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October 2009

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Subject:
From:
Robert Kondner <[log in to unmask]>
Reply To:
Date:
Wed, 21 Oct 2009 09:44:05 -0400
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If you use a piece of kapton tape to hold the QFN up you get a better reflow
under the QFN.

Some PCB design don't provide that mm or so of pad on the outside of the
QFN, that makes rework really difficult.

Bob Kondner

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of prashant chopra
Sent: Wednesday, October 21, 2009 8:43 AM
To: [log in to unmask]
Subject: [TN] PWB Layout spacing queries

Hi,
 
I have a couple of queries with regard to the component spacing ---
 
1. What should be the minimum spacing between the QFP and the nearest chip
components? (Assuming that the in case of failures the QFP will be reworked
manually using the soldering iron).
 
2. What should be the minimum spacing between the QFN package and the
nearest chip component? ( I am attaching a datasheet of QFN above).
 
3. What should be the minimum spacing between the 0402 component and the
other nearest 0402 component or chip component?
 
Also, I have one query reagrding the QFN rework process (datasheet attached
above) --
 
1. Ideally, in case of replacing the old QFN, the rework procedure should be
similar to the BGA rework procees and using the rework station and nozzle. 
But, one of our manufactuirng location is not having the rework station.
They rework it manually, using the hot air gun to remove the component. Then
they place the new QFN component and solder the small portion visible on the
side edges (as shown in side view in data sheet) using the soldering iron.
But I believe, this will not allow much solder to flow below the part and
form contact there as major portion of the pad is below the body of the QFN.
Is this process reliable? Will it have any impact on the performance over a
period of time? is there any other reliable manual process which can be used
for this rework?
 
Appreciate all your help and response. Thanks in advance.
 
Rgds
Prashant


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