We have a PCB that needs to remain as flat as when it left the fab, but it is bolted down to a metal base. We think the base is not flat enough. Before we call in a mechanical consultant, do you guys and gals think it makes sense to put a pattern of concentric rings on the PCB and then check for fringing with a lens? I just got the Edmund optics catalog in the mail, so I got to wondering.
Have you checked it with blueing?
What makes you think it was flat enough when it left the fab?
What makes you think that its thickness is consistent enough so that when it's bolted to said metal base it'll be flat enough?
If you think it's the base, why not measure the base?
I suspect that the board surface will be rough enough that fringing won't be a big help -- but it won't hurt to try. Getting a metrology expert to do a consult may be money well spent, particularly if you budget for at least one fool before you find one who can get the job done right.
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The newton rings are a check for optical flatness, which I doubt you will achieve with a pcb. You'll need an optical surface as well. A milled surface is hardly optical, meaning a point source reflex is still a point source. It shouldn't be too difficult to check a milled surface for flatness , just giving a quantitative answer is not that trivial.
Rene
-- Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
dial indicators with stands cost about 20-30$ from Harbour freight and other low cost maching supply companies.
Steve Roberts
Seriously, the level of flatness from a PCB, is going to be so 'bad' in optical terms, that Newton's rings would be pointless. However for normal mechanical 'use', a well designed board should be pretty flat. The key though is in the design. If (for instance), you have a large ground plane area on one side, and not the other, the board _will_ be bent when it 'leaves the fab'....
Best Wishes
Thanks to all, much to consider. Upper management is resistant to spending money on a mechanical guy...
Others have already answered your question pretty well (unambiguously with "No", I suppose). Let me just add that Newton's rings aren't a pattern that you put on things; rather they are generated by interference when a spherical glass surface touches a gainst a flat one (very simply stated).
robert
instead of the PCB get a steel plate with the same dimensions and mount it. rw
Just how flat does it have to be?
You can get a class B surface plate and DTI pretty inexpensively, which will allow you to check to within a fraction of a thou.
Best regards, Spehro Pefhany
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Homer's blueing idea with a machinist's "surface plate" is pretty good, but even a simple straight edge will reveal deviations from flatness on the order of 2-3um.
Best, James Arthur
Homer's blueing idea with a machinist's "surface plate" is pretty good, but even a simple straight edge will reveal deviations from flatness on the order of 2-3um.
Best, James Arthur
Homer's blueing idea with a machinist's "surface plate" is a pretty good gauge of flatness, but even a simple straight edge will reveal deviations from flatness on the order of 2-3um.
Best, James Arthur
Homer's blueing idea with a machinist's "surface plate" is a pretty good gauge of flatness, but even a simple straight edge will reveal deviations from flatness on the order of 2-3um.
Best, James Arthur
Homer's blueing idea with a machinist's "surface plate" is a pretty good gauge of flatness, but even a simple straight edge will reveal deviations from flatness on the order of 2-3um.
Best, James Arthur
Homer's blueing idea with a machinist's "surface plate" is a pretty good gauge of flatness, but even a simple straight edge will reveal deviations from flatness on the order of 2-3um.
Best, James Arthur
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