Paul, I don't think we are on the same wavelength. Are you telling me you can drop a part 50mil onto a board to avoid pressing it down into the paste? I cannot imagine doing that and getting any placement accuracy. That would be nonsense. It is in the force sensor where the machine detects board contact. Also, watch the board deflection as a part is placed, you would be surprised. The MyData's allowed me to slow down placement to where you can actually see this deflection. Bob K. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards Sent: Tuesday, October 11, 2011 12:09 PM To: [log in to unmask] Subject: Re: [TN] [DC] QFN layout Normally we set the machines to a height placement point for QFNs to fix the problem BUT you can do that on all machines and in ALL cases but when you can it eliminates most of the problems... Paul Paul Edwards Process/Quality Engineering Surface Art Engineering -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner Sent: Tuesday, October 11, 2011 8:51 AM To: [log in to unmask] Subject: Re: [TN] [DC] QFN layout Karen, Sadly I know more about the problems and not as much about the solutions! :-( For example the placement force on most Pick and Place (PnP) machine has a minimum setting. If you place a small part with a small number of leads the "Pressure" (Force / Area) on the paste is very high and can squish paste into places you don't want it. Take a look at a board as if comes off a PnP machine. Lift of some small parts and you will see that leads have been pushed down through the paste to the PCB copper. This works great for gull wing parts, it helps insure a good reflow. But guess what happens on a small QFN? Actually the paste on the large center thermal pad helps act like a standoff. But, how flat is the PCB as it sits on the PnP and where did the operator decide to place board supports? So much for the part coming down flat onto the PCB as it is placed! Ugh, so many problems and so few solutions! Bob K. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Karen Tellefsen Sent: Tuesday, October 11, 2011 10:39 AM To: [log in to unmask] Subject: Re: [TN] [DC] QFN layout How does one do these calculations, what are the rules? Is there a web site that can provide guidance? QFN's are difficult to solder well Karen Tellefsen - Electrical Testing [log in to unmask] 908-791-3069 Re: [TN] [DC] QFN layout Robert Kondner to: TechNet 10/11/2011 10:29 AM Sent by: TechNet <[log in to unmask]> Please respond to rkondner Pete, I agree. Sometime it is less of an agenda and more of a case of never having reflowed these parts. My observation with QFNs has suggested having a good .5 to 1.0 mm of pad outside the package. It acts like a solder reservoir area allowing solder to flow out from under the leads as the component reflows and settles down. So often I see suggest QFN footprint with no such area. Just a non-qualified observation. Bob K. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete Sent: Tuesday, October 11, 2011 9:32 AM To: [log in to unmask] Subject: Re: [TN] [DC] QFN layout I never trust the manufacturer's suggested footprint, always do my own calculations. They have a different agenda than I need to have. Lately, though, I'm finding their footprints much closer to mine. Pete --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 16.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives For additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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