PCB Thermal Design Hack Chat

Keeping the magic smoke inside you boards

Wednesday, March 30, 2022 12:00 pm PDT Local time zone:
Hack Chat
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Mike Jouppi will host the Hack Chat on Wednesday, March 30 at noon Pacific.

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Most of the time, designing a printed circuit board is a little like one of those problems in an introductory physics course, the ones where you can safely ignore things like air resistance. With PCBs, it's generally safe to ignore things like trace heating and other thermal considerations in favor of just getting everything placed sensibly and routing all the traces neatly.

But eventually, the laws of physics catch up to you, and you'll come across a real-world problem where you can't just hand-wave thermal considerations aside. When that happens, you'll want to have a really good idea of just how much a trace is going to heat up, and what it's going to do to the performance of your board, or even if the PCB is going to survive the ordeal.

Digging into the thermal properties of PCBs is something that Mike Jouppi has been doing for years. After working in the aircraft industry as a mechanical engineer, he started Thermal Management LLC, which developed software to make the thermal design of PCBs easier. He'll stop by the Hack Chat to answer your questions about PCB thermal design considerations, and help us keep all our hard work from going up in smoke.

  • Hack Chat Transcript, Part 2

    Dan Maloney03/30/2022 at 20:19 0 comments

    Mike Jouppi12:55 PM

    Mike Jouppi12:55 PM
    Ok, here is the board stack up

    Sven F12:55 PM
    OK, thank you very much.

    Your links are already helping me!

    Do you know any surface mountabe heatsinks manufacturers, too?

    So far I used some BGA-heat sinks and mounted them to the PCB with some custom made metal frame.

    But that is quite labour intensive and complex. A simpler approach would be appreciated (like those "standard" heatsinks with M3-drill holes and a rail for the mounting clips?

    Mike Jouppi12:56 PM

    Mike Jouppi12:56 PM
    Here is one of the trace configurations on top of the image

    Mike Jouppi12:57 PM
    It's a serpentine configuration so we would know that same current is in all the traces

    Mike Jouppi12:57 PM
    Source wires at the beginning and end, along with sense wires at various traces.

    Mike Jouppi12:58 PM
    We had four copper planes in one configuration and the no traces in the other.

    Mike Jouppi12:58 PM

    Mike Jouppi12:58 PM
    This data has no planes

    Mike Jouppi12:59 PM
    The board is suspended in air, same as in IPC-2152. We follow the test procedure in IPC-TM-650

    Mike Jouppi12:59 PM

    Mike Jouppi12:59 PM
    This data is for the board with copper planes.

    Mark J Hughes12:59 PM
    That's interesting -- almost like there's two equations to fit the transitions to steady state...

    Mike Jouppi1:00 PM
    Current is on the right side axis and the blue lines are current.

    tinfever1:01 PM
    If I'm understanding this correctly, the current still still only being applied through the layer 1 serpentine 10 mil trace but there is an added electrically isolated plane in the PCB for the second chart?

    Thomas Shaddack1:01 PM
    Why thermocouples and not thermal imaging?

    Mike Jouppi1:01 PM
    I will be adding content back to my website soon at I have already added my papers used to create the charts in IPC-2152.

    Mark J Hughes1:01 PM

    Mark J Hughes1:01 PM
    What's happening -- the transition peaks as I'd expect when the current rises. But then as you let it go for a long period of time, the rate of heat transfer looks almost linear.

    Mike Jouppi1:02 PM
    No thermocouples

    Thomas Shaddack1:02 PM
    Random thought... could the resistivity-temperature dependence of copper be used to employ the traces themselves as temperature sensors? Could be handy for assessing the inaccessible ones in the internal layers of the boards.

    Mike Jouppi1:02 PM
    The method uses the resistance of the trace.

    Thomas Shaddack1:02 PM

    Thomas Shaddack1:03 PM
    Was confused by the blue connectors on the board.

    Dan Maloney1:03 PM
    We're at the end of our normally allotted hour now, but it seems like the discussion is still plenty active. I say keep going if you have the time, Mike.

    Mike Jouppi1:03 PM
    Random thought... could the resistivity-temperature dependence of copper be used to employ the traces themselves as temperature sensors? Could be handy for assessing the inaccessible ones in the internal layers of the boards. Yes, as long as you know the resistance of the trace before heating, then the temperature can be calculated

    Mark J Hughes1:03 PM
    Hey Mike -- didn't you say you're looking for a company to sponsor your continuation with the work?

    Mike Jouppi1:04 PM
    I'm looking for a new home for all the software, data, hardware that my company created.

    Dave Blundell1:04 PM
    I frequently find myself with very short power traces. Like < 25mm. I am paranoid about thermals, often ending up in ABS enclosures. I'll tend to throw 3oz and wide AF at everything, but i wonder how much I'm over-compensating

    Dave Blundell1:04 PM
    throwing layer after layer of 0.5oz isn't really in the cards for most of the designs I deal with

    Mike Jouppi1:05 PM
    I join your paranoid feelings about thermals. It was an area of study that I have yet to complete

    Dave Blundell1:05 PM
    3oz external 2 layer or 4layer w/ 3oz external / 0.5oz internal is typically all I can get made

    Thomas Shaddack1:05 PM
    "Overcompensating" is an ugly name. Better say "enhanced margins".

    Mike Jouppi1:05 PM

    Read more »

  • Hack Chat Transcript, Part 1

    Dan Maloney03/30/2022 at 20:19 0 comments

    Mike Jouppi joined  the room.11:50 AM

    Dan Maloney11:51 AM
    Hi Mike! Welcome aboard!

    Mike Jouppi11:51 AM
    Hi Dan, thanks

    Dan Maloney11:51 AM
    We'll get started in just a few minutes

    Mike Jouppi11:51 AM
    I came in early to see how this all works

    Dan Maloney11:52 AM
    Oh sure -- good idea to see how the interface works.

    Mark J Hughes11:52 AM
    Hi Mike! Hi Dan!

    Mike Jouppi11:52 AM
    Yes, too easy

    Mike Jouppi11:52 AM
    Hi Mark

    Dan Maloney11:52 AM
    Hi Mark, welcome back!

    Mark J Hughes11:53 AM
    Pssh -- miss this? NEVER!!!!

    Dan Maloney11:54 AM

    Mark J Hughes11:54 AM
    So -- what's everyone been up to lately?

    Nicolas Tremblay joined  the room.11:54 AM

    Mark J Hughes11:54 AM
    Also Dan -- I don't know if you want to promote it -- Mike just gave a talk with me here:

    Mike Jouppi11:55 AM
    Mark, I was going to ask you for that link. So....thanks

    Dan Maloney11:56 AM
    Well, that's quite the rogue's gallery in the thumbnail...

    Mark J Hughes11:56 AM
    Some say it's the best hour of youtube this year. I say no. It's the best hour this century!

    Mark J Hughes11:56 AM
    (Thos other people also say I'm a bit hyperbolic, whatever that means)

    Mark J Hughes11:57 AM
    It was a lot of fun though.

    Mark J Hughes11:59 AM
    Woohoo! It's go-time!

    Mark J Hughes12:00 PM
    (Also Dan, unrelated to Mike, I forgot to share this link with you:


    Dan Maloney12:00 PM
    OK folks, let's get started. Welcome to the Hack Chat, I'm Dan and Dusan and I will be moderating today as we welcome Mike Jouppi to talk about thermal design consideration for our PCBs.

    Mike Jouppi12:00 PM
    Thanks Dan

    Dan Maloney12:00 PM
    Hi Mike, welcome! Can you start us off with a little about your background?

    Mark J Hughes12:01 PM
    Hi Dan & Dusan & Mike!

    Grogdor the Burninator joined  the room.12:01 PM

    Mike Jouppi12:01 PM
    Sure, I'm a mechanical engineer that specializes in heat transfer. I'm semi-retired and lately I've been trying to get the word out about IPC-2152

    mandresc joined  the room.12:02 PM

    Mike Jouppi12:02 PM
    IPC-2152 is an IPC standard for sizing electrical traces in PCBs

    Mark J Hughes12:02 PM
    Why Mike, I too love IPC standards. But I find IPC-2221 so burdensome and dated. Tell me more about IPC 2152!

    Mike Jouppi12:03 PM
    I refer to it as a baseline for trace sizing. By that I mean that it represents as basic a configuration as possible.

    Dusan Petrovic12:03 PM
    Hi everyone!

    Mike Jouppi12:03 PM
    Just traces in a pcb. Not copper planes, no mounting and the board is suspended in air or vacuum (for space environments)

    alol_abax joined  the room.12:04 PM

    Alexander Olsen12:04 PM

    Mark J Hughes12:04 PM
    What amount of current do most engineers need to start worrying about trace heating?

    Mike Jouppi12:04 PM
    People think that that is unrealistic for a pcb

    Mike Jouppi12:05 PM
    The best way to assess trace heating is by the power dissipation of the trace. I^2R

    Alexander Olsen12:06 PM
    I suppose that changes quite alot based if it placed on an inner or outer layer as well, and the stack-up in general

    Mike Jouppi12:06 PM
    I think that the standard is unrealistic as well, just to clarify. I think that the copper planes and mounting configurations need to be considered to provide a realistic temperature rise for a given trace size and applied current.

    Mike Jouppi12:06 PM
    Inner and outer run very close to the same temp.

    Mike Jouppi12:07 PM
    Outer run a little hotter in the configuration used for the testing.

    tinfever12:07 PM
    Is there a way to calculate power dissipation of planes that doesn't require super fancy and expensive simulation software?

    Alexander Olsen12:08 PM
    Interesting! I've allways been paranoid and made some significant extra margins for the inner layers

    Mike Jouppi12:08 PM
    You can make rough calculations depending on the amount of cut outs and odd geometries

    Mike Jouppi12:09 PM
    That's why I'm out talking to people.

    mandresc12:09 PM
    Mike, when you say power on pcb tracks, are you refering as the tranverse area right?

    ryan.clement joined  the...

    Read more »

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