QFN 2/3, where the sun never shines

Werner Almesberger werner at openmoko.org
Sat Jan 22 22:06:57 EST 2011

The truly dark side of QFNs is the central pad underneath the chip.
Here, we have:

- the size of the pad on the chip,
- the size of the land pattern underneath, and
- the pattern (!) of the solder paste.

NXP have a good introduction to the topic [1].

Before we get to the central pad itself, other noteworthy information
in [1] includes:

- for 0.5 mm pitch QFNs, the placement tolerance is 30-50 um (page 16,
  table 4)

- the solder paste area on copper pads (besides the center pad) should
  leave a 25 um border of uncovered copper on each side (page 14, fig
  14), according to NXP.

Now to the pad. The goal is to determine the pad size. Here are our
usual suspects (see the previous posting for data sheet references):

28QFN 5x5 mm
			Center pad	Center land
qfn.fpd (old)	(*)	-		-/3.25/-
qfn.fpd (new)	(*)	-		-/3.404/-
SiLabs C8051F326	2.9/3.15/3.35	3.2/-/3.3
Intersil L28.5x5	2.95/3.1/3.25	-/3.1/-
Wolfson 28QFN 5x5 A	3.2/3.3/3.4	-/3.4/-

32QFN 5x5 mm
			Center pad	Center land
qfn.fpd (old)		-		-/3.4/-		(for AT86RF231)
qfn.fpd (new)		-		-/3.554/-	(for AT86RF231)
Atmel AT86RF231		3.2/3.3/3.4	-
Atmel ATmega32U2	3.5/3.6/3.7	-
NXP SOT617-1 [1]	2.95/-/3.25	-/3.1/-
NXP SOT617-3 [3]	3.45/-/3.75	-/3.5/-
Intersil L32.5x5 [4]	2.95/3.1/3.25	-/3.1/-
Intersil L32.5x5B [5]	3.15/3.3/3.45	-/3.3/-
Intersil L32.5x5D [6]	-/3.5/-		-/3.5/-
Intersil L32.5x5E [7]	-/3.7/-		-/3.7/-
Intersil L32.5x5G [8]	-/3.5/-		-/3.5/-
Wolfson 32QFN 5x5 A/C	3.2/3.3/3.4	-/3.4/-

(*) old: my original definition, commit
    new: the footprint suggested by Adam (make the pad 5 mil wider
         at each side), commit 8703fc1a0beab2e458ae2f583e76d2ed98b8528b


- Intersil always use the nominal size of the center pad for the land
  pattern. Wolfson use the maximum size of the center pad, while NXP
  and SiLabs stay close to the nominal size, sometimes with a slight
  shift towards the maximum.

- the new sizes exceed the maximum values of documented patterns in
  all cases, which is probably a bad sign.


- set the land size for the C8051F326 to the maximum size recommended
  by SiLabs, 3.3 mm

- return to the old value of 3.4 mm for the AT86RF231

- introduce a 3.7 mm central land for the ATmega32U2

[1] http://www.nxp.com/documents/mounting_and_soldering/AN10365.pdf

(Third part - the solder paste - will follow in a bit. It's a tricky
subject which can wait until I'm done with the prototypes.)

- Werner

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