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February 2008

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Subject:
From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Fri, 22 Feb 2008 15:50:21 -0500
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Hi Kevin,

We noticed that the pad rotation increases starting from the center of the PTH where there is no rotation, to maximum rotation on the outer layers. One point to remember is that if you see pad rotation in a microsection it is because there has been plastic deformation of the dielectric material during thermal excursion. To be able to observe the rotation the material has to have lost some elasticity and is now deformed at ambient. The rotation at temperature will be more severe than observed at in a cross section at ambient.

There appears to be a bending moment about .0005" to .001" in from the barrel along the copper foil of the pad. I recently saw a plot of stress forces in a PTH in a FEM simulation at 260°C and the forces show a downward then upward stress pattern .0005" into the foil. The pattern could be described as a lazy "S" shape, on it's side,  oriented down the axis of the foil. This is exactly the same shape we observed when interconnects have under gone catastrophic failure to the degree they separate from the barrel of the PTH and are displaced out of plane to their original connection. This stress pattern explains the layer two and N-1 foil cracks we saw back in the '80s and now again with lead-free assembly.

Our company's reliability testing measures damage at internal interconnections. I do not know if other tests can enunciate interconnect failures with the degree of sensitivity we have but you can imagine that a resistance change at the interconnection could be measured and plotted over thermal cycles. This resistance plot shows damage onset, accumulation and acceleration through the life of the test.

Sincerely,

Paul Reid

Program Coordinator

PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1

613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>


 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: Friday, February 22, 2008 11:53 AM
To: [log in to unmask]
Subject: [TN] Inner Layer Separation / Land Rotation

Following my previous response to Jack regarding Werner's PCB paper, I started doing a little reading, and was hoping someone could further explain the phenomenon of "torque loads on the inner layer connections due to land rotation".  How is this caused?  I feel I fully understand the radial stress on the vias from resin expansion, but how is rotation incurred?  Also, is there specific testing to determine if a PTV barrel interconnect is weak within the PCB itself, explaining any resulting ILS from soldering process?

 

Thanks,

 

Kevin Glidden

Manufacturing Engineer

Luminescent Systems Inc.


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