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Metal in the Board: A Return to the Future
September 26, 2012 |Estimated reading time: 12 minutes
Editor's Note: This article originally appeared in the June 2012 issue of The PCB Magazine.
Printed wiring boards replaced wiring for point-to-point interconnections in electronic circuits. As more and more functionality has been embedded in solid-state devices, board technology has proven to be adaptable, not only enabling the interconnection of the many different types of components and packages that have evolved, but also providing controlled environments for managing signal characteristics as well as a robust and versatile mounting platform.
Now, electric power is moving into the solid-state/surface mount arena and the PWB is once again rising to the challenge, providing conduction for hundreds of amperes and dissipation of dozens of watts of power through adoption of technologies that can be grouped together as metal in the board (MiB), as shown in Figure 1.
Figure 1. High-current assembly. (Source: Andus Electronic)
The study of the interrelationship between device packaging and interconnection has provided some very useful early warning insights as the industry has transitioned from analog to digital and from pin-in-hole to microBGA and beyond. The combination of package developments in power applications and a focus on energy efficiency is giving rise to major growth opportunities for MiB around the world—opportunities such as motor control and power management where the current and/or thermal challenges are substantial. At the same time, MiB is also enabling handling of heat and power on much smaller scales.
The thermal dissipation capabilities in particular are making MiB an essential contributor to the solid-state lighting (SSL) revolution. Recent developments in LEDs include orders of magnitude improvements in light output per watt, and dramatic increases in working lifetime are transforming how light is designed, produced, and managed. This is a story of astonishing growth as LED technology surges through illumination markets. Developed over the last century or so, as incandescent bulbs became more efficient and electricity more generally available, these well-established markets are set to be radically transformed as the nature of the source itself shifts from a single bright bulb to distributed, intelligent networks of light. "Distributed, intelligent" implies opportunities for the interconnects necessary to control and power the network, i.e., opportunities for MiB technologies.
This story is seen repeating itself in automotive, E-mobility, green power, wireless communications, industrial controls and power management applications. Each of these is an exciting marketplace that needs the combination of digital control and enhanced thermal/power management that is provided by MiB technology. The wide range of characteristics and requirements that these markets represent is being matched by an expanding array of MiB techniques.
Let's take a look at one or two MiB approaches of the more than a dozen BPA has defined to illustrate the challenges presented to fabrication and examine how each of these various MiB techniques fit the needs of rapidly growing markets.
Insulated metal substrate (IMS) technologies have been around for a long time, servicing niche applications where the circuit demands could be met by a single layer of interconnections and the board itself could provide the heatsink. LED applications are changing this, with substrate area expected to surpass 1 million square meters per month within five years. IMS approaches are also diversifying in response to thermal, power, and interconnection challenges, and these developments are examined in some detail in BPA's imminent “Metal in the Board” report.
One type profiled is the basic construction consisting of a metal baseplate (typically 1.5mm thick type 5052 or 6061 aluminum, or copper sheet for special applications) with a copper circuit layer bonded to the aluminum using a glass-reinforced, FR-4 epoxy dielectric. This configuration, now found extensively for mounting medium power LEDs and simple motor controllers, is the lowest-cost version of the IMS series, as shown in Figure 2.
Figure 2. IMS board, front and back view. (Source: Shirai Denshi)
Although the structure of the board is simple, fabrication is not. In comparison to conventional PWB manufacture, an IMS substrate weighs nearly twice as much as FR-4, and machining the aluminum base requires considerably different tool geometries, feeds, and speeds from epoxy-glass, as do the alumina-filled higher conductivity dielectrics used to obtain lower thermal resistance in other IMS types. The aluminum base needs to be protected through chemical processing, requiring a masking step which adds cost whether automated or manual. The advantages of the IMS approach are mechanical robustness and excellent substrate dissipation characteristics. However, while meeting the demands of a large part of the massive LED business analysed in the report, this version leaves a number of requirements unmet that offer higher value-added opportunities to PWB fabricators. What BPA calls "discrete wiring" is an interesting case in point.
Discrete Wiring: DWPWB in the BPA MiB Classification
This MiB type builds discrete elements, in the form of copper strips or wires, into the board structure to provide both current carrying capability and potential pathways for thermal conduction.
Figure 3. DWPWB section.
There are at least two discrete wiring technologies currently available in production quantities: WireLaid from Schweizer Electronic using basic technologies from Jumatech GmbH, and HSMTec from Häusermann GmbH (Austria). These techniques bond wires or copper strips to copper foil or foil-clad innerlayers to provide selectively enhanced power management.
The WireLaid technique bonds silver-coated copper wire or strips to copper foil, which is then laminated together with prepregs and inner cores to form a multilayer structure.
Häusermann's proprietary HSMTec process can bond unclad copper wires or strips to either copper foil or inner cores. The buried wires or strips are accessed by arrays of blind or PTH thermal vias, or through selective removal of the dielectric.
The selective nature of the discrete wiring approach enables medium current capability (60 - 100A depending on the technology, geometry of the discrete wire sections, and allowable temperature rise) between individual nodes on the board, limiting the added functionality to only those nets where it is needed. The increased copper cross-section also serves as an effective thermal pathway. Both strips and wires provide the mechanical characteristics necessary to support formed applications, where tabs or sections of the board can be bent into position during assembly. Figure 4 shows a close-up of an HSMTec board formed to support the optical requirements of a street-lighting application.
Figure 4. Formed section of DWPWB LED assembly. (Source: Häusermann GmbH)
Apart from the manufacturing know-how involving positioning, metalbonding, and encapsulation processes necessary to precisely embed the discrete wire components, the additional functionality offered by this technology requires a different approach to board design, layout, and stackup. This offers a further opportunity for skilled manufacturers capable of supporting their customers with value-added design engineering services.
MiB Characteristics
The DWPWB presents the following advantages:
- Standard materials and processes are used in most process steps
- Embedded copper profiles provide discrete thermal management pathways
- Multilayer boards provide flexible design solutions
- Power and control electronics can be combined on one board
- Simplified assembly
- Formable double-side or buried plane constructions can be bent (semi-flex)
- More compact than rail or grid designs
There are also a few disadvantages:
- The convective heat dissipation capabilities of the board are limited
- Encapsulation of thick embedded strips and wires requires specialized processing
- Serial process/discrete wiring has to be placed/bonded one at a time
- Requires external heatsink for optimum thermal performance
Where could DWPWB be used?
The selective nature of the MiB structures in DWPWB recommends it for the following applications:
- Mixed power and control circuitry
- Controlled HB (high brightness) LED (color arrays)
- Formable HB LED arrays for PAR lamps, streetlights
- Intelligent motor drivers
- Drive control amplifier
- Mixed industrial and test electronics
- Frequency inverter power supplies
- Servo drive power amplification
These characteristics define potential markets, and BPA has done this for all the developed MiB types. Confining the discussion for the moment to automotive, DWPWB lends itself to applications where the formable characteristics of the substrate may be used to decrease overall sub-assembly volume and/or increase functionality, such as electronic power steering (EPS), position sensing transducers and logic, and intelligent, decorative and safety external lighting.
While based on time-tested materials and processes, many of the technologies covered in the report are recent developments and are designed to address future program needs as defined by OEM and Tier 2 industry roadmaps. BPA expects DWPWB to see an accelerating level of interest and program wins over the next year or two as the benefits of the technology meet the needs of industry. In the case of success with EPS, DWPWB penetration is expected to be less than a million units in 2016, but then ramping to a significant portion of the worldwide market by the end of the decade as the technology proves itself a cost and performance effective alternative to current module/board assemblies.
Product development and NPI program activity for DWPWB in the lighting sector holds promise for commercial introductions within the next 18 months, followed by a cascade effect as the technology proves itself. BPA thinks that, by the end of the decade, DWPWB will take a small but established position in the then-massive LED marketplace. Specialty applications will be overshadowed by the huge bulb market, but with overall volumes measured in billions of units, the "small" share which emerges from the forecast model is a significant opportunity.
MiB: Technologies Follow the Applications
The exciting new applications of this decade are changing the issues that define the potential markets for PWBs. This holds true as well for the necessary thermal and power management characteristics.
For example, in SSL, the solid-state device, which is the LED itself, does not need heat to generate light. The energy necessary to cause electron excitation and photon emission is not provided by heat, as occurs across the resistive filament of an incandescent lamp. Light-emitting diodes produce photons through electroluminescence: the recombination of electrons with electron holes within a suitable semiconductor material. Heat is a by-product of the production of light, in addition to thermal buildup due to the small percentage of light refracted within the LED chip, the semiconductor junction at the heart of the LED has a discrete resistance causing a voltage drop and heat buildup according to Ohm's law.
For LEDs, the power-to-light relationship is much better balanced compared to incandescent, where some 80-90% of input energy is dispersed as heat. Nonetheless, heat is generated and the efficiency and lifetime of the LED device is strongly influenced by the temperature at which it runs. Incandescent bulbs get very hot—anyone who has tried to change a freshly burned-out bulb can testify to this—but the primary thermal pathway is convective across a large area (the bulb itself). Since the heat generated by LEDs is primarily resistive and originates in a very small area, more efficient conductive pathways are needed to get the heat out. Conduction, in which thermal energy flows through solids or liquids similar to electrical energy, can be much more efficient than convective methods and is required when cooling an LED device, as shown in Figure 5.
Figure 5. Board level conductive pathway. (Source: Bergquist)
Lower temperatures mean longer life and better operating efficiencies. For example, illuminating a freezer ensures an excellent life for an LED device—possibly 50,000 hours-plus, equivalent to more than five years at continuous operation. At the same time, the efficiency and lifetime of the LED will also depend on the current loading.
On the West Coast of the U.S., Cree is driving the light output per device ever upwards, taking the lead in luminous output for HB LEDs, while Philips, the major lighting supplier worldwide with a reputation built on the lifetime performance of their products, seems to be taking a more conservative strategy. The replacement market for traditional incandescent bulbs—measured in the billions of units—is in its infancy, marked by a number of different design approaches with widely varying numbers and ratings of devices. About the only thing in common is the E27 base. Mounting methods share one universal requirement—to get the heat out—and providing the most efficient pathway is leading to a proliferation of the "IMS" MiB category.
Current output capabilities expressed in lumens per watt have led to the definition of what might be called the "70-watt barrier," where generating a luminous output equivalent to an incandescent 70-watt bulb in the same volume as the incandescent produces a level of heat flux requiring inventive ideas in design of the conductive thermal pathway. The power densities necessary today to achieve 70-watt-equivalent luminosity (about 12 watts) have led to development of active cooling approaches. One contender, Switch Lighting from San Jose, California, is demonstrating exciting ingenuity by immersing the LED array in a transparent cooling liquid that is contained in the bulb.
The market forces are massive; with more than 90 million 75-watt bulbs sold per year in the U.S. alone, and a population of sockets measured in the billions worldwide with growth rates driven by upwardly mobile populations, the pressure is on to replace inefficient incandescent bulbs. Twists and turns can be anticipated as the technology evolves, with the MiB opportunity changing with each move.
Electromobility
Another MiB opportunity set to explode is electromobility. The development of the lithium battery and improvements in electric motor technology are enabling the design of ever higher-performance electric bikes, scooters, and motorcycles. The low cost, flexibility, and non-polluting characteristics of these modes of transport have stimulated the development of massive markets. Already there are hundreds of millions of electric bikes in use in China. The continuing concentration of the population in urban centers, together with strong political pressure to reduce pollution and carbon footprint, will continue to drive these markets around the world. For smaller bikes of 200 watts or less, both drive-circuit complexity and power management are not issues requiring MiB, but for higher performance bikes, electric scooters, and motorcycles, the characteristics of MiB which integrate power control, power management, and thermal dissipation with compact electronic packaging methodologies will be essential.
Figure 6. MiB power module.
Printed wiring board technologies have accompanied the transition of electronics from analog circuits to the microelectronic world of digital logic ("Who needs a current when one electron would do?") Now, electronic systems are increasingly more concerned with power in an expanding array of exciting new markets developing out of the need for greater energy efficiency and mobility. Automotive and electro-motive technology, green energy, industrial and power control, wireless communications—most of these markets are building on the control capabilities of digital systems to achieve the expected improvements in performance, presenting challenges of managing both single electrons and currents cost effectively within the same system or subassembly. In most cases, each represents a completely new market for MiB technologies that provide enhanced thermal and power management capabilities.
Owner and Principal of LPC Ltd, William (Bill) Burr collaborates with BPA as senior consultant. A Photocircuits alumnus, Bill was CTO of one of Europe's top ten for over 20 years and a pioneer of copper direct laser technology. As Past President of the EIPC and Vice Chairman of EFIP, Burr continues to contribute internationally to the electronic packaging and interconnection industry. Contact Burr at w.burr@bpaconsulting.com.
Nick Pearne co-founded BPA in 1977, focusing on microelectronic packaging and interconnections, and providing valuable insights to the industry in anticipation of the major changes that have swept microelectronics throughout the past 30 years. In addition to his responsibilities with BPA, Pearne is a director of several companies primarily involved in technology marketing and business development. Contact Pearne at n.pearne@bpaconsulting.com.