We use “pattern classes” and “drill classes” as convenient shorthand to measure the manufacturability of the PCB. This controls whether the board can be pooled or not and has an impact on the pricing.
The pattern class covers
· The minimum sizes for copper track (conductor) and gap (isolation) for Outer and Inner layers:
- Outer layers OTT = Track to Track , OTP = Track to Pad, OPP = Pad to Pad and OTW = Track Width
- Inner layers ITT = Track to Track , ITP = Track to Pad, IPP = Pad to Pad and 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.
Minimum IPI value is always IAR +75µm (3mil)
The smallest of these values determines the pattern class.
The drill class is based on the smallest production tool size on the board.
For more information see the current Classification table on the next page
IMPORTANT: Annular Ring calculations are done from the production TOOLSIZE for the holes, not from the finished hole ENDSIZE. --> For the conversion rules for ENDSIZE to TOOLSIZE see Holes section 6.
1. The Classification table shows the lower limit values of any given class.
2. 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 (12mil). As NPTH holes have no plated barrel, a smaller annular ring may lift during soldering or break away even during normal operating conditions.
3. The thickness of the starting copper foil determines the minimum pattern values that are possible. This means that the highest pattern class possible depends on the copper thicknesses. Thicker copper needs a wider isolation for reliable etching – see the Classification table.
Not all copper thicknesses are available in all services. See our services overview document for guidance.
4. RECOMMENDATION. Do not design up to the limits of any given classification. Always keep a small margin above the classification limits. This may be needed where the CAD output does not exactly match the design data due to rounding or matching errors caused by different units or grids