> -----Original Message-----
> From: Ian Wilson [mailto:[EMAIL PROTECTED] 
> Sent: Tuesday, March 16, 2004 4:26 AM
> To: [EMAIL PROTECTED]
> Subject: [PEDA] An example why IPC footprints are often sub-optimal
> 
> One to stir up the hornets nest a little...and a little off 
> topic maybe

Ian, we just had a thread on common PCB footprints so its probably well
on topic.

> http://www.considered.com.au/ProtelFiles/images/Phycomp_vs_IPC.gif

I knew they were oversized but never quite appreciated the difference
until I see this, with them side by side, have always been use to
drafting my own.

> shows the Phycomp (the old Philips, now part of Yageo) reflow 
> 0402 footprint versus the 0402 footprint from the Altium 
> P2004 Chip Resistor library (in the ../Library/PCB folder) 
> which I think is based on IPC.
> 
> You can see the ridiculous difference.  The one on the left 
> is based on reflow with a +/-0.15 mm placement accuracy.  I 
> need maximum packing density - IPC in this case is not on for 
> this application.

That's a generous tolerance, guess you know the target machines.
 
> The problem with one size fits all (and an oversize like the 
> IPC postage stamp footprints) is that assemblers and others 
> can grab onto it as a pseudo-standard and say "we only accept 
> IPC footprints".  Instead of attempting to understand the 
> pressures on the product and adapting processes they simply 
> take the easy way out.  Sure, using small footprints may 
> reduce yield and increase costs - in some applications this 
> is appropriate.  By *blind* use of overgenerous footprints I 
> think designers are loosing the ability to optimise their 
> products globally - they are reduced to local optimisation 
> only.  And yes, this is probably a skill that is developed 
> over time and with experience - but newcomers to the industry 
> should be told, in no uncertain terms, that "IPC footprints 
> are an appropriate starting point and since they are designed 
> to cope with many soldering processes are necessarily not 
> optimum for any.".

Well said
 

John


> I am not keen on any library that thinks wave footprints are 
> the same as reflow.  Does SMTplus makes the distinction?
> 
> Ian
> 
> 
> 


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