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Calculating Plated Through-Hole Size and Pad Diameter
When designing a through-hole PCB footprint, the correct size of the plated through-hole and its associated pad is critical. Getting these dimensions correct will help ensure that there are no issues when assembling the PCB.
If the size of the hole is too small, it is likely that the pins of the component cannot be inserted into the hole, resulting in the need to redo the PCB layout.
If the distance between the edge of the pin and the wall of the hole is too small, the solder will not flow smoothly from one side of the board to the other. If the distance between the lead edge and the hole wall is too large, the solder will not remain and fill in the hole. In both cases, because of insufficient solder, the components may not be soldered firmly.
If the pad diameter is too small, it will also cause poor soldering connection due to insufficient solder.
In case of the above situations, additional manpower will be required to solve these issues taking additional time, and thereby affecting production schedules.
According to the IPC standards, there are 4 steps to properly design plated through-hole size and pad diameter:
1. Determine the performance classes and density of PCB design
2. Determine the lead diameter
3. Determine the plated through-hole size
4. Determine the pad diameter
1. Determine the performance classes and density of PCB design
It is recommended to initially determine performance classes and density levels of producibility for the intended design.
There are three performance classes in IPC:
- Class 1: General Electronic Products
- Class 2: Dedicated Service Electronic Products
- Class 3: High Reliability Electronic Products
There are three design density levels of producibility in IPC:
- Level A: General Design Producibility -Preferred
- Level B: Moderate Design Producibility -Standard
- Level C: High Design Producibility -Reduced
The performance classes and density level will determine the plated through-hole size and pad diameter.
2. Determine the lead diameter
Most components have round leads, but some of them have square leads, and others have rectangular or even oval leads.
For round leads: The lead diameter is pin lead thickness. For Square/rectangular/oval lead: The lead diameter is the pin diagonal size.
Figure1: Cross-section of through-hole component pin leads
3. Determine the plated through-hole size
To determine the plated through-hole size, we must first understand LMC (Least Material Condition) and MMC (Maximum Material Condition). Refer to IPC-2222B. 9.2.2, page 19
There are two extreme situations to consider when mounting a though hole lead into a hole on PCB.
Smallest lead
Largest hole
LMC: Lead is at smallest, hole is at largest
Largest lead
Smallest hole
MMC: Lead is at largest, hole is at smallest
Figure2: Least Material Condition and Maximum Material Condition
After we determine the density level of the PCB design and know the shape of the leads, we can calculate the size of the hole based on IPC-2222B (Page 19, Table 9-6) to determine the plated through-hole size.
Figure3: IPC-2222B (Page 19, Table 9-6)
4. Determine the pad diameter
From the hole diameter we calculated above, we can determine the pad size based on IPC 2221B.
The worst-case pad-to-hole relationship established per IPC-2221B (9.11, page 95) is as shown below:
Land Size (Minimum Pad Diameter) is equal to a+2b+c, where:
a: Maximum Hole Size: Calculated from step 3.
b: Minimum Annular Ring (IPC-2221B, 9.1.1-2, page 97-99)
Figure4: IPC-2222B (Table 9-2)
c: A standard Fabrication Allowance (IPC-2221B, page 96)
Figure5: IPC-2222B (Table 9-1)
For example, In the case of Level B/External Pad, the formula is: Minimum Pad Diameter = Maximum Hole Size + 0.1mm + 0.25mm
Let’s take this example one step further.
In the case of a square lead, considering Level B and External Pad (Unit: mm), this is the calculation:
Figure6: Example of a square lead
Minimum Lead diameter in LMC: 1.23-0.04=1.19mm
Maximum Lead diameter in MMC: 1.23+0.04=1.27mm
Maximum plated through-hole size in LMC: 1.19+0.7=1.89mm
Minimum plated through-hole size in MMC: 1.27+0.2=1.47mm
Minimum Pad Diameter = 1.89mm + 0.1mm + 0.25mm=2.24mm
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