Wireless charging is often demonstrated with two exposed coils centered neatly on a bench. Production products are less cooperative. Plastic walls vary, adhesives build thickness, coils shift during bonding and users place devices at angles. A system that looks strong at nominal alignment can become slow, hot or unstable at the edge of those tolerances.
For that reason, wireless charging coil alignment belongs in the mechanical specification, not only in the electronics test report. We want to know where both coils can be, how far apart they can move and what performance is acceptable throughout that window.
Alignment has three dimensions
Teams often discuss only lateral offset, the X and Y movement between coil centers. The Z direction is equally important. Every layer between the transmitter and receiver adds distance: top cover, paint, decorative film, adhesive, air, receiver housing and internal support features.
Angular error also matters. A receiver can be centered but tilted because of a camera bump, curved surface, product foot or uneven cradle. That tilt creates a smaller gap on one side and a larger gap on the other.
A useful alignment specification therefore includes:
- X and Y center-position tolerance
- Nominal and maximum coil-to-coil Z-gap
- Allowed angular tilt
- Coil flatness and housing warpage
- Receiver orientation and rotation
- Movement expected during normal use
These values should be based on worst-case stack calculations, then confirmed on physical samples.
Choose stable mechanical datums
The coil should be dimensioned from features that the assembly process can control. A cosmetic outside edge may shift relative to internal bosses after molding. A removable cover may not locate consistently. If the receiver docks against two internal ribs, those ribs may be a better datum than the outside silhouette.
On the drawing, identify the coil center and polarity or winding orientation. Show the ferrite outline separately. Define how the coil is prevented from sliding while adhesive cures. For a flex coil, add the tail direction and bend keep-out, because a tail under tension can pull the coil away from its intended center.
In the factory, we also need an inspection method. A tight tolerance that cannot be measured economically is not a production control. Camera fixtures, locating gauges or marked reference samples can be more practical than manual caliper checks on a hidden coil.
Do not treat the coil as a circle only
Diameter is not the only characteristic that influences coupling. Winding distribution, inductance, resistance, ferrite size and nearby conductors change the magnetic behavior. Two coils with similar outside dimensions may not provide the same operating margin.
The receiver-design guidance from Texas Instruments notes that coil and shielding selection affects efficiency, temperature, coupling and FOD. This matches what we see during integration: a coil substitution that fits the drawing can still require new tuning and validation.
The approved coil should therefore be controlled by manufacturer, part number, electrical limits and construction. If a second source is needed, qualify it as an engineering change rather than a purchasing-only substitution.
Z-gap is a stack, not a single wall thickness
Product drawings often list the top-cover thickness and call it the charging distance. The real Z-gap includes much more. On the TX side there may be coil adhesive, a support plate and clearance to the inner cover. On the RX side there may be another adhesive layer, ferrite, battery clearance and the receiver enclosure.
Create a stack table with nominal, minimum and maximum values. Include paint, labels or soft-touch coatings if they sit in the magnetic path. If the product uses a protective case, decide whether the case is part of the guaranteed operating condition.
Air gaps caused by ribs or cosmetic curvature are especially costly. Plastic can be relatively benign magnetically, but unnecessary distance still weakens coupling and increases the effort required to deliver power.
Magnets improve placement but add new controls
Magnetic alignment can make the user experience more repeatable. It does not eliminate tolerance work. Magnet diameter, thickness, grade, polarity, concentricity and retention all need control. A reversed or shifted magnet ring can create an obvious functional failure even when the electronics are correct.
Magnets and metal parts also affect loss and FOD behavior. Test the intended magnetic assembly, including adhesive and any steel retention pieces. Do not validate with one magnet grade and quietly substitute another after pilot production.
Assembly fixtures should prevent reversed polarity. A simple go/no-go polarity check can be built into the station before the housing is closed.
Build a position-and-load map
Our preferred validation method is a grid or defined set of mechanical limit positions. At each point, run more than a charging-start check. Record useful output or device behavior, input power, temperature and stability over time.
The map should include:
- Nominal center position
- Positive and negative X/Y limits
- Corner or combined offsets
- Maximum Z-gap
- Maximum allowed tilt
- Representative light, medium and full loads
- Normal and elevated ambient conditions where relevant
For products that move during use, add dynamic tests. A vehicle charger, robot dock or handheld cradle may experience vibration and repeated repositioning. The system should recover without cycling endlessly between start and stop.
Link alignment to thermal results
Misalignment often increases loss before it creates a complete charging failure. The user may still see a charging icon while the product runs hotter or delivers less power. That is why pass/fail based only on detection can be misleading.
Plot temperature and power at nominal and limit alignment. If firmware derates at the limit, define whether that is acceptable. The product claim should reflect sustained real-world performance, not the best moment at perfect alignment.
For a sealed receiver, monitor battery temperature as well as the RX electronics. Heat from the coil and power stage can flow directly into the cell depending on the internal stack.
Convert the design window into factory controls
Once the limits are validated, decide which variables require production checks. Typical controls include coil-position fixtures, adhesive thickness, magnet polarity, ferrite placement and a functional test at one or more limit positions.
A golden sample represents nominal performance. Limit samples are also useful: they show the acceptable boundary for position, cosmetic fit or charging behavior. Both should be revision controlled.
If field returns show intermittent charging, retain the assembly before pulling it apart. Measuring the as-returned coil position and stack can reveal a mechanical cause that disappears after disassembly.
What to provide for an alignment review
Send a section drawing, coil-area dimensions, enclosure materials, nearby metal and magnets, expected receiver positions and the required charging power. If the industrial design is still flexible, mark the surfaces and features that cannot move. We can then focus changes on the remaining space.
CowinLink’s TX/RX engineering process includes mechanical-stack and coil-position review before a design is locked. Our industry application overview also shows how alignment requirements differ across medical, furniture, automotive and industrial products.
For a project-specific review, send the cross-section, target power and available tolerance window through our engineering contact page.
The practical conclusion is straightforward: specify a three-dimensional alignment window, validate performance throughout it and control the critical stack variables in production. That is how a successful bench demonstration becomes a repeatable wireless charging product. A controlled wireless charging coil alignment window is therefore part of the product specification, not just a sample note.
