Thickness and Roughness of Coatings

MID applications are now widely used for signal transfer, electrical circuitry in sensor applications, and for antennas. A low power requirement is common to all these fields. This settling of past and present MID applications at the low end of the power range has a great deal to do with possible plating thicknesses associated with chemical metallization. 

Electroless metallization is characterized by deposition rates of only a few pm per hour. This obviously means a certain dwell time in each bath, and this in turn is partly responsible for the costs of metallization. Copper plating thicknesses of well over 10 pm are theoretically possible with conventional industrial metallization equipment. However, the question of economy arises at this point. Standard practice in applications calling for thicknesses in excess of 10 pm is to resort to galvanic buildup, applying copper at deposition rates of around 1 pm per minute. 

Much the same applies for the other metals that can be deposited on a conductive underlayer of copper. The deposition rate for nickel in chemically reductive metallization baths is roughly 6 to 12 pm per hour. It takes about 20 to 30 minutes to deposit a gold finish with a maximum thickness of 0.1 pm. Because the coatings are thinner, beaker metallization is much more closely comparable to other metallization configurations. 

A circular die geometry in combination with a sequence of overlapping embossing strokes promises the most success in purely mechanical embossing. Reduction of around 50% in roughness has been achieved in research. Reliable wire bonding, however, is still not sufficiently viable [185], 

One way of further reducing laser-structured roughness is to combine mechanical die-embossing pressure with ultrasonic energy transfer. Suitable die geometries transfer the ultrasonic vibrations selectively to the surface. Research has shown that there is no appreciable benefit to be obtained by employing ultrasonics. Differences in the smoothed surface were due entirely to die pressure, not ultrasonic power [185]. 

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