Reading time ( words)
Several interfaces were typically encountered in microvia processing, although it was preferred to minimise their number. Interfaces tended to be the weak links. They could occur at drilling, desmear, electroless copper or direct metallization, flash plating, via fill plating and conformal plating, and each presented an opportunity for reliability issues. Of the many factors influencing interfaces, features such as oxidation and residues, materials of construction, differences in thermal expansion coefficients and differences in grain structure were significant. Bowerman examined each one in detail and commented on how it could influence reliability. Reliability could be optimised minimising the number of interfaces and implementing, monitoring, and controlling best plating practices for each applied layer.
Andre Bodegom, managing director of Adeon, discussed the benefits of parameterised AOI in aid of traceability. He listed some of the challenges experienced by companies developing AOI systems to meet current and future industry demands. Designs often had different materials and technology levels per layer or materials of different brands with different contrast. Acceptance levels could vary, according to the customer specification or the IPC definition. There was also the need to maintain design-embedded intelligence from CAD/CAM to the AOI system, include key component and feature information, and to automatically attach specific inspection parameters.
Further desirable features were the ability to create a “black box” machine language and automatically select magnification levels, illumination and greyscale settings, together with an automatic and intuitive set up procedure for operators. To maximise equipment utilisation, it was necessary to have the flexibility to accept a high mix of different jobs, to combine applications such as IC substrates, PCB inner and outer layers, and laser vias, and to cope with a wide range of material variants, all on shorter cycle times. Traceability was an essential requirement, as was the ability to communicate with all CAM stations and industry formats, to inspect, measure, record, report and to communicate with the outside world.
Quite a shopping list! So how could all of these objectives be achieved? Bodegom maintained that the answer lay in parameterising optical inspection, with automatic set up and material recognition, a calibration library, black-box data preparation, and an intuitive graphical user interface. Parameterisation was about translating information into intelligence, automatically populating AOI parameters, identifying types of defect and their impact, and recording and reporting all of the data. Additional options were an enhanced metrology function to enable local height measurement and 3D profiling, finished board inspection capability and the ability to verify and classify defects off-line.
“Be flexible. Go digital.” Was the headline of the presentation from Rita Torfs, project manager at Agfa Materials, proposing inkjet as “more than the traditional way to apply solder mask.”
Agfa had an extensive background in inkjet in wide-format printing systems and industrial printing, and a substantial portfolio of inkjet products for the PCB, printed electronics and chemical milling industries. Although there remained a steady demand for phototool films, they saw great growth potential in PCBs for etch resist, solder mask and legend inks, and saw the change from analogue to digital imaging techniques as bringing benefits in cost, environmental friendliness, speed and flexibility.
But there were major challenges to be overcome in formulating inks for jetting compared with screen printing. In particular, inks for jetting were much lower in viscosity and needed pigments and filler particle sizes to be in the nanometre, rather than micron, range. Whereas aqueous-based inks were good for printing on paper and solvent-based inks were suitable for printing on PVC and other substrates, PCBs required solvent-free UV-curable formulations.
Torfs explained the complex interactions that were taken into account in optimising jetting performance: the design and geometry of the print-head, the viscosity, dynamic surface tension and high frequency elasticity of the ink, and the waveform of the print-head electronics. And all of this was before the ink arrived at the surface of the PCB, where a further series of complex interactions had to be considered: the surface energy of the substrate and the ink, the physics of wetting and flowing, the coalescence of successive drops, the photochemistry of UV pin-curing and the UV or thermal post-cure. Images were created in single or multiple passes, and could be selectively built up depending on the print strategy and the system electronics.
Principal components of a solder mask ink were monomers, photoinitiators, pigments and additives such as surfactants and adhesion promoters. Pigments had to be ground fine enough to give a stable dispersion and good jetting behaviour. And the resulting formulation had to meet the specification requirements of coverage, image quality and edge definition, with fast curing and excellent adhesion and hardness.
Torfs showed examples demonstrating the coverage and imaging capability of Agfa’s solder mask inks and listed test results confirming that they satisfied all of the requirements of IPC-SM-840.
Agfa having discussed inkjet from the formulator’s perspective, it was interesting to learn from the experiences of the equipment manufacturer. Carsten Schimansky, founder and CEO of Notion Systems, described new technologies for solder mask application. He began by referring to the standard process sequence for liquid photoimageable solder mask, and indicating the substantial savings that could be made in floor space, materials, maintenance costs, labour costs and energy costs by changing to an inkjet process. There were significant technical benefits as well, especially the ability to deposit material only where it was required, with no mask in holes or on pads, and to control deposit thickness.
The natural edge profile of an ink-jet solder mask pattern was an advantage where it came close to a component pad, because it avoided any entrapment of subsequent process chemistry. Alternatively, the mask could be brought in contact with the edge of the pad and form a seal. Schimansky showed details of the range of equipment his company could supply, and described some of the finer points of ink-jet printing technique, how keeping the distance between print-head and substrate to a minimum gave higher accuracy, less satellites and less drop deviation by airflow. The provision of an integrated drop-watcher enabled real-time monitoring of main drop volume, satellite volume, drop velocity and drop angle, and was of great assistance in fine-tuning the printer for different inks. An interesting feature was the ability to adjust the reflectance level of a solder mask simply by changing the print strategy, to suit customer preference.
With 25 presentations over two days, covering a huge spectrum of the state of the art in PCB technology, this had been a memorable 50th anniversary conference for EIPC. Indeed, I learned that the call for papers had been oversubscribed and the final selection had involved some difficult choices. But it was a great learning experience for all who attended—and more than that, a splendid networking opportunity for the leaders of the European PCB industry. There is always a wonderfully friendly and sociable atmosphere surrounding EIPC events, and it is always a great pleasure to have the opportunity to attend.
Alun Morgan wrapped-up the proceedings, thanking delegates for their attention, speakers for sharing their knowledge and sponsors for their generous support. Special thanks and good wishes to Michael Weinhold for his enormous contribution over the years, and to Kirsten Smit-Westenberg and Carol Pelzers for their calm and professional organisation and management of another superb event.
“We are 50 years old, and many things have changed. But some things remain the same, and we are still here to support the European PCB industry!”
My thanks to Alun Morgan for kindly sharing his photographs with us.
- EIPC 50th Anniversary Conference Day 1: The Past, the Present and the Future, Part 1, by Pete Starkey
- EIPC 50th Anniversary Conference Day 2 Part 1 by Pete Starkey
- EIPC 50th Anniversary Conference Day 2 Part 2 by Pete Starkey
- EIPC 50th Anniversary Conference by Barry Matties