A highlight of the productronica Forum in Munich, Germany was the technical presentation of Sun Chemical's Dr. Karl-Heinz Ognibeni, who described the development and evaluation of the newly-announced SunFlex family of screen printable flexible circuit coverlay materials.

Although SunFlex SD had been developed, first and foremost, to offer flexible circuit fabricators a lower-cost alternative to traditional laminated coverlay foils, it had technical, as well as economic, benefits--most notably, that all of the processing and performance problems associated with the adhesive layer were eliminated.

Taking advantage of Sun Chemical's in-depth technical experience of supplying screen printed resists to the printed circuit industry, together with the abundance of background knowledge on resins and pigments within the group, the SunFlex formulation was based on a chemically stabilised polyamide-imide system, and had similar rheological characteristics to those of a typical solder mask. Printing through a 43T mesh screen gave a 10 micron dried film thickness, and thicker deposits could be built up by a series of print-and-tack-dry operations. No special equipment or conditions were required over-and-above those of a normal screen-print shop. The material dried tack-free in 15 minutes at 80 to 100ºC, and was cured by baking at 150ºC for one hour. No special precautions were necessary in drilling and plating--there was no adhesive layer, so there was no need for plasma desmearing. And SunFlex easily withstood finishing processes such as HASL and ENIG.

Evaluation and qualification had been carried out with the co-operation of development partners Andus, Ilfa, AT&S and Contag, and SunFlex had completely satisfied their performance requirements. Although the material had not been designed for dynamic flex applications, it was showing remarkably good test results, and there was potential for future development in this application. Furthermore, in its part-cured state it could function very effectively as a no-flow prepreg, and fully-cured it fulfilled all of the requirements of IPC-SM-840 as a traditional high-reliability solder mask.

In the area of materials and process economics, SunFlex offered spectacular cost benefits. As well as savings in purchasing and maintaining an inventory of a range of materials and sizes, all of the tricky profiling, handling, laying-up and alignment operations associated with conventional coverlay foils were eliminated, and individual coverlay press-bonding cycles were unnecessary--even complex multi-layered builds needed only a single cure cycle. Dr. Ognibeni quantified these savings using authentic examples provided by the development partners.