Designing Electronics to Fit an Existing Product: Checking Mechanical Fit Before You Order

A marine electronics company needs to place a dual-motor trim-tab controller inside an existing hull compartment. The bench-tested board fits a flat sketch, but the molded cavity tapers, slopes near the lid, and shares space with a drain hose.

The company had already reserved a board run. The twin 24 V actuators moved the trim tabs independently, and the controller reported position over CAN. The board needed to sit beside a bilge-pump fitting inside a fiberglass compartment with a tapered wall and a sloped lid. A board order before fit review could turn a small production run into twenty parts that could not be installed.

Import the hull before moving a connector

The product engineer imported the hull and compartment STEP models into Flux. The usable space narrowed from 148 millimeters at the opening to 132 millimeters near the back wall. The lid sloped down by 9 degrees. Two molded posts accepted M5 screws, and a drain hose passed along the lower edge.

The controller board measured 92 by 68 millimeters and used four layers. It carried an STM32G4, two DRV8876 brushed-motor drivers, a TJA1051 CAN transceiver, current-sense amplifiers, and protected 24 V input. The board had two motor outputs, two position-sensor inputs, a CAN port, and a service connector.

Flux built the schematic and board from the motor and interface requirements. The product engineer checked the driver ratings against the trim-tab actuator datasheets and reviewed the current-sense paths. She updated the BOM with components that had current availability. The board connected the dual motor channels to screw terminals along the hull-facing edge, where the service technician could reach them through the opening.

Check component height against the real lid

The initial board placement left the largest driver heat sink beside the sloped cover. In top view, the board outline fit. In X-ray view, the heat sink came within 0.4 millimeters of the lid at the narrow end. The product engineer asked the mechanical designer, the mechanical engineer, to include the actual screw terminals and connector models instead of relying on generic boxes.

The mechanical engineer designed a 6061 aluminum tray that used the two molded M5 posts. A 2-millimeter tray carried the PCB on four M3 standoffs. The enclosure lid closed against a 2-millimeter silicone gasket. The service connector faced the hull opening, and the motor terminals faced the drain-hose side without crossing its path.

The team moved the driver heat sink 6 millimeters toward the wide end and changed the two motor terminals to a lower-profile version. The new terminal body cleared the sloped lid by 2.1 millimeters. The mechanical engineer added a 4-millimeter chamfer to the tray corner near the drain hose and left 10 millimeters for the cable bend.

The 3D assembly also exposed a problem with the hull's molded posts. They were 2 millimeters farther apart than the old drawing showed. The mechanical engineer lengthened the tray slots but kept the PCB's four mounting holes fixed. A strength check confirmed that the shorter tray ends still supported the board under the expected cable load. The product engineer reviewed the current paths and confirmed that no copper moved closer to the mounting screws.

Three requests from the product engineer's fit review

Design a twin trim-tab controller for two 24 V brushed actuators. Keep independent motor outputs and position inputs, report status over CAN, and fit inside the supplied hull compartment.

Use the STM32G4, two DRV8876 drivers, and TJA1051 on a 92 by 68 millimeter four-layer board. Keep current-sense paths beside each motor channel and expose the service connector at the hull opening.

Import the hull, sloped lid, drain hose, and M5 posts. Check the driver heat sink and terminal blocks in X-ray view, preserve 10 millimeters for the cable bend, and place the board on a 2-millimeter aluminum tray.

A fit check changed the order before fabrication

The product engineer and the mechanical engineer completed the board and enclosure review in an afternoon. They checked the imported hull, lid angle, drain hose, connector bodies, board heights, standoffs, and mounting slots together. The Flux agent kept the BOM and mechanical model aligned after the terminal change. The product engineer approved the revised pinout and signed the board release.

The final electronics used the STM32G4, two DRV8876 motor drivers, TJA1051 CAN interface, current-sense circuits, and protected 24 V input. The mechanical package included the aluminum tray, M3 board supports, hull-post slots, silicone gasket, terminal clearance, and drain-hose relief. The manufacturer received Gerbers, BOM, pick-and-place data, and STEP files for the tray and assembled controller.

The board order went ahead after the fit check. When the first assembled controller arrived, the mechanical engineer mounted it in the hull mock-up and confirmed the lid closed without pressing on the heat sink. The terminal blocks were reachable, the motor leads followed the drain-hose side, and the service connector faced the opening. The team tested both trim-tab motors independently before the boat's electrical installation.

The product engineer added the accepted hull STEP revision to the release package. The production team could see which molded posts and cover angle the controller fit. That mattered because the same hull platform had two compartment revisions, and the old drawing did not show the taper.

The design review prevented a late enclosure change and a second board order. More importantly, the electronics did not become a separate object that production had to force into the boat. The board, case tray, ports, cable bends, and hull volume were checked together before the supplier started the run.

A marine electronics fit checklist

• Import the current hull and compartment STEP revision before releasing the PCB.
• Model connector bodies, component heights, lid angles, and hose paths.
• Check the board in X-ray view against the real cover, standoffs, and tray.
• Preserve access to terminals and service ports after installation.
• Review current paths, heat-sink clearance, gasket compression, and cable bends.
• Order boards only after the mechanical assembly matches the production hull.

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