Electronics Protection - Winter 2015 - (Page 8)

Feature Thick Print Copper Technology Increases Thermal Reliability Ryan Persons, Technical Services Engineer, Heraeus Electronics Dr. Paul Gundel, Product Manager, Heraeus Electronics To meet the increasing demands of the power electronics market for high reliability under harsh environmental conditions, designers are striving to maximize the thermal performance of high-power applications. Currently, direct bonded copper (DBC) is the principal technology used for building these circuits. However, DBC poses several disadvantages in the circuit building process. DBC has weaknesses in its thermal mechanical reliability and lacks the flexibility to allow multiple copper thicknesses for power and signal functions in the same design. Recently, an alternative technology, thick print copper (TPC), has been developed that utilizes screen-printable copper pastes on a variety of substrates including Alumina (Al2O3), Aluminum Nitride (AIN), Zirconia Toughened Alumina (ZTA), and Beryllia Oxide (BeO). When processed, these materials look and perform similar to DBC, but demonstrate excellent reliability and design flexibility. Thick film technology is especially useful for applications requiring high reliability under harsh environmental conditions such as aggressive or corrosive chemicals, extremely high- or low-temperatures, and mechanically stressed circuits. The robust characteristics of thick films make them well suited for demanding applications in automotive, power and outdoor lighting electronics where high reliability is essential. The benefits of the TPC system have been shown through data generated during passive thermal shock tests in comparison to high-end DBC. Thick print copper technology also offers the possibility of replacing high-end MCPCB technology through the use of higher thermally conductive dielectric substrate materials such as Al2O3 and AlN. These advancements will allow designers to drive LEDs harder and ultimately produce LED modules with higher power densities. Screen Printing Advantages Thick film materials are most widely used in screen printing electronic circuits. The thick film paste can be printed on various substrates such as ceramic, metal or glass. A typical thick film paste has conductive, resistive or insulative properties which are chosen depending on the end application. During the screen printing process, thick film pastes are dried to evaporate the lower vaporization solvents at temperatures less than 150°C. The printed thick film layer then enters a high-temperature firing step (500°C - 1,100°C) to sinter and form a bond to the substrate. Firing results in a dense body which acts as a signal controller (conductors or resistors) or isolator (dielectric or overglaze). This process can be repeated multiple times, alternating a conductive layer and a dielectric layer, to create a multilayer thick film circuit. Circuits built using thick print copper (TPC) technology provide a more reliable alternative to DBC, direct plated copper (DPC), or MCPCB substrate technologies for high power applications. As with standard thick film materials, the copper paste is screen printed on the substrate and dried at 125°C for 10 minutes; followed by firing at 900°C to 925°C in a nitrogen-blanketed muffled furnace. Through repeating the above steps, fired-film thicknesses Figure 1. Typical steps followed to achieve up to 300 microns can be achieved. By varying the screen design on differmultiple thicknesses on the same substrate ent layers, multiple thicknesses can be achieved for different parts of the through the use of TPC thick film matericircuit (Figure 1). als and standard screen printing processes. To create a circuit layer, DBC and MCPCB use copper foils at a specific thick- Thru-hole connections can be made for the option of two-sided boards via stencil printing ness. The layer thickness is constant across the entire substrate and cannot be a compatible high viscosity, copper, thru-hole easily varied. For DPC, the copper layer thickness is built up through the plating thick film material. process to one consistent circuit layer thickness across the entire substrate. With TPC, the copper thickness on a circuit can be varied easily by screen printing. Each layer is individually added, allowing for varying thicknesses by selective deposition, as illustrated in Figure 1. The versatility of TPC provides design engineers with more flexibility and quicker design optimization. TPC substrates utilize the same assembly materials that DBC, DPC and MCPCB use including Pb-free solders, conductive epoxies, wirebonding materials (Au, Al, and Cu), ribbon-bonding materials (Al and Cu) and silver sinter materials with very little optimization needed. 8 Winter 2016 * www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Winter 2015

Electronics Protection - Winter 2015
Editor's Choice
Thick Print Copper Technology Increases Thermal Reliability
Slashing Printed Circuit Board Design Cycle Time Using Real-Time PCB Thermal Analysis Tools
Thermal Management of Electronic Devices Utilizing LHS Materials
Industry News
Calendar of Events

Electronics Protection - Winter 2015