Ashok,
The minimum solder joint thickness depends on 5 parameters:
(1) the size of the QFN,
(2) the CTE-mismatch with the PCB,
(3) the delta-T,
(4) the reliability requirement in terms of number of cycles, and 
(5) the reliability requirement in terms of acceptable failure probability.
How all of this hangs together, can be found in IPC-D-279; alternatively, you can take my workshop on fundamentals of solder joint reliability.
Werner


 


 

 

-----Original Message-----
From: Ashok Dhawan <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, Jul 5, 2010 12:02 pm
Subject: [TN] QFN : what is minimum solder thickness to rework


I am debating on what is minimum acceptable solder joint thickness for QFN-32 

packages ? Just for clarity , QFN-32 is bottom termination part with no toe 

joint due to exposed copper on toe. 



We did cross sectioning and found that solder thickness varies form 15 microns 

to 45 microns - different footprints. We are wondering what should be minimum 

acceptable thickness before we need to rework ?  Any graph or relationship to 

determine reliability of joint with respect to thickness of joint ? 



We are building Class3 products for automotive industry. No information found 

on thickness of solder joint Except for IPC-7093 (unreleased draft copy) where 

the solder thickness is mentioned as 50 microns. 



Anybody to share what should be used as minimum thickness before 

reworking ? 



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