Applying IPC Through-Hole Standards In PCB Design

April 2, 2021 Cadence PCB Solutions

Key Takeaways

  • Learn what IPC through-hole standards are.

  • Find out why it’s important to comply with IPC  through-hole standards.

  • Figure out how to calculate pad size with  IPC through-hole standards.

If there’s one skill that I wish I could master, it’s sewing. I never really got started as I have difficulty threading the needle. Either the eye of the needle is too small or the thread is too thick. However, perhaps it’s just my inability to work on something so tiny that’s the real problem.

I have better luck assembling through-hole components on a PCB and soldering them, although there can be some challenges to navigate. While it’s considerably easier than threading a needle, there are still some parameters that ought to have accurate tolerances to prevent problems in your PCBA.  

Thankfully, you can depend on existing IPC through-hole standards to ensure that the pad or via parameters are set at the optimum value. 

What Are IPC Through-Hole Standards?

IPC through-hole standards ensure the manufacturability of PTH components.

IPC through-hole standards provide guidelines for land and drill size for PTH components. 

As a PCB designer, you’ll come across the term IPC often. IPC was formerly known as the Institute of Printed Circuits. It’s a trade organization that was formed to regulate various aspects of the PCB design lifecycle. You’ll get detailed information on how to work on technical aspects in a PCB layout, including matters regarding plated through-holes.

There are two standards—the IPC-2221 and IPC-7251—that contain guidelines for through-hole elements in a design. The IPC-2221 is a generic standard that covers electrical and manufacturing requirements for a PCB. Section 9 of the IPC-2221 is dedicated to holes and interconnection, which serves as a good reference for PTH design.

IPC-2221 provides detailed guidelines on the minimum annular ring size, land requirements, location tolerance, and other relevant requirements for through-hole designs. The document also provides images as examples of how the holes ought to be drilled and fabricated.

IPC-2221 is supplemented by the IPC-2222 document, which contains standards on rigid organic printed boards. IPC-2222 contains guidelines for calculating hole size based on density level. 

You can also find more detailed guidelines in the IPC-7251 document. IPC-7251 is a dedicated standard for through-hole design and land patterns. It contains more specific guidelines, such as component tolerance for various types of through-hole leads, joint tolerance, and footprint dimensioning of the components. 

Parameters specified in the IPC-7251 are often designated for three levels of producibility:

  • Level A: General design producibility 
  • Level B: Moderate design producibility 
  • Level C: High design producibility 

Why Are IPC Through-Hole Standards Important?

IPC through-hole standards are important for solderability

Annular ring and drill size affect solderability.

It’s a mistake to assume that through-hole designs are easier than surface-mounted ones. Without complying with regulatory standards, it’s easy to make mistakes that are irreparable during assembly. 

For example, getting the drill size right for a component’s pad is a tricky affair. If the drill size is too small, you’ll find it hard to fit in the component’s lead. Squeezing the component into pads that are too small will either damage the component or rip off the plating. 

On the other extreme, a pad with a drilled hole with a large tolerance results in the component being too loose after placement. This can affect solderability and PCB repair efforts. 

Drill size aside, the annular ring’s dimension is an important parameter in through-hole designs. An annular ring is the copper area that extends beyond the drilled hole on the PCB. There’s a minimum value for the annular ring that needs to comply with. If an annular ring is too small, the copper around the hole may be easily detached when heated during repair or when subjected to mechanical stress. 

Calculating Pad Size with IPC Through-Hole Standards

IPC through-hole standards ensure optimized pad sizes.

Get the right pad size with IPC-2221 land size recommendations.

You’ll need to get the pad size right to avoid the problems we’ve discussed. Thankfully, IPC-2221 has spelled out clearly how pad sizes should be calculated. Use the following formula the get the right pad size for through-hole components:

Minimum pad size = maximum lead diameter + (2 x minimum annular ring requirements) + standard fabrication allowance 

The maximum lead diameter can be derived from the component’s datasheet while the other two parameters are available from the respective tables on the IPC-2221 standards. 

Once you’ve got the pad sizes calculated, you’ll need to configure them accordingly in a robust PCB design and analysis software. Try using OrCAD, as its user-friendly interface makes setting through-hole parameters easy. It also has a bunch of other tools to help ensure your designs are successful the first time. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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