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Led Engin layout

Former Member

Hi,

 

I'm new to designing footprints and I'm working on the layout for the LED Engin LZ1-00U600. I'm having trouble figuring out how to design the solder pad layout for the thermal pad of this LED. I'm attaching a picture of the layout provided on the datasheet. Can anyone guide me on how to do the central disk of this footprint?

 

Thank you.

 

image

  • Where did you get that pad layout - it's quite different from the one in the data sheet I downloaded from Farnell and both of them look quite wrong.

     

    You need some way to get the heat from the centre thermal pad to a heat sink or a much bigger copper area on the board. The best way to do this is to have a large number of small vias though the pad (making sure that the CAD system doesn't put thermal breaks in). Each 0.5mm diameter via gives you 1.67mm of copper (its circumference) to conduct heat away to other layers. on a 3.2mm round pad you might expect to get at least 12 such vias in which would give you an 20mm length of copper conducting heat away. The 0.76mm dia hole you have only gives a 2.5mm length of copper to carry away the heat - the multiple vias can be solder filled and will be at least 8 and probably 10 times more effective than the single hole. (These calculations assume 1oz (35u) copper and finished hole diameters.)

     

    I can see no point in the splits in your round pad at all.

     

    I'm not an Eagle user so some one else may be able to tell you how to make  around pad with 12 holes in it.

     

    (If you have a lot of these LEDs on one board running at full power you will need to take the heat out of the board by clamping it to a heat sink or using heat pipes - without more information it's not possible to advise.)

     

    It just occurred to me that you may be planning a ceramic on metal type board in which case the via strategy would be different.

     

    MK

  • The datasheet that I have comes from here: http://www.ledengin.com/files/products/LZ1/LZ1-00U600.pdf

    I checked the one on Farnell and I believe it's the same one though.

     

    I was planning on an FR4 PCB. I'm still very confused on how to make the layout for this emitter. Is the datasheet wrong then?

     

    Thank you.

  • in reply to Former Member

    Interesting - I googled the part number and got  a link to Farnell part number 1901493 which has a similar but different data sheet.


    I think the footprint on the data sheet you have is probably intended for ceramic clad metal substrates and will not be good for FR4. Either talk to the manufacturers of the part or get ready to do experiments. The data sheet (either of them) does not, in my opinion, give a footprint suitable for FR4 and full power operation. (Now I see your actual data sheet there isn't a hole in the pad at all so all the heat will need to flow through the fibreglass board -  the LED will fry.)


    If you check up on some surface mount power devices you will be able to get an idea of the sort of techniques commonly used to get low thermal resistance from chip to ambient.


    MK