[SI-LIST] Causes of Phase Differences in stripline construction

Hello, 
 

I have a 2 part problem. Our customer has informed us they are seeing phase
errors in the assemblies we build for them. The requirement is all channels
need to have a phase delta of < 14.5 degrees. In other words, the max phase
- the min phase across 48 channels.

 

Part 1 of the problem is a phase delta in excess of 30 degrees. We have
found the root cause of this problem to be PCB delamination.

 

Part 2 of the problem is the phase delta around 15-18 degrees. This is
usually brought back into tolerance by changing an SMT capacitor. 

 

Finding the root cause of part 2 is proving to be rather challenging.
Originally the specification was < 8 degrees. When the customer started
seeing failures, they increased the specification to < 14.5. When they saw
even more failures in addition to the delam, they finally brought it to our
attention. 

 

In my opinion the stripline construction "should" be rather forgiving. It's
1/2 oz, 19.4 mils wide surrounded by ~18 mil dielectrics on each side.
Though one of my concerns is each trace length is around 14.5 inches and is
serpentined. In addition, no two serpentine designs exhibit the exact same
pattern. 

 

Another concern I have is the channels are spread across 3 different layers
in the stackup (16 channels each on layers 2, 7, and 11). The PCB is 12
layers. 

 

Layers 2 and 11 have the same stackup:

==================== Ref

     18 mil core

-------------------- Signal

     5.8 mil pp

     6 mil core

     5.8 mil pp

==================== Ref

 

Layer 7 is slightly different:

==================== Ref

     12 mil core

     5.8 mil pp

-------------------- Signal

     18 mil core

==================== Ref

 

Also to toss another variable in the mix. This board is completely round
with each channel beginning at the perimeter of the PCB and equidistant from
the next channel. So with that said, I am aware of the trace orientation in
regard to weave/glass/resin now becomes a factor as well.

 

So my question to the list is: What are some causes of an increased phase
spread? Also, any reference material recommendations are welcomed.

 

Thank you,

Craig Sullivan

Manufacturing Engineer

MPL Incorporated

P: 607.266.0480

F: 607.266.0482

csullivan@xxxxxxxxxx

 

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