EIPC Technical Snapshot: Business Outlook

Estimated reading time: 10 minutes

Impact of Millimeter-wave 5G on PCB Interconnection Technology
Erich Schlaffer, R&D programme manager with AT&S in Austria (specialists in high-tech PCBs and IC substrates), gave a pragmatic introduction to the probable impacts of millimeter-wave 5G on PCB interconnection technology.

Why use 5G? Schlaffer sees it as a technology enabler and an essential innovation driver for tomorrow’s megatrends, although it will involve a fundamental transformation in IC packaging and substrates and have a high impact on existing interconnection concepts.

Successful implementation of 5G requires a fundamental transformation in PCB technologies, combining developments in thermal management, loss reduction, miniaturisation, and integration. The transition from sub-6 GHz to millimeter-wave 5G will necessitate enhanced double-sided organic or ceramic-based systems-in-packages and the integration of antennae, front-end-modules, power amplifiers and radio-frequency ICs, together with new low-loss materials and advanced EMI-shielding.

Schlaffer showed an example of a side-facing 5G millimetre-wave transceiver/antenna-in-package module from the iPhone 12, with a 16-layer any-layer stacked PCB consisting of eight high-density layers with 50-micron vias and 40-micron line-and-space for routing 150-micron pitch flip-chip die, plus eight low-density layers with 100-micron vias for antenna structures.

He explained the structural shift from the 4G LTE standard to millimeter-wave 5G. Whereas the earlier concept was to have two standalone PCBs—an antenna board and a digital multilayer, linked by cables and connectors for millimetre-wave 5G frequencies of 28-39 GHz and beyond—the two PCB functions need to be integrated into a single module with a more sophisticated form of interconnection “because cables and connectors don’t work anymore!” His illustration showed an integrated radio front-end module for a massive-input-massive-output active antenna system. The main radio-frequency board had integrated active and passive components, with SMT components on the external surface. The antenna board and the main board were interconnected both electrically and thermally in the Z-axis using silver sinter-paste.

He described a series of alternative enabler technologies for module integration—embedded component packaging, 2.5D cavity and suspended stripline solutions—and discussed various heat-spreading techniques. His concept example for a 5G radio base station showed a high-layer-count multilayer of 3.2-mm finished thickness on halogen-free high speed base material with an asymmetric build up, embedded gallium nitride active components, thermal pads in the Z axis and the antenna on top. The concept example for an integrated RF front end was a symmetrical build up with 20-micron lines and spaces, 50-micron laser microvias and 120-micron component pitch, and demonstrated a heat-spreading concept for an embedded RF chip, using a pyramidal array of copper-filled microvias cascading outward through the layers.

Schlaffer’s presentation provoked a very active discussion in the Q&A session. It is clear that PCB fabrication for millimetre-wave 5G applications will present significant challenges to manufacturers and material suppliers, and that only a few specialists are yet fully equipped to engage in the technology.

Taiyo America: Solder Mask
The final speaker was Taiyo America’s Midwest sales manager Don Monn, renowned for his entertaining and informative presentations at EIPC conferences. He gave an enlightening appraisal of the history and current trends in solder mask, first winding-up his audience by implying that, although they couldn’t see him, he could see them and what they were up to, prompting some anxious checking that their video was disabled!

He set out to answer the question: “What have and what do people want from solder mask?” He listed improved registration, faster processing, robustness to final finish chemistries, OEM acceptance, a rainbow of colours and surface finishes, high reflectivity and colour stability of white formulations and, unsurprisingly, cheaper.

The history began with his comment, “Years and years ago, when many of us were younger...” as he related it to screen printing. Liquid photoimageables were introduced in the late 1980s, followed by laser direct imaging, then direct imaging with multiple-wavelength UV-LEDs, and culminating in inkjet application, which Monn considered the direction for the future.

He reviewed the limitations of traditional screen printing—registration, uneven thickness, skips, smudging, bleed-out, operator fatigue toward the end of the shift—and explained how these had been progressively overcome by newer technology developments. The elimination of artwork by direct imaging was a major advance. The cooperation between ink formulators and equipment manufacturers to optimise the relationships between exposure wavelengths and photoinitiators was another. One remaining shortcoming of coating processes for liquid photoimageables was “mask in holes” because the material was applied over the whole panel then selectively removed by a photographic developing process, and difficulties could be experienced in removing residual material from holes.

Although 40% of solder mask currently used in North America was direct-imaged, either by laser or LED, there was a growing trend toward inkjet imaging. Not only did inkjet offer a fully additive process with the benefit of placing solder mask exactly where it was needed with no waste and the option of varying the thickness as required, it offered a much shorter process sequence, and mask-in-holes problems were avoided.

Monn concluded that the advantages of inkjet printing were too numerous to ignore, and that both PCB fabricators and OEMs would benefit from the trend, particularly as board values were increasing and yields were more important than ever. He noted that inkjet installations had doubled in each of the last two years.

In addition to the developments in application and imaging techniques, there were ongoing advancements in the physical properties of solder masks; examples being the ability to withstand higher operating temperatures and the capability to act as heat dissipators. 

Alun Morgan moderated the busy Q&A session, and then thanked the audience for their attention and the speakers for their excellent presentations. In particular, he acknowledged Walt’s contribution of over 50 years to the industry and his long-term commitment to the EIPC as board member and keynote provider of market research, business analysis and forecasting. Walt assured us that he really is retiring now, and we sincerely wish him well.

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