Starting a Board From the Mechanical Drawing: Outlines, Mounting Holes and Keep-Outs

An electronics lead receives a DXF and STEP model before a board outline exists. The enclosure has already been designed around cable glands, a front service opening, and a steel support rail. The board must fit that geometry without forcing a case redesign.

The first email included a note about a 74 by 50 millimeter PCB, but the drawing showed four holes, two connector cutouts, a heat-spreader pad, and a cable path that narrowed the available space. The electronics lead chose to treat the mechanical drawing as a set of real design inputs, not an illustration that could be approximated after routing.

Read the drawing at its reference points

The electronics lead checked the drawing origin, units, and revision before importing it. The DXF defined the enclosure floor and hole centers; the STEP file showed the 2.5-millimeter aluminum wall, DIN-rail bracket, and two cable glands. She aligned both files to the same origin and measured the usable board area between the enclosure ribs.

The holes sat 66 millimeters apart horizontally and 42 millimeters vertically. A 4-millimeter keep-out around each M3 screw head left room for a 74 by 48 millimeter board. The front opening reserved 18 millimeters for a service connector, while the two cable glands needed 12 millimeters of bend space behind the terminal blocks. The electronics lead kept the wall's heat-spreader pad clear of connectors and tall components.

The board used a STM32G0B1, an ADS1115 analog converter, a MAX3485 RS-485 transceiver, four 4–20 mA inputs, and protected 24 V power. It had four layers and connected to the enclosure's terminal blocks through removable plugs. The electronics lead grouped the analog inputs near the lower glands and the serial connector at the front opening.

Let the connector edges guide placement

The Flux agent developed the schematic and board layout from the IO list and imported mechanical files. The electronics lead checked the part datasheets, set the four mounting holes to match the DXF, and confirmed that the board edge remained inside the usable floor. She ran design rule checks before locking the component positions.

The first 3D view revealed that the terminal-block levers could not open because a raised enclosure rib sat behind them. The electronics lead rotated the blocks 90 degrees and moved them 3 millimeters toward the board center. The wires now entered through the glands with a 12-millimeter bend radius. A low-profile USB service connector faced the front opening, while the RS-485 termination jumper remained reachable after installation.

The mechanical engineer reviewed the board against the STEP model. She added a small relief in the plastic end cap rather than changing the aluminum enclosure. The board used four M3 standoffs, and a 0.8-millimeter thermal pad contacted the case's heat-spreader land below the 24 V regulator. In X-ray view, the regulator sat directly over the pad and the terminal-block bodies cleared the side wall by 2.4 millimeters.

The team also checked the front opening from the outside. The USB connector was centered 1 millimeter below the panel label, and its plug could be removed without taking the unit off the DIN rail. The mechanical engineer changed the label plate position by 2 millimeters to preserve finger access. The bracket and case stayed the same.

What to model before routing

For a board that starts with a mechanical drawing, the order of checks matters:

1. Confirm units, datum, revision, and hole coordinates in the DXF and STEP files.
2. Create the board outline from the available floor, not the nominal enclosure width.
3. Add keep-outs for screw heads, ribs, cable bends, connectors, and heat-spreader areas.
4. Place connectors at their enclosure openings before routing internal signals.
5. Import component STEP models and verify the closed case in 3D and X-ray views.
6. Run board checks after the mounting holes and edges match the mechanical references.

These are not six generic layout steps. They are the constraints that shaped the electronics lead's controller and can be applied to another design where the mechanical package arrives first.

Requests the electronics lead used for the drawing-led layout

Create a 74 by 48 millimeter controller board from the supplied DXF. Match the four M3 holes, keep the front service connector inside its opening, and preserve the 12-millimeter cable bends behind the terminal blocks.

Use the STM32G0B1, ADS1115, and MAX3485 for four 4–20 mA inputs, protected 24 V power, and RS-485. Keep the analog inputs by the lower glands and place the service connector on the front edge.

Import the aluminum enclosure and DIN-rail bracket STEP files. Clear the raised rib by 2 millimeters, align the regulator to the 0.8-millimeter thermal pad, and show the closed cover in X-ray view.

Close the assembly before releasing the board

The electronics lead and the mechanical engineer completed the layout and enclosure review in five hours. They checked the four holes, terminal levers, cable paths, USB access, heat pad, bracket, and label position in the assembled model. The Flux agent carried the rotated connectors into the board and BOM. The electronics lead edited the IO labels and approved the pin assignment.

The final design used a four-layer 74 by 48 millimeter board with a STM32G0B1, ADS1115, MAX3485, four current-loop inputs, and protected 24 V power. Its mechanical package included the aluminum enclosure, plastic end cap, DIN-rail bracket, thermal pad, terminal blocks, cable glands, and label plate. The team exported Gerbers, BOM, pick-and-place data, and STEP files for assembly.

The first assembled board slid onto the enclosure standoffs without rework. The terminal levers opened, the service connector fit its cutout, and the heat pad touched the regulator. The electronics lead gave the installation notes to the panel technician before the refrigerated conveyor test.

The drawing-led process helped the team avoid routing a board to nominal dimensions and discovering later that a rib or screw head occupied the useful space. With the holes, keep-outs, and connector openings set first, routing became a circuit task inside an agreed mechanical envelope.

A board-from-drawing checklist

• Confirm drawing revision, units, origin, and mating STEP model.
• Locate mounting holes and keep-outs before placing components.
• Put connectors at cutouts and verify cable bend space.
• Check component heights, ribs, lid clearance, and heat-transfer contacts in 3D.
• Preserve access to service ports and terminal levers after installation.
• Export the board and enclosure package from the same approved revision.

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