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The application of fine structures within a reliable process is enabled by the inkjet technology, a digital print method. Eliminating the process steps of pre-drying, exposure and development, steps which are essential in the processing of similar high resolution photoimageable solder resists, this method of inkjet application brings a tremendous potential for savings. Thanks to the direct digital realisation of layouts, it is possible to flexibly and rapidly modify the layouts; even small and very small series can be implemented in a cost-effective manner.
One of the technical benefits of inkjet printing is the fact that different layer thicknesses can be applied on one PCB side, thus permitting a high edge coverage along with low layer thicknesses in critical areas. Vias and other areas, if necessary, can be directly saved out from the printing process to avoid the risk of defects as they may be generated by incomplete curing or the free development of photoimageable inks.
Nevertheless, the developer is faced with particular challenges as far as the formulation of high functional inkjet inks is concerned, since the basic prerequisites such as a very low viscosity and small particle size, distinctly reduce the range of suitable raw materials.
Special requirements are also imposed by the print method as such. The printer and, what is more, an appropriate print head must be chosen carefully, in view of fulfilling the specific demands in terms of resolution and process speed. Special skills are needed by the operator to develop suitable print strategies for perfect coating results, as well as for the coating process as such.
Peters’ latest Elpejet formulations reach good properties for the solder resists: With the respective print head adjusted, they provide an outstanding print image and an edge coverage precise to specification, along with a high resolution. Among the typical application parameters, there is pinning with UV LEDs immediately after printing, an additional UV bump if necessary, or thermal curing to achieve relevant final properties such as “no bleeding,” adhesion, scratch/crack resistance etc.
The Elpejet solder resists were tested with various print heads such as the KM 1024i by Konica Minolta or the Dimatrix Samba G3L by Fuji Film, which are both drop-on-demand systems (DoD) using the piezo technology. The R&D department is still working on the final fine tuning. Visit www.peters.de.