Global Impact on European PCB Fabrication
EIPC Summer Conference 2017
Electronics industry professionals from 13 countries, mainly from Europe and Scandinavia, others from the USA, but some from as far away as India and Japan, gathered in Meriden, the centre of England, for the EIPC Summer Conference.
EIPC Chairman Alun Morgan welcomed all present, and acknowledged the support of trade and press sponsors before digging into his archive of industry history and reflecting upon the inaugural Printed Circuit World Convention, in London in June 1978, of which EIPC was a sponsor and a young Rex Rosario, with dark hair and a splendid moustache, was a member of the organising committee. Protégé of Dr Paul Eisler and founder of Graphic in 1968, rewarded for his services to the PCB industry with the Most Excellent Order of the British Empire in 2001, Rex had not missed a single World Convention and had for the last three years been Secretary General of the World Electronics Circuits Council. Past chairman of EIPC, Rex Rosario was delighted to be called forward to receive Honorary Fellowship of the Institute from his current counterpart.
After much applause and flashing of cameras, Rosario returned to his seat as the conference programme got under way with Walt Custer’s business outlook on the global electronics industry, with emphasis on Europe. Although some current market statistics would not be available until a few days after the conference, Custer gave the comprehensive analysis for which he has become legendary, against a background of geopolitical concerns and copper foil shortages. His leading indicators showed improving business conditions and a global resumption in demand, with expansion in most sectors of the world electronics supply chain. Although exchange rates affected growth calculations, and figures varied depending on what base currency was used, it was clear that the European industry had enjoyed a good first quarter and continued growth was forecast in end markets, particularly in the automotive and medical sectors.
Looking at the structure of the European PCB industry, with acknowledgement to Michael Gasch, an interesting analysis was that of Europe’s top 74 PCB fabricators by revenue, representing 1852 million Euros in 2015, half the revenue was shared by the top 20 in the proportions: 1-5: 26%, 6-10: 11%, 11-15: 8%, 16-20: 7%. Gasch had calculated that 2016 figures would be slightly less than those from 2015 and commented that the largest ongoing concern in the industry was the shortage of copper foil which would result in price increases and supply bottlenecks for laminates.
For once, Custer did not need to offer the “unpleasant truth or comforting lies” alternative, commenting “The truth is pretty good right now!” Europe was strong and business conditions were improving globally. He forecast that the world PCB industry would grow by 2% in 2017 and by the same amount in 2018, although the geopolitical situation remained a major worry.
Recently returned from the Electronic Circuit World Convention in Korea, where he had received an award for Best Paper, Thomas Hofmann, President of Hofmann Leiterplatten in Germany gave a technical keynote presentation entitled “Sharing experience in Embedding of Active and Passive Components in Organic PCBs for more Reliability and Miniaturization.”
Founded in 1989, the objective of Hofmann Leiterplatten had been to provide innovative fabrication support to electronic designers and development engineers in large OEMs and EMS companies. Amongst many successful developments, the technology of embedding active devices in PCBs was now being widely adopted and was attracting a great deal of attention, ironically from some who had dismissed the concept 20 years ago on the basis that “nobody needs it”. Thomas Hofmann described how embedding technology had evolved since the 1960s with thick film resistors, through the 1970s with proprietary buried-resistor materials laminated into PCBs, to the first examples of Hofmann Leiterplatten’s “Active Multilayer” (AML) in the 1990s.
An essential requirement was to completely encapsulate the embedded device with resin, then the benefits of improved thermal properties, environmental and mechanical protection could be realised. Some early concerns had been expressed regarding possible damage to components during the multilayer pressing operation, but these had been demonstrated to be unfounded and a depth of knowledge had been established over many years’ experience of resin flow, press conditions and curing behaviour, together with in-house surface-mount assembly capability. He showed many application examples and a series of practical design guidelines.
Hofmann stressed the importance of cooperation between the PCB fabricator, the electrical designer, the PCB lay-out-engineer, the laminate manufacturer and the component supplier, as well as the assembly specialist and the test engineer, in order to achieve high first-pass yield. In effect, to achieve the full benefits of embedded device technology, it would be necessary to reconsider the structure of the total supply chain. But he made it clear that this should not be regarded as a disruptive technology, more an evolution of existing PCB fabrication techniques combined with new design, fabrication, assembly and testing methods.
The second technical keynote presentation came from Dr Despina Moschou from the University of Bath in the UK. Well-known for her work on Bio-MEMS and Lab-on-PCB microsystems, Dr Moschou described an inter-university collaborative CHIRP project supported by the British Council and the Scientific and Technological Research Council of Turkey to develop a child-friendly pre-diabetes diagnostic test patch for mass-population preventative screening. The prevalence of diabetes in Turkey was twice the global average, and with a rising incidence of childhood obesity there was a need for a painless, reliable, disposable device. Currently available solutions for diabetes screening were either low-cost but invasive, or non-invasive but high-cost. Lab-on-PCB technology offered a realistic non-invasive, disposable alternative. The proposed system used an array of hydrogel microneedle for the painless extraction of interstitial fluid and a sensing platform based on a flexible PCB. Sample transfer from the microneedles to the biosensor was via plasma-treated hydrophilic microfluidics. Inorganic alternative to enzymes were being investigated for improved reliability of glucose measurement, and a sensor based on copper oxide nanoparticles inkjet printed onto a gold electrode was showing favourable results.
“In the good old days, frequency was not an issue” was EIPC Technical Director Michael Weinhold’s opening comment as he introduced the session on high frequency and power. The first presentation came from Martyn Gaudion, Managing Director at Polar Instruments in the UK, discussing the effects of copper roughness on insertion loss and how to account for it in field-solver modelling. Whereas a DC current was carried uniformly through the cross-sectional area of a conductor, at frequencies of 10MHz and above AC current flowed mainly along “skin” of the conductor. At frequencies around 10GHz, the effective skin depth was less than 1 micron, so the surface roughness of a copper conductor could have a dramatic effect on insertion loss. Electrodeposited copper foil was “treated” during its manufacture to give a favourable surface for bonding to laminating resin, and during multilayer PCB fabrication various chemical processes were used to prepare inner layer copper surfaces prior to lamination. So there would always be some degree of surface roughness to be taken into account when modelling insertion loss. But how to measure it and give it a quantitative value in a calculation to model insertion loss? “All models are wrong but some are useful!” was the often-used quotation attributed to statistician George E. P. Box, to be kept in mind as Gaudion reviewed traditional surface roughness estimates related to equivalent numbers and depths of scratches, but these were only valid for relatively low frequencies. “The higher the frequency and the longer the line, the higher the loss. Maybe not a big issue in a smartphone, but a big problem in a backplane.” Currently the best model was the one proposed by Huray, which considered the copper surface as a series of piles of snowballs in order to explain how incident electromagnetic waves were reflected and absorbed. It could be simplified by assuming all the balls were the same diameter. But it still remained to determine the best way to physically measure the micro-topography of the surface.
Approaching the impact of copper roughness from a more practical angle, Mutsuyuki Kawaguchi from MEC Company in Japan introduced a new bonding treatment which maintained a smooth surface. Traditional bonding chemistries depended on producing a more-or-less micro-roughened surface to improve the mechanical bond to the laminating resin. The process he described achieved the bond chemically by forming a copper-tin-nickel alloy layer 100-200 nanometres thick on the conductor surface followed by an anti-tarnish coating, and this combination then reacted with the resin during the multilayer pressing cycle to form a covalent bond. Even after 10 reflow cycles, peel strength remained slightly higher than that achieved with roughening treatments, for a range of high frequency laminates. And transmission loss at 50GHz was about 10dB/m less than that for other treatments. Kawaguchi explained the bonding mechanism by revealing that the anti-tarnish contained a silane coupling agent which reacted with hydroxyl groups at the alloy surface and cross-linked to the resin chemistry. A new copper foil with this chemical adhesion mechanism was under development and first samples would be available in Q4 2017.
IPC-4562 defines three classes of copper roughness: “standard”, “low profile” and “very low profile”. But the foil manufacturing industry has gone far beyond these definitions, with “ultra low profile” and “almost no profile” copper foils becoming available. Thomas Devahif, from the R&D department of Circuit Foil in Luxembourg, described the development of these foils for use in very low loss material. Because the roughness of “very low profile” foil was around 3.0 microns, loss became significant at frequencies close to 1 GHz. It was necessary for roughness to be less than 1.25 microns to achieve acceptable results at frequencies above 20 GHz. This had now been achieved whilst maintaining good adhesion to low-loss resins. Devahif explained how the use of organic additives in the copper sulphate plating electrolyte had resulted in much smoother deposits. The drum-side surface quality of the foil had been further enhanced by an improved technique for polishing the titanium electrodeposition drum, and it was now possible to produce foils as thin as 6 microns. Zinc-chromate passivation gave excellent thermal and chemical resistance, and had less impact on insertion loss than other metallic passivation treatments. Mechanical measurement of surface roughness was not effective on ultra-low profile foil because on the limitations of probe tip geometry; 3D white light interferometry was a good non-contact technique. A new-generation no-profile foil was in development, with a silane-based adhesion promoter, and would be available at the end of 2017.
In addition to his duties as EIPC Chairman, Alun Morgan is a facilitator for the High Density Packaging User Group, and it was from this position that he reported the results of their in-depth investigation of the impact of new-generation chemical treatment systems on high-frequency signal integrity. The focus of the project was primarily on loss rather than adhesion. It set out to create a test vehicle for smooth copper that could be used in future projects to correlate copper roughness with signal integrity properties and to understand the differences between roughness measurement techniques, as well as evaluating the thermal shock results of the candidate adhesion promoters.
The test plan had been to assess six different inner layer pre-lamination copper surface treatments using three different surface roughness measurement techniques, and to use two different techniques to measure insertion loss. Morgan described the design rules, layout and stack-up details of the six-layer test board. Copper surface roughness measurements were made by white light interferometer, 3-D laser scanning confocal microscope and white light vertical scanning interferometer, and signal integrity measurements by stripline and SPP techniques. It was observed that new chemical treatment systems offered some nominal improvement in signal integrity and allowed for a tighter performance band than was currently possible with the existing alternative oxide systems. Existing alternative oxide systems had the capability to provide some nominal signal integrity improvement through tight control of the micro-etch process, but significant effort was required to generate incremental improvement and to maintain day-to-day consistency.
Much discussion has been centred around the fundamental dielectric and loss properties of PCB materials, but how could these be meaningfully determined? Alexander Ippich, Senior Signal Integrity Engineer OEM Marketing at Isola Group, gave a presentation on practical aspects of the comparative testing of electrical performance of PCB base materials. If the wrong assumptions were made, comparing materials properties could be like comparing apples and oranges – although there might be some basic similarities, there were remarkable differences in detail.
He reviewed the available test methods for electrical performance, for Dk and Df testing on etched-off dielectric and for insertion loss testing on transmission lines, and commented on the pros and cons of alternative approaches. Testing of blank dielectric was quick and easy but the influence of copper was not included and the coupon was not representative of actual design, whereas although testing of transmission lines involved time-consuming design and fabrication of test vehicles, the influence of copper was included and the result bore much more relevance to a real design. The test method itself could introduce variables associated with cables, connectors and probes, and choices had to be made whether to test single or differential trace length, and whether to use a time domain reflectometer or a vector network analyser to make the measurements. PCB fabrication processes had considerable influence on results, and even small etching inaccuracies had a significant effect on loss values. Furthermore, if it was attempted to compare results on test vehicles that had been made by different PCB shops, the results could be skewed by process differences rather than actual differences in the properties of the materials. “It’s not like taking a ruler and measuring a piece of wood!”
Summing-up, Ippich made it clear that for data to be useful, the test method and conditions needed to be fully and exactly understood, and the test vehicle fit for use. When comparing materials, best results were obtained if the same test vehicle, the same PCB manufacturer using the same processes, and the same test method were used. The tests should ideally be carried out back-to-back and randomised, with more than one data point per material and a statistically significant sample size.
Marc Ladle, from Viking Test in the UK, ended the session with advice to PCB fabricators on keeping current with their equipment suppliers, commenting that, although their manufacturing equipment was generally expected to last at least 10 years, the technology of their products tended to change more quickly and process development to match product evolution was a driving force for new equipment purchase. In his experience there was enormous variation in the way different companies addressed equipment purchase. Some had done their research, had a clear idea of what was available and who was the best company to supply it. Others would just order a machine and apparently only be interested in price and delivery. But in general there was input from the customer and the material supplier with some recommendations from the equipment supplier. Ladle was keen that equipment suppliers and equipment buyers should work more closely together. And a good place to hold a meeting was next to the scrap bin! Suppliers could set up visits to see machinery in another factory, or to the equipment manufacturer, to assist the buyer in making an informed decision. The day an existing machine had critical failures and needed urgent replacement was not the right time to start looking – it paid to have a good idea of what was available and who was best at making it.
The session on thermal management was moderated by Oldrich Simek, owner of Pragoboard in the Czech Republic, whose first presenter was Andrew Piotrowski from ICL Industrial Products in the Netherlands discussing a new phosphorus-based curing agent for printed circuit applications. He explained that ICL was a major supplier of both bromine-based and phosphorus-based flame retardants, and the trend was away from bromine and towards phosphorus. Most halogen-free flame retardants were derivatives of 9, 10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO).
He made it clear that flame retardants did not necessarily make materials non-inflammable, but did enable them to comply with flammability tests like UL94-V0, and most flame retardants had a negative effect on the physical and electrical properties of the printed circuit board. The main challenge for flame retardants was finding materials that satisfied all of the electrical, thermal, chemical, and mechanical properties required for optimal device performance. Phosphorus-based flame retardants were more efficient than bromine based flame retardants based on the active element content whereas the latter had excellent hydrolytic stability and low water uptake, which remained a challenge for many phosphorus based flame retardants. There was an increasing market demand for laminates with low Dk and very low Df, and such properties were difficult to achieve with reactive flame retardants. At a given frequency, the Dk and Df of a material depended mainly on dipole strength, the number of dipoles in one mole of the material, and dipole mobility, and in order to achieve low Dk and Df it was necessary to use polymers with low-polarisability bonds. Piotrowski explained that the goal had been to develop a molecule meeting the criteria of Dk less than 3.0 and Df less than 0.005 for the flame retardant itself, and the Dk and Df of the final product not being adversely affected by the curing chemistry, additional requirements were high chemical resistance, low moisture uptake, long storage life, good adhesion, low stress, high hardness, low shrinkage, high tensile modulus, low coefficient of thermal expansion and high thermal stability.
These objectives had been realised with a phosphorus-based aromatic polyester multifunctional curing agent known as E15-152T, which had a high reactivity with epoxy and cured at a similar rate to that of a classical phenolic but without the formation of unstable polar hydroxyl groups, resulting in enhanced thermal stability and very low Dk and Df. And it enabled a V0 flammability rating to be achieved with phosphorus levels less than 2.5% in unfilled epoxy systems.
Next to speak was Padraig McCabe, Business Development Engineer with Schoeller-Electronics in Germany, with a presentation on thermal management solutions. Defining the thermal management problem in terms of heat flux, he explained that substrate materials were poor heat conductors, whereas copper had much higher thermal conductivity. Depending on the copper distribution, the heat flux in a circuit board was normally better in the x-y plane than in the z-axis and a power or ground layer had a big influence on the heat flux. Heat flux and direction were mainly influenced by the thermal conductivity of the materials and the temperature gradient in a given area. Tracks on or in a PCB made little contribution to heat conductivity because their cross-sectional area was too small. Many component packages were designed with a predetermined thermal pathway within the package, and the component footprint on the PCB generally incorporated a thermal pad with thermal vias to conduct heat away in the z-axis. But thermal via arrays had limited thermal conductivity and local copper coins offered an alternative with typically ten times the thermal conductivity for a given area.
McCabe showed comparative thermographs demonstrating that a copper coin could lower the device temperature by 10°C, resulting in a potential doubling of the component lifetime. He reviewed a range of thermal management options: pre-bonded or post-bonded heat planes, adhesive bonded coins, embedded coins and press-fit coins, with examples of appropriate applications for each, commenting that local copper coins could reduce weight and cost compared with conventional attached heat sinks and provide the opportunity of assembling components on both sides of the PCB, simplifying the assembly process and increasing first pass yield. The integration of local copper coins into PCB constructions was a well-established process, designs could be tailored for specific components and requirements, and PCB’s with integrated local copper coins had been demonstrated to be reliable and robust.
FR-4 laminates were familiar to all, but what was FR-15? EIPC Board Member Emma Hudson, from UL VS in the UK, gave the details. Driven by demands from the automotive, LED and high temperature power supply sectors for high performance epoxy materials, the Standards Technical Panel had reached consensus in January 2017 to add a new UL/ANSI grade to Tables7.2 and 7.3 of the 6th edition revision of UL746E.
This was FR-15, the new higher temperature FR-4 type material for UL Recognised Boards. In parallel with FR-4.0 and FR-4.1 designations, FR-15.0 was a brominated material and FR-15.1 was halogen-free. Other constituents and performance values were the same as for the equivalent FR-4 materials. The main differences were in temperature rating, where for thicknesses of 0.63mm or greater FR-15.0 and FR-15.1 required an Electrical and Mechanical Relative Thermal Index of 150°C. The benefits were that no additional testing was required for copper-clad laminates, and reduced testing for PCBs through the MCIL/CCIL programme. No file review was required for laminate or PCB, no testing was needed to convert FR-4.0 or FR-4.1 recognized laminates that have been previously evaluated for FR-15 Relative Thermal Indices, and minimal testing was needed to upgrade existing PCBs using a material converted from FR-4 to FR-15.
There followed an open session on issues facing the PCB materials supply chain, with Isola’s Karl Stollenwerk and Ventec’s Thomas Michels and moderated by Alun Morgan, a sequel to the discussion held at the EIPC winter conference in Salzburg. It was generally agreed that, although the copper foil shortage was presently not as critical, and a seasonal drop in demand in Asia had allowed laminators to replenish their stocks somewhat, the crisis was not over and the pressure would probably return within the following few weeks. And although base copper prices were declining on the London Metal Exchange, conversion costs were rising and further foil price increases were expected. No immediate shortage was seen in glass fabric, but price rises were anticipated. Thomas Michels repeated his earlier advice to PCB manufacturers to resist the temptation to panic-buy, and to work closely in partnership with their suppliers to plan their future requirements.
A full and technically intensive first day concluded with a visit to the nearby National Motorcycle Museum, followed by a convivial conference dinner, generously sponsored by Ventec.
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 of Ventec Europe got proceedings of 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 remarkable 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 9 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 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 8 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 5 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 flex 26 metres long in a large US 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 inner layer 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 micron. 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. Mann’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 of 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 365nm for surface cure and one at 405nm 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 355nm, 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 Microsection 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 -40 +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-initated 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 350nm to 440nm 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 365nm, 385nm and 405nm 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 on February 1st and 2nd 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.
The Editor wishes to thank Pete and the peerless publishers of I-Connect007 for their benevolent permission to publish this matchless article in Speednews. All shall gain.
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