---------------------------- Forwarded with Changes
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From: Engelmaier::(ENGLMRAA) at ~FABRIK
Date: 11/18/96 9:06PM
To: Andrew P Magee at Rogers-MCD
*To: TECHNT at ~FABRIK
Subject: Re: Interconnect Failure
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Mr. Engelmaier,

Why do you say that the stress of z-axis expansion is greatest near
the 
center? 

Thermal expansion is measured in terms of expansion per unit length
(strain) 
per change in temp. ( in/in/C ). If two dissimilar materials like
copper and 
epoxy are bonded to each other then they expand as a composite
structure 
based on:

Comp. Strain= dT x Sum( Ei x Ai x CTEi) / Sum( Ei x Ai)

Where
Ei = Tensile Modulus of layer i
Ai = Area of layer i
CTEi = Coefficient of Thermal Expansion of layer i
dT = Change in Temperature

The difference between each layer's free strain and this composite
strain 
generates the stress in the material. From Hooke's Law:

Layer Stress = Ei x (Comp. Strain - dT x CTEi)


As you can see there are no length terms involved in determining the
stress.
What do you propose is the mechanism for your observation that
fractures 
typically occur near the center? Perhaps the uniformly generated
stress is 
being concentrated by something like differential etchback or thinner
copper 
plating at the center of the PTH.

     
     Andy Magee - Applications Engineer
     Rogers - Circuit Materials
     Tel: (602) 917-5237
     Fax: (602) 917-5256
     E-Mail: [log in to unmask]
     Website: http//www.rogers-corp.com/cmu
     
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______________________________ Forward Header
__________________________________
Subject: Re: Interconnect Failure
Author:  Engelmaier::(ENGLMRAA) at ~FABRIK
Date:    11/18/96 9:06 PM


From: [log in to unmask]
Date: Mon, Nov 18, 1996 9:06 PM
Subject: Re: Interconnect Failure
To: Magee, Andrew P; TechNet
The stress of z-axis thermal expansion is greatest near the center of
a 
PWB--that is why PTH barrels fracture there. The reason why inner
layer 
separations occur predominantly on the outermost inner layers is the 
combination of the radial stresses from the expanding resin and the
bending 
moments from the land rotations on the inner layers because the resin
can 
more freely expand in the z-direction away from the PTH.
     
Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting 
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-Mail: [log in to unmask]
     

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