Component Courtyards and Keepouts: Space the Process Needs

Every component on a board needs space around it that carries no circuit. That space is what lets the stencil print, the nozzle place, the camera inspect and the technician rework, and it is the first thing a dense layout gives away. The difference between a board that assembles reliably and one that needs constant attention is often measured in tenths of a millimetre of unused area around the parts.

Courtyard, Keepout and Clearance

Three different boundaries surround a component, and confusing them causes most of the problems.

The pad clearance is an electrical rule. It is the gap between two adjacent copper features that keeps them from shorting, and it is set by the fabrication capability.

The courtyard is a mechanical and assembly rule. It is the area the component occupies on the board, including its body, its tolerance for placement and the space the assembly process needs around it. Two components whose courtyards overlap will interfere physically or will make one of them impossible to place accurately.

The keepout is a prohibition. It is an area where certain things must not be placed at all, for reasons that may be electrical, mechanical or thermal: under an antenna, around a crystal, beneath a shield can, inside the swing area of a connector latch, or around a mounting screw head.

Manufacturers publish courtyard dimensions with their footprints, usually as an excess around the body, and standards define density levels that trade area against assembly difficulty. A design that follows the least dense level is easy to assemble and uses more board area; one that follows the tightest level saves space and shifts more risk onto the process.

Why the Space Matters

The courtyard exists because five different processes need to get near the component.

The stencil needs support and a defined aperture, and paste that smears into a neighbouring aperture or a mask dam is a defect. The placement nozzle needs vertical clearance to descend and release the part without touching anything already placed. The optical inspection camera needs a sight line to the joints, which usually means a shallow angle from several directions. Rework needs room for a hot air nozzle and a pair of tweezers, and a part buried between two tall components is effectively not reworkable. And the component itself needs thermal isolation from its neighbours, both during reflow and in operation.

Typical Values

The excess around a component body is set by the placement accuracy and the size of the part, and the numbers in common use are close to the following.

• Small chip components such as 0402 and 0603: about 0.25 mm of excess on each side is a normal working figure, and 0.1 mm is a tight design that limits nozzle access.
• Larger chip components and small integrated circuits: about 0.5 mm, since the placement tolerance and the nozzle footprint both grow.
• Fine pitch packages and connectors: often 1 mm or more on the side where the nozzle approaches, because the placement head and any inspection camera need to reach the joints.
• Tall or heavy components: enough space on every side for a rework nozzle, which in practice means several millimetres and no neighbours taller than the part itself.

Where the space is genuinely unavailable, the trade should be made deliberately. A design can be tightened below the recommended courtyard, but then the stencil, the nozzle and the inspection angles all become constraints, and the assembly house should be asked whether the layout can be built before the design is released rather than after.

Keepouts for Specific Components

Some components impose a keepout that has nothing to do with assembly, and those are the ones that get forgotten.

Antennas. A printed or chip antenna needs a defined area of the board free of copper, and often free of any metal including the enclosure and the battery. The dimension depends on the design and the frequency, and it is usually the largest keepout on the board. Our article on PCB antennas covers how the clearance area is specified.

Crystals and oscillators. A crystal should be placed away from high speed switching nodes, from the edge of the board and from any trace that carries a fast edge, since coupling into the oscillator is a direct route to clock jitter.

Switching converters. The switching node and the inductor generate a magnetic field. Keep sensitive analog circuits, sensors and clock traces away, and keep the loop area of the current path small.

Sensors. A pressure, humidity or gas sensor may need an opening in the enclosure, no conformal coating, and a keepout around a vent. A magnetic sensor needs the absence of ferrous material nearby.

Connectors. The mating action needs space outside the board, and a latch or a cable bend needs room that is not always obvious from the footprint drawing.

Mounting hardware. Screw heads, standoffs and the flat area under a metal chassis have their own keepout for copper and for components.

Routing Inside a Courtyard

A courtyard is not the same as a copper keepout. In most cases traces may pass underneath a component, and doing so is normal on a dense board. What matters is which component and which trace.

Routing under a package is usually fine, provided the trace is covered by solder mask, the impedance is not affected by the package, and the component does not generate or need to avoid a field. Routing under a crystal, a switching inductor or an antenna is a different matter, and routing under a part that is bonded to the board with an adhesive or a thermal pad is generally impossible.

Routing under a connector is often unavoidable and usually acceptable, as long as the trace carries no signal that the connector will disturb and the connector body does not press on it. The risk is mechanical rather than electrical: a connector that is inserted and removed repeatedly can wear through the mask and the trace underneath.

Two practical rules follow. First, distinguish between the assembly courtyard, which is about space, and the electrical keepout, which is about interference, and record them as different things in the CAD data. Second, where a trace must run under a sensitive component, decide that deliberately and note it, because a later revision that moves the trace may not realise the constraint existed.

Height and Mechanical Interference

Copper is only one dimension of the problem. Components have height, and a board that is electrically correct can still fail because a part does not fit into the enclosure or under a chassis feature.

The practical approach is to identify the height zones before placement. Where a board slides into a case, the highest components need to be near the edges or in a dedicated region. Where a shield can or a heatsink sits over an area, the local height limit is what the can allows, not what the component list suggests. And where a connector protrudes through a panel, its position is fixed by the mechanical design and the board layout has to accept it.

Two habits reduce the surprises. Keeping a height map of the assembly and checking it against the mechanical model is the first, and it is particularly important for a first prototype. The second is to leave the sensitive areas of the layout until the mechanical design is settled, rather than routing them and then discovering that a mounting boss sits exactly where the tallest component needs to be.

What Happens When the Space Is Missing

The consequences of an over-dense layout show up as specific problems on the assembly line.

Paste smears between adjacent apertures when there is no room for the stencil to seal, which produces bridging that no profile change will fix. The placement nozzle collides with a neighbouring part, which either shifts the component being placed or damages the one already there. The inspection camera cannot see the joints at the angle it needs, so defects that would have been caught optically reach electrical test instead. And rework becomes impractical, because there is no space for a nozzle and no way to heat one joint without disturbing its neighbours.

There is also a thermal consequence. Components packed tightly together share their heat, so a part that runs warm raises the temperature of everything around it. On a board with several power devices this can be the difference between a design that meets its thermal specification and one that does not, and the fix is layout space rather than a bigger heatsink.

Design Checklist

• Place every footprint with its courtyard visible, and check that courtyards do not overlap.
• Confirm the density level the design is using, and accept the assembly difficulty that comes with the tightest level.
• Separate assembly courtyards from electrical keepouts in the CAD data, so both are visible and neither is lost.
• Reserve the antenna keepout, the crystal area and the switching converter loop before routing begins.
• Check the height map against the mechanical model, including shield cans, heatsinks and case features.
• Leave rework access on tall or expensive components, and on any part that is likely to need tuning.
• Keep test points out of courtyards where a probe would collide with a component, and out of areas covered by a shield.
• Ask the assembly partner to review the placement density before the design is released.

FAQ

• Can traces run under a component? Usually yes, provided the component is not sensitive and the trace is covered by mask. Check the specific part before assuming.
• Is the courtyard the same as the component body outline? No. It is the body plus the placement tolerance and the space the process needs.
• How much courtyard does a 0402 need? About 0.25 mm per side is a normal working figure, and less becomes an assembly constraint.
• Do courtyards matter for hand assembly? Less, but they still matter for inspection and for rework access.

Summary

A courtyard is the space a component needs in order to be printed, placed, inspected and repaired. It is not decoration and it is not the same as an electrical clearance, and treating the two as one is how a dense design ends up unmanufacturable.

The rules are simple to apply. Use the footprint courtyard from a reliable library, check that courtyards do not overlap, reserve the keepouts that specific components demand, and check the height map against the mechanical design. Then, where the design has to be tight, make the trade knowingly and confirm with the assembly and layout review that the process can still reach the joints, because the cost of recovering the space later is a redesign rather than a process adjustment.