I once read that young Eddie Rickenbacker got pranked by his older co-workers when they sent him around to ask for a tool that doesn’t exist. Later on, Rickenbacker would become a race car driver in the 1900’s, an automotive designer, an American air ace in World War I, a government consultant, and eventually the head of Eastern Air Lines. Yet as a junior apprentice in that automotive shop, he had to learn the basics of his trade the same as the rest of us do. He did that by asking a lot of questions.
If you are just starting out your journey as PCB thermal relief designer, you probably also have a lot of questions. This is a good thing, so keep on asking. I have found though that there are some questions that new designers hesitate to ask. This is often because these questions seem so basic that they feel that it’s something that they should already know. One of those topics is about PCB thermal relief pads.
What are thermal relief pads and why are they necessary? When should I use them? How do I create them? If these are some of the questions that have been in the back of your mind as you’re learning how to design circuit boards, then read on. I’ll do my best to give you an overview on this subject to get you started. And don’t worry, I’m not about to send you out looking for a left-handed monkey wrench like Rickenbacker’s co-workers did to him.
Thermal relief and heat pads are necessary.
What is a thermal relief pad?
A multi-layered circuit board will usually have internal power and ground planes. These internal planes are often created as negative planes within the CAD system. A negative plane is a reverse image of what will eventually be fabricated by the board shop. The images on a negative plane represent the absence of metal, and they are used to create clearances around holes that don’t connect to the plane. A thermal relief pad is used to connect the hole to the plane, but it uses spokes and voids to restrict the metal area of the connection. This is important for soldering thru-hole parts in the board.
When the leads of a thru-hole part connect directly to an internal power or ground plane without a thermal relief pad, the metal plane can act as a heat sink during soldering. The large area of metal in a plane will cause the heat to dissipate, and it will take longer before the solder will reflow. With some leads of a partial taking longer to solder than others, it may be difficult to get all of the parts leads to solder correctly. This can cause bad solder joints which could eventually break causing intermittent contact or an open connection.
To counteract these effects, thermal relief pad components are used on negative planes. The thermal relief pad creates small metal voids around the hole so that the connection is made through small spokes of metal. With the connection being restricted to the small spokes, there is less thermal contact with the rest of the plane. This allows the power or ground lead of a thru-hole part to heat up and solder at the same rate as the other leads of the part.
Working with negative planes don't have to be a negative experience
When should I use a thermal relief pad when designing circuit boards?
A thermal relief pad should be used anytime the lead of a thru-hole part is connected to a negative plane. The number and width of the spokes in a thermal relief pad should be based on the power requirements for that pin. For instance, a power pin that needs a 40 mil trace to connect to the plane would require a thermal relief pad with four 10 mil wide spokes.
Another question that often comes up is if PCB vias require thermal relief pads. Remember that the purpose of a thermal relief pad is to counteract the heat-sinking behavior of thru-hole leads soldering into internal planes. Since a via is not getting a lead soldered into it, it typically doesn’t require a thermal performance relief pad and can have a solid connection to the plane.
How do I create a thermal relief pad?
Early CAD systems used to require that thermal relief pads be constructed manually. In my career, I have used arcs, lines, and polygons to construct thousands of thermal relief pads. Although this option is still available in many CAD systems today, it usually isn’t used unless a custom thermal relief pattern is required for a PCB Design.
Thermal relief pads in most modern CAD systems are created by rules. A menu is provided allowing you to specify the shape of the pad (round or square), the rotation of the pad, the number of spokes, and the width of the spokes. These rules can then be applied to specific pins, parts, layers, nets, or net classes. You will also typically have the ability to set up multiple rules for various situations, such as different net classes, and then set a priority for those rules.
There is a lot more to thermal reliefs than you may have originally imagined. We’ve only covered the basics here, but it should give you a good starting point. Learning how your CAD tools work with thermal relief pads will be the real key to assuring your success with them.
PCB design software, like Altium Designer, gives you extensive rules-based functionality for the creation and use of thermal relief pads. In addition, the new and enhanced design environment of Altium Designer 18 will help you to successfully complete all aspects of your next surface mount PCB design.
Would you like to find out more about how Altium can help you with your printed circuit board design needs? Talk to an expert at Altium.
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