QFN 3/3, pasty islands in a copper sea

Werner Almesberger werner at almesberger.net
Thu Mar 31 05:58:39 EDT 2011


Solder paste is a bit of a sticky subject when it comes to QFNs. While
the peripheral pads are reasonably easy, publicly accessible
specifications for the center pad are scarce and the ones I found
differ wildly.


For the peripheral pads, the consensus seems to be to make the solder
paste area slightly smaller than the copper pad, but not to try to
compensate for any potential layer registration issues.

In [1], NXP specify a distance between the copper border and the
border of the solder paste of 25 um. I used 1 mil (25.4 um) in atben
and atusb.


For the center pad, the story is quite different. If one were to
simply cover the entire center pad with solder paste, the gas produced
when heating the solder paste would expand and splatter molten solder
all around the pad, probably shorting peripheral contacts.

Some companies have devised rather fancy patterns to combat this
problem, e.g., the star pattern on page 13 of [2].

Luckily, there are also simpler patterns that seem to work. A very
common type of pattern is to have an array of small square solder
paste deposits in the middle of the pad:

    +----------------+
    |                |--- Copper pad
    |  +---+  +---+  |
    |  |   |  |   |  |
    |  +---+  +---+  |
    |                |
    |  +---+  +---+  |
    |  |   |  |   |--|--- Solder paste
    |  +---+  +---+  |
    |                |
    +----------------+

We can characterize the solder paste pattern with the following
parameters:

    |<---Pad size--->| (P)
    +----------------+
    |                |
    |  +---+  +---+  |--
    |  |   |  |   |  | ^
    |  +---+  +---+  | |
    |                | | Paste zone (Z)
    |  +---+  +---+  | |
    |  |   |  |   |  | v
    |  +---+  +---+  |--
    |                |
    +----------------+
           |<>|<->|
            G  Solder paste island size (I)
            a
            p (G)

If we have an N x N pattern, we get Z = N*I+(N-1)*G

Looking for specifications for chip of sizes similar to the one we
need and with publicly accessible solder paste specifications, I found
Silab's C8051F326 [3], NXP's SOT788-1 [4], and SOT818-1 [5]:

Package		Pad	Paste zone	Islands	Island size	Isl. gap
		(P, mm)	(Z, mm)		(N x N)	(I, mm)		(G, mm)
SiLabs		3.25	3.1		3 x 3	0.9		0.2
SOT818-1	3.4	1.75		2 x 2	0.75		0.25
SOT788-1	4.0	2.4		3 x 3	0.6		0.3

NXP recommend in [1] that

- paste zone = 35% of pad area, or
  Z*Z = 0.35*P*P

- paste coverage = 20% of pad area, or
  N*N*I*I = 0.2*P*P

On the other hand, SiLabs aim for 67% paste coverage in [3].

Applying the above formulas to the above specifications, we get

Package		%zone	%paste
SiLabs		90%	69%
SOT818-1	26%	19%
SOT788-1	36%	20%

Since the chips in ATBEN/ATUSB don't have to drive any major loads,
thermal conductivity of the center pad is not important. I did not
try to solder the center pads in my prototypes, so having any solder
paste at all can only improve the effect the center pad has on RF
performance.

I therefore favoured low risk of solder paste spills over having the
center pad tightly connected, and used NXP's formulas. I also looked
for similar footprints to make sure also parameters NXP didn't specify
in [1], such as the number of islands, the island size, or the gap
size, are sane.

In ben-wpan/modules/qfn.fpd, we have the following QFNs:

Footprint	Center pad  	Closest NXP (with complete land pattern)
QFN28-Atmel	2.4 mm		for further study
QFN28-SiLabs	3.3 mm		use SiLab's specification (for now)
QFN32-VHHD-2	3.7 mm		between SOT818-1 and SOT788-1
QFN32-VHHD-6	3.4 mm		SOT818-1

For ATBEN/ATUSB, we need only QFN32-VHHD-2 and QFN32-VHHD-6, so I'm
leaving a closer study of QFN28-Atmel and QFN28-SiLabs for a later
time.

Finally, these are the parameters I picked (all dimensions in mm):

Package		Pad  Paste zone  Islands  Isl. size  Isl. gap  Zone  Paste
		(P)  (Z)         (N x N)  (I)        (G)
QFN32-VHHD-2	3.7  2.15        3 x 3    0.55       0.25      34%   20%
QFN32-VHHD-6	3.4  1.95        2 x 2    0.75       0.45      33%   19%


[1] http://www.nxp.com/documents/mounting_and_soldering/AN10365.pdf
[2] http://www.wolfsonmicro.com/documents/uploads/misc/en/WAN0118.pdf
[3] http://www.silabs.com/Support%20Documents/TechnicalDocs/C8051F326.pdf
[4] http://www.nxp.com/package/SOT788-1.html
[5] http://www.nxp.com/package/SOT818-1.html

- Werner




More information about the discussion mailing list


interactive