When One Board Won't Fit: Splitting a Product Into Connected Boards

A camera operator asks where to connect a second HDMI source while an AV team reviews a crowded portable video interface. The first board carries two HDMI connectors, an audio codec, a display, controls, and video-processing parts; it fits only if the front-panel encoder overlaps an HDMI shell.

The interface needed two HDMI inputs, one HDMI output, stereo audio, a headphone jack, and a front-panel level control. The electronics lead led hardware engineering at an AV equipment maker. He proposed separating the video circuitry from the audio and user controls. The interface could keep its fast video paths on one board and gain a smaller control section, joined by a flexible cable.

Keep HDMI pairs on the video board

The video board used an ADV7611 HDMI receiver, an ADV7511 HDMI transmitter, and an STM32H743 for system control. It measured 105 by 64 millimeters and used six layers with controlled-impedance TMDS pairs. Both HDMI connectors and the display output stayed on that board. The electronics lead kept the high-speed differential paths short and away from the switching power input.

The second board carried a PCM3168A audio codec, the front-panel controls, headphone output, and status display. It measured 88 by 56 millimeters and used four layers. A 24-pin, 0.5-millimeter FFC carried I2S audio, I2C control, reset, status, and power between the boards. The ribbon did not carry HDMI signals.

The Flux agent developed the schematic and layouts from the interface list. The electronics lead checked the ADV7611 and ADV7511 datasheets, maintained the BOM, and reviewed the TMDS routing. He moved the HDMI output connector 3 millimeters toward the rear edge to keep its cable clear of the audio board. Design rule checks caught a gap between the video-board edge and the connector's shield tabs.

Stack the boards without trapping the ribbon

The mechanical engineer imported the extruded aluminum housing, HDMI connector models, headphone jack, and front-panel encoder. The enclosure measured 218 by 126 by 42 millimeters. The video board sat on a lower 2-millimeter aluminum tray, and the audio-control board mounted above it on four M3 standoffs.

The first assembly model put the FFC connector at the far end of the video board. Its cable crossed the HDMI shell and bent sharply against the top cover. The mechanical engineer moved the connector toward the center and added an 18-millimeter radius path between boards. The FFC passed through a 12-millimeter side channel and stayed outside the video-pair keep-out.

The electronics lead wanted the front-panel encoder at the center of the unit, with its shaft clear of the audio board. The mechanical engineer moved the upper board 6 millimeters rearward and shortened its mounting ears. The headphone jack then aligned with its front cutout, and the display was visible without blocking the control label. Two 1.5-millimeter thermal pads connected the video-board regulators to the aluminum top plate.

The housing had two 2-millimeter ventilation slots at each side. The mechanical engineer checked the airflow path around the ADV7511 and power converter. The assembly view showed the lower board, upper board, FFC, case, connector shields, and service screws together. An X-ray view confirmed 2.4 millimeters above the tallest capacitor and 3 millimeters between the ribbon and the lid.

Three requests from the electronics lead's product review

Split this two-input HDMI interface into a video board and an audio-control board. Keep the ADV7611 and ADV7511 with the HDMI connectors, and carry only I2S, I2C, reset, status, and power over the FFC.

Place the PCM3168A, headphone jack, encoder, and status display on the 88 by 56 millimeter four-layer board. Align the controls with the front panel and preserve 18 millimeters for the ribbon bend.

Fit both boards inside the 218 by 126 by 42 millimeter aluminum case. Keep the TMDS paths on the six-layer video board, clear the HDMI shells, and show the thermal pads and side vents in the assembled view.

The split board reached the demo case

The electronics lead and the mechanical engineer completed the electrical and enclosure review in an afternoon. They checked the connector map, TMDS routing, FFC signal list, ribbon bend, encoder shaft, headphone cutout, top-plate heat path, and board heights in one project. The Flux agent kept the board and enclosure views aligned as the mechanical engineer moved the FFC connector. The electronics lead reviewed the fast signal routes and approved the I2S pin map.

The final design used a six-layer video board with the ADV7611, ADV7511, and HDMI connectors, plus a four-layer audio-control board with the PCM3168A, display, encoder, and headphone output. A 24-pin FFC joined the two. The mechanical package included the aluminum tray, upper standoffs, case, front panel, thermal pads, and ventilation slots.

The assembled prototype fit the demo housing with the HDMI cables attached. The camera operator could change inputs, set the audio level, and monitor the headphone output without removing the cover. The video link stayed on the board designed for its controlled-impedance routing; the FFC carried the lower-speed audio and control signals between boards.

The split also gave the manufacturing team two boards that could be assembled and tested as separate subassemblies. The electrical team could revise the audio-control section without changing the HDMI layout. The case drawing kept their connector alignment and board stack tied to the same product revision.

The mechanical engineer marked the FFC pin-one corner on both boards and the assembly sheet. That label prevented a 180-degree insertion during the demo build and gave the contract assembler a clear orientation reference. She also added a simple continuity check for the audio data, reset line, and shared ground before closing the case.

A two-board product checklist

• Group circuits by signal speed, noise, and service function before splitting.
• Keep high-speed video paths and connectors together on the controlled-impedance board.
• Define every FFC signal, pin, orientation, and bend radius before layout.
• Import the case and connector models to check stacked-board access.
• Review component heights, thermal pads, airflow, and fastener access in X-ray view.
• Export both boards and the housing from one approved assembly revision.

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