The purpose of this document is to clarify the issues relating to clearance needs from hole to pad using figures based on standards developed by the Institute for Interconnecting and Packaging Electronic Circuits (IPC). The figures quoted are based on IPCâs Performance ãClass 3ä for high reliability electronic products and Design ãLevel Cä for high design complexity boards.
General Requirements
Pads must be provided for each point of component lead attachment or other electrical connection such as vias. Circular pads are the norm, but it should be noted that other shapes may be used to improve producibility. These may include, for example, filleting to create additional pad area at the conductor junction or keyholing to create additional pad area along the axis of the incoming conductor.
Examples of Modified Pad Shapes
Filleting 
Key Holing
All pads and therefore annular rings should be maximised wherever possible, consistent with good design practice and electrical clearance requirements.
Annular Rings Requirements
The minimum annular ring on a supported (plated) external layers is the minimum amount of copper (at the narrowest point) between the hole and the edge of the pad after plating of the finished hole. IPCâs class 3 calls for a minimum annular ring of 0.05mm (0.002").
Minimum Annular Ring
It is important to have some annularring left on the pad after manufacturing to avoid problems such as holewall pullaway, partial voids, partial solder joint, reduced clearance and soldermask in holes.
Annularring problems
Pad Requirements
IPCâs ãDesign Standard for Rigid Printed Boards and Printed Board Assembliesä (IPC-D-275) paragraph 5.3.2.1 states: To meet the annular ring requirements specified, the minimum land surrounding a supported (plated) hole shall be determined by the following equation:
Minimum pad size = a + 2b + c
where:
- = Maximum diameter of finished hole.
- = Minimum annularring requirements.
- = A standard fabrication allowance, which considers production master tooling and process variations required to fabricate boards.
Allowing a 0.1mm positive tolerance on hole sizes for (a),0.05mm minimum annular ring requirement for(b) and 0.45mm fabrication allowance for pane lsup to 450mm long for (c),comes to a total of 0.65mm, meaning that a commonly specified 0.9mm hole requires a 1.55mm pad.
This 0.65mm hole-to-pad clearance requirement is fine for 0.3mm (0.012") track & spacing designs running 1 track between pad on a conventional 2.54mm (0.1") pitch. It is, however, 0.05mm too high for the denser 0.2mm (0.008") track & spacing designs running 2 tracks between pads. Denser designs usually utilise surface mounted components where there are no hole to pad clearance problems other than for vias where the hole size is usually reduced to accommodate the smaller pad size.
Examples of design technologies
0.3mm track & spacing 
0.2mm track & spacing
Summary
Whilst 0.65mm is the preferred minimum hole-to-pad requirement, we accept down to 0.5mm so long as it is recognised that there is a potential risk of not achieving the minimum annular ring specified by IPC under Class 3. Redesign, replot or reducing the hole size are the only courses of action possible for boards where breakout is not tolerable.
As stated it is possible to get hole breakout with a 0.5mm clearance, if all of the allowable variables are on maximum and in opposite directions. These variations include hole size over nominal by 0.1mm, pad width loss of 0.05mm, pattern location movement and/or film expansion of 0.1mm (to the left) and hole location movement of 0.05mm (to the right).
Hole with & without variations
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