I have been informed that my original post came out as gibberish. Since
many of you are not gibbers, here is what I said in English (I hope).
Andy,
Typically, eddy current testing is done on a free standing ferrous coupon,
not on the assembly itself. You are inducing currents in the metal and I
don't know that you want that on an electronic assembly.
Fritz Byle of Astronautics Corporation gave an outstanding paper at the
2009 Apex conference on optical methods for measuring coating thickness
using a Zeiss microscope. I have copied the abstract of that paper here.
Doug Pauls
Abstract
Conformal coatings are used in high reliability electronics to protect the
circuits from environmental contaminants. They are applied by a variety of
methods, and in varying thicknesses. Confirming that the thickness meets
specifications called out by documentation or customer can be problematic.
Mechanical, ultrasonic, electrical (capacitive, eddy current), and various
optical techniques are available, but all involve incurring significant
limitations/penalties in capability, capacity or cost.
For optically transparent, and some translucent coatings, it is possible
to accurately measure the thickness using optical (focal) techniques. This
paper presents data on an innovative coating measurement process based on
commercially-available low-cost optical equipment modified to make the
measurements. The modified equipment is capable of making measurements on
films as thin as 25µm (0.001?) and thicker than 1000µm (0.040?) with high
repeatability. The method does not require a free edge and is not
dependent on before/after coating differential measurement. The process
has been fully developed and is used in a production environment.
The paper presents an overview of the equipment and method, Gage R&R data
for the process, as well as comparative information on other available
techniques. The focal technique is applicable to measurement of all types
of optically clear coatings and films, and is appropriate for
moderate-volume measurement applications where direct, non-contact
measurement of coated parts is desirable and where measurement in small
areas is required.
andi1978 <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
06/27/2011 03:23 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
andi1978 <[log in to unmask]>
To
<[log in to unmask]>
cc
Subject
[TN] The best method to measure acrylic conformal coating thickness (dry
film) over populated PCB.
Dear TechNet gurus ...
Our company is looking for equipment to verify the conformal coating
thickness on fully populated PCB after the product is received from our
subcontractor (measure on fully dry conformal coating).
Our assemblies are heavily populated so that standard tool (calliper or
similar) are useless in this case.
We were considering to use eddie currents technique (measuring tool with
5mm tiny probe with 45 degree angle) but we are unclear on the measuring
method use by this system.
Basically on their website
http://www.conformalcoating.co.uk/Conformal-Coating-Thickness-Measurement-Systems.php
they said: measurement is taken over the ferrous or non-ferrous material
which in my opinion means the measurement is taken through the conformal
coating + solder resist to the closest/first copper layer inside the PCB.
By using this tool and to correctly calculate the conformal coating
thickness, we have to add the solder resist thickness (with tolerance)
plus we must be sure that the ground plate/layer is located just
underneath the solder mask/measuring point.
In our case (complex assembly) this condition will not be always be
achievable (read: no ground plate in the measuring area), complex build.
Have you guys any idea what would be the best, accurate tool/technique to
verify the acrylic coating thickness on populating product.
Please bear in mind our boards are FTI equipment and conformal coating
parameter is critical from product functionality/reliability point of
view.
And again, there are not much "real estate" and the sensor to measure the
thickness must be small in diameter as possible.
Much appreciate your advise,
Andy
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 16.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
For additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------
______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 16.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
For additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
