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Are thin copper tracks not allowed on Aluminium substrate PCBs?

Discussion in 'Circuit Simulation & PCB Design' started by Flyback, Mar 22, 2017.

  1. Flyback

    Flyback Well-Known Member

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    Hello.

    Is it true that Aluminium core PCBs for LED mounting should always be almost totally covered in copper pours?

    We are doing a LED PCB using an aluminium core. It has 35um copper tracking and 100um isolation dielectric between tracking and aluminium substrate. The aluminium core is 1.5mm thick. (other dimensions are 110mm by 46mm)

    The attached shows our copper tracking on this PCB? Please advise if it is totally wrong, and should instead have loads of copper pours.
     

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    Last edited: Mar 22, 2017
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Please include the part number for the LED.
    What I am trying to find out is; Is the middle leg isolated? Does the heat come mostly out of the middle leg?
     
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  3. Flyback

    Flyback Well-Known Member

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  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I would go to the sch and connect the middle pad to ground. (all LEDs) (it is isolated)
    Fill the board with a ground plane. Turn off "thermals".
    The reason for turn off thermals is to connect the middle pad to ground with a thick trace.
     
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  6. Flyback

    Flyback Well-Known Member

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    thanks, this is an aluminium core PCB, and as you know, there can be no ground plane.
    I was thinking of having a thick trace out of the thermal pad (instead of the "necked" down one that i have) but worried about wicking the heat away during soldering.
     
  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    You are using aluminum core and you are concerned about heat getting away from your part during soldering. It will if you hand solder one part at a time. (with or with out wide traces)

    I think you will heat the entire board, aluminum and all, solder all parts at one time, and there will be no problem. I have demonstrated that there is not a problem. In my case I may have several (internal) layers of copper ground/power plain(s) and parts soldered to the top and bottom which is 85% copper. All we did was add some seconds to the heat up time in the oven. (which you would do if there are large parts)
     
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  8. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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  9. Flyback

    Flyback Well-Known Member

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    Hello,
    Thanks, do you think the attached is better? There is now a single copper pour connecting all LED Tabs. The isolation between this pour and other copper is 0.3mm. I hope this is OK for MCPCB? (the distance between the LED pads is in fact less than 0.3mm and is unavoidable) I presume that there will be no problems with heat being 'wicked' away by this extra thermal conenction to the LED TABS. As you say, the whole thing will be heated up ready for soldering, so heat getting 'wicked away isnt so much of a prolem for MCPCB boards?

    All topcopper is 70um.
     

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  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I like what you did. The heat should, through copper, spread out away from the LEDs. Then the heat should transfer into the Al.
     
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  11. Flyback

    Flyback Well-Known Member

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    The layout is now as in the pdf in post #8. As you can see, there is a large single thermal copper pour connected to all the thermal tabs of each LED. This large copper pour is in fact floating. Should i tie it to some electrical node so as to stop it from floating up to some high potential, whereby it may then break over the isolation between tab and electrodes in the LEDs?


    I note that our MCPCB is also actually floating, and i wonder if it is necessary to ground it with a ring terminal which gets screwed to the MCPCB, and then wire this terminal to ground?
     
    Last edited: Mar 25, 2017

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