Classification

Introduction

We use “pattern classes” and “drill classes” as convenient shorthand to measure the manufacturability of the PCB.

This will define whether your board can be Pooled or not, it also has an impact on the pricing.

Pattern Classes

The pattern class covers the minimum sizes for copper track (conductor) and gap (isolation) for Outer and Inner layers:

Outer layer

  • OTT = Track to Track
  • OTP = Track to Pad
  •  OPP = Pad to Pad
  • OTW = Track Width

Inner layer

  • ITT = Track to Track
  • ITP = Track to Pad
  • IPP = Pad to Pad
  • ITW = Track Width

The minimum copper rings on outer and inner layers

  • OAR = Outer Annular Ring
  • IAR = Inner Annular Ring

The minimum IPI (Inner layer Pad Isolation)

  • IPI is the clearance between the edge of any unconnected hole (plated or non-plated) and the nearest copper (plane, track, pad).
  • This is measured from the production tool size (actual drill or rout bit diameter).
  • The minimum IPI value is always IAR + 0.075mm (75µm or 3mil).

IMPORTANT
  • The smallest of these values determines the pattern class.
  • The Classification table shows the lower limit values of any given class.
  • The thickness of the starting copper foil determines the minimum pattern values that are possible.
  • Thicker copper needs a wider isolation for reliable etching.
  • The highest pattern class possible depends on the copper thicknesses.
  • Not all copper thicknesses are available in all services.

For more information please see our current Classification table on below.

Drill Classes

The Drill class is based on the smallest production drill size (TOOLSIZE) used to manufacture your board,.

This is not to be confused with the finished size hole size (ENDSIZE) that you have defined in your design/CAD system.

For more information on holes sizes and how to calculate the TOOLSIZE vs the ENDSIZE please refer to our PCB Design Guidelines – Drilled Holes page.

IMPORTANT
  • The smallest of these values determines the pattern class.
  • The Classification table shows the lower limit values of any given class.
  • The Annular Ring values OAR and IAR in the Classification table are for plated holes (PTH).
  • For connected non-plated (NPTH) holes we recommend a minimum annular ring of 0.30mm (300µm or 12mil).
    • As NPTH holes have no plated barrel, a smaller annular ring may lift during soldering or break away even during normal operating conditions.

For more information please see our current Classification table on below.

Recommendations

  • Do not design your board up to the limits of any given classification.
  • Always allow for a small margin above the classification limits, this will allow for any rounding errors etc.
  • for more information please see our PCB Design Guidelines – Input Data Requirements page.

Classification Table Classification Table 16 March 2020


Page Update History

21/01/2020 – Modified – Classification Table – Service indicators for Drill Class

17/03/2020 – Modified – Classification Table – Changed minimum IAR value for Pattern Class 9 & 10

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