Recently fine line Flexible Printed Circuits (FPC) are used in many applications such as Thermal Head Printers and Faximille. In these applications very precise control of plating thickness is required and leads us to difficult problem both in design and in measurment of plating thickness.
The conventional β-ray method and Fluoro X-ray method are not sufficient to answer the above requirement. The Electron-Probe X-ray Micro Analyzer (EPMA) is recently developped surface measuring method which is especially suitable for these purposes.
We applied this new technique to analyze the degrading problem in gold plated surface of FPC. Also the authors summarized the circuit resistivity effect on plating thickness control.

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The experimental studies on the breaking process of film conductor in printed circuits are described. The experiment was carried out on the model printed circuit, which has a thickness of 35μm, widths of 0.5 to 5 mm and a length of 10cm. The current and terminal voltage impressed on the film conductor were measured by a dual beam oscilloscope. The breaking section and the structure of the film conductor were observed with a microscope and an X-ray microanalyzer.
By application of the current, the film conductor of the printed circuit was torn off from the plastic-board and the ripple marks were formed on the film conductor. As the applied current was increased, the film conductor was deformed into a string-like form. The breaking current density of the film conductor was about 90 kA/mm². At the breaking section of the film conductor, copper crystals of the order of 2-10μm were observed.
These phenomena are explained as follows: the copper crystals grew up as a result of the temperature rise of the film conductor by the application of the current. Formation of the ripples and shrinking of the film conductor into a string-like form were caused by the heating stress. And most of the breaking of the film conductor occurred by the heating stress caused by applied current.

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The relationship between mixed potential and crystal states was confirmed from a TEM observation of deposits and a measurement of electrochemical mixed potential for varing additives of a full-additive electroless copper plating bath. The plating conditions of longer transition time and less cathodic mixed potential made the crystal particle of deposits larger, however in the case of shorter transition time and more cathodic mixed potential, the size of crystals became fine and the particle density of deposits increased, respectively.
The measurement of mixed potential and the TEM observation of deposits during long term electroless plating was performed and disscussed on the basis of the relationship between mixed potential and crystal states obtained from the above experiments. The relationship between variation of mixed potential and TEM results for long term bath performance was corresponding well to the above results of additive effect. Furthermore, the mixed potential measurement for long bath operation was indicated to become the monitoring sensor for long term bath control system.

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[in Japanese]

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