The Institute of Circuit Technology Annual Symposium 2018

Estimated reading time: 13 minutes

Neil Chamberlain’s presentation set a scene for Joost Anne Veerman, MuTracx International’s inkjet printing specialist, to discuss the results of an investigation of the effects of conductor edge smoothness on impedance. Veerman explained how a conductor image was created as a composite of a series of ink droplets and demonstrated the characteristic differences between X-direction and Y-direction features. The nature of the inkjet process was that, however accurately it was controlled, the edges of features would always show some evidence of waviness as a consequence of the residual geometry of successive drops. By comparison, direct imaging with a photoresist resulted in more sharply-defined edges. To some extent, the etching process has a smoothing effect, but there remained a distinct difference in conductor-edge geometry between the two technologies. The questions to be resolved were whether this difference was significant in the context of controlled impedance and whether inkjet imaging was appropriate for high frequency PCBs.

A test board had been designed using a composite of standard IPC coupons in 0°, 45° and 90° orientations in a 10-layer stack-up, featuring 100-ohm edge-coupled differential strip-lines with 100/150 micron and 100/100 micron track/gap, and single-ended strip-lines with 175 micron 50 ohm, 75 micron 75 ohm, and 195 micron 50 ohm tracks.

Two panels were manufactured using the MuTracx inkjet printer, and two using LDI and photoresist. Track widths and edge waviness were measured on innerlayers before and after etching. Impedances were measured with a time-domain reflectometer, and microsections were made for measurement of dielectric and copper thickness. Veerman showed comprehensive test data.

From the results it was clear that there was no strong correlation between cross-section data and impedance. Impedance correlated with average track width and, significantly, no difference was observed between LDI and inkjet imaging technologies in terms of the effect of track-edge waviness. Dielectric thickness had the most dominant effect, together with the effect of resin-to-glass ratio on dielectric constant.

Before the commencement of the afternoon session, the indefatigable efforts of Len Pillinger over many years of acting as the link between the Institute and the standards committees were recognised in the award of Honorary Fellowship, presented by ICT Chairman Professor Andy Cobley.  

The programme re-commenced with a discussion of advances in direct imaging technology from Michel van den Heuvel, imaging group director at Ucamco.

The trend had been to move from high-pressure mercury lamps to UV-LED light sources, particularly for solder mask exposure. The state-of-the-art was a broad-multi-wavelength system, with peaks typically at 365, 385 and 405 nanometres, operated simultaneously and with the ability to control individual intensities to optimise the mix to suit individual solder masks.

The shorter wavelengths favoured the polymerisation of the surface of the solder mask, the medium wavelengths penetrated deeper into the thickness, and the longer wavelengths ensured good polymerisation at the copper surface. The optimised combination gave a very uniform through-cure, resulting in very straight sidewalls after development and the capability to reliably resolve 50-micron solder dams without undercut, as well as shortening the exposure time and hence increasing throughput. The system also gave very good results with black solder masks.

Van den Heuvel explained that the Ledia equipment supplied by Ucamco had many advanced features: its autofocus capability could compensate for panel warpage of up to 800 microns; its automatic imaging-head calibration facility ensured the position of each imaging head in relation to the other five heads, resulting in perfect stitching of the image and compensated for minute dimensional changes in the machine due to temperature variations. It also had a range of innovative automatic alignment features to compensate for substrate distortion. To fulfil the requirement for complete traceability, there was the facility to mark every individual PCB image with a unique identity, human- or machine-readable, which could also incorporate scaling information if required.

Peter Alliston, VP sales for Orbotech West, discussed the trends currently driving PCB technology, and described how leading suppliers of imaging and inspection tools had responded with developments in the capability of their equipment. He identified four market sectors: Internet of Things and 5G infrastructure, smartphones, automotive, and Industry 4.0 and robotics.

High-frequency PCB designs for 5G wireless broadband operating in the millimetre wave spectrum would demand strict impedance control at the 5% tolerance level, and low-loss materials. It was forecast that there would be 30 billion connected devices by 2020 and 75 billion by 2035.

Smartphones were leading mass production technology, with trends to 20X faster processing and communication rates together with increased battery capacity, to support augmented reality, artificial intelligence, and biometric and medical sensing. As batteries got larger, the space available to accommodate the PCBs got smaller, forcing an increase in interconnection density towards 20-micron lines and spaces that could not be achieved by established HDI processes. The technology was trending towards substrate-like-PCB (SLP), produced by modified semi-additive processing (mSAP). The use of flex in smartphones was increasing—up to 20 flex circuits in every unit, and there was a move towards roll-to-roll manufacture for more cost-effective production

Rapid development continued in automotive electronics, and the requirements of safety and autonomous operation were driving advances in sensor technology—radar, lidar, cameras, long range radar, and ultrasonics, many of which needed 5G connectivity. The use of HDI was increasing, and the automotive sector was forecast to represent 15% of the total HDI market by 2020.

Enormous investment was being made in automation, smart data exchange and cyber physical systems, and the global robotics industry was forecast to expand from $34 billion to $226 billion by 2021.

So how did a company like Orbotech respond to these trends and provide solutions for the PCB industry? Alliston described latest developments in direct imaging for HDI, mSAP and solder mask, with particular emphasis on achieving registration accuracy and high throughput. He discussed new concepts in AOI with integrated 2D metrology, and how the needs of Industry 4.0 were supported with a smart factory metrology package offering full product traceability for root cause analysis and life cycle control.

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