[gta02-core] PCF50633 footprint and footprints in general
werner at openmoko.org
Sun Jan 10 07:06:05 CET 2010
Dave Ball wrote:
> I've had another go, with the solder paste layer and using the PDF
> you suggested.
Much better !
> Is there a proper method for working out how much
> paste should go on the central gnd pad?
NXP's AN10365  has a few useful advices on pages 14 and 15:
a) "The solder paste should cover approximately 20 % of the total land
b) "the solder paste pattern, including the spacing between the
deposits, should have a coverage of 35 % of the land area"
c) "A HVQFN48 with an exposed pad of 5.1 mm × 5.1 mm, for example,
should have nine solder paste deposits that are arranged in a
We can also use analogies. E.g., the HVQFN48 package has a central
pad of 5.1 +/- 0.15 mm squared  and the recommended footprint
has a central pad of 5.1 mm squared . The PCF50633's HVQFN68
has a non-square central pad of 4.8 +/- 0.15 mm times 4.3 +/- 0.15
mm, so the footprint's SLx/SLy should be 4.8 and 4.3 mm as well.
(You used the upper bounds 4.95 and 4.45 mm.)
Comparing  with  and , we get: b) 3^2/5.1^2 = 34.6%
(recommended 35%) and a) 9*0.75^2/5.1^2 = 19.5% (recommended 20%),
so NXP do indeed as they say.
Using c), since 4.8 and 4.3 mm are a bit smaller than 5.1 mm, we
should probably also have a 3 x 3 array - like HVQFN48 - instead
of a 4 x 4 array.
Your pattern has an overall size SPxTot/SPyTot of 3 mm squared.
9 mm^2 are 43.6% of 20.64 mm^2 (40% of 22.0275 mm^2), which is
a bit larger than the 35% recommended in b). 2.65 mm (34.0%) or
2.7 mm (35.3%) would be closer.
Your area for a) came up out perfectly, with 16*0.5^2/4.8/4.3 =
19.4%. Taking into account the change to 9 pads, we could use
0.65 mm (18.4 %) or 0.7 mm (21.4 %).
Hmm, I think I have to teach fped to "print" calculation results,
so that we can check such percentages without using "bc".
> The figures I've used so far
> are just what seemed reasonable!
Not too shabby for dead reckoning :-)
Ah, and please keep the silk on copper issue in mind.
By the way, in the last few days, I made several usability
improvements in fped's GUI. E.g., there are now tooltips on
almost all items, and quirks such as false drags when clicking
(but not dragging) a draggable point are gone.
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