Hi 'jk,'
The composite CTE of the PCB is important--at least as a sanity check, and of 
course, the lay-up and lamination details are very important.
However, for PCBs during soldering processes and in severe operational 
environments that is not enough. The only way to determine the local loading 
conditions is to run FEA simulations--and these are only as good as the input 
paramaters used; in fact, the can be useless or even misleading, unless the best 
possible information is used.
In fact, there are 3 types of resin layers in any PCB: 'laminate' layers, 
'prepreg' layers, and 'just' resin layers. This is important, because these 
layers have diifferent properties;   and because it has been observed over and over 
again, that barrel cracks occur where the 'prepreg' layers are in the PCB.
The reason is clear; 'prepreg' layers have a significantly higher resin 
content than 'laminate' layers and are also less fully cured. That results in a 
higher CTE(z) for the 'prepreg' layers--I estimate that 'prepreg' layers have a 
CTE(z, <Tg) ~10 to 15 ppm/C higher than the 'laminate' layers.
The 'just' resin layers are not x,y-constrained and thus have the isotropic 
CTEs of the resin in all directions, but because they are thin, do not have any 
real impact.
I received the information I need direct from Nelco.
N4000-13
X axis CTE (-40 to +125C.)=16.9 ppm/°C
Y axis CTE (-40 to +125C.)=4.9 ppm/°C
Z axis CTE (below Tg)=69ppm/°C
Z axis CTE (above Tg)=283ppm/°C
This is, I am quite certain, for 'laminate' layers. For 'prepreg' layers I 
would estimate CTE(x,y) to be about the same, but Z axis CTE (below Tg)~79ppm/°C 
and Z axis CTE (above Tg)~298ppm/°C. For the 'just' resin layers CTE (x/y/z,<
Tg)~30ppm/°C and CTE (x/y/z,>Tg)~100ppm/°C.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com


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