You are in dire straits: Quality For Nothing and your chips are free.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
Sent: Monday, February 25, 2013 9:14 AM
To: [log in to unmask]
Subject: Re: [TN] QFN reliability

Wayne,
Be careful of what you ask for with spacers. Usually solder mask can be used but in the case of chips it results the majority of reflow solder balls and tombstone defects with small ships. If the QFN package has a ground pad and solder mask between I/O pads then it can act as a gasket trapping flux volatiles resulting in part hopping or movement during reflow as those flux volatiles boil off and can result in open solder joints. 

John maxwell

Sent from my iPad

On Feb 25, 2013, at 6:54 AM, Wayne Thayer <[log in to unmask]> wrote:

> Getting joint height without spacers on QFN is a challenge.
> 
> Worst case:  There's a serial flash in a QFN package:  Has 8 small pads with no base pad at all.  Impossible to get joint height without spacers.  Ditto for most chip caps, where the end fillets help suck the part down to no height.
> 
> Please provide paper references on spacers for QFN.
> 
> Thanks,
> 
> Wayne Thayer
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
> Sent: Monday, February 25, 2013 9:36 AM
> To: [log in to unmask]
> Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.
> 
> Sorry for the confusion.
> The statement "there's no real solder in the joint--all intermetallics" is not quite true. It all depends on the aperture size, type of component, etc, etc. Any amount of solder deposit is going to provide much more solder than the IMF alone, which is only a few microns thick at best. However, the solder in between the IMF on the belly pad of the part and the IMF on the pad is important only in its modulus of elasticity. The more you have, the better. But one can only print about a 45-50% reduction of the pad dimension, as otherwise you will have solder balls, etc.
> Oftentimes a spacer is added. It has been shown to greatly increase the reliability. Many papers on that subject out there.
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Gregory Munie
> Sent: Monday, February 25, 2013 8:14 AM
> To: [log in to unmask]
> Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.
> 
> OK. Now I know the answer about QFN reliability: It's Yes. And No.
> 
> Starting off Monday more confused than finish of last Friday . . . 
> 
> :-)
> 
> Greg Munie PhD
> IPC Technical Conference Director
> 630-209-1683
> [log in to unmask]
> 
> 
> 
> 
> http://www.ipcapexexpo.org/
> http://www.ipc.org
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
> Sent: Monday, February 25, 2013 7:46 AM
> To: [log in to unmask]
> Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.
> 
> QFNs are here to stay, and like Chinese semiconductors, are now or soon will be used in all of our military systems.  Years from now, while picking through the rubble of our civilization, anthropologists will speculate on what it was which made us all so stupid.  

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