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
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______________________________ Reply Separator 
_________________________________
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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