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TPS51200 - Reducing Ground Vias

Adam Jankauskas
Prodigy 525 points
Other Parts Discussed in Thread: TPS51200, TPS51200EVM

From customer:

"I am using part TPS51200DRCR on very thick pcb with a 22 layer stackup.  I have ( at least 6 layers) 2oz copper ground planes and at have ground pours on top and bottom layers.  I want to reduce or eliminate the ground vias on the solder slug so I can remove the part for manufacturing rework.  I pulled down the data sheet and it referenced a SLUA271 pdf (SLUA271A June 2002 Revised September 2007) I see on section 3.4.1 (Page 7 of 21)Thermal Pad Via Design that I can ask a TI representative if I can remove the vias.  Can you point me in the right direction to someone who can help with thermal pad design for thick/multiple ground plane pcbs?

Ideas.

  1. Can I remove all the thermal vias?
  2. Can I use a special via that is only connected to external ground planes?  Then use thermal relief on the pad on both sides. 
  3. Can I reduce thermal vias and keep the same inner layers?  Will it help the part solderability

This change is only for rework when I want to remove a part and then solder a new part with hot air reflow.  The pcb will go through our oven process for top and bottom and lead free paste for the initial solder process.  On the rework we would apply hot air at 400C to top and bottom.  It appears the part is heated way beyond its acceptable range to place these parts (the way I suggest)?  Is this a bad idea?"

  • 0
    TI__Mastermind 49320 points

    Hi Adam,

    TPS51200 is a DDR LDO controller.The thermal performance of an LDO is greatly depends on the PCB thermal pad underneath the body. I prefer the idea #2 since TPS51200EVM used two layers and 2 oz copper PCB. The EVM thermal performance looks good. Please refer to the thermal design section in the datasheet for power dissipation consideration. I don't think it is a good idea by applying hot air at 400C to top and bottom. Short time might be ok.

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    TI__Prodigy 525 points

    Customer Response:

    On the thermal performance below; I need to go dig into the data sheet but I want real numbers.  I want to be able to calculate the thermal pad necessary to meet the part’s requirements.  I would like to calculate the junction and case temperature and show the change in temperature based on my thermal pad/ vias.  Can someone help me?  I was hoping for a spreadsheet using the inputs.  I don’t do this mechanical thermal analysis a lot so I was just hoping for some reference guide. 

    Thermal Question for QFN part.

    Inputs and variables to enter into a spreadsheet.

    1. QFC part.  Temperature range.  Industrial (-40 to 85C) or commercial (0 to 70C)
    2. Part power dissipation.  Power from Voltage drop and Current.
    3. Ambient temperature.
    4. Forced air cooling Y or N.  (flow rate if available)
    5. Data sheet values Theta coeficient.  Degrees C/ Watt.
    6. Calculate copper area.  2oz/1oz/1/2oz etc

    Tj= Ta + theta * power dissipated.  I think the formula should be more complicated and I am going from memory here.  It should include the case temperature and temperature delta across the thermal pad.   I remember it is a bit tricky getting this right but I need to get something accurate enough to make these changes to my board.

  • 0 in reply to Adam Jankauskas
    TI__Mastermind 49320 points

    Hi Adam,

    I don't have any spreadsheet for thermal analysis. Could you please contact TI package group directly regarding your question?

    Thanks,

    Nancy