EIPC 2017 Winter Conference Review of Day 1

Estimated reading time: 19 minutes

The afternoon proceedings began with a session entitled “Advanced component and attachment technologies for high first pass yields,” moderated by Christian Behrendt.

The first presentation came from Wolfgang Schmitt of Heraeus Electronics in Germany, who described a sinter paste which provided a pressure-less low temperature alternative to soldering for nickel-gold PCB substrates, primarily designed for automotive and power electronics applications. The material had been created originally for the Asian market but was now available in Europe. It offered potential advantages in manufacturing costs, process costs and material costs.

He explained the principles of sintering: a metallic powder coalescing into a solid or porous mass by diffusion alone, without going through a molten phase, under the influence of heat and usually pressure. A range of pastes was available, but the one of interest, based on micron-sized silver powder, was capable of being sintered at 200°C without the application of pressure. The resulting material was not completely solid; the residual interstices between the silver particles effectively amounting to about 35% porosity, and this gave benefits in resilience, with a low elastic modulus. Bond strength to nickel-gold was excellent and under shear stress the failure mode was cohesive rather than adhesive. Thermal conductivity and electrical conductivity were superior to solder. The only disadvantage in production was the time taken for the sintering operation, which could be as long as three hours. IR sintering offered a route to shorter process times and current work indicated that a 40-minute process was feasible. 

“Future electrical property needs drive semi-additive processes into high volume PCB manufacturing” was the title of the presentation of Uwe Altmann from Orbotech in Belgium, a regular contributor to EIPC conferences. He reviewed industry trends in hand-held devices, focusing on PCB needs for 5G operation, where 75.4 billion end units were forecast to be in use by 2025. Operating frequencies greater than 6GHz were anticipated, so close-tolerance conductor geometries and impedance control would become increasingly important, and with component pitches trending to 0.3mm and below, lines and spaces of 30 microns and less would be required. These advanced HDI designs were beyond the capability of existing panel-plate tent-and-etch fabrication processes and an alternative approach was required, with the capability to produce precision fine-line PCBs cost-effectively in high volume.

Altmann described the modified semi-additive process mSAP which is becoming adopted as the mainstream process to carry HDI into the next generation. Starting with 3-micron copper, the sequence was: laser drill, electroless copper, pattern plate, strip resist, flash etch. Orbotech had developed the enabling imaging technology in their Nuvogo Fine series of direct imaging systems, which gave optimal line definition down to the 10-micron line and 15-micron space level and registration accuracy of +7.5 microns, with multiwave laser and large scan optics for maximum throughput. 

Stephan Kunz from Schmoll in Germany talked about registration, improving yield and reducing costs of the PCB manufacturing process.

From their long-term background in CNC drilling and routing machines, Schmoll had progressively and strategically extended their range of equipment to include innerlayer exposure, post-etch punching, x-ray drilling, direct imaging, soldermask exposure and scoring, all of which drew on one common theme—registration. The company was becoming increasingly focused on software and Kunz described how building databases of preventive knowledge could improve the quality of registration and consequently increase yield. Critical data feedback drawn from these systems could be implemented for continual improvements minimising fabrication tolerances.

Having discussed the options for effective panel identification, he went step-by-step through the manufacturing sequence, at each stage listing how quality improvement could be achieved, traceability could be maintained, registration accuracy could be maximised and tolerances minimised, and useful data generated, collected and fed back for ongoing process optimisation. Putting the sequence together, he demonstrated how all the elements of the process-tolerance chain added up, and showed how the chain could be optimised and tolerances reduced by direct imaging, optical innerlayer registration and CCD drilling. 

Theme of Session 3 of the conference was developments in soldering and solderable finishes, and the session was moderated by Martyn Gaudion from Polar Instruments in the UK.

Despite concerns about corrosion, electroless nickel/immersion gold (ENIG) remained the most popular metallic solderable finish for PCBs. Chris Klok from MacDermid Performance Solutions in Germany discussed advancements in electroless nickel/immersion gold chemistry, with particular reference to recent studies to characterise, understand and control electroless nickel corrosion effects.

He explained that, in fact, the deposition process for immersion gold on electroless nickel relied on a controlled-corrosion galvanic displacement mechanism, and that too much, or too much localized, galvanic displacement could lead to excessive corrosion and increased risk of solderability failures through weak solder joints. Corrosion levels and acceptability remained a grey area and could be the cause of dispute when solderability failures were experienced. It had been found that three main factors needed to be addressed in order to deliver consistent and low electroless nickel corrosion: control of the electroless nickel deposit phosphorous content, control of the galvanic displacement reaction in immersion gold chemistry, and control of the gold metal thickness and distribution.

Higher phosphorous content electroless nickel deposits had been recognised as having increased corrosion resistance over lower phosphorous containing deposits, but it was difficult to build higher gold thickness if the phosphorus content was more than 10%, and deposits were more prone to gold adhesion failure. Traditional ENIG systems relied on pH control of the electroless nickel plating rate, but as the chemistry aged, increase in pH resulted in a reduction of the phosphorous content which reduced the corrosion resistance of the electroless nickel deposit. MacDermid had optimised the electroless nickel chemistry to maintain a consistent phosphorus content about 9%, and had incorporated surface-active agents into the immersion gold chemistry to inhibit the corrosion effect and improve the uniformity of gold thickness.

Klok commented that the IPC-4552 ENIG specification now included a rating system for nickel corrosion, which would provide guidance and uniformity when corrosion was under dispute and a methodology for process control and monitoring. 

Page 3 of 4

• < Previous Page
• Next Page >