Sue,
 
If the .038 diameter hole is your finished hole size, then you will definitely have issues at AOI.   To obtain a 38 mil finished hole size you must drill the hole about .042 to allow for plated copper and tin/lead thickness ( yes, even todays HAL adds about .001 reduction to hole sizes sporadically across a board). With this in mind you now have a functional annular ring of .004 per side.  Accounting for drilling and image accuracy a manufacturer will be pressed to hole even .001" finished annular ring 100% on all pads.  
 
Now, if you try and AOI that outerlayer after etch you will have a serious issue with the remaining annular rings being seen on the AOI system as line width violations. The only way I have found to "override" these callouts is to electronically mask out the hole/pad areas and tell the machine not to look at the pad areas.  This will still allow for the detection of  nick, pinholes, opens , shorts and other line with issues on the function line but if defeats the purpose of inspecting annular rings.  However, if your design is if fact a 38 finished hole size, I can guarantee you a high potential for break out has already been designed into this board. 
 
Regarding the LPI,  hole to pad ratio has nothing to do with LPI imaging. If your mask clearance is at least .056 this should not every be an issue. However, I have seen many people confuse annular ring issues with LPI issues when they see an apparent hole to pad registration issue and associate it with LPI imaging.
 
Hope this helps.
 
Ed Cosper
Director Q/A and Product Engineering
American Board Company 
-----Original Message-----
From: Susan James <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, August 17, 1999 4:37 PM
Subject: [TN] Board Layout for AOI and LPI

Dear Experts,

One of our board suppliers is telling us they can't do AOI or LPI on our older board layouts.  These boards are very low tech being 2-sided, 100% thru-hole.  Their issue is with 38 mil holes in 50 mil round pads.  They say this doesn't give them enought tolerance for AOI or LPI.  They want to increase our pads to 60 mils, which for our applications is pretty much out of the question.  They claim they are getting nothing but errors from the 50 mil pads.  From my limited knowledge about AOI, shouldn't they be able to accomplish this as designed by changing the design rules programmed into their Camtek system?  Their equipment is a Camtek 2NVT for AOI, and I don't have any info on their LPI equipment.  They have been supplying these boards for us for about 8 years.  Recently they bought AOI and LPI equipment and we would like to see our boards put on this equipment.  We do a lot of business with this company, also.  The board house fabs to IPC Class 2 specs.    I admit to a definite non-guru status and need some advise.  Are our expectations unrealistic?  I hope I've given you enough info for reply.  Thanks for your help!

Sue James
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