When you’re creating footprints for through-hole components, you will need to size the diameter of the plated through-hole correctly in order to allow for solder filling. How much oversizing is needed in these components to ensure sufficient solder fill? This value is sometimes specified as a lead-to-hole ratio, or as a fixed value above the diameter of a through-hole component lead.
What Lead-to-Hole Ratio Should Be Used?
Lead-to-hole ratio is a simple metric that relates the diameter of a plated through-hole to the diameter of the component lead that will be mounted onto the through-hole:
Density level B
Min. hole size = (Max. lead diameter) + 0.20 mm
Density level C
Min. hole size = (Max. lead diameter) + 0.15 mm
Lead-to-hole ratio = (Diameter of lead)/(Finished hole diameter)
It’s often the case that a specific lead-to-hole ratio is actually not specified in design guidelines, particularly in those you will find from manufacturers. Instead, you will sometimes simply find a fixed recommended value for a particular board thickness. For a standard thickness (62 mil) PCB, the typical recommendation is to allow between 8 and 20 mils additional spacing around the component lead.
The relevant industry standards covering this area of design are IPC-2221 (Section 8.3.1, and Sections 9.2.3 through 9.3) and IPC-2222. In IPC-2222 (Sectional Design Standard for Rigid Organic Printed Circuit Boards, Table 9-4), readers will find data specific to through-holes on rigid PCBs, making this the typical standard that will be used to determine an appropriate lead-to-hole diameter. Note that the standard contains a discussion on the proper gap between leads and plated-through hole walls based on product classification (Class I-III).
The reason for the expansion of a plated through-hole diameter in relation to a component lead is due to the need for an area for solder wetting. Pb-free solders do not easily wet the surfaces of through-hole components as easily as PbSn solder or other solder alloys. If Pb-free solders are used, either higher temperature is needed in the solder pot and on the preheated PCB, or a larger lead diameter is needed.
Within the IPC standards there are three density levels that define hole expansions for through-hole components:
Pads Size and Hole Size Tolerance
Although there should generally be some expansion around the lead diameter to provide reliable assembly, this does not preclude the need for properly sized pads in relation to hole sizes. The pads used on vias and through-hole mount locations must be mounted to provide sufficient annular ring for your product class above the required hole diameter needed to accommodate the component lead.
For through-hole parts, we have two possible formula for determining pad sizes for a hole size based on the minimum annular ring requirement and the product classification:
- Pad = (Lead size) + (Hole expansion) + (Standard fab allowance) + (2x min. annular ring)
Note that this depends on the producibility level at which the fabrication house can reliably operate. Most fabrication houses today operate at Class C producibility level with standard fab allowance of 0.2 mm.
What About Lead Length?
The other aspect of through-hole component assembly is lead length. When a through-hole component is mounted in a PCB during assembly, there can be some remaining lead length protruding from the opposite side of the board. Normally these are just clipped by hand after assembly so that they will not interfere with other components or create a short. However, when you scale, you should still specify the allowed leftover lead length if your components will have very long leads for mounting.
These through-hole thick-film resistors will require trimming once placed into an assembly due to their long leads.
For some components, you won’t have the choice to modify lead lengths. For example, pin headers, press-fit connectors, and other fixed through-hole parts can have pin lengths that are not meant to be trimmed, either because they are plated or they are just too thick to quickly cut.
Other components, like axial passives, are intended to have trimmed leads so that the leads do not hang through the bottom of a PCB once the product is assembled. If these leads are excessively long and need to be trimmed, you will need to specify the lead size you can accept in the final assembly. To do this, make sure to include an item in your assembly notes in a PCB assembly drawing.
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