John, IPC folks,
Have they improved XRF to the point where its really that useful?
From my experience, and unless there has been a significant change in
the technology, an XRF wont tell you what is inside the part, only what
is on the outside and visible, the plating material and thickness, and
only on the spot where you point it. If there are any holes in the
plating on that spot; it will show what is in the hole, and nothing
more.
Am I wrong??
Rick Smith
Senior Materials & Process Engineer
Phone: (512)652-3544
Fax: (512)652-3545
Cell: (512)299-6925
Email: [log in to unmask]
ClearCube Technology, Inc
8834 Capital of Texas Hwy N
Austin, TX 78759
www.clearcube.com
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of John R. Sieber
Sent: Friday, January 06, 2006 10:26 AM
To: [log in to unmask]
Subject: Re: [LF] How to use XRF?
Folks,
As an XRF expert, I feel the need to speak up here.
XRF can be used as a very precise and sensitive technique in highly
accurate test methods. Having said that, I must explain its intended
use in the context of RoHS survey analyses.
The IEC TC111 WG3 is developing a standard that will include the use
of XRF for surveys of materials. In this context, the user must
define the analyzed area and use due care in interpreting the
results. Early in the development of this method, the working group
decided to include a 30% safety margin for safety in results
interpretation. This does not mean that XRF is a terrible
technique. Uncertainties in XRF results could be better than 30% or
worse. The safety margin is used to make decisions about when to
apply more specific test methods to be included in the TC111
standard. This is an alternative to the normal analytical approach
of creating an uncertainty budget and estimating all components of
uncertainty in your own implementation of a method. That option was
deemed to be too complicated for users who bought a bench-top or
handheld XRF unit for fast checks. Hence the concept of a 30% safety
margin.
I admit there is a wide range of variation in performance of XRF
spectrometers because there is a wide range of spectrometer
desgins. However, in all cases, the following rule applies: The
more you can tell your method about the specimen, the better the
results will be. Hence, if the specimen is a layered structure, you
will do better to model it as such. That does not automatically mean
you need layered calibration standards, but it would improve the
calibration model. Fundamental parameters programs can, if
programmed for maximum flexibility, use pure metals and compounds to
calibrate instrument sensitivity, then perform the quantitative
analyses. That approach stretches the reliance on the fundamental
parameters, some of which have large uncertainties in their published
values.
Regarding specimens, XRF is at its best when measuring flat
specimens. Results from specimens that are not flat could be subject
to significant bias. Results from layers that are not uniform will
be an average because the software will assume the layer is of
uniform thickness across theTo learn the extent of the bias, one must
validate the test method using reference materials or complementary
test methods, like ICP-OES. One must also be careful that you know
the area being measured by the XRF spectrometer. Within that area,
the depth of measurement will depend on the atomic numbers the
elements and densities of substances. For solder over copper, you
will probably see the copper. If there is Pb in the solder and in
the copper, it may be impossible to distinguish between the two
without a prior analysis of the bare copper to add into the FP
program with the layer structure.
To summarize:
1. TC111 see XRF as a survey tool to help reduce the amount and cost
of labor-intensive high accuracy lab work.
2. The 30% value being bantered about is a safety margin designed by
TC111 WG3 to help define the decision levels for XRF survey
analysis. i.e. when do you believe that XRF show a compliant
material and when do you need more testing.
3. XRF methods definitely benefit by the use of a priori information
about the structure and chemical forms of substances in the specimen.
It is expected that many users of XRF for RoHS testing will be
relatively unskilled in the application of XRF. I sincerely hope the
spectrometer manufacturers and vendors will provide the support these
folks need to become adept at XRF for their applications. I also
encourage the OEMs who have more resources to step in and help their
suppliers do a
better job.
I would be happy to continue this discussion online or answer
questions in private, if necessary. However, I am not in a position
to help everyone solve their analysis problems. Please work with
your customers and the folks who sold you the XRF instrument.
regards,
John Sieber
At 10:06 AM 1/6/2006, you wrote:
>So I guess I have to use the layered approach.
>
>I wonder how do I have to analyze the joints of a populated
>assembly. What would be the layers to generate? Solder+IMC+Ni+Cu?
>
>Best regards,
>Ioan
>
> > -----Original Message-----
> > From: James, Chris [SMTP:[log in to unmask]]
> > Sent: Friday, January 06, 2006 9:52 AM
> > To: (Leadfree Electronics Assembly Forum); Tempea, Ioan
> > Subject: RE: [LF] How to use XRF?
> >
> > XRF is not the saviour of RoHS and is only good as a basic indicator
and
> > has nowhere near enough resolution for 1000ppm precision. Analytical
> > labs use a 40% error offset when using XRF and if it fails then go
to
> > more precision methods.
> >
> > Homogenous is each part, body, leadframe, plating - anything that
can be
> > mechanically separated - and that's in the literal sense not
practical
> > sense i.e. if you could abraded the plating off a substrate then the
> > plating is one homogenous item the substrate another - pretty absurd
for
> > components but that's how it is.
> >
> > Regards,
> > Chris
> >
> >
> > -----Original Message-----
> > From: Leadfree [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
> > Sent: 06 January 2006 14:39
> > To: [log in to unmask]
> > Subject: [LF] How to use XRF?
> >
> > Hi Netters,
> >
> > we've just become the happy owners of a tabletop XRF analyzer. Works
> > fine, shows what we need, we have already rejected non-compliant
parts,
> > etc.
> >
> > But there is a litle something... The soft allows for creating
> > applications, so we can declare that the component has layers, i.e.
you
> > can define Au over Ni over Cu. At this moment the measurements will
tell
> > you what is the thickness of each layer and what is the composition
of
> > each layer. Normally this increases the accuracy of the
measurements.
> >
> > If I x-ray considering layers, I get readings for a SnPb layer that
show
> > 14% lead. If I don't use the layered routine and I shoot through the
> > bulk of the surface, the Pb goes away, the influence of the Ni and
> > mainly of the Cu dilutes the Pb to below RoHS threshold.
> >
> > Hence the question, how should I measure? What is the homogeneous
> > material, each layer or the bulk?
> >
> > Thanks,
> > Ioan
> >
> >
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--------
John R. Sieber, PhD
Research Chemist
National Institute of Standards and Technology
Chemical Science and Technology Laboratory
Analytical Chemistry Division
100 Bureau Drive, Stop 8391
Gaithersburg, MD 20899-8391 USA
[log in to unmask]
Tel: 1.301.975.3920
Fax: 1.301.869.0413
www.cstl.nist.gov/
Identification of commercial items in this document does
not imply endorsement by NIST or that items are
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