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October 2011

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Subject:
From:
Tan Geok Ang <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Tan Geok Ang <[log in to unmask]>
Date:
Thu, 13 Oct 2011 15:34:11 +0800
Content-Type:
text/plain
Parts/Attachments:
text/plain (417 lines)
Hi Paul,
		Like your statement "we don't use MyDatas for QFNs"
should add and "LGA".
		Really hope that MyData able to solve this issue which I
believe through their software control with Z-height head/Component
thickness/Board flat measurement information, etc.

Regards
Geok Ang
(Office: 68712393)

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Wednesday, 12 October 2011 8:22 AM
To: [log in to unmask]
Subject: Re: [TN] [DC] QFN layout

Bob,

It would be if I told you that we released the part after we deformed
the solder paste 30%...

And that's why we don't use MyDatas for QFNs...

Paul

Paul Edwards
Process/Quality Engineering
Surface Art Engineering


-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]] 
Sent: Tuesday, October 11, 2011 11:59 AM
To: 'TechNet E-Mail Forum'; Paul Edwards
Subject: RE: [TN] [DC] QFN layout

Paul,

 Hmmmm so you say you don't deform the solder paste thickness by more
than
30%. Hmmm.

 Since the paste is probably about 4 or 5 mils high that means you
position
the part within about 1.5 mils. Hmmmm

 Considering the PCB thicknesses vary more than that, and part
thicknesses
vary more than that, and board flatness is much further off than 1.5mil
...

   I would suggest your comment is pure hog wash?  :-)

Bob K.

 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Tuesday, October 11, 2011 12:46 PM
To: [log in to unmask]
Subject: Re: [TN] [DC] QFN layout

Bob,

We measure the part and PCB warp and vary the part vacuum release point
based on that to insure we are not deforming the printed solder paste
structure more than about 30%...

Because the MyData can't do that we don't typically use the MyData for
QFNs...

Paul  

Paul Edwards
Process/Quality Engineering
Surface Art Engineering


-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Tuesday, October 11, 2011 9:49 AM
To: 'TechNet E-Mail Forum'; Paul Edwards
Subject: RE: [TN] [DC] QFN layout

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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