Issue link: https://iconnect007.uberflip.com/i/1547450
58 SMT007 MAGAZINE I NOVEMBER 2026 The Process in 60 Seconds Heat staking forms a permanent mechanical joint out of material the enclosure already has. A ther- moplastic stud, molded as an integral feature of the housing, usually on top of a boss, passes through a clearance hole in the PCB. A heated tool softens the stud and forms it into a head roughly twice the stud's diameter, holding it under pressure while it cools and solidifies. The formed head captures the board, much like a rivet head captures a plate. 1 Modern equipment runs an impulse cycle: heat, form, and cool under pressure, then retracts with a cooled tip so the plastic releases cleanly rather than sticking or stringing (Figure 1). When done right, the result is a smooth dome that keeps the board flat against its standoffs for the life of the product. Designers keep choosing it over screws for straightforward reasons: no fastener to purchase, feed, torque, or vibrate loose; no threaded insert; no adhesive and no cure time; and near-zero per- cycle material cost. In vibration-heavy assemblies, such as automotive interiors, appliances, anything that lives near a motor, the absence of a fastener that can back out is the entire point. The catch is that the joint is only as good as the geometry it was given. A screw forgives a sloppy hole, while a formed plastic head does not. The Hole Is a Design Feature, Not a Formality Start with the clearance hole, because it is the cheapest thing to get right and the most annoying thing to fix later. The working rule: Hole diam- eter equals stud diameter plus 10–25%. For a 3.0 mm stud, the hole is between 3.3–3.75 mm. On a rigid FR-4 board, use the top of that range. A plastic mating part has some give; glass-epoxy laminate has none. If the hole runs tight and the stud runs anywhere off nominal (and a molded stud holds about ±0.05 mm on a good day), the board will either not seat or sit preloaded against the stud before the press ever touches it. Whatever the percentage works out to, never go below 0.10 mm of total diametral clearance. Two details matter nearly as much as the diameter (Figure 2). First, the hole edge. A drilled FR-4 hole has a sharp corner, and the root of the stud, where it meets the boss, is the most highly stressed region of the Figure 1: The impulse heat staking cycle. A heated tool softens the ther- moplastic stud, forms the head, and holds it under pressure while it cools, then retracts with a cold tip so the plastic releases cleanly. Figure 2: Stake-hole design in cross-section, shown for a 3.0 mm stud in a 1.6 mm FR-4 board. The clearance hole runs at the top of the D + 10–25% range, the entry edge is chamfered, the hole is non-plated, and the stud's root fillet sits in the clearance rather than against the hole edge.