Mowing the grass was my job growing up, and I used a plain old gas powered mower without powered wheels, a bag, or any other enhancements. You simply filled the tank, pulled the starter rope, and mowed. Then my parents bought us a new mower that was a nightmare. The grass clogged the chute, the bag had to be emptied every 30 feet, and it was so low to the ground that it would constantly stall. All of these enhancements made it a real pain to use, and I was always looking for ways to break it so I could go back to using the old trusty mower again.
For many of us, falling back to something that we’ve always trusted is a normal part of life. In circuit board design for instance, it’s very easy to use the same through hole components that we’ve always used. Sometimes that is a really good idea, but there are also times where it is better to use something different. Let’s take a look at some of the reasons for using, or not using through hole components in a PCB design.
Through Hole Components: Should a Designer Use Them?
Long before surface mount components were around, through hole parts were the only components available for PCB design. Their package sizes were usually very standard, and designers were accustomed to working with them. If asked to add some bypass caps to a board, we would create a PCB footprint with two holes spaced 0.300 inches apart, add a part outline 0.050 inches from the hole centers, and call it good.
While that may have worked great back then, with shrinking board sizes and more congested part placements today, it is no longer a workable solution. The use of surface mount parts on high density designs has become a requirement, and precise components dimensions are a must in PCB CAD footprint patterns. Does that mean that there is no longer a place for through hole components? Not at all. The thru-hole part is still vitally important to PCB design for the following reasons:
Durability: Many parts that serve as an interface must have a more robust mechanical attachment that what can be achieved through surface mount soldering. Switches, connectors, fuses, and other parts that will be pushed and pulled by human or mechanical forces, need the strength of a soldered thru-hole connection.
Power: Components that are used in circuits that conduct high power levels are usually only available in thru-hole packages. Not only are these parts larger and heavier requiring a more robust mechanical attachment, but the current loads may be too much for a surface mount solder connection.
Heat: Components that conduct a lot of heat may also favor a thru-hole package. This allows the pins to conduct heat through the holes and out into the board. In some cases these parts may be bolted through a hole in the board as well for additional heat transfer.
Hybrid: These are the parts that are a combination of both surface mount pads and thru-hole pins. Examples would include high density connectors whose signal pins are surface mount while their mounting pins are thru-hole. The same configuration can also be found in parts that carry a lot of current or run hot. The power and/or hot pins will be thru-hole while the other signal pins will be surface mount.
As you can see, there are still plenty of reasons to use through hole parts on your PCB design. And just as it is necessary to use through hole components for some part applications, the reverse is true too. The smaller surface mount parts are the better choice for cost and signal performance due to their size. Additionally, surface mount parts are usually a better choice for assembly which we will look at next.
Through hole components are important for power circuits
PCB Assembly of Through Hole Parts
The assembly of printed circuit boards is where a lot of money can be spent or saved depending on what parts are being used. Surface mount parts due to their more uniform size, can be automatically placed on the board and then sent through solder reflow. This is a straightforward process that will mass produce circuit board assemblies quickly and efficiently. Through hole parts however require a more complex assembly process:
Component placement: While there are automated insertion systems for thru-hole parts, many assembly vendors will rely on manually installing the parts instead due to the volume of thru-hole parts that they work with. Having these parts manually installed prior to soldering is a more expensive process than the pick and place machines that are used with surface mount parts.
Automated soldering: Thru-hole parts are normally run through a wave soldering process. This is a tried and true process that has been in place for years, and is very reliable. The problem comes when there are surface mount components nearby that can’t be run through the wave. This requires the use of special pallets to mask off those surface mount components so that only the thru-hole pins are exposed to the wave.
Manual soldering: For high density thru-hole parts or areas that can’t be reasonably masked, the PCB assembler will resort to manually soldering those parts. This again is a more expensive process than automated soldering.
As we have already seen, however, the use of through hole components is a very necessary part of PCB design. In these cases, the extra manufacturing costs must be factored into the overall price of the board. There are however some things that you can do as the PCB designer to help.
Through hole components provide durability for connectors as seen in this OrCAD layout
Tips for Working with Through Hole Parts in Your PCB Design CAD Tools
To help with the manufacturing costs of the board, it is helpful to separate your through hole components from the surface mount parts as much as possible. This will help the PCB manufacturer to simplify the different soldering processes that have to be set up for the board. Obviously this kind of part separation isn’t always possible as the PCB still needs to be electrically functional, but the more that you can, the better it will be for manufacturing.
Another important tip when working with through hole components is how they are modeled as footprints in the PCB design CAD system. Make sure that you are working with correct information from the most up-to-date data sheets. Create the part outlines at the maximum material width, and be sure to use the correct finished hole size. Also, make sure to include in the footprint any pertinent keepout zones, clearances, and component heights.
You can help yourself even more by using PCB design tools that are created with the designer in mind. OrCAD PCB Designer from Cadence has the drawing capabilities to create the precise through hole component footprints that you need, as well as a full set of design rules and constraints to guide you in the placement and routing of these parts. Additionally, OrCAD Capture now offers you access to symbols, footprints, 3D models, and part datasheets through an online component database by using the Unified Part Search feature. This allows you to place these components on your schematic while you are working in real time, while at the same time saving the through hole PCB footprints for your layout.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.