Understanding the IPC Through Hole Standards for PCB Layout

In 1859, English naturalist Charles Darwin published “On the Origin of Species” in which he put forth his theory of evolution. This work has served as the basis of our understanding of how plant, animal, and human species change over time. Although largely based on observations, Darwin’s work has led to assumptions and conjectures that have subsequently proven to be incomplete or false. Not the least of which is the emergence of the cognitive superiority of us humans or the fractal patterns observable in plants. These do not diminish the importance of the work, as environmental factors and competition do play a role in the progression of life. However, later improvements upon the original work do demonstrate the need to continually evaluate ideas and theories so their evolution is most accurate and useful.

The brief history of printed circuit board technology demonstrates a similar evolution. The first circuit boards utilized through-hole connectivity. However, trends of miniaturization and multilayer PCBs have promoted the use of SMDs, resulting in a decline in through-hole usage. Nevertheless, just as aligators have survived with few evolutionary modifications for millions of years, there remain situations where through hole components are the best choice. The PCB industry has also evolved from the early days, and now there are guidelines and rules that govern every aspect of board manufacturing. For through-hole components, the IPC assembly standards are of the most concern. Let’s take a look at why understanding these IPC through-hole standards is important and how they affect your PCB layout design.

When and Why Are Through-Hole Components Used?

It is rare to find PCBA designs that employ only through-hole components these days. The same cannot be said for SMD components. In an effort to build smaller devices with greater complexity and functionality, circuit board designers routinely design boards that only include SMDs. The best designs, however, are those that incorporate the component types that are most well-suited for the environment in which the board must operate. For example, extreme environments, such as space and industrial, typically place temperature, pressure, and motion requirements on boards that through-hole components are better equipped to withstand.

For more information on extreme environmental effects on circuit boards, refer to: PCBA Manufacturing For Extreme Environments Temperature Pressure Corrosion. 

In many cases, the best design is the one that incorporates both through-hole and SMT, as there are manufacturing advantages and disadvantages to each. These include reduced board size contrasted with greater design complexity for SMT and high power usage balanced with increased spacing requirements for through-hole technology (THT).

To see when THT may be a better option than SMD for your design, see: When Is The Through-hole Technology Process Utilized?

Whether you opt to take advantage of THT for your board design in part or completely, you need to be aware of and implement the IPC through-hole standards for your PCB layout, as discussed below.

Applying IPC Through-Hole Standards for PCB Layout

In the figure below, a comprehensive flowchart of IPC standards for PCBA development, which includes design, fabrication, and assembly, is shown.

PCBA IPC Standards overview

As shown above, there are quite a number of IPC standards that are applicable to your PCBA development process. The most important of these are the colored sections in the center: yellow for assembly, blue for fabrication, and green for design. These documents provide guidance for some of the most important layout issues for all three IPC-6011 PCB manufacturing classifications for all boards. In most cases, class 2 exceeds the requirements (of class 1) or is sufficient (for class 3) for your board’s application.

For through-hole components, two of the most important standards are IPC-610A and IPC-J-STD-001, which include requirements and recommendations for the PCB layout parameters listed below.

IPC THROUGH-HOLE STANDARDS FOR PCB LAYOUT
PCB Layout Parameter	Description
Drill hole size	For drill hole sizes, it is important to ensure that your CM can make or bore the holes that you need and that aspect ratios meet requirements for your board size so structural integrity is not compromised.
Annular ring size	The size or diameter of annular rings must be sufficient to provide a good solder joint with the connected trace.
Solder mask relief	Solder mask relief is a tolerance parameter that stipulates how much spacing between solder mask and pads or annular rings there should be to accommodate fabrication equipment imprecision.
Solder fill	Although probably preferable, 100% solder fill is not necessary in all cases. For example, 75% fill is typically acceptable for class 2 and class 3 boards.
Clearances	Clearances with other components and traces are important to ensure no solder bridging occurs, while board edge clearance is important for your board’s depanelization.

The IPC standards are not mutually exclusive. Therefore, rules, regulations, and suggestions can usually be found in multiple standards and they routinely reference each other. However, the best way to begin ensuring your compliance with standards for through-hole components is referring to IPC-610A and IPC-J-STD-001.

The PCB layout, which is performed during design, impacts your board’s manufacturing. Therefore, good layout tips for manufacturing should always be followed. For more information on the parameters listed above, refer to the articles below.

For more info on drill holes and aspect ratios, refer to:

PCB Drill Rules Hole Tolerance Aspect Ratio Other DFM Considerations.

For information on through-hole soldering methods, take a look at:

Understanding Soldering - Part 1: The PCB Soldering Process.

For more info on solder mask, see the following:

Choosing The Right Solder Mask Clearance For Your PCB.

Through-hole components are far from being extinct, and it is highly likely that you do (or will find) many occasions to implement them for your designs. To do so, you need to understand and implement important regulations and requirements that are stipulated in IPC standards.