EIPC Summer Conference 2022: Day 1 Review

Estimated reading time: 18 minutes

The theme of the third conference session was material technology for next-generation products. It was moderated by EIPC treasurer Emma Hudson.

Alexander Ippich, technical director, signal integrity and advanced technology with Isola in Germany, discussed comparative testing of insertion loss performance on printed circuit boards.

Four popular ways to measure insertion loss were listed in IPC-TM-650 2.5.5.12A: vector network analyzer (VNA), IBM short pulse propagation (SPP), Intel SET2DIL, and Intel Delta-L. Ippich summarised the details of each, commenting that there is no standardised test structure or coupon and that the results can be influenced by PCB processing and stackup parameters.

But in order to carry out meaningful comparative testing, it is necessary to decide on one test method, use a consistent test coupon, and keep potential differences in PCB processing small (ideally by using the same PCB shop and processes), and create stackups closely matched in dielectric thickness, with similar weave styles, and resin contents and the same copper type.

He showed a practical example, comparing one of Isola’s low-loss laminates with another material in similar eight-layer stackups, with thicknesses and parameters as closely matched as practicable. A second example compared single-ended stripline losses at 28GHz, plotting dB per inch against line width for various grades of Isola’s halogen-free material. A third example compared insertion loss for surface microstrips on RF/MW materials. 

Dr. Manfred Huschka, vice president of AGC Group’s RF Business Unit, discussed base materials with high electrical and mechanical reliability as enablers for millimetre-wave PCBs.

He began by making it clear that in vehicle-to-vehicle communication systems, advances in one area rely on technological growth from other areas of the supply chain. 

Substrates for millimetre-wave PCBs require the lowest dielectric loss and the lowest moisture absorption, together with the best copper insertion loss and lowest copper foil surface roughness.

PTFE laminates have the lowest dielectric loss of all PCB laminates. Huschka showed graphs of Df and Dk vs. frequency for a proprietary ceramic-filled PTFE composite in a microstrip ring resonator, which showed Df approximately 0.002 and Dk approximately 2.96 at 100GHz. These values could almost be reached with the latest developments in thermoset resin laminates. 

The higher the frequency, the more the insertion loss increases with increased moisture absorption of a laminate, whereas effective Dk decreases. Clearly, laminates with the lowest moisture absorption perform best. Huschka listed moisture absorption figures for three substrate types: unfilled PTFE laminates 0.03–0.03%, ceramic filled PTFE laminates 0.05–0.07%, and PPE/PPO laminates 0.11–0.19%.

Regarding copper insertion loss and copper foil surface roughness, the lower the copper foil treatment profile, the less the treatment becomes embedded in the laminate surface during the bonding process, so there is less to get etched out. Therefore there is less time for the etchant to attack the sidewalls of the circuit traces, so these remained steeper, resulting in better insertion loss. Huschka demonstrated the effect by plotting insertion loss vs. frequency for a low loss laminate clad with ultra-low-profile, rolled-annealed, reverse-treated, and very-low-profile copper foils. 

He completed his presentation by summarising the mechanical and thermal properties of AGC’s dimensionally stable low loss laminate.

The next contribution to the material technology session came from Andreas Folge, 5G OEM Marketing Europe for Nan Ya Plastics Corporation, using the example of 5G as the basis of his discussion of laminates for high-tech requirements.

He introduced 5G as the fifth generation mobile network, a global wireless standard invented by the 3rd Generation Partnership Project (3GPP) in 2018. The three main classes of applications for 5G were Enhanced Mobile Broadband (eMBB), massive Machine Type Communication (mMTC), and Ultra-Reliable Low Latency Communication (URLLC).

Although in the short term, much of the 5G network infrastructure deployment will use sub-6 GHz frequencies, millimetre-wave systems will grow in the medium term and the higher frequency systems will require greater innovation and material developments. Some key words for copper-clad laminates are “low loss,” “ultra low loss,” and “high layer counts.”

So how to develop materials with excellent electrical performance? The basic requirements are for a low-polarity resin with a symmetrical structure, spread cloth with low-Dk glass, and very-low-profile copper foil.

His graph of Df vs. Dk for different resins showed the trend to higher operating speed and lower signal loss as the chemistry moved between the extremes of epoxy and PTFE, passing polyimide, cyanate ester, polystyrene, polypropylene, polyethylene, polybutadiene, and polyphenylene ether along the way.

He demonstrated the difference between standard weave and spread-weave glass cloths, the latter having more uniform Dk distribution and better anti-CAF properties. And whereas standard E-glass had Dk of 6.5 to 7.2, and Df of 0.006 to 0.008 at 10GHz, the corresponding values for low-Dk glass were 4.6 to 5.0 and 0.003 to 0.004.

He also showed how improvements in the surface profile, microstructure, and barrier design of the copper foil improved signal transmission speed, decreased electron scattering and optimised insertion loss, heat performance, and oxidation resistance.

Nan Ya has the benefit of producing all of its copper-clad-laminate raw materials in-house, with complete upstream vertical integration, and 5G has driven the development of new laminates for millimetre-wave applications as well as its material ranges directed at high-layer-count, automotive, HDI, and IC substrate technologies. Folge listed the general properties of their PTFE, polyphenylene ether, hydrocarbon, halogen free and phenolic materials in terms of Df and Dk.

The industry was at the early stage of the 5G business cycle. In 2020-2021 the strongest growth segments were in networking, cloud computing, data centres, gaming, and smart consumer electronics. Wired and wireless infrastructure equipment became a key driver for market transformation, and there was more and more focus on special multilayer boards and high-end packaging substrates driven by high-speed, high-frequency, and high-thermal demands. 

5G offered growth opportunities: Folge commented that 5G-related infrastructure and device demand will continue to increase further worldwide, with the pandemic giving additional momentum as a consequence of remote working, teaching, learning, gaming, shopping, etc. Market segments such as data centre, networking, PC, wired and wireless infrastructure, 5G smartphones and portable devices remain strong, and automotive and industrial applications will follow once the 5G infrastructure in Europe is fully deployed. 

The final presentation of the session came from Alan Cochrane, president, North America at ITEQ Corporation, discussing a new high-resin-content construction for loss improvement. 

The starting point of Cochrane’s line of reasoning was to question the assumption that the Dk and Df values of core and prepreg were the same. Clearly they would actually be different due to their different resin contents. And when the Dk/Df difference between glass cloth and resin was larger, the phenomenon would be more obvious. If the resin contents of core and prepreg were different, this assumption would cause the simulation result to be distorted 

He used the equations: 

Dk_composite=Dk_resinx PercentageResin+Dk_glassx(1- PercentageResin) 

and

Df_composite=Df_resinx PercentageResin+Df_glassx(1- PercentageResin) 

to calculate the relative Dk and Df values for core and prepreg and to demonstrate that when cores and prepregs of the same thickness have different resin content, they also have different Dk and Df. He showed the results graphically at 10GHz.

He also showed some examples of high-resin-content ultra-low-loss core materials, single-ply 3 mil and 4 mil, and two-ply 8 mil, on 1035, 1086, and 1078 glass styles, with resin content ranging from 68% to 78%. These were used in an experimental loss-comparison with equivalent standard-resin-content materials in a 12-layer stackup. A series of simulation experiments was also carried out. 

The results demonstrated the benefits of the high-resin-content stack-up, particularly that board thicknesses can be reduced while keeping the same loss performance, and that crosstalk can be reduced. The aspect ratio of plated through-holes was also reduced, with yield benefits in drilling and plating. And half-ounce copper can be replaced with one-ounce for better loss performance.

Following the busy conference programme, the first day was rounded off with a privileged visit to the headquarters of Ericsson Mobile Communications in Kumla, followed by a splendid networking dinner at the Restaurant Frimis Salonger, back in Orebro. 

With grateful thanks to Alun Morgan for the excellent photographs.  

Pete Starkey is a technical editor for I-Connect007.

Continue reading Pete's review of Day 2 of this year's conference.

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