Mounting Holes and Standoffs: Fixing the Board to the Chassis

The mounting holes are usually the last feature added to a layout and the first to cause a problem in the field. They carry the mechanical loads, they set the relationship between the board and the chassis, and they are the place where an electrical design meets a mechanical one.

What a Mounting Hole Has to Do

A mounting hole must locate the board, resist the vibration and shock of the application, and provide a defined electrical connection if the design requires one. Those three functions impose different requirements and are frequently confused.

Locating the board is a matter of one hole being tight and the others being clearanced, so that thermal expansion of the board does not fight the chassis. Where all the holes are tight, the board either bows or the fasteners load up.

The electrical function determines whether the hole is connected to a net. A hole tied to ground through a plated barrel provides a short, low inductance connection to the chassis, and the same hole in an insulated version provides none.

Plated or Non-Plated

A non-plated hole is a mechanical feature only. It should have a clearance larger than the fastener by enough to accommodate the positional tolerance of both the board and the chassis.

A plated hole can be connected to a ground plane, which serves as a chassis connection and as a low impedance return for the board. The plating also protects the hole wall, and it allows the hole to double as a test point or a connection point.

Where the hole is plated and connected, the connection must be deliberate. A hole that touches a plane accidentally through an oversized pad is a connection that nobody intended and that often appears as a ground loop.

The Keep-Out Around a Fastener

The keep-out radius must account for the fastener head, the washer, the tool that drives it and the standoff beneath the board. A common failure is a component placed under the washer, which is discovered when the board is assembled into the enclosure.

The keep-out also has a height dimension. A component that is electrically clear of the washer but taller than the standoff will be crushed or will prevent the board from seating.

The height of the tallest component in the area should be checked against the mechanical drawing. Our fabrication notes checklist describes where such mechanical constraints should be recorded.

Grounding and the Chassis Connection

Where the board must be grounded to the chassis, the connection should be designed as a connection rather than inherited. The options include a plated hole with a large pad on both sides, a dedicated grounding pad with a screw, or a spring contact.

The impedance of the connection matters at high frequency, and a single hole with a long thin trace to the plane is a poor connection where a direct connection to a plane on both sides is a good one.

Our industrial designs notes describe how these connections are handled where the board is installed in a cabinet with other equipment.

Torque and the Strength of the Board

A fastener torqued beyond the specification crushes the laminate around the hole and can crack the barrel of a plated hole. The failure is not always immediate; a cracked barrel becomes an open circuit after thermal cycling.

The torque specification belongs to the mechanical assembly documentation, and it relies on the fastener being used with the washer it was designed for. A smaller washer concentrates the load and reduces the torque the board can take.

Where the board is thin, a larger washer, a metal backing plate or a plated area around the hole all spread the load. Where the board carries heavy components, the mounting holes may also need to be located near the stiffest part of the layout.

Vibration and Shock

Vibration loads the board in the region between the fixings, and the amplitude of the deflection depends on the span and on the mass carried. Adding a mounting point in the middle of a long span reduces the deflection more than the additional hole costs.

The direction of the dominant vibration matters. A board mounted vertically in a vibrating machine may need additional support in the direction of the motion, and the supports should not be placed where they obstruct components.

Where a heavy component sits far from the nearest fixing, a stiffener or an additional support under it is usually the answer. Our thermal design notes cover the related question of how much copper is available to stiffen a region.

Positional Tolerance Between Board and Enclosure

The board holes and the chassis holes are both manufactured to a tolerance, and the two tolerances accumulate. The clearance in the board hole must be large enough to absorb both, plus the positional tolerance of the connector that must align with its counterpart.

Where a connector must mate precisely, the alignment should be provided by the connector housing rather than by the mounting holes, which allows the mounting holes to be generous without affecting the mating.

Our board outline notes describe how the outline and the hole positions are toleranced relative to each other.

Materials and Environments

In a humid or corrosive environment the fasteners are often the first part to corrode, and a corroded fastener can seize or fail. Stainless fasteners in aluminium chassis, and dissimilar metals in contact, produce galvanic corrosion at the joint.

Where the board must be isolated from the chassis for electrical reasons, the isolation must be maintained under vibration. An insulating washer that compresses over time loses the clearance it provided.

A plated hole connected to ground also provides a path for the corrosion current, which is sometimes useful and sometimes not. The decision should follow from the grounding strategy rather than from convenience.

Process Control and Verification

On a design of this kind, keep-out is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, keep-out is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

How many mounting holes does a board need? Enough to locate it and to keep the deflection between fixings within the limit set by the vibration environment, which is a mechanical calculation rather than a convention.

Should mounting holes be plated? Plate them where they must be grounded or where the wall needs protection. Leave them unplated where they are purely mechanical, which is cheaper and avoids accidental connections.

What does gopcb provide for mounting hardware design? We provide keep-out and clearance recommendations matched to the fastener and washer, plated or non-plated holes with the appropriate pad if a connection is required, and the mechanical notes on the fabrication drawing. Where the enclosure interface is tight, we check it against the mechanical drawing before the panel is built.