“Pattern and Drill Specification” are used shorthandly to measure the manufacturability of the PCB: this generate a classification which is related to the PCBoard’s Services. Any way these classes generally permit to control the price of the PCB on demand service.
Parameter of these specification are:
- the minimum width for copper tracks (conductors) and gap (isolation) between them both for Outer and Inner layers;
- the minimum gap (distance) between cu-plated trough hole and lay out copper both for Outer and Inner layers;
- the minimum copper rings (difference between copper pad and hole sizes) both on outer and inner layers;
- the holes characteristic (minimum size and related production process);
- the required or needed hole accuracy (location and align registration);
- the solder mask characteristic (size of clearance related to copper lay out, minimum line size, devolpment in holes)
- mechanical requirements
The smallest or biggest of these values determines the pcb pattern class.
For details is to be related to the PCBOARDS CLASSES document
IMPORTANT: all calculations are done from the provided cad data which are considered the finished required values (ENDSIZE requirement). The conversion rules for END size to TOOL or ARTWORK size are explained in following sections.
a- the total or finished surface copper thickness is not a suitable parameter due to the electro-phisical characteristic of electrolytic plating processes. A good rule is to indicate always the required base copper and the minimum thickness of the copper wall of the cu-plated holes.
b- thickness of the base copper foil determines the minimum pattern values; this means that the highest pattern level ( minimum line or gap or anular ring) depends on the base copper thicknesses. Thicker copper needs a wider isolation for reliable etching. Not all copper thicknesses are available in all services; check our services table as guideline.
c- the Annular Ring values are normally considered for plated holes (PTH). For non-plated (NPTH) connected holes we suggest a minimum annular ring of 0.30mm (12mil): having no plated barrel, a smaller annular ring may lift during soldering or break away even during normal operating conditions.
d- It is recommended to not draw up to the limits of any class: a small margin is suggested to be kept above the classification limits. That may be used during loading data where the CAD output does not exactly match the design data due to rounding or matching errors caused by different units or grids.