Reading time ( words)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: