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Archive December 2020

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INEMI PROJECT REPORTS BENCHMARK NEW AND EMERGING TEST AND MEASUREMENT METHODOLOGIES FOR 5G/MMWAVE MATERIALS

The transition of 4G LTE based communication to 5G/mmWave-based protocols is expected to drive disruptive changes in the communications industry. For mmWave product designers it is critical to start with correct electrical material models. Fast, easy and accurate methods for characterizing materials at mmWave frequencies are critical for enabling functional designs on the first cycle. To date, however, there is a lack of standard reference materials and characterization test methods for materials at mmWaves.

iNEMI members organized the 5G/mmWave Materials Assessment and Characterization project to address this industry challenge collaboratively. The project is developing guidelines and best practices for a standardized measurement and test methodology that can be shared with industry and relevant standards organizations. The initial focus is to benchmark currently available — as well as emerging — test methods and provide pro/con analyses, identify any gaps for extending test methods to 5G/mmWave frequencies, and develop reliable reference standard materials for setup and calibration.

There are currently 26 members in this project, spanning the entire communications industry value chain. The team has completed significant work to date and has published two reports that are now available to iNEMI members. The reports are summarized in this article.

Report 1: Benchmark current industry best practices for low loss measurements

Each new material for mmWave 5G applications requires careful consideration to determine the best measurement methodology, fixturing requirements, sample fabrication methods and test instrumentation. There are dozens of different methodologies that could be used, but which to choose is often not obvious. Report 1 focuses on the following measurement techniques and their merits: rectangular cavity resonator (RCR), split-post dielectric resonator (SPDR), split cylinder resonator (SCR), balanced circular disk resonator (SCR), and Fabry-Perot open resonator (FPOR). Figure 1 maps these available techniques against frequency.

5G Fig 1-1

Figure 1. Selected measurement capabilities versus frequency.

The report also discusses standard reference materials, outlines best practices and identifies key sources of uncertainty. Significant focus is given to the importance of reference standard material and the need for national metrology institutes, such as NIST, to develop and supply new standards reference materials to the 5G sector. The impact of the existing traceability gap is shown in Figure 2. Proposed standard reference materials considered by the project team are shown in Table 1.
 

5G Fig 2

Figure 2. Traceability gap for complex permittivity of 5G materials.

 

Table 1. Proposed Standard Reference Materials

Material Type

Approximate Thickness

Structure

Approx. Er,Tand

Important Notes

Cyclo olefin polymer (COP/ Zeonex®)

100 um

Isotropic, homogeneous

2.3, 5e-4

Can degrade with finger oil - avoid handling unless using gloves

Cross linked polystyrene (Rexolite®)

700 um

Isotropic,

homogeneous

2.534, 4.6e-4

Easy to machine, difficult to make flat, stable with temperature and humidity

PTFE / Teflon®

 

Isotropic, homogeneous

2.1, 2e-4

Easy to machine

Fused silica

500 um

 

3.8 / 1e-4

Easy to machine, preferred by project team

Sapphire

365 um

Anisotropic (c-plane)

9.4 / 11.6, 5e-5

Hard to machine

Alumina

100 um

Isotropic, homogeneous

9.8, 1e-4

Easy to machine, preferred by project team

 

Report 2: Benchmark emerging industry best practices for low loss measurements

The second report discusses emerging measurement techniques that will become increasingly relevant as new spectrum auctions open up additional communication bands above 5G/mmWaves. These techniques are in the research and development stages and can be commercially deployed for next generation material characterization, especially at frequencies >100GHz. Discussions include two separate wafer-level measurement techniques pioneered at Georgia Institute of Technology and NIST, respectively. It also includes information about time domain spectroscopy-based measurement techniques extending the frequency ranges to higher than 100 GHz to 6G and next generation communication technologies. It is important for industry groups to monitor the new research being pioneered at universities and research institutions.

Ongoing work

The project team will be spending the next several quarters conducting extensive round robin measurements, spread amongst volunteer test sites globally, to assess critical concerns of sensitivity and repeatability of the standard reference materials and provide recommendations to the industry. The project team would welcome new participants who are interested in collaborating and contributing to these developments. If interested in getting involved with the project, contact Urmi Ray (urmi.ray@inemi.org).

INEMI TEAM DEVELOPS TEST TO EXPEDITE EVALUATION OF CONFORMAL COATINGS

Conformal coatings are used in electronics assemblies to protect printed circuit boards and components mounted on them from the deleterious effects of corrosive environments that have high concentrations of gases such as sulfur dioxide, hydrogen sulfide, free sulfur, chlorine, oxides of nitrogen and ozone. Particulate matter with low deliquescent relative humidity (DRH) can electrically short circuit features with potential differences across them by forming low resistance bridges (electrical short circuits) when the relative humidity in the air is above the DRH of the particulate matter.

As data centers and electronic devices proliferate worldwide into geographies with high levels of pollution and high relative humidity, the use of conformal coatings becomes necessary and is no longer restricted to mission-critical and/or military hardware. The decreasing feature size of components is also a factor. With decreasing feature gaps that dust particles and corrosion product particles can more readily bridge, conformal coatings are increasingly needed in today’s designs.

Commercially available conformal coatings are available in a wide range of price points, application methods, and effectiveness in protecting the underlying metal from corrosion. The conventional method of testing the effectiveness of conformal coatings is to expose the conformally coated hardware to a corrosive environment for extended periods of time lasting many months and determine the mean time to failure. This means of testing is both inconvenient and slow. Even where the corrosion of the coated components can be monitored, such as in the case of surface-mounted resistors, it can take more than a year to evaluate a coating and the testing is done under very limited conditions of temperature, humidity and environmental corrosivity.

The iNEMI Conformal Coating Evaluation for Improved Environmental Protection project team has developed and demonstrated a time-saving approach for evaluating conformal coatings. It involves coating thin films of copper and silver and monitoring the corrosion rates of the coated thin films while subjected to corrosive and humid environments. Effective conformal coatings protect the underlying metal thin films well.

Using a modified version of the iNEMI flowers of sulfur (FoS) chamber the team exposed conformally coated thin films of copper and silver to a sulfur gas environment. Performances of acrylic, silicone and atomic layer deposited (ALD) conformal coatings were studied as a function of temperature and relative humidity. 

Testing approach

The thin-film test vehicle (Figure 1) used for evaluating conformal coatings consisted of a serpentine metal (copper or silver) thin film (800nm thick) sputtered on oxidized silicon die 15x15mm. The FoS chamber was modified so that there was no forced air circulation and no chlorine gas in the chamber, only sulfur vapor. The resistances of the thin films were measured using potentiostats to pump known values of currents through the thin films and measuring the voltage drops across them. Thin film temperatures were monitored using thermocouples attached to a data logger. Resistance and temperature readings were taken simultaneously every 10 minutes over the 5-day period of each test.

Conformal Fig 1-1Conformal Fig 1-2        Figure 1. Thin-film test coupon enables 4-point resistance measurement and temperature
        monitoring via attached thermocouple.

Conformal Fig 2

Figure 2. Psychrometric chart showing the four temperature-humidity test conditions.

Three conformal coatings were tested: acrylic coating 39-45 um thick; silicone coating 100 um thick; and atomic level deposition (ALD) coating 0.1 um thick. ALD coatings are ultra-thin (1-200 nm), stochiometric, dense and highly uniform in thickness. The ALD process is performed in a vacuum reactor at relatively low temperatures, typically 80-300°C, depending on the material deposited and the substrate thermal budget. To characterize/evaluate the effectiveness of the conformal coatings, the corrosion rates of the coated thin films were measured and compared with uncoated (bare) thin films.

Four tests were run with the chamber under the conditions shown on the psychometric chart of Figure 2: (1) 15% relative humidity, 40oC; (2) 15% relative humidity, 50oC; (3) 31% relative humidity, 50oC; and (4) 75% relative humidity, 50oC. The durations of various electrical and temperature conditions are listed in Table 1.

Table 1. Electrical and Temperature Test Conditions for the FoS Chamber at 40oC

Duration, days

Current, mA

Nominal Film Temperature, oC

0-1.85

100

42

1.85-3

200

52

3-4.16

300

64

4.16-5.32

100

42

 

The effect of the chamber temperature is clear: a 40oC chamber environment is less corrosive than a 50oC environment. On the other hand, higher relative humidity makes the air less corrosive. At higher humidity, the sulfur concentration in the chamber decreases because of sulfur vapor absorption by surfaces with higher amounts of adsorbed moisture. Lower sulfur concentration makes the air less corrosive.

Figure 3 summarizes the corrosion rates of copper and silver serpentine thin films coated with acrylic or silicone and compares them to corrosion rates of bare (uncoated) copper and silver films. ALD coated thin film corrosion rates were not included because their corrosion rates were too low — they were within the limits of the experimental error.
 

Conformal Fig 3

Figure 3. Summary of corrosion rates of bare copper and silver serpentine thin films and thin films coated with acrylic or silicone. ALD coated thin film corrosion rates are not included in these plots because their corrosion rates were within the limits of the experimental error.

The acrylic coating protected copper thin films from corrosion to some extent. In a 40oC FoS environment, the corrosion rate of acrylic coated copper was two orders of magnitude less compared to bare copper. Increasing the FoS environment temperature to 50oC increased the acrylic coated copper thin film corrosion rate by approximately an order of magnitude. Of the three humidity test conditions (15, 31 and 75%), the highest corrosion rate of acrylic coated copper was at 75% relative humidity. Acrylic coatings did not protect the underlying silver thin films. The increase in temperature from 40o to 50oC increased the silver corrosion rate somewhat. 
 
The silicone coating provided no corrosion protection to the underlying copper thin films. At 40oC and 15% relative humidity, silicone coating provided some corrosion protection to silver thin films. At 50oC, silicone coating provided no corrosion protection to the underlying silver thin film over the whole range of relative humidity tested. 
 

Conformal Fig 4

Figure 4. Photographs of serpentine thin films after the 5.32-day test at 50oC and 31% relative humidity.

The images in Figure 4 illustrate the extent of the underlying metal corrosion in agreement with the metal corrosion rates plotted in Figure 4:  
  • The ALD coatings clearly provided excellent corrosion protection to the underlying Cu and Ag films
  • Acrylic coating protected copper to some extent but not silver  
  • Silicone did not protect Cu or Ag films
Another interesting observation is that Ag2S whiskers grew on the bare silver thin films and that these whiskers were prevented from growing by the three conformal coatings.
 
Predicting field performance often requires accelerated testing involving higher temperatures and harsher environments to shorten the test times to convenient durations. A downside of harsher conditions is that they may unduly overstress the hardware, thus changing the failure mechanism from one actually occurring in the field. A better approach is to keep the test conditions close to or the same as the field conditions and to shorten the test time by improving the sensitivity of detection of the hardware degradation. The latter approach was taken in the iNEMI study by characterizing conformal coatings based on the corrosion rates of coated thin films that can be measured with down to +/-1 nm sensitivity. As a result, there is no need to accelerate the test conditions. This and earlier studies indicate that the environmental conditions in a flowers of sulfur chamber at 40oC are adequate for testing conformal coatings for many applications. 
 

Conclusions

iNEMI’s study provides additional proof that conformal coatings can be evaluated effectively by the degree of corrosion protection they provide to coated copper and silver films. The corrosion rates of metal thin films can be very effectively and accurately measured by the 4-point technique and by taking into account the effect of temperature on film electrical resistance. The film temperature must be measured using a thermocouple with fine wires.  The film electrical resistance corrected for temperature is inversely proportional to the film thickness remaining.  The rate of decrease of remaining film thickness is the corrosion rate of the film and is used to characterize the corrosion protection provided by the conformal coating. Test environmental conditions in a modified iNEMI FoS chamber at 40oC are aggressive enough for conformal coating testing. The advantage of not using conditions that are too aggressive is that the degradation mechanism in the test is the same as that in the field. This proposed approach will help evaluate conformal coatings much faster and more effectively than presently used methodologies, thereby enabling continuing advances in conformal coatings development.
 

Join us at IPC APEX EXPO 2021

The iNEMI Conformal Coating Evaluation for Improved Environmental Protection project team will present details of their study at the IPC APEX virtual conference next spring (March 6-11). Be sure to join us if you plan to participate.

UPCOMING INEMI PROJECTS AND INITIATIVES OFFER OPPORTUNITIES TO GET INVOLVED

2020 has proved a very successful and productive year for iNEMI project teams. By the end of the year 5 projects will have been completed, while 7 new ones will have commenced, yielding 14 currently active projects. In addition, member-led teams are actively developing new project proposals. iNEMI projects cover a broad range of topics in electronics manufacturing with projects ongoing or in planning in the areas of smart manufacturing, packaging, PCBs and laminates, sustainability, 5G/mmWave, photonics and interconnect. This article highlights new projects and initiatives that are welcoming new participants.

Warpage

In recent years, a number of iNEMI projects have studied warpage in both packaging and PCBs. Understanding the key factors that contribute to warpage and the impact of warpage on assembly processes and reliability is critical in the design and manufacture of electronics products. Building on the measurement and simulation capabilities developed in earlier projects, iNEMI is continuing work on warpage issues through the Warpage Characterization and Management Program. As part of this program, the Package Warpage Prediction and Characterization project will continue WPLP-warpagecharacterization of dynamic warpage of newer types of advanced packages to help enable higher yields in board assembly and assist in developing a reliable modelling framework to optimize package warpage simulation. The High Density Interconnect Socket Warpage Prediction and Characterization project, which includes membership drawn from socket manufacturers, plans to establish socket warpage measurement metrology and prediction methods for larger socket sizes and to investigate the impact of molding, material properties and design on large size socket warpage. 

Connectors & optical interconnect

Expanding on previous iNEMI interconnect projects, which identified gaps in existing standards and connector reliability testing methods, Phase 3 of the Connector Reliability Test Recommendations project, which includes OEM and connector manufacturer participants, plans to define a test vehicle and a more relevant test framework for evaluating ICT electrical connectors across a wider range of use conditions than are presently considered. The objective is to improve the efficiency and effectiveness of connector qualification across the supply chain. This project will start in January.

Optical interconnect is becoming increasingly important to the electronics industry. iNEMI’s latest project in this area is the Best Practices and Guidelines for Use of Expanded Beam Connectors in Data Center Applications project, which launched in December. Expanded beam connectors are less sensitive to contamination and promise cost savings due to reduced cleaning requirements and potentially higher reuse in data centers. The project will investigate the impact of contamination on optical performance, develop recommendations for cleaning processes for single mode and multi-mode expanded beam connectors, and conduct cost modelling to study the potential operational and maintenance cost savings in data centers using expanded beam connectors.

Single mode expanded beam interconnects have demonstrated optical and mechanical functionality for future system architectures. The PIC Chip & Micro Optics Demonstrator for Edge Pluggable Free Space Connector team plans to assess the manufacturing issues of using single mode expanded beam connector interfaces in board-level optical interconnect and to develop manufacturing guidelines for PIC modules and micro-optics by building and demonstrating a board-level optical interconnect system.

5G/mmWave

5G/mmWave technologies are driving developments in a broad range of applications as demand for faster connectivity and data volume grows. iNEMI projects are now looking to enable the electronics manufacturing community to address the associated design, manufacturing and reliability challenges associated with these technologies (e.g., high frequency). Earlier this year, 5G Permittivity-1iNEMI launched the 5G/mmWave Materials Assessment and Characterization project, which now has 26 organizations working together to address the lack of industry standard test methods of Dk and Df. The project plans to develop guidelines/best practices for standardized measurement and test techniques and propose test coupon designs for industry wide application. (See related article.)

The move to higher frequencies is also impacting PCB design and fabrication. The Mitigation of 5G Signal Loss due to PCB Copper Foil Surface Treatments initiative plans to look at one of the key issues. Treatment of copper surfaces to improve adhesion to resin systems can help with the fabrication and integrity of the PCB, but can have a detrimental effect on signal loss and integrity, particularly for high frequency 5G/mmWave applications which require very low-profile copper foil and low-loss resin systems for electrical performance. This initiative, led by OEMs and material suppliers, plans to characterize various copper surface treatments/resin systems and determine the metrology and test methodology to characterize copper-foil-to-resin bond strength so that better choices can be made to optimize both signal and mechanical performance.

Smart manufacturing

The implementation challenges of smart manufacturing are being considered across the industry. Automated methodologies to collect, analyze and use machine/process data are key enablers for the factory of the future. The Data Management Best Practices for PCB Assembly project, which plans to launch in January, will focus on developing and demonstrating a generic reference data architecture and best practices to enable efficient implementation of smart PCB assembly in a manufacturing environment where diverse supplier equipment is used.

Data Mgmt Best Practices

Another area of focus for smart manufacturing is automated optical inspection (AOI). An iNEMI initiative is forming to define a Smart AOI Inspection project to demonstrate existing capabilities and identify gaps and opportunities for leveraging artificial intelligence to optimize AOI in PCB assembly.

Sustainable electronics

In the area of sustainability, our focus is on how electronics manufacturing can achieve a circular economy. There are presently 2 projects being developed to address the capabilities gaps in the industry.

The Eco-Design Best Practices for a Circular Electronics Economy initiative is looking at the lack of a shared knowledge base for innovative eco-design and lack of eco-design training. It proposes a project to grow the knowledgebase across the electronics manufacturing ecosystem by identifying and communicating the best eco-design practices that have the greatest impact by considering a holistic view of the product in society and environment.

Extracting maximum value from existing devices is critical to the circular economy. There is a lack of standard processes and data for extended reliability assessments of electronics systems and components. Being able to select products to be refurbished or re-assembled for longer use would reduce consumption of raw materials and processes and enable the use of components beyond design life which would contribute to a reduction in waste. The Extended Reliability Assessment for Electronic Components initiative team is exploring a project to develop a procedure for extended reliability assessment and component classification.

For information

If you would like information about how to get involved in any iNEMI projects, please contact Grace O’Malley (gomalley@inemi.org).

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