Free Trials

Download a free trial to find out which Altium software best suits your needs

How to Buy

Contact your local sales office to get started on improving your design environment

Downloads

Download the latest in PCB design and EDA software

  • PCB DESIGN SOFTWARE
  • Altium Designer

    Complete Environment for Schematic + Layout

  • CircuitStudio

    Entry Level, Professional PCB Design Tool

  • CircuitMaker

    Community Based PCB Design Tool

  • NEXUS

    Agile PCB Design For Teams

  • CLOUD PLATFORM
  • Altium 365

    Connecting PCB Design to the Manufacturing Floor

  • COMPONENT MANAGEMENT
  • Altium Concord Pro

    Complete Solution for Library Management

  • Octopart

    Extensive, Easy-to-Use Component Database

  • PRODUCT EXTENSIONS
  • PDN Analyzer

    Natural and Effortless Power Distribution Network Analysis

  • See All Extensions
  • EMBEDDED
  • TASKING

    World-Renowned Technology for Embedded Systems Development

  • TRAININGS
  • Live Courses

    Learn best practices with instructional training available worldwide

  • On-Demand Courses

    Gain comprehensive knowledge without leaving your home or office

  • ONLINE VIEWER
  • Altium 365 Viewer

    View & Share electronic designs in your browser

  • Altium Designer 20

    The most powerful, modern and easy-to-use PCB design tool for professional use

    ALTIUMLIVE

    Annual PCB Design Summit

    • Forum

      Where Altium users and enthusiasts can interact with each other

    • Blog

      Our blog about things that interest us and hopefully you too

    • Ideas

      Submit ideas and vote for new features you want in Altium tools

    • Bug Crunch

      Help make the software better by submitting bugs and voting on what's important

    • Wall

      A stream of events on AltiumLive you follow by participating in or subscribing to

    • Beta Program

      Information about participating in our Beta program and getting early access to Altium tools

    All Resources

    Explore the latest content from blog posts to social media and technical white papers gathered together for your convenience

    Downloads

    Take a look at what download options are available to best suit your needs

    How to Buy

    Contact your local sales office to get started improving your design environment

    • Documentation

      The documentation area is where you can find extensive, versioned information about our software online, for free.

    • Training & Events

      View the schedule and register for training events all around the world and online

    • Design Content

      Browse our vast library of free design content including components, templates and reference designs

    • Webinars

      Attend a live webinar online or get instant access to our on demand series of webinars

    • Support

      Get your questions answered with our variety of direct support and self-service options

    • Technical Papers

      Stay up to date with the latest technology and industry trends with our complete collection of technical white papers.

    • Video Library

      Quick and to-the-point video tutorials to get you started with Altium Designer

    Pad Stack Design And Fabrication—Part 2, Thermal Ties

    Kella Knack
    |  October 2, 2019

    As noted in part one of this article, there are a variety of aspects regarding manufacturability and reliability that come into play as part of the pad stack design process. When it comes to soldering through-hole components into the plated through-holes in a PCB when the pin connects to a plane, you have to thermally isolate those holes from the planes to ensure successful soldering of those components. This is accomplished via a feature called a thermal tie. This article describes thermal ties, their physical characteristics and how they are successfully implemented in a given design.

    What, Where and Why?

    When a component lead is soldered into a plated through-hole to make a connection to a plane, either power or ground, the heat needed to melt the solder is drawn away from the hole by the copper planes in the PCB. This results in a poorly made solder joint and a component lead that is very difficult, if not impossible, to remove.

    This creates a dilemma. How does one electrically connect a component to a plane while at the same time thermally isolating it? The answer is through the use of a thermal tie. Figure 1 illustrates a two-spoke thermal tie. Specifically, it represents the plane and the method used to thermally isolate the plated through-hole from the plane while connecting it electrically.

    Figure 1. An Example of a Thermal Tie

    There are a number of questions that arise relative to the use of thermal ties including:

    • When are they needed?
    • How many spokes or ties are needed?
    • How wide should the spokes be?
    • How long should the spokes be?

    First, thermal ties are needed only when connections are being made from component leads to a plane when the lead is actually being soldered into the hole itself. Surface mount parts do not require thermal ties and they should not be used with these types of parts.

    From a thermal point of view, one spoke, or tie, is preferred. However, this can result in lost connections if the holes are placed too close to one another. Thus, the number of spokes needed depends on how closely the plated through-holes are placed to each other. If the CAD system being used is allowed to do this hole placement, it is possible that the holes will be crowded so close to one another that the clearance pad from one hole will break the connection of a thermal tie in another. When this practice is allowed, the two ties depicted in Figure 1 may not be enough.

    Due to signal integrity reasons, the holes should not be placed so close to one another that their clearance pads touch or overlap as described above because then slots are created. When this condition is satisfied, two ties are always enough.

    Some may question why one thermal tie does not suffice. Figure 2 shows the two-tie solution along with the drilled hole. There are two conflicting goals that must be met when designing a thermal tie-- achieving maximum thermal isolation while maintaining a good electrical connection.

    A simple illustration showing basic thermal relief pad geometry with a drilled hole in the middle designed for a power plane, with the thermal ties, capture pad, and thermal isolation labeled.

    Figure 2. Thermal Tie Design for a Drilled Hole in a Power Plane

    With the foregoing goals in mind, you want to make as few connections as possible and make the connections or ties as long as practical. In most dense PCBs, the size of the clearance pad has been reduced to the absolute minimum needed to account for all of the manufacturing tolerances. As a result, the difference in the diameter of the clearance pad and the capture pad is usually 5 mils per side or 10 mils total. The capture pad is normally 10 or 12 mils larger than the drill diameter and the clearance pad is 10 mils larger than this. In order to increase the length of the thermal tie, the capture pad is made only 5 mils larger than the drill size. This will result in the breakout as shown in Figure 3 if the worst case drill wander happens.

    A simple illustration showing a thermal relief pad geometry with an off-center drilled hole in the middle, causing one of the two copper spokes to become fully severed from the capture pad.

    Figure 3. Thermal Tie Illustrating Breakout

    As depicted in this figure, even when the drill is off-center, one of the ties will still make a good connection with the plated through-hole. This means that the balance between reliability and thermal isolation has been satisfied.

    Additional questions regarding the two-tie thermal tie structure may arise such as:

    • Electrically, are two thermal ties good enough?
    • Is the inductance low enough?
    • Is the resistance low enough?

    Figure 3 is a plot of resistance versus trace width and trace thickness. If the thermal tie in Figure 3 is 5 mils wide, 7 mils long and made from ½ ounce copper, its DC resistance will be approximately 1 milliohm.

    A figure showing trace resistance (ohm/ft) vss. trace width (mils) as a function of foil thickness. The graph shows an inverse relation, with trace resistance decreasing as either trace width or foil thickness increases.

    Figure 3. Trace Resistance vs Trace Width and Trace Length

    This is far less than the resistance of the component lead frame soldered into the hole. A trace of this geometry will have an inductance of approximately 0.14 nanoHenrys. This factor is very small compared to the inductance of the lead soldered into the hole. Therefore, from the electrical point of reference, a two-tie thermal tie is more than good enough.

    In the final analysis, a thermal tie structure containing only two ties provides the best balance between electrical and thermal considerations. This is especially true when connections are made to multiple planes such as ground planes.

    Summary

    Pad stack design is not just limited to the design process. When it comes to all of the elements associated with the number and variety of holes in a PCB and the appurtenant structures and processes associated with them as well as the overall manufacturability and reliability of any given design, there are a number of factors that have to be taken into account throughout the entire design-to manufacturing-to- assembly product development cycle. Doing so at the front-end of the design process ensures that you will have a design that satisfies specified performance parameters, meets cost and deadlines, and will work correctly the first time.

    Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium or read more about designing to meet thermal demands with Altium Designer®.

    References

    1. Ritchey, Lee W., and Zasio, John J., “Right The First Time, A Practical Handbook on High-Speed PCB and System Design, Volumes 1 and 2.
    2. Ritchey, Lee W., “How Many Thermal Ties Are Needed to Connect Power Planes?” Current Source Newsletter, Volume 1, Issue 1, May 2004.

    About Author

    About Author

    Kella Knack is Vice President of Marketing for Speeding Edge, a company engaged in training, consulting and publishing on high speed design topics such as signal integrity analysis, PCB Design ad EMI control. Previously, she served as a marketing consultant for a broad spectrum of high-tech companies ranging from start-ups to multibillion dollar corporations. She also served as editor for various electronic trade publications covering the PCB, networking and EDA market sectors.

    most recent articles

    Back to Home