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Check for PCB flatness with Newton's rings?

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.
 
H

Homer J Simpson

Jan 1, 1970
0
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?

Have you checked it with blueing?
 
T

Tim Wescott

Jan 1, 1970
0
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.
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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
R

Rene Tschaggelar

Jan 1, 1970
0
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.

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
 
dial indicators with stands cost about 20-30$ from Harbour freight and
other low cost maching supply companies.

Steve Roberts
 
R

Roger Hamlett

Jan 1, 1970
0
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.
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
 
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.


Thanks to all, much to consider. Upper management is resistant to
spending money on a mechanical guy...
 
R

Robert Latest

Jan 1, 1970
0
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

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
 
R

Ryan Weihl

Jan 1, 1970
0
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.

instead of the PCB get a steel plate with the same dimensions
and mount it.
rw

--
 
S

Spehro Pefhany

Jan 1, 1970
0
Thanks to all, much to consider. Upper management is resistant to
spending money on a mechanical guy...

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
 
J

James Arthur

Jan 1, 1970
0
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.

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.

http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

Best,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
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.

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.

http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

Best,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
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.

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.

http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

Best,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
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.

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.

http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

Best,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
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.

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.

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.

http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

Best,
James Arthur
 
J

James Arthur

Jan 1, 1970
0
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.

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.

http://msl.irl.cri.nz/training_&_resources/Measurement_articles/How_flat_is_flat.pdf

Best,
James Arthur
 
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