The evaluation of solder paste characteristics using AC impedance spectroscopy
One of the more critical aspects of engineering is often understanding the variety of complex materials that are employed in industry today. This encompasses knowledge of both the physical nature of materials, and how differences in composition and processing affect the properties and performance of the material (or combination of materials) to be utilized. The monitoring of solder paste characteristics during surface mount circuit assembly procedures is ultimately concerned with obtaining a high yield of reliable solder joints. The process control of solder paste systems under manufacturing conditions is, at present, not derived from on-line measurements of the paste. Because these tests are not performed in-process, they cannot detect critical changes in paste behavior caused by varying process conditions, environmental factors, or the loss of rheological characteristics induced by prolonged stress on a screen printer. AC impedance techniques can be a valuable tool in characterizing the properties of materials that possess measurable dielectric behavior. Recent advances in impedance measurement instrumentation (and the evolution of the personal computer) have yielded improved data acquisition and analysis capabilities. This has precipitated efforts to utilize this methodology for the study of more complex materials systems (such as solder paste). The primary focus of this investigation is the implementation of AC impedance techniques as a means of characterizing the behavior of a broad range of solder paste formulations. In the course of this work, it has been determined that it is possible to represent both the dielectric and the electro-chemical nature of these materials in the form of an electrical equivalent circuit model. This modeling yields significant detail concerning the microscopic attributes of these materials, unobtainable from other forms of study. As a process control tool in the manufacturing realm, the use of AC impedance techniques to monitor solder paste has the potential to detect the onset of irreversible changes during screen printing operations. The results of this investigation reinforce the utility of AC techniques as a means of tracking critical changes within solder paste.
Richard Walter Hirthe,
"The evaluation of solder paste characteristics using AC impedance spectroscopy"
(January 1, 1997).
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