EIPC Summer Conference 2022: Day 2 Review
Örebro, Sweden on June 15 brought a bright and early start to Day 2 of the EIPC Summer Conference for those who had enjoyed the previous evening’s networking dinner, but had resisted the temptation to over-indulge or to carry on their long-awaited catch-up conversations with old friends into the small hours. All but a few were in their seats for 9 a.m., awake and attentive for Session 4 of the conference, on the theme of new process technologies, moderated by Martyn Gaudion, CEO of Polar Instruments.
As digital technologies continue to change the shape of electronics manufacturing, Dr. Kai Keller, director of business development with Notion Systems in Germany, put a perspective on the realities of additive digital processes with his reference to the Gartner Hype Cycle, the graphical depiction of a common pattern that arises with each new technological innovation. Beginning with a technology trigger, the curve rises rapidly through a peak of inflated expectation before promptly declining into a trough of disillusionment from which it progresses steadily up a slope of enlightenment and matures toward a plateau of productivity.
Commenting that manufacturing electronics is a wasteful undertaking, particularly regarding the consumption of enormous volumes of water, Dr. Keller set out to explore the potential for additive functional manufacturing of full electronic devices, taking as an example the inkjet printing of OLEDs from its origins in 2004 to the mass production of high resolution OLED monitors in 2021. “It might take long, but additive production processes will prevail in the end, simply because they make sense,” he said.
The focus of his presentation was the inkjet printing of solder mask on PCBs, and again he reviewed the history of its development from 2003 to the present day, when the technology is well proven and market adoption of inkjet solder mask is ongoing.
Keller outlined the principles of the process, discussed data preparation and full workflow integration, and reviewed the results. As well as high repeatability, the benefits of digital inkjet solder mask included no mask in holes, no mask on pads, adjustable thickness, and small and reliable solder dams. He went into detail on their natural bump profile: their wider base gave improved adhesion, with no undercut areas to trap chemicals or dirt. And he showed many real examples illustrating precise dimensional control.
The development team at Notion Systems continues to work on increasing the digital additive process window, and has realised more than 40 different additive production processes for conductives, dielectrics, resists, adhesives, active layers, optics, and coatings.
Novel nickel-free surface finishes for next-generation PCB technologies was the topic of the presentation from Dr. Kunal Shah, president and chief scientist at LiloTree in the United States.
Dr Shah remarked that high frequency PCBs was one of the fasted growing areas in the electronics industry and that a novel approach in surface finish was essential to obtain optimum performance and better reliability of electronic assemblies.
Although electroless nickel/immersion gold (ENIG) was currently the most widely used finish for high-end PCBs, it was not optimal for 5G frequencies because of high insertion loss due to the nickel layer. ENIG could also suffer from hyper-corrosion issues and brittle solder joint failures.
Current alternatives for 5G PCBs included direct immersion gold, electroless palladium immersion gold, and electroless palladium autocatalytic gold, although there were still concerns in terms of higher insertion loss and brittle solder joints. Higher thicknesses of precious metals were not cost-effective and not eco-friendly.
Shah described a novel nickel-free surface finish, with 50 nanometres of cyanide-free gold on copper treated with a nano-engineered barrier layer. The finish was characterised by robust solder joints with thin and distinct intermetallics. It was cost effective because of its significantly lower precious metal thickness than current alternatives. The new finish used a simple plating set-up of four process tanks and four rinse tanks.
The insertion loss of the nickel-free finish was almost identical to that of bare copper over the 0–100GHz frequency range, there was negligible change in contact resistance through six reflow cycles and the nano-engineered barrier layer prevented diffusion of copper into gold, leaving the surface corrosion-free. The process was cyanide-free, halogen-free, and RoHS and REACH compliant. As well as low insertion loss, it enabled robust solder joints with no brittle intermetallic failures.
Dr. Marko Pudas senior engineer and project manager with Picosun in Finland, discussed the technique of atomic layer deposition as a method of applying thin barrier films to PCB assemblies, which had been the subject of a successful project funded by the European Space Agency for mitigating tin whiskering.
Pudas described the principle of atomic layer deposition (ALD) as two or more chemical vapours or gaseous precursors reacting sequentially on the substrate surface, producing a solid thin film. The process cycle comprised the introduction of molecules containing element A, the adsorption of the molecules on the surface, the introduction of molecules containing element B and reaction with element A on the surface, and the completion of one monolayer of compound AB. The cycle was repeated until the desired film thickness was reached.
ALD was a batch-coating vacuum deposition process that had been shown to cover the most demanding 3D structures, down to pore sizes of less than 100 nanometres. It was capable of depositing a wide range of materials; aluminium oxide and titanium dioxide were named as examples, and was extremely repeatable in thickness and quality. Adhesion was chemical and the coating would not peel off. Typical coating thicknesses were in the range of 100 nanometres to 0.5 microns.
Pudas showed several examples illustrating the benefits of ALD in space applications, not only tin whisker mitigation, but corrosion protection, oxidation protection, even coating the insides of gas containers to prevent leakage. In high-reliability electronics it enabled very effective conformal coating of PCB assemblies, and he referenced many performance and qualification test results.
An iNEMI project comparing various test environments for conformal coating evaluation had demonstrated the effectiveness of ALD coatings on silver and copper in extremely corrosive environments. And ALD had been proposed as an alternative to polymer solder mask in critical applications.
Session 5, focused on manufacturing technologies for 5G and beyond, was moderated by Stig Källman from Ericsson.
Dr. Sebastien Depaifve, technical service engineer with Circuit Foil, discussed developments toward the next generation of ultra-flat ED-copper foils for high speed digital and radio frequency applications. He described the progression of improvements in hyper-very-low-profile foils over the last decade to the ultra-low-profile foils of the present day, explaining that in high frequency applications the signal is carried mainly on the edges of the conductor, and surface roughness contributes to significant signal loss.
He reviewed the factors influencing insertion loss: the type and content of silane bonding treatments, the metallic content of nodular or nodule-free foil treatments, the size and shape of nodules if present, the roughness on both side of the base foil and the grain size of the copper.
The electrodeposition process had been progressively improved, with optimisation of the drum surface preparation and the use of specific organic levellers. Grain-refining additives had been used to optimise grain size to one micron, which gave maximum electrical conductivity, and a model had been developed by Circuit Foil to study the influence of nodular treatment shapes on insertion loss. Lower profile copper foil resulted in lower transmission loss, especially at high frequencies, and a metallic-free passivation process had been developed.
With the decrease in roughness, it was necessary to compensate for loss of mechanical adhesion by silane or other chemical adhesion promoters, and it had been confirmed that silane content had no significant influence on insertion loss.
Depaifve concluded by reiterating that optimisation of all the different parameters was required for next generation materials development, and that the optimisation must be conducted in co-development with resin and glass suppliers.
In a logical follow-on to Depaifve’s paper, Jim Francey, Isola Group’s RF business development director in Europe, presented an update on reliability and loss properties of copper foil for 5G applications on behalf of the iNEMI 5G Copper Foil Project. The iNEMI team set out to characterise various copper surface treatments with the objective of mitigating signal loss while still maintaining good adhesion and, hence, durability of the PCB. In addition, the team undertook a comparative study to determine signal loss from copper roughening by oxide-alternative chemical bonding treatments during PCB fabrication.
Traditional measurement methods for copper adhesion such as peel strength were not always indicative of the durability and reliability of printed circuits, and contact profilometry was inadequate for ultra-low-profile copper foils. Noncontact 3-D tools as defined in IPC TM 650 2.2.22, such as white light interferometry and laser scanning microscopy gave more accurate 3D height profiles.
Certain reliability issues associated with ultra-low-profile copper foils had been identified, particularly copper-to-dielectric delamination either on the clad laminate or oxide-alternative side, or dielectric-to-dielectric where prepreg was bonded to an etched laminate surface. This interface was also a site for possible bond-line CAF growth.
The iNEMI project would benefit the industry by formulating a compendium of material performance criteria and cross referencing to a suite of existing standards, enabling users to specify a category of material that would meet electrical performance and reliability requirements whilst minimising excessive costs through over specification. Additional cost and time savings might also be realised by reducing the need for design reiteration and requalification to meet OEM specifications.
Completion of the project would enable commonality in specifying the topology of copper foil and bonding treatments, and provide better assurance for meeting PCB electrical performance characteristics. It would also provide predictability of durability and reliability of copper foil adhesion with respect to signal loss for various copper surface finishes, as well as reducing product qualification costs and associated timescales.
The bonding of PTFE multilayers has traditionally presented many challenges. Helmut Kroener, senior marketing director for PWB materials at Showa Denko Materials, introduced a new low-loss thermoset bonding film for PTFE HDI and any-layer multilayer.
With the rapidly increasing growth and diversity of applications operating at radio frequencies, there is an ongoing need for high-performance substrates with good dielectric and loss properties. Of all the low-loss materials PTFE, although expensive, offers the lowest transmission-loss, and multilayer substrates based on PTFE has excellent low-dielectric properties. But its high melting point makes multilayer lamination difficult, especially in hybrid constructions where the other component materials can’t withstand the bonding temperature.
The low-loss bonding film described by Kroener offered the opportunity to build multilayer constructions at a processing temperature of 200°C, offering a lower-cost higher-speed solution. Functional groups in the thermosetting material interacted strongly with the PTFE material, and it was also capable of excellent adhesion to low-profile copper foil, offering further improvements in transmission loss.
The film was unreinforced by glass cloth but contained an inorganic filler, so its properties were isotropic, with Dk of 3.0 and Df of 0.0023. It was supplied in widths up to 520 mm, in thicknesses of 25, 50, or 65 microns, on a 50 micron polyester carrier film. Alternatively, it was available as a resin-coated copper foil in the same dielectric thicknesses, on 12 or 18 micron copper. The resin had low melt viscosity and good gap-filling ability. Once cured, it could be laser-drilled for microvia formation or drilled mechanically, and plated using normal PTFE processes.
And now for something completely different. The final presentation of the EIPC Summer Conference was a non-chemical process for making roll-to-roll flexible circuits. Dry phase patterning was described by Tommy Höglund, sales and marketing manager with DP Patterning in Sweden.
The process he described could take flexible circuit manufacturing out of the PCB shop and into the EMS shop, feeding direct to the pick-and-place line.
The tooling was a laser-engraved metal cylinder, set-up time five minutes, processing time about one hour, to produce a roller with the required circuit pattern effectively engraved as a sharply-defined negative image and the insulating areas remaining as part of the original surface.
The production line, 18 feet long, had a roll-to-roll transport system for single-sided flexible laminate and the imaging mechanism rotated the prepared tool, known as a cliché, at web speed. This was running at very close clearance against a high-speed rotating cylindrical cutter, with the clearance corresponding to the thickness of the base film of flexible laminate, typically polyester in the range 35 to 75 microns.
The material was fed under tension around the cliché and where it passed through the nip between cliché and cutter. The cladding-metal layer, typically copper or aluminium, was milled away in areas corresponding to the original surface of the cliché, to leave the required circuit. The system appeared to work remarkably well in the production of antennas, heaters, RFID circuits, and conventional flexible circuits with 300 micron lines and 200 micron spaces on 18 micron copper.
Sustainability benefits of dry phase patterning were listed as low carbon footprint, low energy consumption, no chemicals, no water usage and recyclable residuals. Target market segments were communication, automotive, LED lightning, and heaters in general.
Kirsten and Carol
As this conference came to a close, there was unanimous consensus on the success of the event, and universal praise for the efforts of Kirsten Smit-Westenberg, Tarja Rapala-Virtanen, and Carol Pelzers in bringing it all together.
With grateful thanks to Alun Morgan for the excellent photographs.
Pete Starkey is a technical editor for I-Connect007.
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