The Institute of Circuit Technology Annual Symposium 2018

Estimated reading time: 13 minutes

Chris Wall, technical director of Electra Polymers and ICT Treasurer, discussed inkjet as a method of patterning solder mask. Speaking from over three decades of experience in ink formulation, he began by defining the performance requirements of a solder mask, a permanent polymeric coating for a printed circuit board, patterned to allow access to selected areas of copper, and resistant to subsequent processes and to the end use environment, preventing the growth of metal whiskers and preventing corrosion of underlying circuitry. Its primary purpose was to enable mass soldering processes.

There were two principal process routes to creating a solder mask pattern: indirect and direct. The indirect route involved coating all over with a photoimageable material, exposing an image using a photomaster or a laser, developing to remove unexposed material, and final curing. The direct route was to place the material only where it was needed, traditionally by screen printing, or more recently by inkjet printing, followed by a final cure.

The intrinsic limitations of screen printing were resolution and registration, plus the need to make and store artworks and screens. Drop-on-demand inkjet offered a repeatable digital process with potential for high output, with the capability to scale and offset the image to compensate for distortion of the PCB. Reduced waste, reduced energy costs and reduced work-in-progress were other potential benefits. But how to formulate an ink that would jet successfully and meet the material performance requirements?

Wall described the piezoelectric drop-on-demand process and demonstrated how a drop was created and how it could change its shape in flight. The tail formed as it left the nozzle could be absorbed into the drop or could break away as a satellite and be deposited as an extraneous pattern defect. Formulation of inkjet solder masks was subject to several constraints, and there were a limited number of suitable raw materials. The choice was restricted to very low-viscosity resins and monomers, with low or no filler content and pigment particle sizes less than 200 nanometres, compared with 5–15 microns for conventional solder masks. These low filler contents could adversely affect flammability, thermal shock and solder resistance. And the low viscosity and surface tension meant that good jetting performance was constrained by the laws of physics and the type of print head used. There could be a tendency for pigment sedimentation on storage and to spreading, bleed and track edge thinning on printing, with performance heavily dependent on surface preparation.

The mechanism of curing depended on the resin chemistry. UV-cured acrylates polymerised and cross-linked by a free radical chain reaction, epoxies by a cationic reaction, and hybrid epoxy/acrylate systems by a combination of the two. It was normal practice to additionally carry out a final thermal cure.

To help overcome bleed and track edge thinning effects, the ink could be instantaneously pin-cured with low-level UV to fix droplets in place, and multi-layered techniques could be used to initially create dams around pads before subsequent filling to eliminate striping, build up thickness on conductor edges, and give a smooth ink surface with sharper edge definition. Specialised surface treatments could be used to minimize the spread of ink droplets.

Chris Wall made the closing remarks on behalf of the Institute, thanking speakers and delegates for making the event another superb learning and networking opportunity, Bill Wilkie for pulling it all together, and Polar Instruments for their generous support. And to round off the day, everyone had the opportunity to browse one of the finest collections of cars, motorcycles and motoring memorabilia in the world.

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