“It’s what’s on the inside that counts.” Most of us have heard this in wildly different contexts than PCB design. Still, it’s true more generally: diving beyond the surface is likely to reveal the most critical features, attributes, or characteristics of many things, both esoteric and universal. For PCB box design, maybe the phrase should be revised to “what’s on the inside counts most, but don’t ignore the outside.” Designing the board-to-board connections, heat sinks, and integration into greater systems requires the same level of planning as the board itself. Neglecting this crucial aspect can severely impact the reliability of the board itself as well as any adjoining equipment.
PCB Box Design Influences Board Design (And Vice Versa)
Though novice designers may not realize it at the time, box design begins during board design; the board is a literal input for the box. When planning the design rules, incorporating the package height into the land pattern will help ensure dimensioning of the box is suitable for the assembled board. Combined with associated STEP files for 3D modeling, the representative board model can be exported to solid modeling software to dimension, add features, and create an assembly of the finalized box build.
Other general parameters of the enclosure will depend on the role the box must fulfill alongside the board. Generally, these fall along three dimensions:
- Shock absorption/mounting: In an ideal setting, a board is at rest, or at least at rest relative to the enclosure, with component bonding that is within expected g-force levels. Mounting introduces some mechanical stress to the board localized to the mounting screw (this can be spread across the board, assuming equal torque among the screws). Conformal coating can absorb complex vibrations in the board but may interfere with heat exchanges and limit total thermal flux. Grommets are used for many roles to isolate sensitive equipment from shock and act as strain relievers on necessary cabling to prevent damage to connectors.
- Heat flow/dissipation: Whether a passive or more complicated and costly active heat dissipation system, there must be a route for heat to flow out from sources on the board. When more drastic cooling solutions are required, manufacturers can begin to change the composition of the enclosure and add additional cooling features. Switching to metal, which is costlier than plastics, allows for even more benefits in terms of cooling potential; aluminum is a common choice for its high malleability and strength-to-weight ratio. Heat flow simulations can be performed on different enclosure designs to determine the best positioning of critical features, such as openings in the box and fan placement.
- Isolation: When addressing EMI issues on a board, the enclosure is the last place to fix the design. A board often undergoes revisions to fall within allowable limits, as the enclosure does nothing to the underlying EMI production. By acting as a Faraday cage to prevent signals from radiating outside the box, the enclosure can provide additional EMI prevention for a board to pass specifications.
The Importance of Chassis Ground in Enclosures
It may be surprising to hear that the enclosure serves an electrical purpose too. “Ground” in the context of a board alone commonly refers to the theoretical zero level for other voltages; this may occur as a digital and analog ground in a mixed-signal design, but it is important to note that ground is ground (theoretically). In this case, grounds are separated to prevent noise on the more sensitive analog components, but heeding proper layout practices and manufacturers’ data sheets should yield two “distinct” grounds that are tied together at a common point and, therefore, functionally equivalent.
Often with enclosures, there is yet another ground: chassis ground. The chassis ground is the link between the signal ground – commonly referred to as just ground on the board – and Earth ground, providing a safe path for current to flow. All of these grounds fit the idea of the ground as a zero reference point, but their exact role is slightly more complex:
- Earth: Ground in the most literal sense. Earth provides a safe passage for current in otherwise interrupted circuits to flow as a preventative measure against injury and death. Due to its inhomogeneous composition (taken as a whole), Earth cannot be a true zero voltage point. Still, it is highly dissipative, safely draining large currents and voltages within its highly insulative matter. More strictly defined, Earth grounding requires a conductive rod or cable to be buried at a minimum depth of 2.5m/8ft per NEC.
- Chassis: The chassis connects to a board’s metal enclosure. Doing so allows the box to act as a safety feature (preventing shocks to and from the board that could injure personnel or damage equipment) and reduce EMI emissions.
- Signal: The classical definition of ground is a reference voltage for the remainder of the circuit. Much like an altitude measurement, voltage has to exist as the electric potential between two nodes on a circuit; voltage cannot be measured at a point without knowing its relation to reference (or some other established voltage value).
For the best EMI and noise prevention, ground chassis should only be connected at a single point, similar to analog and digital ground overlap. The reason for this is very much the same: a current loop cannot form through a single point, curtailing noise on the ground.
Your Contract Manufacturer Can Box With The Best Of Them
PCB box design marries some essential ideas of electronics and network analysis, providing safe housing for the board(s), protecting operators from injury, and enhancing the installation’s reliability. While it is sometimes treated as an afterthought relative to the board, a design team that embraces the enclosure’s functionality at the initial design stage will likely reap significant benefits in the end product. Whether you’re looking for a box build for your board or are undertaking an electronics NPI, VSE is standing by to offer decades of combined experience among our engineering team. Our engineers build our customers’ electronics and, coupled with our manufacturing partners, optimize designs for unparalleled performance.
