Declan.
I copied the next text from the EMPF home page which you can find at:(I
hope that is OK Toni)
http://empf.arl.psu.edu/products/is4000/
You have to access this home page to be able to see the graphics and more
interesting features of this equipment.
Anyone considering Statistical Process Control of Solder Paste System
should check this out.
If you need further information contact Michael Frederickson
Phone: (317) 655-EMPF(3673), Fax: (317) 655-3699 [log in to unmask]
Alex Basauri
Statistical Process Control of Solder Paste System
Courtesy of Alpha Metals
Manufacturers who monitor solder paste characteristics during surface
mount circuit assembly procedures are ultimately concerned with
obtaining a high yield of reliable solder joints. The solderability of a
given paste system under reflow conditions is, at present, not measured
directly using any one given technique. An existing quality control test
for paste includes the measurement of viscosity as a means of
establishing the thixotropic behavior required for printability. In
addition, the issue of solderability is addressed most directly via the
solder ball test, reflecting the level of oxidation of the powder in the
paste, and the wetting test, which acts as a barometer for the ability
of the flux to create a solderable surface. However, none of these tests
can monitor paste viability in-situ within the flow of manufacturing. As
an alternative, the evaluation of solder pastes using AC impedance
measurements has been found to be feasible in studies conducted at
Marquette University and the EMPF.
The use of impedance techniques to characterize material properties is
widespread. It consists of the application of a small-signal voltage of
varying AC frequency to a given material through suitable electrodes.
The resulting current flow is then measured, and the impedance is
determined as the ratio of the applied voltage to the measured current
at each frequency. Such data is quite useful for materials comprised of
regions of differing electrical behavior, as is the case for solder
paste because each region will impede current in a manner that will
predominate only over a given frequency interval. When the impedance is
plotted, per Figure 1, a spectra is observed which is derived from these
current-limiting attributes (microstructural and/or chemical reactio
n-based).
Figure 1
Further, these attributes can be accentuated using different probe
configurations (2, 3, or 4 probes). Each of these impedance mechanisms
is interpreted as an appropriate combination of resistors and
capacitors, and the entire material is represented as an equivalent
circuit. Once obtained, this circuit, as seen in Figure 2, delineates
the microscopic electrical nature of the paste and acts as a reference
model against which the degradation of paste viability in manufacturing
can be judged.
Figure 2
Alex Basauri
[log in to unmask]
Trimble Navigation LTD
The GPS Solution!!
645 N. Mary Avenue B6
Sunnyvale, CA 94086
(408) 481-8751
(408) 481-6077 Fx
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