PCB mounting bosses usually support the circuit board, while separate bosses secure the lid or accept external foot fasteners. Their appearance is only a clue. Identify each location from the enclosure drawing, then confirm the screw specification before assembly. A screw that fits the lid may split a board support or puncture a blind hole. Supplied hardware is not proof of compatibility with every hole inside the housing.
What you need before choosing screws
Start with the enclosure model, its dimensional drawing, and any hardware schedule supplied with it. The drawing should identify mounting locations, hole sizes, and fastening requirements. If those details are missing, request the screw specification from the enclosure manufacturer.
Have these items available:
- The PCB drawing, including thickness, mounting-hole diameters, and component heights on both sides.
- Calipers for accessible dimensions and a depth gauge for checking blind-hole geometry without cutting material.
- A thread gauge for checking loose screws, where applicable.
- The specified screws, washers, spacers, and a suitable torque-controlled driver.
- A marked sketch separating board supports, lid closures, and external mounting points.
Keep power disconnected throughout mechanical fitting. Remove the board before measuring blind holes or investigating loose hardware.
A boss is a raised mounting feature, not a description of its thread. It may contain a molded pilot hole, a tapped hole, or a threaded insert. The PCB enclosure terminology article explains the broader differences between bosses, standoffs, and mounting plates.
Identify and size the screws
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Map every fastening location before opening the hardware bag.
Label the suspected board bosses, lid bosses, and foot attachment points on your sketch. Follow each hole through the drawing or an accessible view of the empty enclosure. A foot recess may conceal a cover screw, but it may also serve a separate attachment. Do not assume the arrangement from appearance alone.
Lower internal bosses often support a PCB, while taller corner features may receive lid screws. These are common arrangements, not reliable identification rules. Confirm which component each fastener actually retains.
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Check the board against the intended mounting pattern.
Compare hole-center spacing, hole diameter, board outline, and connector access against the enclosure drawing. Then check the clearance beneath solder joints and through-hole leads. Board supports must hold the PCB clear of the base without loading components against the lid.
Do not bend the PCB or pull misaligned holes into position with screws. The PCB enclosure fit article covers selecting a housing before fixing the board layout. That dimensional check should happen before screw selection.
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Identify the receiving hole and its specified screw family.
A smooth pilot hole in plastic may require a thread-forming screw designed for that material and boss geometry. It does not automatically accept a general-purpose self-tapping screw. A tapped hole or threaded insert normally needs a matching machine screw.
Use the following distinctions when reading the hardware specification:
Receiving feature What to confirm What not to assume Molded plastic pilot hole Specified screw family, diameter, engagement, and installation torque Any screw that starts will be suitable Tapped plastic boss Thread diameter, pitch, usable depth, and torque A thread-forming screw can replace the specified machine screw Metal threaded insert Thread system, diameter, pitch, engagement, and insert limits A visually similar metric or inch screw will fit Plain clearance hole Required nut, clip, or separate threaded support The hole itself should grip the screw Calipers can help check a loose screw's outside diameter. They cannot establish its complete thread specification or prove compatibility with a plastic pilot hole.
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Confirm diameter and pitch without using the boss as a gauge.
Match the manufacturer's callout exactly. Metric and inch fasteners can look similar while having incompatible diameters or pitches. Even two screws with similar outside diameters may have very different thread forms.
Do not drive several candidate screws into a plastic boss to discover which one grips. Each attempt can cut or deform material needed by the correct fastener. If the enclosure documentation gives only a hole diameter, ask for the intended screw specification rather than guessing.
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Calculate engagement from the complete clamped stack.
For a conventional non-countersunk screw, length is measured from beneath the head to the tip. Subtract the thickness of everything between the head and the boss face. That stack can include the PCB, washer, bracket, and any spacer above the boss.
The resulting projection is the nominal engagement into the receiving feature. It must satisfy the manufacturer's engagement requirement while leaving the specified clearance at the blind bottom. Hole depth alone is not usable thread depth; drill points, incomplete threads, and the screw tip can reduce usable space.
For example, a hypothetical 0.236 in [6.0 mm] screw through a 0.079 in [2.0 mm] stack projects 0.157 in [4.0 mm]. That projection is acceptable only if the specified engagement range includes it and the tip clears the bottom. It is not a general recommendation for plastic bosses.
Countersunk screws use a different length reference. Follow the drawing rather than applying the under-head calculation unchanged.
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Check the head, washer, and surrounding components.
Confirm that the screw head fits the available mounting area and clears nearby components. Check both mechanical clearance and the PCB's intended copper keepout around the mounting hole. A washer can increase the contact area while also reaching exposed copper.
Do not substitute a countersunk head where the drawing specifies a flat bearing surface. Its wedging action can damage an unsuitable board hole or boss. Likewise, do not add a washer without recalculating engagement and checking electrical clearances.
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Assemble using the specified installation method and torque.
Support the board on its intended bearing surfaces and start the correct fasteners squarely. Install all screws without forcing alignment, then tighten as directed by the manufacturer. Thread-forming screws naturally require forming torque; that is different from forcing a mismatched machine screw.
Use the specified torque and a suitable controlled tool. A driver's lowest clutch setting is not a verified torque value. Stop if a boss cracks, an insert rotates, the PCB bows, or the screw bottoms before clamping. Increasing torque will not correct any of those conditions.
Checks before powering the assembly
Inspect the installed board with the enclosure still open. The PCB should rest evenly on its supports without rocking or visible distortion. Check that each screw head bears on the intended surface and that no loose hardware remains underneath.
Confirm the following before closing the lid:
- Solder joints, leads, and components clear the enclosure base and mounting features.
- Screw tips remain within the documented fastening geometry and do not penetrate an exterior wall.
- Connectors align with their openings without pulling the board sideways.
- Cables have enough movement and bend space without pressing against the board or lid.
- The lid closes normally without becoming an unintended clamp for the PCB.
- Lid screws and foot fasteners occupy their documented locations, separate from the board hardware.
Where access permits, inspect blind-boss areas for whitening, splits, displaced inserts, or distortion. Record the installed screw specification and torque with the assembly documentation. This prevents a later repair from substituting whichever screw happens to be available.
Mistakes that damage bosses and seals
Using lid screws for the PCB. Lid fasteners may differ in diameter, thread form, or length. Keep hardware grouped by its documented function, even when the heads look identical.
Choosing length from total hole depth. A blind hole can be deeper than its usable threaded region. A bottomed screw may feel tight without clamping the board, or its tip may damage the wall.
Treating foot pads as sealing hardware. A rubber foot may cushion the enclosure or conceal a fastener. Neither function establishes that it seals an opening. If a mounting screw crosses an outdoor enclosure wall, follow the separate guidance on mounting screws without enclosure leaks.
Repairing a stripped boss with a larger screw without approval. Increasing diameter changes stress in the remaining boss wall. Adhesives, inserts, and replacement fasteners also need material and geometry checks. Obtain a documented repair method or replace the damaged part.
Drilling new holes before checking alternatives. If the existing bosses cannot support the board, consider an approved carrier or mounting plate. The article on mounting a PCB without enclosure posts addresses that different mounting problem.
When replacement housing selection becomes necessary, carry forward the board outline, mounting pattern, component heights, and required internal clearances. The enclosure selector helps compare usable dimensions; confirm PCB support provisions separately before ordering. Specify the enclosure drawing revision, mounting arrangement, complete screw callout, engagement limits, and installation torque rather than requesting only a screw diameter.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
What are the cylinders with holes inside a PCB enclosure?
They may be PCB supports, lid fastening bosses, or attachment points for another component. Their height and position provide clues, but the enclosure drawing determines their purpose. Check the mounting pattern and receiving-hole specification before inserting any screw.
Can I use the supplied lid screws in the PCB mounting bosses?
Only when the manufacturer's hardware specification assigns the same screw to both locations. Lid and board fasteners can differ in diameter, thread form, head style, and length. A supplied screw that fits one location can damage another.
How do I choose the screw length for a blind PCB boss?
Use the specified screw length and subtract the complete clamped stack to determine projection into the boss. Compare that projection with the manufacturer's engagement limits and blind-bottom clearance. Total hole depth does not necessarily equal usable thread depth.
Do plastic PCB bosses always need self-tapping screws?
Not automatically. A smooth molded hole may require a particular thread-forming screw, while other bosses contain tapped threads or metal inserts. The correct choice depends on the receiving feature, material, and manufacturer specification, not simply whether a screw grips.
