Coil bobbin design: wall thickness, creepage and retention

Three jobs in one moulding

A coil bobbin, also called a former, does three jobs at once. It is the structure that holds the winding, the insulation that separates the winding from the core and from the outside, and the interface that locates the coil in the assembly and carries the terminals.

Those jobs pull in different directions. A thicker wall is stronger and insulates better, but it takes space from the winding window. A taller flange increases creepage, but it also increases the size and can trap air. A tight terminal pocket holds the pin, but it can crack the moulding when the pin is inserted.

The design review should treat the bobbin as part of the insulation system and part of the mechanical assembly, not as a plastic detail drawn around the coil.

Wall thickness and winding pressure

The winding pulls the wire tight, so the bobbin wall sees an inward pressure along the length of the coil. The wall has to resist that pressure without bowing into the core gap. If it bows, the core may not fit, or the winding may sit off centre and rub.

Wall thickness is also a thermal decision. Heat from the copper passes through the wall, the core and the mounting. A thin wall conducts better but distorts more easily. A thick wall is stiffer but adds thermal resistance and reduces the winding window.

For a small bobbin, a wall in the region of 1.5 to 2.5 mm is common, but that is illustrative. The real figure depends on the diameter, the winding tension, the material modulus at temperature and the support from the core. Ribs can add stiffness without adding a full wall, provided the ribs do not create a sink mark or a weld line in a critical area.

Flanges, creepage and clearance

The flange sets the winding height and the creepage path from the winding to the outside. Creepage is the distance along the surface of the insulation. Clearance is the distance through the air. They are different requirements, and a design can pass one and fail the other.

Measure creepage from the edge of the winding, over the flange, to the nearest conductive part or to the outside surface. A wire that builds up at the flange can reduce that path, so the winding margin belongs in the drawing. End play between the bobbin and the core can open a path that was not visible on the bench.

Barriers, slots and extended flanges can increase the path without making the coil much larger. Potting or conformal coating can change the effective path, but only if the coating covers the surface completely and adheres. A partial coating can leave a tracking path under the edge.

The creepage requirement belongs to the finished assembly and the pollution degree, not to the bobbin alone. A bobbin that meets a distance in free air may fail when it is mounted against a metal bracket or inside a contaminated enclosure.

Terminals and retention

The terminal is where the winding meets the outside world, and it is a common source of field failures. The pocket has to hold the pin against push and pull, the winding has to be routed to it without crossing a sharp edge, and the solder joint has to be accessible for inspection.

Insert moulding gives the strongest retention because the plastic forms around the pin. A press-fit pin is cheaper but depends on the hole tolerance, the interference and the material stiffness. A loose pin can rotate, break the winding wire or create a high resistance joint.

Route the start and finish leads in a channel or a slot, not across the flange face. A lead that crosses the winding can be pinched by the next layer. A lead that leaves at a sharp corner can crack the enamel. The terminal also needs a strain relief if the coil will be handled or potted.

Bobbin features and failure modes

The table links each feature to the failure it prevents. Use it during the drawing review, when the geometry is still easy to change.

Material and process

Material choice starts with the hot spot and the flammability requirement. PA66 with glass fibre is common for general coils. PBT is dimensionally stable and has good electrical properties. PET is used where the thermal and mechanical balance suits. PPS is a higher temperature option with good chemical resistance.

Glass fibre content changes the modulus, the shrinkage and the weld line strength. A high glass grade is stiffer but more abrasive and more anisotropic. A weld line across a thin wall is a weak point, especially under a terminal or a flange corner. Move the gate rather than adding wall thickness if the weld line is the problem.

Draft, shrinkage and ejection belong in the first drawing review. A bobbin with no draft will be dragged out of the tool, and the flange edge will show it. A gate on the winding surface can leave a protrusion that rubs the wire. A gate on a sealing face can leak.

Validation before release

Check the bobbin in the assembly, not on its own. Measure the wound outside diameter, the flange height after winding and the end play with the core fitted. A bobbin that measures correctly empty can still fail when the winding builds up.

Run a dielectric test on the finished coil, including the path from the winding to the core and to the terminals. Check the terminal retention with a push and pull test, and inspect the lead route after potting. A thermal cycle shows whether the wall creeps or the terminal pocket opens at temperature.

Finally, wind a pilot lot at production tension and speed. The moulding, the wire and the winding process interact, and a bobbin that works at bench speed may fail when the winder runs at rate.