A Mechanical Engineer's Day: EE–ME Co-Design, PCBA Assembly, and Interference Checks

🌏 繁體中文版：[機構工程師的日常]:電子×機構協同設計、PCBA 組合、干涉檢查

People often think mechanical design means "drawing a nice-looking enclosure." In practice, a mechanical engineer's day revolves around a whole electrical–mechanical co-design process.

These are my notes from asking a senior engineer at a hardware system manufacturer what the job actually involves.

This post covers the standard industry workflow between electrical (EE) and mechanical (ME) teams, and what a mechanical engineer does once the board data arrives: building the 3D assembly and running interference (collision) checks.

The standard EE–ME co-design workflow

On the electrical side, the flow is fairly fixed:

  1. Starting from the customer's Product Requirement Document (PRD), an industrial design (ID) concept is shared with the customer. Then, based on the MCO (Mechanical Control Outline) and PCB outline, the team fixes the positions of key components, keep-out areas, and height-restricted zones.
  2. EE designs the circuit: power, interfaces, power budget, communication protocols.
  3. The schematic is drawn in OrCAD.
  4. EE selects suitable components (comparing datasheets, downloading vendor 3D models).
  5. Then comes PCB placement and routing (layout).
  6. Every component needs a matching 3D CAD model — this is effectively the interface contract EE hands over to mechanical.

The key hand-off point: once PCB placement is done, EE and the layout team must run a 3D assembly and interference check with mechanical. It isn't a one-sided mechanical task; it's a loop of "EE provides data → ME verifies → feedback."

StageEE (electrical)ME (mechanical)
1. RequirementsRead the PRD; break down electrical specs: power, interfaces, power budgetRead the PRD; break down mechanical specs: size, assembly, environment, cost
2. Core designSchematic in OrCAD → component selection → collect a 3D CAD model for each partModel the enclosure, mounting bosses, and openings in 3D; verify once the PCBA data comes back
3. PCBPCB placement → routing— (wait for placement freeze)
4. Joint checkAfter placement freeze, deliver DXF / STEP / component height listImport the PCBA into 3D CAD; run the assembly and interference check
5. ConvergeReceive the interference report → adjust placement or use lower-profile partsFix the enclosure and openings → verify again → freeze

The mechanical side: PCBA assembly and interference checks

Once the EE deliverables arrive, the mechanical engineer's core job comes down to three things:

  1. Import the PCBA model into 3D CAD (Rhino, SolidWorks, Fusion — whatever the company uses).
  2. Design the enclosure, mounting bosses, and openings (USB, terminal blocks, buttons…).
  3. Run the interference check — confirm that when the enclosure is closed, it doesn't press on any component, and every connector opening lines up precisely.

That last point is easy to underestimate and is the most important: passing an interference check doesn't mean "it closes." It means every component keeps at least about 0.5 mm of clearance from the enclosure. Tall SMT capacitors, BGAs, and connectors are the usual culprits for rubbing on real hardware.

In practice: building the 3D assembly from a DXF

What EE hands over usually isn't a complete 3D file. It's typically a 2D DXF board outline + a component height list + STEP files for key parts, and mechanical builds the 3D assembly from that:

2D board data received from the EE side
2D board data received from the EE side
The board rebuilt as a 3D assembly
The board rebuilt as a 3D assembly
StepAction
1Take the DXF, extrude it to board thickness (usually 1.6 mm), and cut the mounting/locating holes from the datum holes. The board solid has to exist before anything else.
2List the tall components likely to hit the enclosure (large capacitors, green terminal blocks, connectors, switches) and get their heights from EE (datasheets or the BOM).
3Download STEP files for the key parts (official vendor 3D models) and place them at their 2D positions from the DXF, building up a 3D assembly.
4Design the enclosure, bosses, and openings (USB, terminal blocks, buttons) around the PCBA assembly.
5Run the interference check: does the closed enclosure press on any resistor, capacitor, or connector? Are the connector openings correctly positioned and sized relative to the connector centers?
6If something collides → report back so EE can move parts or switch to lower-profile components, then repeat steps 4–5.

Common mistakes

  • "I didn't think that capacitor was so tall." Datasheet heights are usually the package outline; after soldering (including the solder itself) a part can sit 0.5–1 mm higher. Either add a blanket ~10% height margin during the check, or ask EE for as-soldered heights.
  • "The connector opening fits exactly." That ignores tolerances. The connector has tolerance, the plastic housing has molding tolerance, and "exactly" means "will jam." Give every connector opening at least 0.3 mm of radial clearance, plus a chamfer to guide insertion.
  • "Forgot the mounting bosses." A PCBA that just sits in place runs hotter and can crack solder joints under vibration. Bosses (or countersunk screws) are part of the structural stiffness, not decoration.

Wrapping up

A mechanical engineer's real skill isn't "drawing enclosures." It's closing the loop: read the EE data → turn 2D into 3D → prove with an interference check that it closes, clears every part, and the connectors line up. When that loop runs smoothly, far fewer problems show up in real hardware — and that co-design process is a big part of whether a product holds up in the market.

This post is adapted from the Traditional Chinese original on Vic's Iteration Notes.

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