Hi Prashant,

The "alternative" repair method is not reliable.
First - the airgun.
You will never know which temperatures hit the board and adjacent components.

Secondly- The connection for a QFN is made under the component, not at the side. Although it migth work electrically at first, this is not a reliable bond over time.

Not only the assembly process should be "in control" , also the repair process. This often overlooked.
You need a decent repair station to achieve this.

Get rid of the hot air gun!
Or use it at home for a paint job.


Jowan Iven 
Process Engineer 
Neways Advanced Applications B.V. 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of prashant chopra
Sent: Wednesday, October 21, 2009 2:43 PM
To: [log in to unmask]
Subject: 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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