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July 2008

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Subject:
From:
Joe Fjelstad <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 10 Jul 2008 11:46:13 EDT
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Excellent list, Paul
 
Thanks for taking the time. 
Joe
 
 
In a message dated 7/10/2008 8:07:30 A.M. Pacific Daylight Time,  
[log in to unmask] writes:

Just a  few thoughts on inner layer shifting.

1. Don't forget Pythagoras and  the proverbial True Hole Position - .001" 
misregistration in the X and .001"  in the Y can be .00141" at 45°. This concept  
  
works with holes, pads and clearances etc.
2. Consider that the drilled  holes, ablated holes and layers are all 
independently tooled and shift  independent of each other.
3. Hole to hole registration can vary due to  drill deflection, drill walk 
etc.
4. Image feature to image feature may  move due to uneven expansion of the 
dielectric or the art work. This uneven  expansion may be due to physical 
deformation or uneven  thermal expansion.
4. Sequential lamination adds the vagrancies of layer  shift by variation 
sublam to sublam registration.
5. Back drill, counter  bore and secondary drill operation add another layer 
misregistration  particularly if the secondary drill is producing tooling 
holes.
6. Over-etch and under-etch can effect line width and spacing that  may 
appear to be a shift in layer when in fact the issue is variation in  feature size.
7. Improper artwork compensation to accommodate process  variation can 
produce what appears to be layer shift but in fact is improper  sizing.
8. Mixing materials, uneven builds, grain direction (warp vs. weft)  all can 
produce registration issues.
9. Nonfunctional pads in every layer  may aggravate drill and image 
registration.
10. Glass style and epoxy  ratios may affect registration.
11. Humidity can have an effect on artwork  and image registration.
12. Tooling methods, round alignment holes verses  slots, aligning in the 
center verses the edge of panel will affect layer  shift.

To know if you have inner layer shift, I believe, you need to  consider 
possible causes of misregistration. For example the inner layer may  not be 
shifted, it may be that the drilled holes are shifted. You have to  choose what you 
consider to be you reference point, drill hole or layer one  image etc. and 
consider the registration to that. 

All that being said,  reasonable etch back should not have a significant role 
or be significantly  affected by, inner layers shifting.


Paul Reid

PWB  Inc.



-----Original Message-----
From: TechNet  [mailto:[log in to unmask]] On Behalf Of Kathy Kuhlow
Sent: Wednesday, July  09, 2008 4:07 PM
To: [log in to unmask]
Subject: [TN] PCB Inner layer  shifting

I was just reading and got to thinking about PCB inner layer  shifting. Now I 
realize that this can be caused by registration issues but  here are the 
questions I can't readily answer: 

1) Is it possible to  cause inner layer shifting on only signal layers during 
the assembly  operations or will this be seen in the PCB fabrication testing?

2) Can  inner layers that create opens or shorts be latent or would the 
effect be seen  immediately? 
3) Would inner layers that do shift all shift the same way?  In other words 
would I get the same opens/shorts? What would be the proportion  of possible 
affected if not all? 
4) Would the PCB stresses caused by  lamination not be released prior to the 
PCB test or is there enough residual  stress still in the PCB at time of 
assembly to cause an issue? 
5) How  would you know if you have inner layer shifting? 

I was just reading  about etch back and some the test data related with the 
process, is this part  of the driver to develop this process? 

Kathy Kuhlow
6980  Professional Parkway East
Sarasota, FL 34240
Phone: 941-907-1000 ext  8248
Mobile: 320-267-5549
Email:  [log in to unmask]


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