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July 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:
Thu, 10 Jul 2008 11:05:34 -0400
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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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