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 [1] has a few useful advices on pages 14 and 15: a) "The solder paste should cover approximately 20 % of the total land area." 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 three-by-three array." We can also use analogies. E.g., the HVQFN48 package has a central pad of 5.1 +/- 0.15 mm squared [2] and the recommended footprint has a central pad of 5.1 mm squared [3]. 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 [1] with [2] and [3], 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". [1] http://www.nxp.com/documents/mounting_and_soldering/AN10365.pdf [2] http://www.nxp.com/acrobat/packages/sot619-1_po.pdf [3] http://www.nxp.com/documents/reflow_soldering/sot619-1_fr.pdf > 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. Thanks, - Werner
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