in the optimal order.

Standoff installation: Standoffs or spacers that support the board in the enclosure should first be installed. These provide a platform to mount the PCB and ensure it doesn't touch the enclosure directly. The PCB is placed onto the standoffs and secured using screws or clips. In the case of slide or snap-in mounts, friction holds the board in place.

Connector and cable routing: Any required panel-mount connectors should be installed through the enclosure body. Using guides or clips inside the enclosure, cables are routed to avoid interference with the PCB or other components. They're often secured using cable ties or clips. For volume production, pre-assembled cable routes or harnesses can be used to guide cable bundles.

Sealing the enclosure: Each portion of the enclosure is brought together to finish the mechanical assembly. This can be done manually or with the help of machines. Screws, clips, or other fasteners are then used to seal and secure the enclosure.

At any point in the above process, there may be additional fasteners or cable routing as the board is built up. The placement of parts, connectors, cables, and mechanical elements on the PCB should not interfere with placement of fasteners or cable routing.

Example PCIe add-in card assembly process, provided by Intel.

Extrusion, casting, or injection molding processes may automate portions of the above list of steps. For example, with injection molding, standoffs or slide-mount rails can be included in the enclosure without manual installation. Certain features will still need to be fabricated and assembled manually.

Once the assembly process is complete and the board is mounted in the enclosure, the device must be placed in its product packaging so that it can be shipped to customers. Additional final steps could include application of labels or printing markings on the enclosure. Final inspection, either by humans or automated, should be performed to check for any visible defects, misalignments, or issues.

Materials

Packaging for electronics assemblies will be either plastic or metal. Some metal enclosures, such as folded sheet metal, can have the lowest costs per part at low volume due to the high fixed costs of injection molds. Sheet metal enclosures are typically the best entry-level option for an enclosure. At higher volume, typically enclosures will be made from injection-molded plastic, extruded aluminum, or cast aluminum.

One option that is very popular for prototyping is a 3D printed metal enclosure. This will give an idea of form and fit for the final PCB assembly, and it can be used to test the assembly process, including with fasteners.

Board Mounting

You can’t just toss a board into a plastic box and expect your product to be reliable. Instead, a board must be mounted in the enclosure in order to keep its position/elevation fixed and to prevent the board from shifting when the product is handled by the user.

Some options for mounting a board into an enclosure, ranked in terms of mechanical strength, are shown below.

Epoxy and glue are not the best materials to use for directly mounting a PCB to an enclosure, but cheaper products that are meant to be thrown away might take this approach. Epoxy and glue are also bad ideas because they can be slightly conductive, so they create some leakage current if the materials bridge two exposed conductors on the PCB. The best, and quote common, option for mounting and fixation is to use screws and posts.

Encapsulation

Many products that are placed into an enclosure could be encapsulated with a potting compound. While a potting compound is not always the best option for supporting or fixating a board (similar to snap-in mounting), it is very often used with mechanical mounting to provide complete encapsulation of the PCB assembly inside the enclosure. Potting compounds completely cover the internal electronics so that they cannot shift or move, and the electronics will not be able to be accessed for inspection and repair.

Epoxy potting compound being used to fill voids in an enclosure.

Application of a potting compound will typically be the final step in mechanical assembly before closing up the device housing.

Typically these compounds are epoxies that solidify as an opaque material, although transparent potting compounds are available. Silicone and acrylics can also be used as potting compounds, but the use of silicones is not always the best choice due to their latent conductivity. The same considerations that apply to conformal coatings also apply to potting materials as they are essentially the same base materials.

Aside from transparency, potting compounds must have some important material characteristics:

• High thermal conductivity
• High dielectric breakdown strength
• Must withstand high temperatures and pressures
• Must withstand exposure to moisture, solvents, oils, etc.
• Ability to flow through an automated dispenser
• Low outgassing, especially for aerospace

The first two items in the list are most important for potting compounds. High thermal conductivity (higher than air) is the biggest advantage, particularly for devices that generate a lot of heat but are housed in plastic enclosures. If a fan is not needed, but heat needs to be removed from all around the device, consider using a potting compound.

Thermal Management in the Enclosure

Some devices will need to have fans, which then demand airflow coming into the enclosure, or an enclosure-integrated heatsink that draws heat to the exterior of the enclosure. There are several options for admitting airflow into an enclosure, whether via natural convection or when driven by a fan. Probably the most common is the circular fan vent seen on many power supplies (see below).

Fan vents like this are common for providing airflow, but other options include vanes or grates.

In other cases, the enclosure could be the heatsink or it could integrate a heatsink into the product. If the enclosure contains a heatsink, it will often be part of the exterior of the enclosure and will likely not affect the mechanical assembly process.

If your enclosure will be the heatsink for your product, then the enclosure will need to dissipate enough heat to cool the electronics, but ideally it should not be too hot to touch. Many systems will incorporate cooling through packaging, which can be aided with a thermal compound bonding the board to its enclosure. But if the enclosure is too hot, your design will not comply with certain industry standards, and the user may not be able to interact with the product.

Off-the-Shelf Options

Although most custom products will not use off-the-shelf enclosures, some of these can be modified if needed. Depending on the extent and type of modification, you could get something closer to a custom enclosure with lower parts and fabrication costs.

One popular off-the-shelf option comes from Polycase, which allows for slide-in mounting of a PCB into an extruded aluminum enclosure. While not a typical option for products produced at high volume, they are a reliable option for low volume production where metal or plastic fabrication options are unavailable. Once the end caps are mounted and screwed in, this type of case will meet IP-66 ratings. The case could also be filled with a potting compound to provide high heat flux to the exterior of the packaging.

The downside of this type of system is that cabling and connectors will not be available through the outside of the enclosure, so some custom machining might be required in order to use an off-the-shelf enclosure. This would also eliminate the IP rating unless any openings are properly sealed (again, potting compound might be applied).

No matter how you plan to design your mechanical assembly, make sure your assembly balances thermal, weight/size, and manufacturing requirements during the design phase. Make sure you qualify your most advanced designs using the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.

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