The leading document for PCB fabrication is IPC-A-610. IPC-610is used by most PCB fabrication shops to manufacture PCB’s for their customersend-use, which are broken up into 3 Classes, 1, 2 or 3.

CLASS 1 –General Electronic Products

Includes products suitable for applications where the majorrequirement is function of the completed assembly. Toys

CLASS 2 –Dedicated Service Electronic Products

Includes products where continued performance and extendedlife is required, and for which uninterrupted service is desired but notcritical. Typically, the end-use environment would not cause failures.Computers & Phones

CLASS 3 –High Performance Electronic Products

Includes products where continued high performance orperformance-on-demand is critical, equipment downtime cannot be tolerated,end-use environment may be uncommonly harsh, and the equipment must functionwhen required, such as life support or other critical systems. Military &Medical

This affects the CAD library Through-hole packages due tothe pad stack construction must meet or beat the IPC-610 rules for AnnularRings, Plane Clearance, Inner Layer Pads, Thermal Relief Patterns as shownbelow.

There are not many IPC-610 Fabrication rules for SurfaceMount Technology. Minimum pad to pad & solder mask swell for registrationmight apply. But the pad size and spacing are most important for the Assemblyprocess for component attachment guidelines identified in the J-STD-001standard.

When defining a pad stack, you first must define the holesize. This is done by taking the maximum lead diameter for round leads and the hypotenuseof a rectangle shape lead. IPC considers that both round and rectangle leadsare the same, but PCB Libraries, Inc. believes that the round terminal leadconsumes more volume in the hole than a rectangle lead and therefore the holeover round lead should be larger than hole over rectangle lead. Library Expertallows the user to change both values independently. The Library ExpertPreference default values are Round Lead + 0.20 = Hole Size and Rectangularlead + 0.15 = Hole Size.

However, IPC-2222 has a table that is inclusive andcalculates the hole size based on Density Levels. See image below.

Once the Hole Size is established, the main point for padsize calculation in the IPC-2221 standard is the minimum annular ring of theplated through-hole pad stack after fabrication tolerances are applied. Here isan image of the minimum annular ring.

There are 2 fabrication tolerances that must be accountedfor. The first and most obvious one is the hole size tolerance that is in thedrill chart on the fabrication drawing. The typical hole size tolerance for athrough-hole terminal lead is +/- 0.075 mm (3 mils) and vias are + 0/- HoleSize. If the minimum annular ring for an external plated through-hole is 0.05mm (2 mils) and the hole size tolerance is 0.075 mm, the pad size annular ringmust be 0.125 mm or a pad size of 0.25 mm over the hole size.

The second tolerance that needs to be applied to calculatethe pad size is the layer to layer registration. This is typically held to0.075 (3 mils) and only occurs when the layer count is greater than 4 layers. A10 – 20 layer PCB could have layer to layer registration issues in everydirection on the inner layers. The minimum annular ring on the inner layers is0.025 mm. For Class 3 boards that are 10 or more layers, the maximum layer tolayer registration should be held to +/- 0.05 mm. Also, the hole toleranceshould be set to +/- 0.05.

Let’s make it simple. 0.025 minimum annular ring + 0.05layer to layer registration + 0.025 hole tolerance = 0.10 annular ring or thepad size = 0.20 over the hole size. But if the fabrication shop cannot hold thelayer to layer registration to 0.05 mm and requires 0.075 mm, then the minimumpad size would be 0.25 mm over the hole.

All of these values can be set in the Library Expertsoftware Preferences to ensure that your through-hole calculations meet or beatyour fabrication tolerance allowances.

The via hole quantity typically out numbers the terminallead holes and PCB designers want to make the via annular ring the minimumvalue per the manufacturing Class that they must meet. Since there is noterminal leads in a via hole, the hole size tolerance can be + 0/- Hole Size.Allow the fabrication process to plate the via holes closed. If an inner layershifts 0.075 in any direction, the annular ring is affected. See the imagebelow.

Another important pad stack calculation is the planeanti-pad. Most fabrication shops require a minimum annular ring of 0.35 mm (15mil) over the hole size. Making the plane anti-pad annular ring smaller couldcause the drill hole to be too close to the copper plane and making it largerremoves important plane copper that carries the signal return path fortransmission lines. Ideally, the plane anti-pad would be smaller than the padsize + the electrical clearance. The electrical clearance is the trace to padspacing rule. All traces on all layers should be over a copper plane and notintrude into the anti-pad area. Therefore, make the anti-pad large enough tomanufacture but small enough to maintain signal integrity.

Note: The OD of the plane Thermal Relief is the samediameter as the plane Anti-pad.

Incremental pad sizes are important for a couple reasons. Smallthrough-hole components have small terminal lead diameters that require smallerhole sizes. Small packages are normally light in weight and they don’t carrymuch current. Parts that require higher currents need larger terminal leaddiameters. High current parts generate more heat and are heavier in weight.These features require a more robust annular ring that will allow more solderto hold the part in place and to handle the heat transfer. The Library Expertsoftware program uses a default multiplier of 1.5 X hole size for the pad sizecalculation. See the image above for the pad size calculation with a 1.0 mmhole verses a 2.0 mm hole. The annular ring doubles in size.

All of the pad stack calculation values in the LibraryExpert software program come default with the best-known industry practices,but they are all user definable to ensure that your plated through-holecalculations meet or beat your corporate or customer standards.