One of the tools used by people who know their way around the kitchen is aluminum foil. It can help your baking from developing a crispy exterior, keep meat from drying out while cooking, and protect delicate foods like vegetables when they are on the grill. If used properly, aluminum foil can make a real difference in your cooking. Its ability to manage and conduct the heat can turn a regular old hot meal into something extraordinary.
Managing and conducting heat is important with a circuit board as well. Most circuit boards will have large areas of metal for power or ground planes, and those planes will typically connect to several component pins. If those pins are the leads of a thru-hole device, then soldering or un-soldering those pins can be very difficult with all of the metal in the planes they connect to. This is where the thermal relief comes in, and there are some PCB thermal relief guidelines that you should know more about.
What Exactly Is a Thermal Relief?
Whether your printed circuit board design is a complex multi-layer board or a simple one-sided board, there will probably be some large areas of metal on it to conduct the power and ground nets. This metal may be a network of wide traces, or it could be a large, filled power plane. Either way, this extra metal can act as a heat sink for the connected pin, causing problems with soldering such as:
- Thru-hole component pins: These pins may not get enough heat to solder correctly, resulting in a cold solder joint. Additionally, trying to unsolder a thru-hole that is soldered directly to large areas of metal could force too much heat to be applied to melt the solder. This excess heat could potentially damage nearby traces or components.
- Surface mount component pins: Smaller two-pinned SMT parts may also have problems if one of their pins is trying to solder directly to a large area of metal. The unbalanced metal between the pins could result in the solder melting faster on one side than the other, pulling the part up and away from the other pin.
No matter what though, these components must connect to the large areas of metal. Compounding the problem is that, usually, design engineers want as much metal as possible for electrical purposes. Power nets need the metal for higher current loads as well as to improve power integrity to provide clean power to the rest of the board. As you can see, this puts the electrical performance needs of the board in direct conflict with the manufacturing needs of the board. The solution to all of this is the thermal relief.
For thru-hole component pins, a thermal relief is a connecting pad with small voids in it. These voids create metal spokes that will connect the metal barrel of the hole to the perimeter of the pad. The voids break up the solid connection, allowing the spokes to restrict the amount of heat that is conducted from the barrel of the hole during soldering to the rest of the metal trace or plane. A surface mount part is similar in that it also uses spokes to connect the solder pad to the metal trace or plane instead of soldering the part directly onto the large area of metal.
PCB Thermal Relief Guidelines You Should Know
Here are some details about thermal relief pads that you should know before you use them:
- Thermal relief pads should be used whenever a thru-hole pin is connected to a fill of metal, a power plane, or any metal that is wider than the component pad.
- SMT parts soldered directly to large areas of metal should have some sort of thermal relief between their solder pad and the area of metal they are soldered to.
- The number and width of the spokes in a thermal relief pad should be based on the amo
unt of power being conducted by that pin. If the power requires a minimum trace width of 40 mils, then your thermal relief pad should have 40 mils of spokes attached to it. This can be accomplished with four spokes that are each 10 mils wide. - Thermal relief pads in most CAD systems are designated by some sort of rules, so make sure that you fully understand and set them up. These rules will govern the size, shape, number, and width of the spokes, as well as minimum allowable spokes. Additionally, the rules are usually able to be applied to connections on individual nets, net classes, pins or component classes.
- In some cases, a thermal relief pad may not be able to get all of its spokes to connect correctly. This can happen if your thermal reliefs are congested together, if they are being used in a split plane, or if the area of metal that they are in is too small. Make sure that your software has rules set up for detecting minimum thermal relief connections, and keep an eye out for these problems as well.
The best way to deal with thermal reliefs on your design is to be armed with the knowledge you need to specify their rules before you start the layout. Using your contract manufacturer for this information can be a very helpful resource for you.
How Your Contract Manufacturer Can Help with Your PCB Design
Your PCB CM should have a lot of experience in building circuit boards. They should understand what is needed with thermal reliefs to create good solder joints. In addition, they should know the problems your specific design will face based on the board materials, layer stackups, engineering requirements, and manufacturing processes that it will go through.
At VSE, our engineering experience and manufacturing expertise will be a real benefit to you in deciding what kind of thermal reliefs that your design will need. With both the knowledge we have from building circuit boards, we can readily identify issues and provide you with viable and cost-effective alternatives on your PCB design. This will keep your project moving quickly and efficiently through the design, fabrication, and assembly process.
