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Special Processing Needs for Thermally-sensitive Designs
December 31, 1969 |Estimated reading time: 7 minutes
By Joe Lynch and Dave Sommervold
Insulated metal substrate is one answer to shrinking form-factor products that clash with increasing power requirements. Effective heat management then emerges as a make-or-break issue.
Thermal management challenges have become increasingly important across a widening range of applications for the basic reason that "watt-density" requirements of many products have escalated well beyond the capabilities of conventional FR4-based printed circuit board (PCB) designs. Advanced thermal management techniques initially came into prominence within power-control and conversion applications in which critical design parameters required packing high-wattage functions into compact devices. For such use, it is vital to minimize the risk of thermal damage to internal circuitry and surrounding components in the final assembly. Additionally, similar challenges are emerging in other important high-watt-density applications, e.g., motor controllers, automotive power modules, etc.
Insulated metal substrate technology has emerged as a key enabling factor for managing thermal properties at a fundamental level by permitting both watt-density and design flexibility optimization. At the same time, the evolution of a new generation of automation-friendly, SMT-oriented interconnect technologies is enabling efficient high-volume production of reliable and cost-effective assemblies that use metal substrates.
Thermal Management Basics Thermal management challenges largely concern the dissipation of heat generated by high-power designs and the simultaneous prevention of damage to closely packed circuitry and components. Also, designs overall must address any differences in the coefficients of thermal expansion (CTE) between various parts of the assemblies. The danger here is of thermally induced stresses that can weaken or break critical circuit traces or interconnect points. Equally important is that effective internal heat reduction is a source of improved product reliability that allows for expanded design "head room" to provide higher functionality, such as more watts per square inch output for power devices.
Conventional PCB substrates such as FR4 inherently are unable to transfer heat efficiently and generally have compelled designers to resort to off-board mounting of heat-generating components, which requires manual off-line assembly. Such thermal management "work-arounds," including rails, heat-sink clips and off-board heat sinks, represent suboptimal extra steps that reduce production efficiency, add unnecessary costs and limit the ability to achieve small form-factor objectives.
It generally is understood that the most effective thermal management begins with fundamentally sound approaches in designs rather than as afterthoughts via add-on devices, which are limited. This means solving probable thermal-control problems at the outset as an integral step in substrate design. Two primary approaches to address thermal issues at the substrate level have used either ceramics or metal-base materials as an alternative to conventional glass-based substrates. However, as the need for heightened thermal management has moved more into mainstream applications, metal substrates are preferred because they are more compatible in higher-volume production environments, i.e., they do not share the fragility and process difficulties associated with ceramic materials. Rather, metal substrates allow for larger-sized panelized board arrays and are more flexible for accommodating various shapes, cutouts, etc. Further, metal substrates can deliver high thermal-dissipation characteristics while approximating the incorporation of many of the high-volume, production-oriented handling advantages of conventional FR4 techniques (Figure 1).
Figure 1. An example of an insulated metal substrate assembly.
Other metal substrate advantages include lower component operating temperatures owing to their capacity to handle greater currents than those of thick-film ceramic materials. The metal units also provide better electrical-isolation characteristics (even with thin dielectric coverings) than that achieved with direct-bond copper (DBC) on ceramic.
High-volume Production and SMTIn addition to managing thermal characteristics, substrate and interconnect technologies must accommodate the intricacies of today's highly integrated designs while meeting the pragmatic requirements of high-volume production environments. Just delivering high-watt densities is not particularly useful unless the devices also can be cost-effectively manufactured and interconnected to other subsections of the module within the overall assemblies. In all cases, the specific requirements of the end applications must drive the design parameters for both insulated metal substrates and their associated interconnects.
With the greater use of metal substrates, the ability to automate interconnect methods has become a major issue. After all, designers continue using well-established interconnect methods, such as wire leads flex soldered to the metal substrate, formed lead frames, card-edge adaptations and custom terminals; all can be labor-intensive and difficult to automate. In fact, because most of today's insulated metal substrate designs necessitate the use of surface mount interconnects, the goal is to develop pure SMT production techniques that can leverage existing automation environments, minimize off-line manual operations and deliver consistent reliability. For example, many DC power conversion devices consist of dual-stacked PCBs that use pins soldered to a metal baseboard, which act also as a heat spreader for the high-wattage power components. The pins then connect to a conventional board, which holds the majority of the logic circuitry, or protrude through the FR4 to act as external through-hole pin connections. The high-wattage characteristic of DC power devices and CTE variations can induce significant stresses to the solder joints. Therefore, consistent (automated) pin alignment is key in ensuring that they provide proper mating of the PCBs and external connections (Figure 2).
Figure 2. The SMT pin design provides a compact footprint, stable solder joints and consistent alignment.
A Concern for RobustnessProviding reliable mechanical robustness also is an important consideration in, for example, automotive or motor-controller applications in which all components and assemblies can be subjected to continuous levels of shock and vibration in relatively harsh environments. Often, automotive power modules must be mounted securely within mechanical structures that may provide neither consistently precise tolerances nor coplanarity. Additionally, the dictates of automotive connector standards require that interconnect designs used in power modules must provide high flexibility for creating robust solder connections to the underlying metal substrates while achieving precise alignment. At the same time, the interconnection methods must leverage existing processes and support efficient high-volume production requirements.
It now is important that SMT solutions designed for use on metal substrates be available. Instead of hand assembly and soldering connections to individual boards, manufacturers must be able to run high volumes of assemblies down standards-oriented surface mount production lines. Because all materials (including metal bases) tend to expand and contract during heat dissipations and thermal management functionings, and because the connectors often represent a board's primary physical interface to external structures, it is critical that the interconnects provide optimal solder joint reliability.
To address that requirement, a new machine-placeable pin design uses a "treadhead" feature to provide improvements in both alignment and pull strength. With traditional (albeit surface mounted) "nail-head" pins, production is hampered by the need to use either inefficient hand-soldering methods with cumbersome fixtures or expensive custom robotics to achieve marginally acceptable yields with adequate alignment. Additionally, nail-head designs typically require an excessively large solder fillet to meet minimal pull-strength requirements.
Figure 3. The treadhead allows for outgassing and an increased solder fillet area, resulting in improved pin alignment and increased pull strength.
By contrast, the new treadhead pins have a specially designed pattern on their bottoms consisting of a round center reservoir with slots extending to the outside edges. The pin-head shape facilitates outgassing during the reflow process and creates a vacuum under the head, which serves to pull the pins tightly into proper alignment on the PCB pads (Figure 3). The design also promotes solder wicking to form compact high-strength solder fillets under the head, which increases solder bond surface area between the pin and the PCB without using excess real estate.
Empirical test results have shown that the new design is capable of automatic self-alignment of pins placed as much as 50 percent off-center from the pad. Tested soldered pins consistently provide more than seven pounds of shear strength and 40 to 50 lbs of pull strength.
A "bow-tie"-shaped foot has been designed to accommodate the tighter board real-estate constraints becoming common in many metallized substrate applications. Such a design can provide a stable foundation on small land pads and promote solder wicking for optimal solder fillets concentrated around the base of the pins. The result is consistent pin-to-pad alignment and high solder joint strength with a minimal horizontal profile to conserve scarce board space, allowing for interconnects especially appropriate for smaller diameter "data pin" connectors (Figure 4).
Figure 4. The SMT pins use a bow-tie shape to provide a stable foundation on a small pad.
Because they can be manufactured using a cold-forming process rather than machining, the new pin designs permit copper as the primary material to further improve conductivity and heat dissipation.
Cool Running and ManufacturableAs metal substrate technologies move further into the mainstream and the range of applications continues expanding, it is clear that comprehensive design approaches are necessary to address new and emerging thermal management issues. System designers will need to work closely in conjunction with both substrate manufacturers and interconnect suppliers to minimize conflicts and to assure smooth interoperability of technologies.
Ultimately, the resolution of thermal management challenges for any design must be handled on a case-by-case basis using a blend of substrate and interconnect technologies appropriate for both product specifications and production strategies. By working with innovative technologies that combine design flexibility and broad production process adaptability, designers of power devices, automotive power modules and motor controllers can assure efficient and compact product designs that are cool, reliable and highly manufacturable.
Joe Lynch may be contacted at Autosplice Inc., 10121 Barnes Canyon Rd., San Diego, CA 92121; (858) 535-0077; Fax: (858) 535-0130. Dave Sommervold may be contacted at The Bergquist Co., 18930 W. 79th St., Chanhassen, MN 55317; (952) 835-2322; Fax: (952) 835-5080.