Chamfer in Mechanical Drawings in Datasheets

Discussion in 'General Electronics Chat' started by mwatson, Nov 9, 2015.

  1. mwatson

    Thread Starter New Member

    Nov 9, 2015
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    OK, I have been searching all over for this. I am trying to make a footprint in Proteus from an ATtiny841 with the 3mm x 3mm VQFN package. Now I know how to draw foot prints, and I know what a Chamfer is, but the way they describe the Chamfer measurements is what I don't understand. I have attached a picture of the drawing. It will say things like "C0.18 (8X)" for one of them and on the other and another "0.3 Ref (4x)."

    For instance what does the 8X and 4x mean? Some kind of magnification? C0.18, does that mean I just take 0.18 and multiply by C? Are they using circles to demonstrate the dimensions with a radius of 0.18? I am so confused by this.

    Any help would be GREATLY appreciated. I do embedded software engineering and I have my own projects going on at home so I am trying to learn the hardware side as much as possible. I get a lot of it, but this threw me off a bit.

    Thanks again for any help,
    Mike Watson
     
  2. eeabe

    Member

    Nov 30, 2013
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    The 8X just means it occurs 8 times in the drawing, but they only show one example.

    ...and with a quick internet search, it looks like the chamfer dimension is the distance from one side to the chamfer corner, same on both sides for a 45 degree chamfer.
     
    Last edited: Nov 9, 2015
  3. mwatson

    Thread Starter New Member

    Nov 9, 2015
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    Awesome eeabe! Thanks so much. It makes sense now.

    --Mike Watson
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    @mwatson
    I don't believe that drawing is intended to be the land (pad) pattern for PCB artwork.. (dimension for dimension anyways)..
     
  5. mwatson

    Thread Starter New Member

    Nov 9, 2015
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    mcgyvr, you just really confused me now. If the artwork in a datasheet isn't suppose to be the land pad pattern for PCB artwork (dimension for dimension anyways).. then how do I know what I can get away with? Because Proteus has a basic qfn20, I think one has a thermal pad as well but there are no chamfers on any of it.. I am guessing I just use the tolerances (max and min of each measurement)?

    So I guess my question really is, could I use something like that and just forget the detailed chamfers? As long as the space between pads (width), the space between the center of the pads, length of the pads, and the pads are all laid out with the width and length from pad to pad are within the mechanical specifications of the drawings, it should solder properly, right?

    I mean little tiny corners chamfers cut off on the chip's pad shouldn't keep the chip from properly soldering to a square pad of the same size or slightly bigger without a chamfer I wouldn't think, now that I am thinking about it. Would this be a correct assumption? I mean it seems like it. I know I read somewhere, in a datasheet or somewhere, that the pad needs to be slightly bigger anyway for thermal expansion I think? I can't remember the reason, just that you needed to add like 5 to 10 percent to all sides.

    Anyway thanks for all you guys help already and any more advice you may give,
    Mike Watson
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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