Global Impact on European PCB Fabrication: EIPC Summer Conference 2017, Day 2
Editor's Note: To read "Global Impact on European PCB Fabrication: EIPC Summer Conference 2017, Day 1", click here.
The second day saw the conference room full once more, the late networking session in the bar having resulted in no significant casualties, and I was pleased to accept the invitation to moderate the first session, on processes and materials for flexible PCBs.
Thomas Michels got proceedings off to a flying start with an insight into a new range of cast polyimide flexible laminates where the polyimide resin, with or without filler, was coated and cured directly onto the foil to make single-sided material, and two single-sided could be bonded together with the same resin system to make double-sided. The laminates showed remarkably good dimensional stability, heat resistance, flexibility and flexural endurance. In anticipation of 5G requirements, the material was already available with polyimide thickness as low as nine microns with 9-micron copper, and even lower thicknesses shortly to be released. A modified polyimide resin system had been developed for low-loss applications, with Dk of 2.8 and Df of 0.005 at 10GHz. And whereas conventional coverlays required a separate adhesive layer, a versatile 25-micron one-layer coverlay had been developed with excellent thermal resistance and filling performance, which enabled thinner, lighter and more flexible constructions to be fabricated.
Well-known for his work on stretchable circuits, Professor Jan Vanfleteren, from IMEC, the Interuniversity Micro Electronics Centre in Belgium, introduced a new dimension in flexible PCB-based technology for randomly shaped circuits. He explained that here was a growing opportunity for shape-retaining free-form circuits that could conform to 3-dimensional surfaces beyond what could be achieved with flexible circuits, for example in ergonomic man/machine interfaces and free-form light sources.
Conventional flex-rigid assemblies were expensive and offered limited design freedom. Laser-structured moulded 3D interconnection devices were difficult to fabricate in more than a single layer, component placement was slow compared with 2D assemblies, and only a limited range of high-end plastics could withstand soldering with SAC alloys. Against this background, Professor Vanfleteren had set out to develop technologies for 3D circuits that would allow easy lab-to-fab transfer, using normal 2D fabrication and assembly procedures with off-the-shelf components and standard lead-free soldering.
He reviewed the principles established in his work on stretchable circuits, particularly the meander-pattern design rules for conductors, and showed how these principles had been adapted by replacing the elastomeric carrier with a flat, rigid thermoplastic polymer carrier. Once the circuit had been fabricated and assembled, it was subjected to a one-time deformation from flat to 3D using thermoforming techniques, resulting in rigid 3D-shaped thermoplastic objects with embedded electronics. Repetitive strain on components and interconnects was avoided, and components were additionally protected by the embedding polymer. There was great potential for industry take-up, and he showed examples of applications, including an omni-directional LED light source for homogeneous three-dimensional light distribution, and a three-dimensional touch-sensitive control panel for a washing machine. Remaining challenges for industrialisation of rigid, free-form 3D circuits were laser structuring of the meanders, high throughput technology based on punching, dedicated circuit lay-out tools and the precise positioning of components after 3D forming.
Next came a variation on two-dimensional flex, but with an effectively infinite X-axis. Philip Johnston, managing director at Trackwise in the UK gave an intriguing presentation on length-unlimited multilayer flex. The background to his story was that Trackwise had been approached by a large UK aero engine manufacturer with a requirement for single-piece flexible circuit more than eight metres in length. After a world-wide search for a supplier, where the rest of the industry had said “No,” Trackwise said “Why not...” and after some development work produced a proof-of-concept 6-layer flex five metres long, by a process they subsequently patented. Johnston stressed that this was a seamless continuous circuit design, not a step-and-repeat.
Having successfully manufactured and delivered the 8-metre order, Trackwise set out on a mission to change the way aeroplanes are wired with their “Improved Harness Technology™”—not an easy task in an industry characterised by its obsession with reliability and cautious about the possible negative consequences of any change from long-established standard practice—and many challenges had to be overcome along the way. Materials had to be sourced in roll format, and equipment suppliers had to be willing to adapt conventional machinery to meet these exceptional requirements. Manufacturing was a combination of roll-to-roll PCB and electrolaminate production techniques. Quality assurance could be a conundrum: “How do you verify the integrity of a PCB 25 metres long? And what about accreditation?” Whatever, the first flight qualification was about to take place, with a flexcircuit 26 metres long in a large U.S. unmanned aerial vehicle and there were many opportunities in the automotive, industrial and scientific sectors.
Whenever the patterns on two or more circuit layers were required to be aligned in manufacture, some form of registration system was required. Bernd Gennat, VP of sales and marketing for DIS in Germany presented a detailed account of lay-up technologies and bonding processes prior to the pressing of the multilayer PCBs, with an emphasis on state-of-the-art direct optical registration. He discussed the pros and cons of riveting and pin-laminating systems before going into detail on optical lay-up technology and coupled induction welding. Listing the benefits of the system developed by DIS, he explained that once the cameras had moved to a particular panel size, they stayed stationary for the entire process. Positioning and clamping took place in the same station, and layers were held in position by a clamping mechanism whilst in view of the vision system. The system easily handled thin cores without damage or distortion, and an arrangement of unique targets per layer ensured idiot-proof lay-up. A vision system maintained the position of the targets during the complete alignment and clamping cycle and alignment was always to the camera, never to the previous layer. A coupled induction welding system allowed aligned panels to be handled horizontally and an additional benefit was that the same lamination and separator plates could be used for a range of panel sizes.
EIPC board member and past chairman Paul Waldner, managing director of Multiline International Europa, gave a more general overview of registration systems, beginning by defining registration as “making things fit each other according to the requirements set forth by the end-user,” and listing the main registration challenges encountered in making printed circuit boards: aligning layers to one another, aligning pads on one layer to pads on other layers, aligning holes to pads on every layer, aligning outer layer images to innerlayer images, aligning solder mask images to primary imaged circuits and aligning components to the bare board. He then produced an equally long list of factors that could affect registration: material characteristics, environmental influences, mechanical process influences, mechanical process accuracy, mechanical process precision and, probably most important, good design based on an in-depth knowledge of materials, environmental influences and mechanical capabilities. He also suggested that a statistical process control system that tracked environmental and process changes and correlated those changes to changes in dimensions of the various images during the manufacture of a completed circuit should be an important tool in the manufacture of multilayer circuit boards.
He clarified the difference between mechanical accuracy and precision: accuracy being the capability of a process expressed as a range, from a nominal value, of the possibility of results, precision expressing the range of results from each other, independent of the nominal value. Designing for good registration required good data, and it was necessary to collect and process data with the use of targets at each process. Waldner gave some good design guidance with a series of practical examples. He then explained the evolution of alignment systems, from simple eyeball alignment, through punching and pinning, to automatic camera alignment. Considering multilayer registration systems, he described a range of options and advised what was appropriate for different levels of difficulty. The most difficult registration was for large, high-layer count multilayers, with up to 50 layers and layers as thin as 18 microns. These were the exclusive domain of pin-lamination using sophisticated post-etch punching and very thorough factory data gathering and SPC systems.
Switching role from presenter to moderator, Paul Waldner introduced the final conference session, on solder mask and conformal coatings, and his first presentation came from the ever-popular Don Monn from Taiyo America, discussing solder mask for direct imaging. Monn’s word of the day was “Yields.” And what was affecting yields? “Registration! If you optimise registration in every department in the shop you would immediately increase your throughput and, more importantly, your profits!” For liquid photoimageable solder mask, direct imaging was the best option for achieving optimum registration, but what were the right UV wavelengths to suit the photoinitiator system? A typical LPI solder mask required a peak at 365 nm for surface cure and one at 405 nm for through-cure, and for flood exposure this could be achieved with a gallium-doped mercury lamp. But early direct imaging systems used a laser source with a peak at 355 nm, and it was necessary for the solder mask supplier to formulate his photoinitiator system specifically for this wavelength. More recently, direct imaging systems using laser diodes gave more flexibility, and dual source machines enabled the laser output to be tuned to suit the photoinitiator which could then be formulated to achieve the optimum balance between surface- and through-curing of the solder mask. This balance had a direct effect on sidewall geometry, and Monn showed real examples of solder-dam microsections illustrating the importance of getting this balance right.
Don Monn’s presentation set the scene perfectly for Uwe Altmann, Orbotech’s specialist in direct imaging systems, to discuss higher performance solutions for a wide range of solder mask applications. The obvious benefits of direct imaging of solder mask were the elimination of photo-tools and accuracy in registration. But the big challenge was the number of different solder mask inks on the market—ten suppliers and literally hundreds of ink variants, with different colours, different surface brightness, different photoinitiator systems and formulated for different coating methods. Consequently, there was no “one set-up fits all” opportunity. A different energy set-up was required per manufacturer and per application method, coating thickness, developing process, final finish requirement and reliability specification. When designing a direct imaging system for solder mask, it was necessary to consider imaging speed, adhesion of fine dams in the developing process and final finishing, good line quality and sidewall profile, registration accuracy and high depth of focus, as well as the flexibility to image many different solder mask types. Orbotech had developed a direct imager specifically for solder mask and Altmann gave a detailed description of its technical features and performance capabilities. In summary, the system featured a one-pass digital micro-mirror engine with twelve optical heads to enable one-shot exposure, a dual-table transport mechanism so that one table could be loading and registering while the other was imaging, patented high power LED optics with wide depth of focus, and a partial scaling option.
From solder mask to conformal coating, Stefan Schröder from Lackwerke Peters in Germany discussed requirements for conformal coatings today and in the future. In automotive applications, the objective was protection from moisture and electro-corrosion under thermal cycling conditions with dew formation, and typical testing subjected comb-type coupons, coated and uncoated, to alternating temperature and humidity stress. It was expected that in future, temperature stress limits would increase from -40º +125 °C to -65 +155°C or higher, with increased requirements for media and noxious gas resistance. In addition, there was a drive towards lower energy costs for application and curing, and reduced solvent emissions.
Present-day conformal coatings fell into two general classifications: oxidative curing and physical drying. Oxidative curing coatings had the benefit of sufficient resistance to media and good adhesion, but the crosslinking reaction was slow and difficult to monitor, electrical insulation against moisture was limited, and low-molecular-weight substances tended to separate during crosslinking. Physical drying coatings were rapid drying, with very good moisture insulation and resistance against hydrolysis, but tended to be low solids content, thermoplastic and with limited resistance against media. And some of the solvents used could present health risks.
For the next generation, thixotropic conformal coatings would offer improved coverage of sharp-edged component leads and a good balance between edge coverage and flowing. Optimal edge coverage would give improved climate resistance. But these materials would need to be applied by spraying. Dam-and-fill conformal coatings would offer the ability to micro-encapsulate fine-pitch-pins and build barriers to prevent migration. Schröder discussed the formulation options for two-component polyurethane and polyacrylic resin systems. Next generation UV technologies would offer solvent-free silicone thick film coatings or solvent-free silicone-free thick film coatings with a dual curing mechanism: fast UV curing, together with moisture-initiated reaction in shadowed areas.
Back to solder mask imaging for the final presentation of the conference. Michel van den Heuvel, from Ucamco in Belgium, discussed broad multi-wavelength UV-LED technology for direct imaging of solder mask. He described the mechanism of digital micro-mirror projection devices, used in conjunction with multi-wavelength UV-LEDs in the 350 nm to 440 nm range, to give a combination of high power and long life. Expanding on Donn Monn’s comments on the need to balance the absorption and transmission of UV energy through the solder mask to achieve the right combination of surface cure, through cure, adhesion to substrate and sidewall geometry, he explained the benefits of mixing 365 nm, 385 nm and 405 nm wavelengths. A meaningful analogy was a hamburger: a single wavelength UV laser cooked the outside but left the inside raw, whereas a multi-wavelength LED laser cooked it all the way through! The balance of wavelengths could be tuned to suit the photoinitiator system of a particular solder mask and, using a Stouffer step-wedge as a measure, van den Heuvel demonstrated that a 3-wavelength UV-LED mix gave 1-2 steps higher than a 2-wavelength mix for the same exposure power.
Wrapping-up the proceedings after two days of top-class technical presentations, interactive question-and-answer sessions and networking opportunities, Alun Morgan thanked the sponsors for their generous support, the paper selection committee for putting together such a comprehensive and balanced programme, the moderators for managing the conference sessions, the presenters for sharing their knowledge and experience, and the delegates for their attention. Particular thanks were due to EIPC Executive Director Kirsten Smit-Westenberg and Event Manager Carol Pelzers for their faultless all-round organisation of an outstanding event. Looking to the future, Morgan announced that the Winter Conference would be held in Lille in France, February 1–2, 2018, and to expect a memorable celebration of EIPC’s 50th anniversary in September 2018.
I am once again indebted to Alun Morgan for kindly allowing me to use his photographs.
Wrap-up video interview: