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April 2008

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Subject:
From:
Dennis Fritz <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 29 Apr 2008 11:03:30 EDT
Content-Type:
text/plain
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In a message dated 4/29/2008 4:19:24 A.M. Eastern Daylight Time,  
[log in to unmask] writes:

I need  to be able to predict the fatigue life of solder joints under
different  thermal cycling and vibration load conditions. The PCB will
usually be FR4,  the solder tin/lead, the components everything from
BGA's to chip passives.  Ideally I need to provide an estimate on cycles
to  failure.


While many of us on TechNet can help you some, I too do not think there is  
exact empirical software. 
 
Fortunately, you are using tin/lead where there is a lot more data, and  
working models.  I can give you the following personal observations from  both the 
IPC Relaibility Conference two weeks ago in Boston, and from our  project 
advising Crane Naval Depot on repair of interconnections in the face of  the 
lead-free avalanche:
 
1. Werner Engelmaier (and other consultants) can use projections from  
fatigue equations to estimate first failure, assuming the input data is  correct.  I 
am assuming you want estimated first failure, not some  statistical 
representation like when 10% failure or 50% failure.  His  presentation is posted from 
that conference, but since it was for a fee by IPC,  the presentation posting 
on the IPC site is their property. 
 
2. From our project analysis of many Weibull plots, we generalize that the  
smaller the solder joint volume, the earlier the failure.  BGAs and worse -  
QFNs - are now getting a lot of publicity, because they are the first to fail in 
 thermal or vibration situations. 
 
3. Vibration is a less exact science than Thermal cycle.  What kind of  use 
condition are you dealing with?  We are quite concerned about military  
electronics in high vibration conditions where stiffer Lead-Free solders may be  
used.  Tests show that component location on boards is very important, as  BGAs 
can either be almost instantly failed, or can perform admirably, depending  on 
assembly location on a vibrating board. 
 
4. FR4 is not always plain old FR4 in either thermal cycling or vibration  
testing.  Much of the newer, high temperature laminate is FR4, but has  
different modulus and possibly even different moisture absorbsion, that effects  
mechanical properties of the solder joints attached.  Fortunately  for  Crane, most 
military repair is on older 140 Tg boards that are not as stiff as  the newer 
laminate that can sustain highter soldering temperatures. 
 
The prior recommendations for consultants and working consortia could be  
quite helpful.  
 
Dennis Fritz
SAIC, Inc. 



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