Eyelet and Turret Terminal Staking on PCB Assemblies: 6 Rules
An eyelet is a small metal tube that is pushed through a plated hole and rolled over on the far side so that it is held mechanically as well as electrically. A turret terminal is the same idea with a post above the board, so that a wire, a test probe or a clip can be attached and removed many times without damaging the laminate. Both are installed by staking, and both are then soldered.
They are used where a connector is too large or too expensive, where a wire has to leave the board at a defined point, where a service loop has to be terminated by hand, and where a test point must survive repeated probing. Because they are installed in a press and then soldered, they sit between two processes, and most of the failures they suffer come from the handover between them.
Where These Terminals Earn Their Place
A staked terminal solves a problem that a plated hole cannot. The copper in a plated hole is thin, and a wire wrapped around it, or a probe pressed against it, will eventually crack the barrel or lift the pad. The terminal carries those forces in its own metal and spreads the load over the full length of the barrel and the flange on the top surface.
They are also a repair aid. A pad that has been damaged during rework can sometimes be recovered by fitting a terminal that makes a mechanical connection to the remaining copper and provides a stable point for the wire. That is an accepted repair when it is done to a defined procedure, and it is one of the few repairs that improves the mechanical strength of the finished joint rather than reducing it.
Hole and Barrel Requirements
The hole diameter has to match the terminal shank closely enough to grip it, and loosely enough that the plating is not scraped away during insertion. A hole that is too small shears the plating on the way in and the terminal is loose in a hole with a damaged barrel. A hole that is too large lets the shank float, the staking tool rolls the flange into air, and the pull strength drops to whatever the solder can hold.
The barrel length has to suit the board thickness. A terminal that is too short will not protrude far enough to roll over properly, so the swage is incomplete and the mechanical grip is lost. The terminal barrel, the board thickness and the finished swage height should be checked together against the supplier drawing before the first article, not after a batch has been staked.
Staking Force and Tooling
Staking is a controlled deformation. The tool presses the shank flange down against the top of the board while an anvil on the far side rolls the protruding barrel outward into a flange. Both halves of the tool have to be aligned, or the flange forms on one side and not the other.
The press force is the parameter that decides whether the result is right. Too little force leaves the terminal loose and the swage incomplete. Too much force cracks the barrel, crushes the laminate around the hole or bows the board under the tool. A press with a settable stop and a force readout is worth far more here than a hand press with a lever, because the same setting can be repeated on the next batch. Where a hand press is the only option, a go and no-go gauge on the swage height gives the operator something objective to check.
Swage Height and Seating
The finished swage should show a continuous rolled flange that touches the board surface around the full circumference. A gap under the flange means the barrel was too short, the hole was too large or the force was too low, and the terminal will move when a wire is pulled. A flange that has been rolled so hard that it has thinned to a sharp edge has been over-staked, and the barrel is likely to be cracked underneath.
The top flange should also sit flat against the board. Terminals that sit on a mask-covered or uneven surface are loaded on one side only, and that is where the laminate crushes first. Where the surface is not flat, a thin washer or a deliberate mask keep-out around the hole makes the seating predictable and is far cheaper than sorting staked boards later.
Soldering the Terminal
The terminal is a large thermal mass compared with a normal through hole joint. A turret post draws heat away from the barrel quickly, and an operator working with an iron that is correct for a chip component will hold the tip on the joint far too long. The result is a joint that looks soldered but has not wetted the barrel, or a board that has been locally damaged around the flange.
The right approach is a larger tip, a higher thermal capacity and a short contact time, with the solder fed into the joint rather than onto the tip. Where the board can take it, preheating from below shortens the contact time considerably. Where the terminal is on a ground plane, the thermal demand is the same as any heavy ground connection, and the joint should be treated as a thermal problem rather than a technique problem.
Mechanical Support and Wire Routing
A soldered wire puts a bending load on the terminal every time the harness moves. The joint itself should not be asked to carry that load. A service loop, a tie point or a clamp within a short distance of the terminal takes the strain, and the wire should leave the terminal in a direction that does not work the joint sideways.
Where several wires terminate at adjacent terminals, they should be routed so that a pull on one does not load the others. Where a wire is solid rather than stranded, the bend radius matters more, and the wire should not be bent hard against the terminal body. The routing belongs in the assembly instruction, because it is the part of the process that no inspection afterwards can restore.
Inspection and Pull Testing
Visual inspection confirms that the flange is continuous, the terminal is seated, the hole is filled and the wire is properly wetted. It cannot confirm grip. The only direct check on the mechanical joint is a pull test on a sample or on a first article, using the direction and rate stated in the specification.
A pull test on a staked terminal should load the terminal rather than the wire, and the failure should be recorded with the load at which it occurred and where the failure happened. A joint that fails by pulling out of the hole is a staking problem; a joint that fails by the wire breaking is a wire gauge problem; a joint that fails by tearing the copper out of the laminate is a design problem. Those three outcomes point at three different people, and the test is only useful if the outcome is written down.
Defects and Their Causes
A cracked barrel is nearly always over-staking, and it often shows only as a hairline under magnification. A loose terminal with a good solder fillet points at an oversized hole or a short barrel. A cold joint on the terminal points at insufficient heat or a contaminated surface, and it behaves like any other cold joint in a high thermal mass location.
Flux trapped under the flange is common when the staking is done after soldering or when too much flux is applied before staking, and it can lead to corrosion later. Where the terminal carries a wire that will be handled during final assembly, the risk of an over-filled joint shorting to an adjacent terminal is real, and the usual bridge prevention rules apply with the same discipline.
Documentation and Verification
The terminal part number, the hole size, the board thickness, the swage height and the press force should all appear on the assembly drawing or the work instruction. Without those numbers the operation is a craft rather than a process, and a change of operator will change the result.
First article verification should cover the staked height, the seating gap and a pull test, and the results should be kept with the build record. A terminal that is missing altogether is a different failure, and the usual missing part check is what catches it before the unit is potted or conformally coated. Where the same function could be served by a press-fit pin, the press-fit assembly route avoids the soldering step altogether and is worth considering at design review.
FAQ
Can a staked terminal be replaced on a finished assembly? Yes, but the removal usually damages the hole, so the replacement normally needs a larger terminal or a repair eyelet with a wider flange. The repair should be recorded and the hole should be inspected for barrel damage before the new part is fitted.
Is solder enough to hold a terminal in place? No. The solder makes the electrical connection and seals the joint, but the mechanical load is carried by the swaged flange. A terminal that relies on solder for grip will loosen as the joint is flexed, and the failure will look like a solder defect when the real cause is staking.
What pull strength should a staked eyelet hold? The value comes from the supplier drawing and the product specification, not from a workshop rule of thumb. What matters in production is that the same value is measured the same way on a sample from each setup, so that a change in the press or the tooling is detected before a batch is shipped.
