Rigid-Flex Steering-Sensor PCB Engineering Example

Keep the sensor datum independent of the flex connection

A rigid-flex steering-sensor concept can place the sensing circuit in one rigid region and supporting electronics in another. The flexible connection accommodates the packaging arrangement. It should not be assumed to locate the sensor accurately or to absorb an undefined assembly displacement.

This is a board-level engineering example, not a qualified steering sensor or evidence of a delivered product. A four-layer rigid-region proposal and a polyimide flex connection are possible design inputs. They do not define the number of copper layers in the flex, its bend life or the accuracy of the assembled sensor.

Identify what establishes the measurement geometry

For a magnetic-angle sensing concept, define the magnet, shaft axis, sensor location, mounting datums and relevant gaps in one mechanical coordinate system. The sensor's active location may differ from the package outline. Use the actual device drawing and magnet specification, then show how the rigid board region is constrained in the housing.

TI's angle-sensing application note discusses the effects of magnet selection, sensor placement and mechanical alignment on measured angle. It supports treating geometry as part of the measurement chain. It does not establish accuracy for this proposed steering-sensor arrangement or replace a system-specific calibration and verification plan.

A hypothetical mechanical tolerance budget

Assume three independent drawing contributions are bounded as follows: PCB locating feature ±0.10 mm, housing locator ±0.15 mm and sensor placement ±0.05 mm along one selected direction. A conservative worst-case sum is ±(0.10 + 0.15 + 0.05) = ±0.30 mm. For a nominal 0.80 mm geometrical gap in this exercise, the calculated interval is 0.50 to 1.10 mm.

This is a one-direction mechanical budget, not a magnetic model or an angle-error prediction. It does not include shaft runout, thermal expansion, board deformation or a statistical distribution. Its practical use is to ask whether the proposed location scheme needs tighter control before magnetic simulation and testing. Do not convert the 0.30 mm result into an angular accuracy claim.

Review the flex region and transition separately

Würth Elektronik's RIGID.flex 1F rules describe a particular construction with a single copper layer on externally located flexible polyimide material and explicit transition and coverlay definitions. That specific example demonstrates why rigid and flex region names alone are insufficient. Its layer construction and numeric rules are not adopted for this hypothetical board.

The proposed fabrication drawing should identify each region's layer stack, flex contour, coverlay openings, stiffeners where needed and the boundary to the rigid material. Define whether the connection is formed once during assembly or experiences motion. Evaluate the routing and return paths across the transition, with vias and component lands located according to the agreed construction.

Check whether the flex moves the sensor island

In the installed model, inspect the direction and support of the flex connection. If its restoring force tilts or displaces the sensor region, an acceptable flat-board dimension may not describe the installed sensing geometry. Connector mating and screw tightening can also change that relationship, so they belong in the assembly sequence.

Compare the proposed rigid-flex arrangement with a separate interconnect if it simplifies controlled sensor location. The rigid-flex PCB review should address the actual region drawing, assembly access and packing state. The objective is a reproducible geometry, not simply removing a cable from the parts list.

Verify geometry and sensing as distinct outputs

The system owner defines angular acceptance, environmental coverage and any functional-safety assessment. Neither board continuity nor a mechanical gap measurement proves steering-system suitability.

Hand over the coordinate system and tested assembly

Provide fabrication and assembly files, region stackups, flex and coverlay data, stiffener definitions and final installed drawings. Include the sensor and magnet revisions, active-element location reference, fixture datums, calibration version and test method. Record whether sensing data were taken before or after the final mounting operation.

Begin a review with the sensor principle, housing layout and intended motion of the connection. Use the engineering RFQ to provide the available design and define the project-specific manufacturing and evidence requirements.