Power Dev Power Dev' - Oct 2012 : Page 10

OCT OBER 2012 ISSUE N°7 Y OLE ASKS Silver sintering materials are a kind of mAgic Thomas Krebs, head of development for sintering materials at Heraeus Materials Technology in Hanau, Germany, explains what the company’s silver-based mAgic die-attach products bring to power electronics. Yole Développement: Can you tell us what Heraeus’ mAgic sinter materials are? Thomas Krebs: The mAgic product family contains two product groups, sinter adhesive and sinter paste. The sinter adhesive is an epoxy-based conductive adhesive providing electrical and thermal conductivity in the range of a lead-free solder paste. The sinter paste provides a pure silver connection after processing. This die attach connection provides the advantages of silver, like its high thermal and electrical conductivity as well as a melting temperature of 961°C. YD: What are the key die attach challenges in power electronics packaging today, and how do Heraeus mAgic sintering materials meet them? TK: In comparison to solder, which is usually used for the die attach in DBC modules, the mAgic materials provide a marked increase in lifetime. Solder fatigue worsens thermal performance of the die attach layer over the lifetime, which also increases degradation of wire bonds, as they lift off from the die. This occurs faster at higher operation temperature. For conventional solders a reasonable lifetime can be reached at operation temperatures of 125°C. Improved solder alloys like SAC-X can be used at operation temperatures of 150°C, maybe 160°C. Devices manufactured using mAgic adhesive can be used at operation temperatures up to 200°C. The sinter paste can even be used at temperatures above 200°C. Another item that has to be considered for solder materials is the cleaning of fl ux residues as well as solder splatters. Flux cleaning is a relatively dif fi cult and costly process. It’s needed to ensure robust wire bonding of the devices. The cleaning equipment, the cleaner itself and the fl ux system of the solder have to be adjusted to ensure an optimal cleaning result. Furthermore solder splatters are an issue for the manufacturing of DBC devices. Solder splattering could occur during the re fl ow process, especially if a vacuum re fl ow is used. In cases where solder splatters can be found in a wire bonding area the solder has to be removed from the substrate. Removing the splatters from the DBC circuit is a manual and costly process. Because of mAgic sintering materials’ properties fl ux cleaning or removing of splatter is not needed. In addition to these topics, Heraeus has set up an application lab for the sintering process. The relevant application and analysis equipment, as well as the know-how, are available to our customers. This enables a joint process development to shorten development time and reduce costs. YD: an you give any examples of where silver sintering materials’ advantages are most evident? TK: There are two situations where mAgic sinter materials are best suited. In the fi rst, customers need increased lifetime. Such circumstances include, but are not limited to, applications where maintenance is dif fi cult, time consuming and costly. In converters for offshore wind parks a high number of semiconductors will be used in the windmills and at the converter platform. To ensure high system availability each single die attach connection has to provide a high reliability and lifetime. Silver sinter materials allow increased lifetime and reliability because they don’t suffer fatigue. In the second situation, an increase in power density is needed to save space and reduce costs. Space is saved and costs reduced with by handling a given power with a device of lower volume. On the other hand increased power density will result in increased power dissipation. The die attach layer has to withstand the increased operation temperature and has to have a low thermal resistance to support the heat transfer from the semiconductor to the environment. Silver sinter pastes provide both: a high thermal conductivity and a no notable aging at operation temperatures of 200°C Thomas Krebs, Head of development for sintering materials, Heraeus Materials Technology “By the power module generation after next several sintered devices will be seen on the market,” explains Thomas Krebs. Thermally conductive: The thermal performance of packages using mAgic adhesive is equal to soldered devices. (Courtesy of Heraeus Materials Technology) POWER Dev’ 10

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