Manufacturers and buyers of LED and ultra-bright LED lighting along with other power electronic products are demanding steady improvements in component performance. On the other hand, component designers have only limited space available for any compelling design to flourish and they also must complete their projects within tight budgets. It is the classic case where success in resolving one large obstacle often presents one or more additional large and equally challenging hurdles.
Sometimes the attention focuses on the component’s PCB or substrate, tasked with delivering improved electrical, thermal, and mechanical performance within a shrinking footprint and with formidable cost constraints. Data sheets, design guidelines and the experience of co-engineering and manufacturing partners can serve to enable designers to optimize component performance, reliability, manufacturability – and lower project costs. Still, many issues linger and many other issues may remain.
For instance, elevated performance requiring added power tends to build thermal loads constantly. The issue is rapidly compounding into a major one involving, most notably, LED lighting and ultra-bright LED substrates.
This paper examines three products borne of the material sciences which are helping LED lighting designers address and resolve ever rising heat loads and heat transfer issues, improve their own team’s heat management efficiency, and attain satisfactory performance levels.
Application Example: Thermal Gap Filler
PCB, substrate and overall LED lighting manufacturing can involve mass production. One thermal interface material geared for LED mass-production applications (Laird Tflex™ HR400 thermal gap filler) is a high performance, compliant and low modulus interface pad conforming to component topography. Its conformability results in little stress on components, mating chassis or parts. Its softness relieves mechanical stress from high stack-up tolerance and absorbs shock, thus resulting in improved component reliability. Its recovery properties for those applications requiring any material rework produce continuing mechanical integrity, even after the component has been re-worked or re-assembled.
The gap filler is naturally tacky on both sides and requires no additional adhesive coating inhibiting thermal performance. The tack is designed to hold the pad in place during assembly and during any transport of the component itself. Tflex HR400 has 1.8W/mK thermal conductivity and performs in temperature ranges from -50° C to 160° C. It is available in ceramic-filled silicone sheets in thicknesses from 0.020-inch through 0.400-inch and meets regulatory requirements including RoHS and REACH. Options include DC1 proprietary tack eliminating coating, or, a fiberglass version in the 0.20-inch and 0.030-inch thicknesses.
Application Example: Thermal Phase Change Material
LED lighting, along with automotive headlamps, microprocessors, chipsets, and laser applications are all primary applications for low thermal resistance, low outgassing phase change materials. One silicone-free, screen-printable phase change material (Laird Tpcm™ 200SP) is designed to meet the thermal reliability and outgassing demands of LED lighting optical applications specifically. It dries quickly to the touch, enabling it to be pre-applied to components for future assembly.
Cost-effective, re-flow compatible Tpcm 200SP aligns well with high-volume manufacturing because of its ease of use. It has a thixotropic index greater than 3, as well as high thermal reliability, as evidenced by the material’s minimal pump out after long-term repeated cycling. It has 20 Pa-s viscosity, specific gravity of 3.2 without solvent, thermal conductivity of 1.5 W/mK without solvent, and a low thermal resistance (0.07 at 10 psi; 0.049 at 20 psi; 0.027 at 50 psi). Tpcm 200SP’s operating temperature range is -40° C to 125° C and the product’s softening temperature is 45° to 60° C. Overall, the material features low total cost of ownership.
Example: Thermally Conductive PCB Substrate
Let us turn to and delve more deeply into growing designer concerns of achieving proper cooling of LED lighting printed circuit board substrates. Ultra-bright LED substrates, along with network DC/DC power converters and battery-powered equipment, are examples of applications requiring significantly higher thermal conductivity compared to FR4. Thermal conductivity is the key to keeping ultra-bright LED substrates and other components cool.
Certain applications require the best possible thermal performance and resistance to thermal cycling. Optimal thermal conductivity becomes one of the designer’s chief goals. Just one example is a thermally conductive printed circuit board substrate (Laird Tlam™ SS 1KA) consisting of a copper circuit layer bonded to an aluminum or copper base plate with Laird’s 3 w/mK 1KA dielectric.
Tlam SS 1KA has eight to 10 times better thermal conductivity for LED lighting applications when compared to FR4. Materials are processed through standard FR4 print and etch operations. The substrate has a UL 746B electrical/mechanical RTI as high as 130° C and is lead-free copper compatible, compliant for low bond stress and RoHS compliant. Tlam boards run through standard pick-and-place SMT and manual wire bond processes. Users quickly discover this solution reduces the stress on solder bonds with ceramic devices. Standard constructions of this substrate are developed with one- or two-ounce copper and 0.040-inch and 0.062-inch thick aluminum. Also available are custom constructions featuring heavier weight circuit copper and thicker aluminum and copper base plates. Equally significant to many, Tlam SS 1KA is environmentally green.
The Broader Perspective
Insulated metal printed circuit board substrates such as Tlam reflect technology originally developed in Japan in the late 1970s as IMST, Today, these substrates are used extensively in LED lighting, automotive electronics, power amplifiers and power supplies as well as motor controllers.
Typically, the substrates simplify LED lighting system architecture, resulting in performance, size, reliability and cost advantages that extend beyond the substrate or board to encompass the complete assembly - and end product.
Their basic construction includes a thin dielectric layer between copper foil tracks and a metal base plate. The advanced technology is found in the dielectric material. It must provide good thermal conductivity and good electrical isolation. Use of superior thermal fillers and resins is essential in quality dielectrics. The goal is achieving exceptional thermal performance with thin dielectrics and with high thermal filler content. The better substrates are built around two basic dielectric systems:
1) 1KA with high thermal conductivity, unique low modulus for severe thermal cycling, and low thermal resistance applications.
2) HTD with high Tg/RTI for high temperature, high-voltage, and fine-line applications. The better materials on the market feature many other secondary electrical and mechanical advantages over alternate materials.
To help reach more goals more quickly, LED lighting designers often rely on design guidelines from manufacturers. The guidelines help users capitalize on any unique performance advantages of the substrates. They normally contain design information for manufacturability including recommended dimensions, tolerances, materials and assembly processes, as well as suggestions for lowest cost system design. Other available application information includes fabrication guidelines and thermal multilayer applications.
Designs always seek improved thermal performance while retaining good dielectric isolation at low cost. The highest quality thermally conductive filler systems are essential. The systems minimize filler content and maintain electrical and mechanical integrity of the dielectric layer. In designing with the more popular substrates, designers know the importance of capitalizing on the thermal advantages without adding unnecessary complexity or costs. Thermal advantages can reduce component size, track width, thermal and mechanical hardware, as well as electrical and thermal interconnects.
Excellent thermal conductivity of substrates has both direct and indirect advantages including:
Improved heat transfer from components that improves component reliability, reduces component size and cost, eliminates component heat sinks and hardware, reduces PCB or substrate size, and increases component density and power density.
Higher current densities in traces, vias, and connectors are possible because a substrate should remove the heat and lower the operating temperature. Standard PCB current density rules are limited by temperature rises. The use of standard PCB rules for substrates can unnecessarily increase the size and cost of your final design. Abide by specific product rules to capitalize on its full advantages.
Better thermal and power management. This advantage applies to the total system. Key factors are maximum power, maximum junction/component temperature, maximum dielectric temperature, maximum ambient temperature, and the thermal resistance of all links between the heat source and ambient. In complex systems with multiple power sources and heat paths, Finite Element Analysis or thermal testing are the only ways to make an accurate final thermal assessment. In higher power applications or higher power modules, the heat is typically transferred by conduction to a heatsink or a metal mounting surface. The temperature of that surface may be known or can be calculated as a function of power dissipation, size, shape, ambient temperature, and air flow. These parameters are application specific. Be sure to obtain application assistance for special products and applications.
Improved isolation of dielectrics. The dielectric strength or dielectric isolation voltage is a measure of the substrate’s dielectric’s ability to withstand high voltages between copper foil and base plate on the substrate, as well as between foil layers or final construction of substrate.
Thermal Via Applications. Multiple vias between copper foil layers can significantly enhance thermal conductivity between those layers. Thermal vias can be much more effective in substrates than in standard PCBs, because the former can provide a means to transfer the heat from the lower layer to a heatsink, bracket or ambient. The improved thermal dissipation not only cools the vias and tracks, but also significantly reduces the thermal resistance for power devices soldered to the upper foil pads. The technique is useful in removing heat from both packaged devices and chips, and is especially effective in complex, multi-layer board applications, such as single board computers and motor drive boards. Thicker copper foil or soldered copper heat spreaders can greatly increase the effective area of heat transfer.
LED lighting design continues to face myriad challenges. Collaboration and product innovations resolving mounting thermal management issues are the correct steps along the road to success.