Folks

There are obviously many variables associated with this issue, some of which will vary widely with the particular application. One in particular is time at temperature. I suggest that we isolate this variable by correlating the material rating to specified equilibrium temperatures; i.e., a particular material package will be certified to a particular equilibrium temperature. A temperature ramp would also need to be specified. When temperature equilibrium is achieved, the induced strains also come to equilibrium. This will of course place an additional burden upon the material suppliers since it is a severe test , but I would argue this is where the burden belongs. 

There are also several first order geometric parameters, associated with the package, that govern the stress at a material boundary in a free expansion. At the last EXPO meeting I presented a paper that has a first principals development of the issue using a strength of materials model. The ratio of the package's thickness to the planer dimension was one of the most important.

Best regards

Lee

J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389  
  ----- Original Message ----- 
  From: Werner Engelmaier<mailto:[log in to unmask]> 
  To: [log in to unmask]<mailto:[log in to unmask]> 
  Sent: Tuesday, December 13, 2005 1:05 PM
  Subject: Re: [TN] Qualification of the Pb-free capable manufacturers


  Hi Valerie,
  I only partially agree--we need BOTH the time/temp rating by material [you
  have to set a base line somewhere and establish some basic capability] as well
  as verification of the board design.
  Actually, this is not particularly a new thing, with the LF-soldering
  temperatures it just has gotten significantly more critical.




  Werner

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