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QFP footprint -extended outside pad?

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#65036

QFP footprint -extended outside pad? | 15 September, 2011

I also posted this in the design section...

G'day folks - haven't been around for some time but still soldering on. Question for y'all.

One of my design engineers has shown me a Creative PCI audio card with a TQFP48 attached. 0.4mm pitch. Let's say the pin width is 0.2mm and the pad design has the pads at 0.2mm width also. On each of the 8 outside pads the pad is wider to the "outside" of the pin. Let's guess at 0.05mm or less.

Does anyone know the reasoning behind this? There is a very crude drawing attached to explain. I removed a few cogs from the Spirograph to do the drawing.

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#65037

QFP footprint -extended outside pad? | 15 September, 2011

Excellent drawing Darby. You haven't lost your touch. Spirograph jeez

Do you and that beautiful woman still get good parking at Watto's? http://www.sydneypubguide.net/pubs/Watsons_Bay_Hotel_(Doyles).aspx

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#65038

QFP footprint -extended outside pad? | 15 September, 2011

HEY DAVE! Jeez - you're quick on the trigger. Glad to see you are still around my friend. You should see my handwriting.

Yes - I still have my miraculous park-pulling powers at Watto's but it's getting harder. You should've seen Di Caprio trying to reverse park that Duesenberg last Sunday. He can't park for tripe.

After I hurled out the bodygaurds and parked it for him I slotted mine in and ran down the wharf and caught a little snapper for him for lunch. pic attached. That's Sydney Harbour in the background. Doyley cooked the snapper in a spatlese "jus". A tad sweet to my taste but Leo seemed to enjoy it.

Oh yeah. The beautiful wife is still with me as well. (Now that is true).

Have I gone off topic? Sorry.

Any idea on the footprint? The reasoning doesn't come immediately to mind for reflow and they're asking me if we should try it. I dunno, I don't see the point. But I don't know everything.

How's Jimbo's?

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#65040

QFP footprint -extended outside pad? | 16 September, 2011

We see no advantage to extending the toe-end of pads. Hell, 90 percent of the time the numb-end of the toe of the component lead isn't plated. So, it doesn't take solder. Further, solder on the toe adds little, if anything to the strength on the solder connection.

We moved to the Tampa area. So, the hilarity of Jimbo's is a long drive.

That is a good-looking fish. I've never caught a snapper that size. They were over-fished around here. So, conservation measures were needed.

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JAX

#65041

QFP footprint -extended outside pad? | 16 September, 2011

Some people believe that increasing the outer dimension will aid in centering the device during reflow...

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#65046

QFP footprint -extended outside pad? | 16 September, 2011

I recently did a layout for a Freescale accelerometer and the mfg recommended pad layout did something similar. They where very specific in the pad layout dimensions. They stated the reason was "With the correct footprint, the packages will self-align when subjected to a solder reflow process" and that translated to "optimal zero-g offset after PCB mounting".

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#65048

QFP footprint -extended outside pad? | 16 September, 2011

#65050

QFP footprint -extended outside pad? | 18 September, 2011

Thanks ISD, I had a look at those papers from Freescale regarding the LGA "optimal zero-g offset after PCB mounting". Had me going for a while until I realised it was in relation to the finished product. Hmmmmm. LGA/QFN type. They can be a little tricky. Might have a look at that

See your still around too JAX, good to see. You've seen a lot of boards come out of ovens - you think it works on a QFP? Any links to white papers etc. I've looked - can't find zip.

We have some Italians here at work Ryan so I'll have that translated shortly.

I'm half tempted to say "go ahead" so that the cad lads can get a nice fuzzy feeling.

Can anybody see a downside that I'm missing if I tell them to try it?

Sorry to hear about your snapper Dave. That pic is actually Bay Of Islands in New Zealand. We have lots of little snapper in Sydney Harbour but they move offshore early in their life. You'd never see one that size inside the harbour. They come into the Bay Of Islands in their thousands to breed. Fish soup. It's crazy. The Kiwis have managed their fishery very well to ensure an ongoing recreational and commercial fishery. That fish is around 3kg. I've caught them to 5kg and seen an 11kg. They got some BIG snapper there. The prime season starts in a few weeks but bummer, I'm not going this year.

Thanks for the input folks.

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JAX

#65056

QFP footprint -extended outside pad? | 19 September, 2011

No white Papers... although you might want to look at Renesas Semiconductor. They have a nifty little formula that seems to not take enough variables into consideration.

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