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SMT electronics assembly manufacturing forum.


footprint formula

jacqueline Coia

#1686

footprint formula | 2 March, 2000

Could anyone please tell me if there is a quick simple formula for working out the ideal SMD footprint for a 52 pin QFP using the dimensions specified on the manunfacturers component drawing. I know of the IPC 'SMD land patterns' standard, which I eventually intend to get, but at the moment I need a quick solution. Thanks in advance.

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Jacqueline Coia

#1687

Re: footprint formula | 2 March, 2000

Could this QFP be easily wave soldered if placed on solder side of PCB, if so are there two different footprint formulas that take both wavesoldering and reflow into consideration. Thanks again. Jack

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#1688

Re: footprint formula | 2 March, 2000

Did you try the online-calculator at the IPC-page. If you have the datasheet it should be no problem. For wave soldering I can�t tell if it�s suitable for that particular part, but you should add extra pads at the end of each column to solderbridging and give a little more lenght to the pad . You could add extra pads only and the end of the component if you know the direction the PCB goes through the wave.

M2C Wolfgang

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RDR

#1689

Re: footprint formula | 2 March, 2000

Jack, a couple of questions first, 1. What pitch are these leads? 2. What are the lead footprint dimensions? 3. Orientation of component?

I personally have never had much luck soldering QFPs in the wave unless they are 50 mil pitch or greater, we oriented at 45 Deg. and added a single large pad at the trailing corner. it somewhat worked (shorting)but I wouldn't brag about it or even try to do it if you absolutely don't have to

Russ

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jacqueline coia

#1690

Re: footprint formula | 3 March, 2000

I have tried the IPC calculator and they do not have this type of component registered. When trying to select 'other' on menu, no connection can be made. The polarity if the QFP has not been decided yet (pre-PCB design) the pitch size of the component is 1.0mm and the lead footprint itself is 2.2mm x 0.4mm. Cheers

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#1691

Re: footprint formula | 3 March, 2000

Hi Jacqueline, - what a name in this technical world - I just tried the calculator and for example choose the CQFP and then selected other and an input mask appeared where you have to fill in the required data from the components datasheet. I have to admit that without the explanations of IPC-SM-782 and for the first time it�s hard to know what to fill in but nevertheless it�s been a great help for our designers and today it�s practice to check all suspicious ( nonproven) patterndesigns with this calculator. For the placement on the bottom side I can�t give any advice without knowing the capability of the specific wavesolder environment ( machine ) but I think it�s been done and working. If I would have to decide I would check with the assembly first or have them make tests.

M2c

Wolfgang

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#1692

Re: footprint formula | 3 March, 2000

Jack: "Calculator/Other" works for me. But you know what, I'll bet they block Scotts from using it, because they're reputed to be too cheap to buy the standard. ;-) (Your component is in the standard. ;-)) If you can't get the supplier to help and are total (not just a little) frustrated, e-mail me a number, and I'll fax you something that has nothing to do with pad layout. Ta. Dave F

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Jacqueline Coia

#1693

Re: footprint formula | 8 March, 2000

Finally got using the calculator guys, (used another server connection). Not sure about the data that is entered for 'solder joint analysis', any helpful hints? To touch on the subject of Scots people being 'cheap', I cant understand these rumours, we are the opposite! these people obvisously have not been out for a night on the town with a Scots person. Cheers.

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#1694

Re: footprint formula | 8 March, 2000

Hi Jacqueline,

for the "manufacturing allowance" IPC says FAB.TOL. is the board manufacturing tolerance and PLACE ACRCY is the pick and place accuracy. For 0,8 mm pitch they give for both values 0,1 mm. Solder joint design goal data comes from empirical information that has been determined over the years for the derivation of good land patterns. For example again 0,8mm pitch: J/T Toe Min 0,17mm J/H Heel Min 0,42mm J/S Side Min 0,0

Your Toe fillet may be a lot greater if you design for wavesoldering. I don�t see how this calculator distinguishes between design for reflow and wave, I guess it mainly covers the reflow aspects.

M2C

Wolfgang

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#1695

Re: footprint formula | 8 March, 2000

Jack: I'll think about the latter point over my next McEwan's. Dave F

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#1696

Re: footprint formula | 8 March, 2000

Wolfgang: I agree that the IPC pads are reflow / top side wave solder. We run bottom-side PCLL at 45� to the wave with fairly involved solder thieves. Ta. Dave F

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