Mechanical Layer

Introduction

It is important to always include a Mechanical Layer data file with as part of your PCB data as it is vital to the production of a PCB.

Generating the Mechanical Layer Data file

The Mechanical Layer should contain the mechanical information necessary to produce your PCB.

It must contain at least the following information:

  • The exact PCB contour (profile), ideally with dimensions (mm or inch).
  • The exact position and size of any internal slots or cut-outs, ideally with dimensions (mm or inch).
  • Any specific tolerances required.

It may also include as necessary:

  • Customer Panel.
  • Drill Map.
  • Drill position Symbols.
  • Layer Sequence.
  • PTH and NPTH indication (if these are not supplied in separate drill data file).
  • Reference Hole.
  • Special Build requirements.
IMPORTANT
  • DO NOT provide actual milling layers for PCB contour or internal cut-outs or slots as it is production dependent as we cannot use this data if provided.
  • When no dimensions are given, we will always take the centre of the contour lines to be the exact outline of the board, regardless of their thickness.
  • DO NOT scale your Mechanical layer, it should be 1:1 and reflect the exact dimensions of the PCB.
  • DO NOT mirror or rotate the Mechanical layer.
  • The Mechanical layer is (as for all layers) viewed from top to bottom through your PCB.
TIP

It is best practice to create the outline using a small line for example 0.50mm wide – where the centre of the line represents the exact outline of the board.

The PCB Contour

This should be drawn accurately to represent the exact 1:1 size and shape of the PCB contour and should contain all relevant dimensions and any specific tolerances, preferably from a Reference Hole inside the PCB.

The PCB contour must be a closed polygon, that is starts and finishes at the same point.

mechanical-layer-Image1
mechanical-layer-Image2
Internal Corners

The PCB contour is normally milled using a Ø 2.00mm tool. This means that any internal corner will have a minimum radius of 1.00mm.

If you require a smaller radius on internal corners this must be clearly indicated in the Mechanical Layer.

mechanical-layer-Image5

Internal Corners with No Radius

To achieve a sharp 90-degree internal corner without a radius you must either place a NPTH (of the correct size) at the exact position of the internal corner on the PCB contour or design the contour in such a way that a create a sharp/90-degree corner.

mechanical-layer-Image6

It is also important to identify which internal corners need to be sharp and without a radius.

Cut-outs and Slots

Cut-outs and slots can be completely inside the PCB contour or overlap it to make a more complex contour, the definition of each is below.

Cut-out – any shape of opening that cannot be created by a drill.

Slot – a cut-out of a constant width opening that is equal to the diameter of a round cutting bit and is defined as a single movement from start to end point.

Drill – a circular hole that can be created using a round drill bit.

Cut-outs

These should be drawn accurately in the Mechanical Layer to represent the exact 1:1 size and shape of the cut-out. All the relevant dimensions and any specific tolerances are included preferably from a Reference Hole inside the PCB contour.

It is best practice to create the outline using a small line for example 0.50mm wide – where the centre of the line represents the exact outline of the cut-out.

Cut-outs must be a closed polygon, that is starts and finishes at the same point.

Internal Corners of a Cut-out

The cut-out is normally milled using a Ø 2.00mm tool. This means that any internal corner will have a minimum radius of 1.00mm.

If you require a smaller radius on internal corners this must be clearly indicated in the Mechanical Layer, like the above for the PCB contour.

Internal Corners of a Cut-out with No Radius

To achieve a sharp 90-degree internal corner without a radius you must place a NPTH (of the correct size) at the exact position of the internal corner of the cut-out contour, like the above for the PCB contour.

Slots

These should be drawn accurately in the Mechanical Layer to represent the exact 1:1 size and shape of the slot. All the relevant dimensions and any specific tolerances are included, preferably from a Reference Hole inside the PCB contour.

They should be drawn with a single chain consisting of lines and arc as necessary to achieve the required shape of the slot.

The smallest slot width is 0.50mm finished size (ENDSIZE).
Slots with a copper on the top and bottom layer will treated as PTH and those with NO copper on the top and bottom layer will treated NPTH unless otherwise stated.

NOTE

Slots can be defined at the PCB level or as part of a component footprint.

Board Level – Slots that are defined as part of the contour of the board, or on a mechanical layer and do not appear in a drill file supplied to the manufacturer.

Component Level – Slots that are defined as part of a component in the PCB design tool, and that should be included in a drill file supplied to the manufacturer.

Types-of-cutouts-diagram

Customer Panels

If you supply panelised data include proper indications for break-routing and/or scoring (V-cut).

Drill Map

mechanical-layer-Drill-Map-1
mechanical-layer-Drill-Map-2
IMPORTANT

We ONLY use a drill map to cross-check that the drill file(s) are correct.

A drill map will NEVER be used to generate a drill data for production as it gives too many opportunities for error.

Drill Position Symbols

Positional indication of all drills using symbols (=drill map).

Use different symbols for each different drill size.

Layer Sequence

A clear layer sequence or buildup drawing for all copper, soldermask and legend layers.

Include any additional layers such as peel-off or carbon.

PTH/NPTH Indication

Indicate which holes and slots are PTH and which are NPTH.

Reference Hole

The distance from one drill hole in X and Y to the PCB outline.

This is particularly important when you only have NPTH holes without copper pads.

Provide the correct sequence as viewed from top to bottom and with the correct corresponding data file names.

Special Buildups

If the build of your PCB requires material thicknesses, copper thicknesses or multilayer buildups which are different from our standard values then add this information to the layer sequence or buildup drawing.

This would result in your boards being non-Poolable and may result in a higher price.

Standard Mechanical Tolerances

Our standard mechanical tolerances are:

  • PCB Contour
    • Dimension +/- 0.20mm
    • Positional +/- 0.20mm
  • Cut-out
    • Dimension +/- 0.20mm
    • Positional +/- 0.20mm
  • Slot
    • Dimension
      Length +/-0.20mm
      Width +/-0.20mm
    • Positional +/-0.20mm

If no tolerances are specified in your data, we will produce according to our standard tolerance specifications.

If you require tighter tolerances these should be clearly indicated in the mechanical layer and tool list.

They will increase the cost of the PCB so should only be used if absolutely necessary.

IMPORTANT

These tolerances may change without prior notice, please always check our  Tolerances on a PCB page for the latest available tolerance information.


Page Update History

22/06/2021 – All Section updated

Back to Top