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October 2011

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Subject:
From:
Robert Kondner <[log in to unmask]>
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Date:
Tue, 11 Oct 2011 12:48:46 -0400
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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

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