Thickness measurement by electron-probe micro analysis (SEM-EPMA)

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Supervisor:
Dr. Gordon Péter Róbert
Department of Electronics Technology

Abstract

In my thesis I dealt with thickness measurement of immersion tin and silver coatings, the most commonly used selective coatings of PWBs’ (Printed Wiring Board) soldering surfaces (pads). During literature review I studied wide range of thickness measurement methods and I performed the feasible ones: thickness estimation from the relative characteristic X-ray intensities (k-ratios) measured by EPMA, mass measurement based thickness estimation, electrochemical dissolving, SEM and AFM investigation of cross-sections at the Department of Electronics Technology; XPS and AES investigation during layer sputtering at the Department of Nuclear Physics; furthermore thickness-calibrated XRF measurement and anodic deplating at Hitelap Zrt. (a PWB manufacturer in Budapest).

I performed the scanning electron microscopic measurements and I set the parameters in consideration with the technological and physical properties of the coatings (e.g. to optimize the extent of the information volume I set the accelerating voltage to 15 kV in case of immersion silver and to 30 kV in case of tin samples).

I grounded the grading opportunities of the boards with immersion silver coating produced by our PWB laboratory. No thickness measurements were used yet, but now we can choose from three methods that can be performed at our department: electrochemical dissolving, mass measurement based thickness estimation and an EPMA based thickness estimation using the free software GMRfilm. With these methods it is possible to investigate the effect of the old electrolyte to the thickness of the generated silver coating. Furthermore I brought to light how the time spent in silver bath influences the thickness of the generated silver coating in case of boards having large copper area (about 510 cm2).

A secondary result of my work is that now we know in which electrolyte silver and tin are soluble. In possession of this information copper-tin and copper-silver intermetallic layers’ topography can be examined by SEM after selective electrochemical dissolution. These investigations already have started at the department and promise significant results.

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