Andrew,
     
     I think you will find a common occurance(typically in thicker boards) 
     that the center of the product does not heat up at the same rate as 
     the outer sections due to the thermal conductivity of the laminate 
     being used.  I feel we must take into account the TIME that is 
     required for thermal expansion. If the thermal expansion of the 
     laminate is quicker and greater than that of the copper then we may 
     see stress cracks in the center of the board. The secondary concern is 
     of course the tensile strength, elongation and ductility of the copper 
     being plated.  The thickness of the plated copper also plays a very 
     large role in this due to the fact that the thicker the copper the 
     higher percent of elongation. As you have pointed out on most thick 
     product the plating is thinner in the center of the hole.
     
     Regards,
     
     Alan B. Cochrane
     Multek Inc.


______________________________ Reply Separator _________________________________
Subject: Fab: Interconnect Failure
Author:  [log in to unmask] at INTERNET_GATEWAY
Date:    11/19/96 10:33 AM


---------------------------- Forwarded with Changes 
---------------------------
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
---------------------------------------------------------------------- 
---------
     
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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