Your question needs to be reviewed in two separate failure modes:

1) Drill offset is usually a parallel dislocation of holes to image. In
effect all the target pads are out of position relative to the drilled
holes, by the same distance and relative position. This is usually caused by
tooling offset error, artwork offset and could also be front to back error.

2) Scaling addresses material movement of the naturally occuring shrinkage
of laminates at lamination. This effect results in pads that are not out of
position by a uniform distance or position. This would result in holes that
are mis-registered say towards the left side of the pad on the left side of
the panel, to the right side of the pad on the right side of the panel,
while in the middle the pad and hole may be perfectly aligned. Typically
artwork is 'scaled' oversize to compensate for this natural effect such that
the post-laminated-shrunken-pad locations are actually back 'on grid'. The
above example assumes you are using a centerline tooling system such as that
from Multiline. It is also evident from this description, that the further
away you are from the centre of the panel, the more significant the scalar
value of the misalignment. This is why the "peripheral area of a laminated
printed circuit board experiences the most scaling problems".

At Circo, we routinely use the periphery of the panel very successfully for
registration assessment. This system however, only works when the basic
elements listed above have been tightly controlled. From your question, it
appears to me that both errors are present and this will confound your
analysis. The effects of both types of failure at the same time is truly
synergistic.


Dave Rooke
Circo Craft - Pointe Claire

__________________________________
>                    Dear synergists:
>                    The peripheral area of a laminated 
>                    printed circuit board experiences the 
>                    most scaling problems.  Yet this is the 
>                    area of the panel that is x-rayed in 
>                    order to measure for xy drill offset.
>                    Xray data suggests our current scaling 
>                    error is not predictable.   
>                    Also, our shop sees that the measured 
>                    offset, especially in Y, usually results 
>                    in an overcompensating shift in the 
>                    drill offset.  
>                    Has anyone else seen a Y offset 
>                    measurement problem?  
>                    More importantly, how is data from the 
>                    peripheral area justifiable?  
>                    
>                    Best regards
>
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