Designing Electronics Around Mechanical Constraints

Put both designs in one coordinate system, review them as one assembly, and write which features of the prototype shell are allowed to be fake. Every figure below is hypothetical teaching maths. None of it is a measured stack, a tolerance study from a tool, or a BrahmWorks result.

Agree the origin, the units and which file wins when the outline changes. Export the outline, the mounting holes and the keep-outs. Do not retype them.

Height is a map, not a single number. Some regions have a boss, a rib, a battery, or a hand. A flat "maximum component height" copied into the layout tool hides the tall connector under the rib and the short region where you actually had room. Section the assembly at the tall parts and at the fasteners. If the mechanical model does not contain the board and the tall parts, the section is a drawing of a wish.

The hardware product development process reviews enclosure and electronics together for that reason. Signing the housing in one meeting and the board in another is how the keep-out becomes a suggestion.

A connector has a direction, a mating cycle, and a relationship to a drafted wall. The wall moves as the toolmaker adds draft. The connector does not. Decide who floats: a slot that is generous, a connector on a tail, or a shell that is still allowed to change. A rigid connector aimed at a drafted opening, with no float and no agreed draft, will be shaved on the bench and then tooled that way by accident.

Cables inside the shell need a bend, a retention method, and a path that assembly can actually follow. A flex can remove a connector and its height. It adds a bend radius, a stiffener, and a place the flex is allowed to fold during assembly. It is not free height. Strain on a cable that leaves the product is a mechanical design with an electrical symptom: intermittent contacts that pass on the bench because the bench does not pull.

A keep-out is a volume, not a sentence that says "keep metal clear". It names what may not enter: copper, screws, a cell, a cable, a plated boss. An antenna keep-out that ignores the cell and the cable is an empty rule. A light pipe, a microphone port and a pressure path are the same kind of constraint. They need a hole in the board or a hole in the shell, aligned, and they fail if a component or a rib closes the path.

Heat needs a path into material that can take it, or an admission that the path is air and the temperature will rise until losses match. A copper pour under a hot part does little if the pour ends at the board edge and the shell is plastic with a gap. Write the path. If there is no path, do not draw a pour and call it thermal design.

Fasteners are electrical. A metal screw through a plated hole into a boss can tie a plane to a shield, or it can short a trace you ran too close. An unplated hole can still crush a nearby via. Bosses eat height on the far side of the board. Put the screw circle and the boss diameter into the height map before the layout fills that circle with parts.

A seal that must survive dust or water changes which test points you may leave exposed and which connectors may pierce the wall. A programming header that breaks the seal is a prototype feature. Say so, and say what replaces it on the unit you sell. Leaving it "for now" is how the production seal is designed around a hole you meant to delete.

Walk the assembly order with the real board, even if the shell is still printed. Screws that trap a flex, a battery door that must close before the connector is mated, a lid that covers the only way to reset the product: these are design defects, not training items. A note in a work instruction will not reach a step the fixture makes impossible.

Invented handheld. A cavity, a drafted wall, a USB-C plug that must leave that wall, a cell under a door, and an antenna keep-out along the top edge. Teaching dimensions, not a tool drawing: internal cavity height 8.0 mm over the connector zone; PCB thickness 1.6 mm; air under the board 0.4 mm; air you want above the tallest part 0.5 mm; connector body 4.8 mm above the board surface; a rib-boss of 1.2 mm intruding into that same zone because the ID model placed a screw there late.

The keep-out is a teaching volume 8 mm deep from the top inner wall, empty of copper, cell and cable. The cell door is on the back, outside that volume. Those statements are the constraint. The arithmetic checks whether the connector zone obeys them.

Heights that must add up in the connector zone:

0.4 + 1.6 + 4.8 + 0.5 = 7.3 mm

Cavity 8.0 − 7.3 = 0.7 mm spare, before the boss. With the boss: 7.3 + 1.2 = 8.5 mm, which exceeds 8.0 by 0.5 mm. The schematic did not fail. The screw did.

Two teaching escapes, not yet chosen. Move the screw out of the zone: the boss leaves, the 0.7 mm spare returns, and someone must check that the new screw does not enter the antenna volume. Or replace the rigid connector with a short flex to a connector parked where the cavity is taller. Teaching cost of that flex, invented: INR 70 per unit against INR 25 for the rigid connector, difference 45. At a teaching 1,000 units, 45 × 1,000 = INR 45,000, plus a flex drawing and a bend radius you must prove in the printed shell. The INR 45,000 does not buy back the 0.5 mm if the flex fold still occupies the zone during assembly. Stack the fold, not the pretty mated picture.

Draft on the wall is a separate sum. A teaching draft moves the opening by 0.3 mm at the outer face relative to the inner face. If the connector is located from the inner cavity and the drawing calls the outer opening, the plug kisses the wall. Float, or a looser opening, has to be at least that shift plus the location tolerance you have not pretended to close. 0.3 mm is an invented draft effect for the lesson, not a toolmaker's rule. The lesson is to put the number in the model once, not to discover it with a knife.

• Mechanical and electrical models share an origin, units, and a rule for which file wins.
• The outline, holes and keep-outs are exported, not retyped.
• Height is a map, including bosses, ribs, cells and hands, not one global maximum.
• The tall parts and the fasteners have been sectioned in the assembly.
• Each connector has a direction, a float or a waiver, and a drafted wall that matches.
• Cables and flexes have a bend and a path assembly can follow.
• Antenna, light, acoustic and pressure paths are volumes, and the forbidden contents are listed.
• Heat has a path, or the lack of a path is written down.
• Screws and bosses are in the electrical keep-out check.
• Seals and programming access do not contradict each other.
• The assembly order has been walked with a board and a shell, even a printed one.
• Prototype holes that will not ship are marked temporary.

Together, once an outline exists and again before either side is tooled or sent for a serious bare-board order. Separate sign-off feels faster and produces two products that do not fit. The shared section at the tallest part is the artefact to bring, not two screenshots.

It can remove the connector body from a short zone and spend that height on bend radius, stiffener and the fold during assembly. Stack those, in the closed product and in the assembly fixture. If the fold collides, the flex did not save the height. It moved the collision to a step nobody modelled.

Whatever the note lists: copper, components, fasteners, cells, cables, plating. "Keep metal clear" does not say whether a plastic rib with a metal screw is forbidden, and it does not say over what height. A keep-out without a volume and a list is a comment. Layout tools will route through comments.

They consume height, they can short a trace or stitch a plane to a boss, and they set an order of assembly that can trap a cable or hide a test pad. Put the screw circle into the board review at the same time as the netlist. A fastener added after layout is a new schematic whether or not anyone redraws it.

Bring the section through the tallest zone, the keep-out volumes, and the assembly order. A review can move a screw, or it can stop a board release that already does not fit the shell you intend to make.

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