Component Staking and Adhesives: Fixing Parts for Vibration
An adhesive applied between a component and the board is a small addition that solves several problems at once: it holds a part that is too heavy for its joints, it damps a resonance that would otherwise fatigue a lead, and it protects a connector from the load applied by a cable.
What Staking Achieves
The primary purpose is mechanical. A large component on a vibrating board acts as a mass on the end of its leads, and the leads experience a bending stress that grows with the mass and with the deflection of the board.
Bonding the component body to the board transfers the load from the leads to the adhesive, which is spread over a much larger area and has a much larger fatigue capacity.
The secondary purposes are thermal and electrical. A thermally conductive adhesive provides a path from the component to the board, and a conductive adhesive provides a ground connection that a lead cannot.
Choosing the Adhesive
The requirements are the cure schedule, the hardness after cure, the coefficient of thermal expansion, the temperature range and, where relevant, the thermal or electrical conductivity.
A soft, compliant adhesive absorbs differential movement between the component and the board, while a hard adhesive transfers more load and can crack at the interface. The choice depends on the component mass and on the thermal excursion.
Where the adhesive must conduct heat, the filler content makes it more abrasive and harder to dispense through a small needle. Where it must conduct electrically, the filler is metallic and the material must be used before its working life expires.
Cure and the Temperature Limit
An adhesive that cures at room temperature avoids thermal stress on the assembly but takes longer and may not reach full properties. A heat cured adhesive is faster and stronger, at the risk of subjecting the components to a second thermal cycle.
The cure temperature must respect the most sensitive component on the board, and the same considerations that apply to baking apply here. Our coating notes describe the related problem of curing in shadowed areas.
The cure schedule must be verified rather than assumed, and the usual verification is a hardness check on a sample cured alongside the production boards.
<img src="https://www.gopcba.com/wp-content/uploads/2024/10/HDI-PCB-min.jpg" alt="Pull test performed on a staked component” />
Where the Adhesive Goes
The adhesive is applied at the interface between the component body and the board, along one or more sides. It must not flow under the component and lift it, and it must not reach a joint that must remain flexible.
The bead should be sized so that it forms a fillet at the interface. A bead that is too small provides no load path, and one that is too large spreads onto neighbouring pads.
A common failure is adhesive that reaches the connector contacts or the inside of a mating surface. Masking or careful placement is required where the part must remain clean. Our design release checklist notes where these constraints belong.
Interactions With Repair
Anything bonded to the board is more difficult to remove, and the adhesive must be cut or softened before the component can be reworked. That operation risks damaging the pads and the adjacent parts.
Where rework is likely, the adhesive should be chosen for removability, and the design should keep the staking away from the pads that will be reworked. A removable adhesive is weaker and the trade must be made deliberately.
Our component reliability notes describe how the mechanical requirement is estimated, which is the basis for deciding whether staking is needed at all.
The Vibration Case
Where vibration is the driver, the analysis identifies the frequency at which the component and its leads resonate. Staking the body to the board raises that frequency and adds damping, and both effects reduce the stress.
Applying the adhesive at the end of the component furthest from the leads is often the better arrangement, since it adds damping without adding stiffness at the point where the lead must flex.
Our board design notes describe how the board deflection is reduced, which is the complementary measure to staking.
Process Control
The variables that matter are the dispense volume, the position, the cure and the working life. All should be recorded with the lot, as should the batch of adhesive used.
Adhesive that has been open beyond its working life behaves differently, and material that has partially cured in the needle produces a bead with an uneven cross section. Frozen material must be thawed completely and mixed as specified.
Our solder defects notes describe the joint failures that appear where the staking transferred load back into a solder joint through an unexpected path.
Verification
The verification is a vibration test on the assembly, performed with the staking applied, and a comparison against the same board without it. The test should run long enough to reach a fatigue conclusion rather than merely to detect an immediate failure.
A pull test on a sample of staked components confirms that the adhesive has cured and bonded, and it is quick enough to run on every lot.
Both measurements should be recorded, because a change in the adhesive supply or in the cure will show up first in the pull test and later in the vibration result.
Process Control and Verification
On a design of this kind, pull test is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
Process Control and Verification
On a design of this kind, pull test is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
FAQ
Does every large component need staking? No. The calculation compares the mass, the lead stiffness and the expected vibration against the fatigue strength of the joint.
Can staking replace a mechanical fixing? No. It supplements the solder joint; it is not a structural fastener and should not be used to carry a load that a screw or a clip could carry.
What does gopcb provide for component staking? We provide adhesive selection for the temperature and conductivity requirement, dispense volume and position trials on the first article, cure verification by hardness and pull test, and vibration testing with and without the staking. Where the calculation shows no benefit, we say so and save the process step.
