> -----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 > > > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
