2020 EIPC Winter Conference, Day 1

Estimated reading time: 21 minutes

Mustafa Özkök, global product manager with Atotech, discussed the manufacturing challenges that 5G and millimetre-wave technology imposed upon the PCB industry, in terms of materials, processes, and design considerations. He explained that the main driver for 5G was the massive forecast increase in the number of connected devices and the quantity of mobile data. For example, an autonomous car was expected to create 4,000 GB per day. Millimetre-wave would drive new hardware, and the wireless infrastructure would need the enormous geographic density of small cells in order to achieve connectivity of everything. The move to 5G would enable high-frequency, high-speed data transfer, big volume data traffic, and high-density connection.

Investment in 5G infrastructure was forecast to exceed $300 billion by 2025 with a large proportion related to upgrades of data centres and public clouds, which would demand substantial growth in the production of servers, routers, and switches. New-generation substrate materials would be required for PCBs and ICs, and designs would regard signal integrity, impedance and capacitance, thermal management, and EMI shielding as critical issues.

Dielectrics formulated for high-speed, high-frequency operation, mixed dielectric builds, and advanced adhesion promoters would all contribute to minimising signal attenuation. Advances in metallisation and plating processes could offer compatibility with hybrid builds and give uniformity of copper distribution. Final finishes designed to minimise signal loss and withstand aggressive environments and extended storage would be required. Many of these material attributes already existed or were in development. Thermal management and new options for EMI shielding were also important considerations.

The session on inkjet and other coating technologies was moderated by EIPC board member Emma Hudson. Her first speaker was Dr. Andreas Albrecht from Cicor Group in Switzerland, with an enlightening presentation entitled “Printed Electronics: Pushing the Limits.” Discussing the principles of fabricating printed electronics devices, Dr. Albrecht described how a functional material—which might be a metal, a conductive polymer, a dielectric, or a sensor material—was converted from the bulk state to nanoparticles dispersed in a solvent mixture or polymer lacquer to form a printable functional ink. He stressed that Cicor did not make inks themselves, but were happy to cooperate with several specialist ink manufacturers and to evaluate and select products to suit their own specific requirements.

Most delegates were familiar with inkjet printing techniques, but aerosol jet printing was emerging as an alternative contactless direct-write method of producing fine features on a wide range of substrates, which could be used for creating conductors, active and passive components, actuators and sensors.

Dr. Albrecht explained the basic procedure. Ultrasound was used to break the ink into droplets, the size of which was governed by ultrasonic power and temperature, and an add-back of solvent ensured stability. A deposition head focused a jet of droplets, carried as an aerosol in a stream of nitrogen, onto the workpiece at a separation between 1–5 mm, as a 10-micron spot. The workpiece was mounted on a stage capable of movement in up to five axes and heated to dry the ink. The deposition head could itself be programmed for free-form writing if required. Pneumatic atomisation was a more complicated alternative to ultrasonic.

Compared with inkjet printing, aerosol jet printing offered some technical advantages: higher resolution, higher printing thickness, and a larger material portfolio. Because of the large and variable stand-off distance, it was capable of printing increasingly complex devices on three-dimensional surfaces. Dr. Albrecht also reported that Cicor had invested in a technology centre for printed electronics at their Bronschhofen site in Switzerland, where a team of application engineers was working on the development and industrialisation of new additive manufacturing processes and connection technologies.

Dr. Albrecht’s introduction to aerosol jet printing provided a fascinating vision of an alternative procedure, but inkjet technology had become adopted as a selective digital imaging process in the printed circuit industry and recently established as an effective means of applying solder mask.

Chris Wall, technical director of Electra Polymers, gave a detailed description of the development and processing of inkjettable solder mask and the benefits of its use in PCB manufacturing. He went through the logical sequence of stages in the development of a new ink: understanding the inkjet application process, identifying the formulation constraints for an inkjet solder mask, identifying and sourcing suitable candidate raw materials, and meeting solder mask performance requirements and external compliance requirements.

Wall summarised the types of head available: drop-on-demand piezo, recirculating and non-recirculating, with examples of each, and described how the head type influenced droplet size and resolution. He further illustrated the way droplets were generated, their shape in flight, and the consequences of satellite formation.

When it came to formulating a jettable solder mask ink, Wall reviewed the constraints of raw material suitability, resin viscosity, pigment particle size, and filler content, and discussed the effects of surface tension and contact angle of the resulting ink formulation. Curing mechanisms were generally a combination of preliminary UV pinning and a final thermal cure. He explained resin and photoinitiator chemistry in sufficient depth to convey the principles without confusing the non-chemists in the audience and described how new products were formulated, evaluated, and submitted for independent testing for compliance to standards, such as UL 94V0, RoHS, IPC-SM840 and NASA outgassing, as well as automotive standards and customer-specific requirements.

Wall reviewed the benefits of inkjet solder mask as compared with liquid photoimageable products, which lay principally in a shorter fully-additive process route and no solvent emissions or developer effluent, and discussed the characteristics of image edge-profiles. The inkjet process enabled custom print strategies for specific designs, such as selective thickness control, multilayer deposition, and different surface textures.

Recognised for his consistently informative and entertaining presentations, Don Monn from Taiyo America gave his update on the progress of inkjet solder mask over the five years since 2015, without standing still. He focused on practical aspects of image definition and geometry, with particular reference to solder dams and solder mask defined pads, using a whole series of “then” and “now” cross-section photographs of actual solder mask features.

Monn demonstrated how edge definition and flow-out could now be controlled to maintain the physical height of dams such that whereas a nominal 5-mil dam in 2015 would slump to a height of barely 0.5 mils and an effective width of almost 10 mils, the equivalent feature now would have a height of almost 2 mils, while maintaining a width of almost exactly 5 mils with a sharply defined edge.

Taiyo America had submitted samples of their inkjet-printed solder mask for third-party testing and measurement of thickness over copper and over laminate, adhesion of solder mask dams, of solder mask over copper and of legend over mask, resistance to electroless nickel immersion gold, to solvents and to solder, and for print quality and registration. Monn showed comprehensive test results confirming compliance with standards in all respects. Of particular note were thickness measurements proving that coating thickness was maintained at track edges, and the results of thermal cycling tests: 1000 cycles -40°C +125°C, 500 cycles -40°C +140°C, and 100 cycles -65°C +125°C, with no deterioration of adhesion, surface hardness, or dielectric strength. “It’s a technology worth investigating!” was Monn’s closing comment.

Continuing on the inkjet theme, Uwe Altmann reviewed the evolution of the technology within Orbotech, from their first inkjet for PCB applications in 1999, their joint venture with the subsequent acquisition of a new system in 2007, to the present day with over 520 legend-printing machines installed worldwide. He discussed the key benefits of Orbotech’s patented DotStream Pro technology, which featured multi-LED based UV for perfect ink-drop pinning, and superior depth-of-focus for the best printing on challenging topographies.

Their NAT technology demonstrator had been used for several years in collaboration with a leading ink vendor to develop inkjet solder mask, resulting in the launch at the recent IPC APEX EXPO of the new Orbotech NEOS solder mask inkjet printer, offering a shorter, simpler and eco-friendly alternative to traditional methods. In addition to their proven DotStream Pro technology, the NEOS featured structural printing technology, which optimised feature sharpness with unique 3D algorithms and included automatic calibration procedures and consistent drop volume control.Page 2 of 5

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