Emerald Group Publishing Limited
Copyright © 2011, Emerald Group Publishing Limited
EIPC Seminar at SMT Nuremberg 2011
Article Type: Conferences and exhibitions From: Circuit World, Volume 37, Issue 3
Imaging and Registration of PCBs
The European Institute of Printed Circuits (EIPC) returned to Germany for their technical seminar on the theme of imaging and registration of PCBs, in conjunction with the SMT-Hybrid-Packaging Exhibition 2011 at the Congress Centre in Nuremberg.
EIPC Executive Director Dr Konrad Wundt welcomed delegates and introduced an outstanding programme of presentations which provided PCB fabricators with state-of-the-art knowledge on the realities of tolerances and registration, to enable them to improve their manufacturing yields with existing equipment and materials, and to understand just what could be achieved with the latest developments.
The scene was set by the first presentation, from EIPC Technical Director Michael Weinhold, entitled “Trends in PCB requirements”, encouraging the audience to realise that it was in the hands of the PCB industry itself to determine how the technology would develop into the future. Recent market data indicated Europe’s annual production of rigid and flexible PCBs to be ∼ US$2.5 billion, but this represented only 5.7 percent of world turnover, and the only way forward for the European industry to maintain and grow this market share was to continue to be innovative. Weinhold quoted examples of PCBs for automotive, industrial, medical, and network electronics which were designed, specified and prototyped in Europe, even though the volume requirement was supplied from China.
Reviewing standards for PCBs, he drew a distinction between manufacturing standards and safety and security standards, stressing the need for the European industry to be actively involved in the writing of standards in accordance with the needs of the industry, and that the costs of establishing and maintaining approvals be justified from a business point of view. One example he used for illustration was he UL QMTS2 FR-4 file review, which was ongoing in the USA with little European participation, and which would generate a requirement for an enormous amount of testing. It was significant that of 270 recognised FR-4 materials, 81 did not match the new terms of reference.
Trends for the future were identified in the new IPC International Technology Roadmap, which Weinhold explained with reference to Alice in Wonderland reaching a fork in the road and asking the Cheshire Cat which way to go. “Where do you want to go?” was his response. “I don’t know”, Alice answered. “Then”, said the Cat, “it doesn’t matter”. It was necessary always to look into the future, to recognise where the next generation of technology was going, and to carefully manage the changeover period.
Laminate expert Alun Morgan, Director of OEM Marketing for Isola Europe, gave an extremely clear and logical account of the factors which determine the dimensional stability of woven-glass reinforced laminates and multi-layer builds, beginning by describing the natural stresses resulting from the laminate manufacturing process – the causes of shrinkage and what could be done by the laminator to mitigate their effects – and the areas where variability could be introduced by the multi-layer fabricator during the bonding process, such as asymmetric thermal profile, asymmetric pressure application, tooling issues, excessive resin flow and multiple layers of thin prepreg, as well as all of the problems which originate from the copper distribution within a multi-layer design. Starting from first principles, he explained how the nature of the glass yarn and the mechanics of the weaving process influenced the characteristics of the woven fabric, and how major improvements in dimensional stability, as well as CAF resistance and electrical performance, could be achieved by the use of low-twist and zero-twist yarns.
Paul Waldner, MD of Multiline International Europa, focused on the mechanical fidelity and dimensional stability of silver halide film and discussed the attributes of different commercial products. He demonstrated that film is a remarkably complex product with a whole lot of engineering solutions built into its structure to ensure that it meets the requirements of processability and process repeatability when used under properly managed conditions. Waldner was confident that, although laser direct imaging was well established and growing in the world market, it would not have a significant impact on the continuing growth of the film business. Film offered robust capability with high resolution and low cost. The escalating price of silver metal was an area of concern, but he saw this as an opportunity and incentive for film users to pay closer attention to reclaiming metal from their spent process chemistries and rinse waters.
Michael Weinhold returned to moderate a session on equipment and processes, beginning with a paper by Stephan Kunz of Schmoll Maschinen on how yield improvements could be made through the latest technical developments in drilling. The basic accuracy of drilling machines had improved enormously over the last 20 years, with the resolution of measuring scales progressing from 2.5 to 0.5 μm and significantly better machine temperature management being achieved. Pressure-foot aperture had been shown to be an important consideration in reducing burring. Pressure feet were now available with a sliding insert, selectable for an 8-mm aperture for standard drill sizes and 2-mm aperture to minimise dust ingress under the entry material when drilling small holes. Likewise, it was important that vacuum extraction was controlled to avoid lifting the entry board. He considered the most critical feature of a drill spindle to be the state of the collet – dirty or worn collets had a dramatic effect on drill run-out. And scratches on entry material could be a major cause of positional inaccuracy for small holes. Regarding cutting speeds, the ideal speed for FR-4 was 150 m/min, and for a 0.1-mm drill this would require a spindle speed of 400,000 RPM. In Europe, it was typical practice to use a 200,000 RPM spindle for holes in the range 0.3-0.1 mm, although drill wear would tend to increase at the smaller diameters. As the accuracy of the drilling process had improved, it was increasingly important to qualify, check and measure the drilling machine at regular intervals.
Hans Fritz of Sense Advanced Technology discussed an advanced phototool registration technique developed by Hakuto, which offered a unique capability to compensate for non-linear distortion of the multi-layer panel. The film was deliberately plotted slightly undersize, then held in the exposure unit within a frame with an array of programmable clamps which effectively stretched it to fit the work-piece, linearly or non-linearly as appropriate. Using glass masters as reference, it had been demonstrated that a panel could be defined consistently by eight peripheral targets, and that scaling at the edges of the panel gave corresponding accuracy within the panel. Using this method, it was possible to scale an outer layer or soldermask phototool to exactly match the pattern distortion on the panel. A batch of boards could be scaled individually, or throughput could be increased by averaging the scale factors of the first few boards and applying the average factors to the rest of the batch. Manually loaded or fully automatic equipment was available.
Some of the practical challenges of maintaining registration in a high-technology manufacturing environment became clear when Alexander Süllau, Production Manager at ILFA Feinstleitertechnik, showed examples of some of the complex flex-rigid multi-layers his company was making for aerospace and communications applications. “How to build boards that are not manufacturable!” as Michael Weinhold commented during the panel discussion. They encountered just about every known material in the designs they built: polyester, polyimide, PEN, PEK, LCP, and PTFE, all of which had their own dimensional characteristics and idiosyncrasies. And Süllau’s examples had fine lines, asymmetric builds, mixed materials and combinations of through, plugged, blind, and buried vias. So ILFA relied heavily on 30 years experience to address the requirements of layer-to-layer, drill-to-pattern, outer-layer-to-drill and solder-mask-to-outer-layer registration, and Süllau reviewed the tolerances which could be achieved by eyeball, pin, CCD and LDI tooling at all stages of production. ILFA were the first PCB fabricator to use the Hakuto film-stretching system described by Hans Fritz, and reported successful results particularly on solder mask patterns which required to be registered to panels with non-linear distortion. For their various flex materials, ILFA used a failure modes and effects analysis to establish best registration. By incorporating the estimated dimensional characteristics of base material, layout and stackup, drill process, press process, exposure process, and inner-layer process into a law-of-error-propagation equation, they were able to determine Δγ best registration figures and build up a database in the form of a stack-up library with stretch factors. As jobs were fabricated they would carry out layer analysis using X-ray, and continuously refine and update their database.
I-Connect007 Editor Pete Starkey moderated the session on alternative imaging technologies, and the first speaker was Orbotech’s LDI specialist Uwe Altmann, with an interesting application of the registration data generated by laser direct imaging of outer layers entitled “Solder mask imaging made easier”. In the situation where the outer-layer image of a multi-layer was exposed by LDI, the LDI machine scaled the image data to fit the drilled-hole pattern, which was probably scaled already to compensate for inner-layer distortion. Making a subsequent solder mask image register exactly with the outer-layer pattern was a straightforward task for an LDI system, but if LDI capacity was limited and the cost could not be justified, it was standard practice to contact print the solder mask using a phototool. The challenge was to dimensionally compensate the phototool to fit, or else suffer yield loss through misregistration. One scale factor did not necessarily fit a whole production batch, and to prepare a separate film for each panel was counterproductive. Orbotech had developed a technique called Group Scaling, whereby during imaging of the outer layers the LDI machine assigned each panel in a batch an identity according to one of nine groups into which its particular scaling parameters fell, and at the completion of the batch exported scaling data back to the film plotter, sorted into the groups which represented best fit for different panel groups. Thus, the solder mask could be imaged with a film whose scaling factor closely matched the average for its particular group, and panels did not need to be measured individually to decide scaling factors. So far as the LDI machine was concerned, the group scaling process was automatic and had no effect on throughput.
Stuart Hayton of Mutracx reported on the development of the soon-to-be-launched Lunaris inkjet system for production-scale inner-layer imaging, and made it clear that there was no intention to compete in the market with LDI which, according to his figures, represented only 4 percent of the world’s PCBs production and was seen as specialist technique for critical applications. The Lunaris system was aimed at the mainstream requirement where it offered the benefit of eliminating 11 of 15 process steps and enabling a job to proceed from CAM data to the etching line in a matter of minutes. And the process guaranteed a defect-free etch resist image with its online AOI capability.
Several suppliers had in the past tried and failed to produce practicable inkjet etch resist systems, and Hayton in his presentation “Etch resist for PCBs – the inkjet Holy Grail” explained how the Lunaris system differed from its antecedents. First, Lunaris was a development of principles established by Océ in the Colorwave graphic arts printer, which had been the outcome of a $200 million project and of which there were already over 1,000 installations. Second, the inks and the printheads came from the same source, Océ, and had been specifically designed to work in combination. Third, Mutracx’s proprietary print strategy and predict features made it possible to exercise precise control on image integrity. Mutracx were currently evaluating the Lunaris system in cooperation with 20 leading PCB fabricators in Europe, North America, and Asia.
From the background of a leading manufacturer of photoplotters, and drawing on many years of practical experience of inkjet legend printing on PCBs, Jean-Paul Birraux from First EIE gave a down-to-earth view on the realities of digital imaging for marking and serialisation using inkjet. He covered a range of inkjet facts including resolution, registration, print quality, panel warpage and adhesion, and how these had been addressed in the development of First EIE’s equipment. In terms of registration and resolution, the mechanics of the equipment could achieve a positional accuracy of 3 μm. “But an ink drop is not a laser beam […]”. Birraux reminded the audience, as he discussed the finer points of drop velocity and drop direction, and recommended a design clearance of 40 μm legend-to-copper, to avoid the risk of ink on pads. Panel warpage was an issue which First EIE had overcome by a combination of a reverse-clamping system a continuous laser measurement of panel topography whilst printing. EIE’s new drop-on-demand printhead was based on 512 jets, operating in “single-wall” mode allowing to fire any combination of 512 jets at the same time, whereas typical “shared-wall” heads could only fire alternate jets at the same time. This enabled a high-printing speed to be achieved with a smaller number of jets, reducing the risk of jet failure. Typical panel throughput was 70/h. The printhead was compatible with several types of UV or thermal curable legend inks, and its no waste recycling ink system made it very economical in operation when used for serialisation or 1D or 2D bar coding of PCBs.
An alternative digital technique to aid identification, traceability and serialisation of PCBs was presented by Sybille Schmelzinger from HOLP, who explained what could be achieved by 2D laser marking. Of the range of lasers available, CO2 lasers were most suitable for marking circuit boards and were capable of producing a white image on green solder mask. She described a machine based on the principle of a stationary workpiece and a servo-controlled flying laser which could very rapidly incorporate a large amount of information into a tiny data matrix, as well as producing text and conventional bar codes. As an example of the application of laser-marked data matrix codes, Ms Schmeltzinger referred to the traceability system published by ZVEI, based on two channels of communication: control, meaning process control with request and response, and unit data, meaning following the process and warning if anything was moving out of control, together offering great benefits in process and yield improvement, whilst providing internal and external traceability. Unlike additive processes such as screen or inkjet printing, laser marking did not involve inks or chemicals and was a truly “green” technology.
In the final session, introduced by Konrad Wundt, Paul Reid, Interconnection Stress Testing Program Coordinator with PWB Interconnect Solutions, gave a highly informative presentation on the effects on reliability of printed circuit design and tolerancing, as measured by IST. Reid described reliability as “when the strength of an object is stronger than the forces applied”, and drew a clear distinction between reliability and quality in a printed circuit context. “Quality is the talk, and means the board will meet requirements. Reliability is the walk, and means the board will survive assembly and will work”. Reliability testing established which variables were critical and which variables had little influence, and IST offered an elegant means of measuring a large range of variables within a few hours. Reid explained the details of the test method and how test coupons could be customised to meaningfully represent particular design features, before describing a series of failure modes with the aid of animated graphics. He ranked copper quality, material robustness and design as the main influences on reliability and commented that design was increasingly becoming an issue, particularly since the advent of lead-free soldering. By understanding failure mechanisms, designers could anticipate potential reliability issues and design in anticipation of the product being robust. Reid ranked, in decreasing order of robustness of interconnect, microvia, plated through-hole, blind via, buried via and mixed structures, then discussed the effects of hole size, grid size, non-functional pads and etch-back on mode of failure, and commented that new failure mechanisms arise with decreasing pitch. Line width and registration appeared not so influential. Again using animated graphics, he showed how surface finishes based on electroless nickel could either promote or inhibit crack formation. Always prepared to provoke a lively debate on the interpretation of failure modes, Reid engaged the audience in a very constructive interactive session, and certainly justified his statement. “You learn from failure!”
EIPC can be relied upon to organise and manage events with topical relevance and content of the highest quality, and are once more to be congratulated. The Nuremberg seminar presented delegates with the ideal opportunity to understand current and future industry trends, to share knowledge of the latest technical developments and to network with their contemporaries.
Pete StarkeyI-Connect007May 2011