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Date: | Fri, 14 Mar 97 17:13:08 MST |
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Hello,
EDS (There is a trade name EDAX as well) measures the x-ray
emission resulting from the electron bombardment in a SEM. If the EDS
accelerating voltage (which you can set) is such that the electrons
penetrate less of the bulk than the incident x-rays in your
fluorescence unit then you will be measuring two different "samples" :
the surface and the bulk solder. The surface solder tends to become
enriched with lead. This migration takes time. Heat speeds it up.
Additionally crystalline patterns that look like flowers or snowflakes
form on the surface. These are lead rich and depending on how small
the raster is set on the SEM will make the sample appear to have more
lead.
If on the other hand you have a reasonably homogenous sample, or
the penetration is equal then the question is what standards did you
use to calibrate the EDS. Standardless EDS is semiquantitative.
Creating and keeping standards for the EDS may be difficult due to the
reasons mentioned above.
Is you pad dimension at least 2X your collimator? If there is bleed
over of the X-rays into the board material then bromine fire regardant
can add to the tin or lead peak (I forget which).
If you have run a gage R&R study on your fluorescence unit and it
passed to your satisfaction then I would trust its numbers and not the
EDS.
Rich
------------------------------------------------------------------
Richard Olsen Telephone (602)-232-9154
Continental Circuits Corp. Main (602)-268-3461
3502 East Roeser Road Fax (602)-268-0208
Phoenix Arizona Pager (602)-310-6245
85040-2905 Internet [log in to unmask]
USA I make circuit boards,what do you do?
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Subject: Accuracy of SnPb comp. meas.
Author: [log in to unmask] at INTERNET
Date: 3/14/97 4:36 PM
Technet,
We use an X-ray flourescence scope to measure solder
thickness and composition. Recently, we sent samples
to a university to have the composition measured by
Edacs (I probably spelled that wrong). In case I
spelled that wrong and you don't know what I mean -
Edacs composition measurements are a by-product of
scanning electron microscopes. Anyway, the tin-lead
percentages we got from Edacs were way different from
the results we got on our X-ray. I'm thinking this
might be because the Edacs measures an extremely small
surface area whereas the X-ray measures a 5 to 10 mil
diameter circle.
Does anyone have any experience with SEM and/or Edacs
in relation to solder composition? We would appreciate
any info to confirm my theory or refute.
Thanks,
Dave Hall
[log in to unmask]
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