Why mounting deserves thought
Holding a board or a motor in place seems like the dull end of a project, but poor mounting causes many mysterious faults. A board that flexes can crack solder joints. A loose screw can drop onto a powered circuit and cause a short. A part glued permanently in the wrong spot is hard to fix. Good mounting is secure, keeps parts electrically and thermally where you want them, and still lets you take the project apart.
Screws: the basics
Screws are described by their diameter, length, thread type and head shape.
- Machine screws have a uniform thread that mates with a nut or a tapped hole. Metric sizes such as M2, M2.5, M3 and M4 are common in electronics, and imperial equivalents exist in some places. Always check the mounting hole size on the board or part.
- Self-tapping or wood screws cut their own thread into soft material such as plastic or wood.
- Head styles include pan, button, countersunk (flat) and cheese heads. Countersunk heads sit flush, but need a matching tapered hole.
- Drive types such as Phillips, slotted, hex socket and Torx differ in how well they resist slipping.
Pick a length that engages enough thread without poking through or touching something behind. A rule of thumb often cited for threaded engagement is at least about one screw diameter in strong material, and more in softer ones, but check the guidance for your material and load.
Be wary of over-tightening. Small screws in plastic or through circuit boards are easily stripped or can crack the board, so tighten until snug and firm rather than as hard as you can.
Standoffs and spacers
A standoff is a short post that lifts a board above the surface beneath it. This avoids contact between solder joints or leads and the case, leaves space for air, and gives room for components on the underside.
- Threaded standoffs screw into the base and take a screw on top. They come in male-female and female-female types, and you can stack them to get more height.
- Unthreaded spacers slip over a screw and are simply compressed.
- Plastic standoffs and snap-in supports push into holes and clip a board in place without tools. They are useful but may loosen with vibration or repeated removal.
- Printed posts are part of a custom case, tailored to the board’s hole pattern.
Material matters. Metal standoffs are strong, but they can short against the board if a mounting hole is connected to a ground or power plane or if a nearby trace is exposed. Many boards intend mounting holes to be grounded, others do not, so read the board documentation, much like reading datasheets for electrical details. Where in doubt, plastic spacers or insulating washers are a safer choice.
Place standoffs close to the corners and near heavy connectors, so the board does not flex when you plug a cable in. A tall connector pushed repeatedly into a board with no support underneath is a common way to crack a joint.
Fixing into plastic
Printed and moulded plastic parts need extra thought because threads cut into them wear down.
- Self-tapping screws work for a few cycles, but a case opened often will strip.
- Heat-set inserts are small threaded metal sleeves pressed into a slightly undersized hole while warm; they give a durable machine thread in plastic. Follow the insert maker’s guidance for hole size and heating, and take care with the hot tool.
- Captive nuts sit in a slot or hex pocket in the part, which stops them turning.
- Through-bolts pass fully through the part and tighten with a nut on the far side, which is simple and strong.
When designing printed parts, make holes slightly larger than the screw for clearance, or slightly smaller for self-tapping, remembering that printed holes usually come out undersize, as explained in the discussion of print tolerances.
Adhesives and other methods
Glue is the right answer in some places and the wrong one in many.
- Epoxy and structural adhesives give strong bonds for dissimilar materials but are hard to reverse.
- Cyanoacrylate (super glue) sets quickly, though it can be brittle and some formulations fog nearby surfaces.
- Hot glue is quick and removable, but it softens with heat, so it is poor for warm components or battery packs.
- Double-sided tape and foam pads are convenient and cushion vibration, though adhesion depends on surface and temperature.
- Hook-and-loop strips, zip ties and cable clips hold batteries and cables while remaining adjustable.
- Threadlocking compounds keep screws from backing out under vibration; use the type and amount recommended by the maker.
Read each product’s safety information, ventilate properly while curing, and keep adhesives away from skin and eyes. Some adhesives and solvents attack certain plastics, so test on a scrap piece first. Keep conformal coatings and glue away from connectors, switches and moving parts.
Vibration, heat and movement
Different environments stress mounting in different ways. Vibration works screws loose and fatigues wires, so use locking washers, threadlocker, or lock nuts, and secure heavy parts such as batteries with more than a blob of glue. Heat softens many plastics and adhesives, so parts near hot components deserve better materials, which ties into the temperature limits of filament materials. Parts that expand and contract with temperature may need a little slack.
Do not pile weight on a circuit board. Heavy items such as large capacitors, transformers and batteries should be fixed to the case or chassis, not just soldered to the board.
Keep it serviceable
A good test is whether you could replace the part in a year, with the tools you have, without cutting anything. Favour screws over glue, use connectors rather than hard-wired joints, and make sure every fastener can be reached. Note which screw sizes you used so you are not hunting for odd parts later; fewer sizes make assembly simpler. The same thinking applies when you plan how a board sits inside project enclosures.
If you are working with batteries or mains-connected parts, mounting is also a safety matter: a cell must not be pierced by a screw, and live parts must stay insulated and secure. Follow the maker’s instructions and, for mains work, rely on a qualified person.