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Tolerance stack

Brandon Richards
Prodigy 30 points


Looking at the drawing,  I can add the profile (0.15) + total height (2.96/2.62)  OR I can add the profile (0.15)+solder ball height (0.45/0.35) + base to lid (2.55/2.23) and get different tolerance stack results.  What tolerance path should be used?  Does this apply to pre or post reflow? Lastly, should I assume nominal value is the middle of these shown max/mins or does the nominal lean one way towards the low or high side?

 

  • 0
    TI__Mastermind 42515 points
    Hi Brandon,

    should there be a picture in your post? If so please try to attach it, the pasting do not work in this forum.

    Regards,
    Yordan
  • 0 in reply to Brandon Richards
    TI__Mastermind 42515 points
    Hi Brandon,

    I have forwarded the question to experts for comment.

    Regards,
    Yordan
  • 0 in reply to Yordan Kamenov
    TI__Guru 55497 points
    1. All numbers on the drawing are at component level prior to mount on PCB and reflow.

    2. The 0.15mm is Seating Plane. Imagine you put this package on a flat plane.
    Not all the solder balls will touch the plane due to different ball height.
    The 0.15mm is the height difference between the tallest and shortest ball.
    It has nothing to do with the package stack up height calculation.

    3. We always use nominal value when running any calculation.

    4. Here are some of the numbers on this package.
    0.80 (Ref) is the estimate height of the top portion of the heat shield
    2.23-2.55 is the package height without the solder balls (nominal is 2.39)
    2.62-2.96 is the total package height with solder ball (nominal is 2.79)
    0.35-0.45 is the solder ball height (nominal is 0.4)

    5. There are two ways you can calculate the total package height and both come up the same number.
    A. Take the nominal value of total package thickness including solder balls  2.79mm
    B. Take the nominal package thickness without solder ball, add the solder ball nominal height
    2.39mm + 0.4mm = 2.79mm

    Regards,
    Kyle
  • 0 in reply to kcastille
    Prodigy 30 points
    HI Kyle,
    Thanks for responding. However, this does not answer my question. I am looking for the correct tolerances. This drawing is over dimensioned and does not allow me to understand the proper path to follow to get a min/max height from a PCBA (which I will refer to as dtm A). My goal is to understand the max/min height off of dtm A so that when I have a heatsink that mounts to dtm A, I know the required height of the standoff to make sure I do not crash into the chip. I also want to know my largest gap, so I can do my thermal analysis. If you are free and want to set up an email chain and/or webmeeting so we can trade pictures and tolerance stack calculations, please let me know. Thanks!

    -Brandon
  • 0 in reply to Brandon Richards
    TI__Guru 55497 points

    Brandon,

    You should use the total package thickness which is 2.62 – 2.96mm.

     

    If you are going to add a heat sink on top of this package, you should also consider the following to determine the final package thickness:

    - Stencil thickness

    - Solder paste shrinkage

    - Total solder ball height after reflow

     

    The solder ball attached to the BGA package will shrink about 10-20% during the board reflow. It will depend on the flux activity level.

    Regards
    Kyle