Hi everyone, this is my first post. Please go easy on me :-).
So after reviewing the Attiny4313 http://www.atmel.com/Images/8246S.pdf datasheet, I came across the dimensionality of my MLF/VQFN IC (page 16 to be exact). The problem is the dimensions don’t add up. After googling around I found some people suggesting that min or max is better to use than nom. Do you guys know why that is? I’m trying to create an lrb for this IC. The particular IC I have is the ATTINY4313-MU.
For a more detailed look at the package, you can go to the JEDEC website, sign up for a free account, and download the actual spec.
In general when you are designing a reliable system, you need to accommodate the worst case performance/dimensions of the part you are dealing with. In the case of making a foot print for an IC package, you need your pads to be slightly larger than the leads themselves so you design around the MAX values so that no matter what end of the tolerance range your part is, your board will accommodate it.
In response to the questions in your picture, “e” is the pitch or centerline spacing of the leads on the package. It’s stated from the centerlines because that is the location that does not change where the actual width of the pad “b” may vary within the MIN and MAX values. You’ll note that the measurement is “BSC” or Basic Spacing between Centers which makes it a theoretically true dimension with no tolerance.
To calculate the distance between the two lines in your picture, you need more information than what is provided there so you need to refer to the JEDEC drawing for the package. From that drawing you will see that packages with an odd number of pads on a side have their middle one on the centerline of the part so the total distance from the center pad to the edge of the device is 2mm (based on a 4.00 BSC package dimension). Since you have 2 pads to get from #3 to #1, you have (2*0.5mm) = 1mm between the centerline of #1 and #3. The maximum width of the pad is 0.30mm so add half of that to the 1mm from the previous step and you have 1.15mm. Subtract that from the 2mm midpoint to edge dimension and you find that the top edge of Pad #1 is 0.85mm from the edge of the package. The JEDEC datasheet says the maximum length of a pad is 0.45mm so the distance between the two lines in your picture is just the difference between these values or (0.85mm - 0.45mm) = 0.4mm when using the MAX tolerances.
-Bill
Here is the JEDEC link to the package. You want to be looking at the “K” revision. http://www.jedec.org/standards-documents/results/MO-220
-Bill