Lead Free Soft Solder Die Attach Process for Power Semiconductor Packaging

Due to increasing regulatory and consumer pressure on the industry to remove lead from semiconductor assembly, the search for a lead-free solder – used to attach the semiconductor chip to the substrate (Leadframe) – has become an important production factor. ESEC, together with OMG-dmc division and the University of Darmstadt (both Germany) has accomplished an extensive development program intended to identify a drop-in solution for currently commonly used Pb-based alloys. Using computer added development tools together with experimental methods the group was able to develop a set of Bi-based solders that not only meet most of the criteria for soft solder die attach applications but exceeds those in some cases. Investigated and presented are the physical properties of the new alloys like, wetting behavior, mechanical deformation behavior, heat dissipation and results like thermal cycling behavior and void rates obtained by producing and investigating real devices. Introduction: The replacement of Pb in electronics is originally motivated by environmental concerns about the use of Pb and by the search for advanced solder materials to fulfill the everincreasing demands in modern electronic devices [1-3]. Although the total use of Pb in electronics is relatively limited the widespread distribution of small amounts of Pb in consumer products and the contamination of landfills is a significant problem. [4] Consequently the legislation in side is continuing to restrict the use of Pb. The latest the guideline of the European Community from April 2001 effectively bans Pb from many applications until 2006, although exceptions are made in areas where technical solutions are not in hand. Lately the consumer interest to purchase ‘green’ electronics has been added to the motivations for Pb-free alloys. As a consequence many manufacturers have announced their road map to replace Pb in their products. Two significant obstacles are currently preventing truly Pbfree products form entering the market. • The technical aspect. Historically Pb-bases alloys like PbSn5 or PbSn2Ag2.5 are used to die attach applications. [5] The alloys have a melting temperature high enough to survive later temperature treatments, they are reliable enough for most applications and they a commercially competitive. Different from the board level where a number of alternatives for the commonly used PbSn37 are available there are no Pb-free alloys known suitable to replace Pb-based alloys. • The commercial aspect Currently about 150 tons/year of high Pb-solder is used for die attach applications most of it as solder wire, ribbon of preform. The solder die attach is predominantly done in highly sophisticated, fully automated production lines. An alternative to the Pbcontaining materials should be acceptable to the production conditions of these lines to avoid major capital expenses. As a consequence the industry has the desire to implement a drop in replacement to minimize the financial impact and therefore improve the chances to successfully market the product. Requirements for a drop-in solution Lead is actually an ideal material for soldering electrical contacts – it melts easily, has excellent wettability, maintains a reliable connection, and is cheap to use. Another advantage is the high melting temperature (>260°C) for the reflow process. Unfortunately, it’s also toxic. The alternative alloys should meet most of the properties of Pbcontaining materials; if possible exceed some properties, particularly the reliability performance. • non toxic The new lead-free has to be free of all toxic elements (especially lead) to meet the WEEE regulations legislated by the EU for an environmentally friendly production process. • Better than lead The new lead-free solder has to have properties similar to the commonly used lead-solder in terms of ductility and wettability. The melting point of the lead-free solder has to be also >260°C to withstand the 260°C reflow process. Also, the thermal reliability of a soldered bond using the lead-free die attach solder must have almost the same as with lead-based solders [6] • Better than tin So far, all lead-free solders in the die attach field are tinbased (e.g. SnSb8, J-alloy), making them very brittle [6]. This makes not only the production and handling very difficult, the thermal reliability of the bonds with tin-based solders is also very low. All existing tin-based solders have a melting point below 240°C, making them useless for products needing a reflow process. • No new investment in new equipment Furthermore, the lead-free solder should be workable on the existing soft solder die bond equipment. That means, there no need neither for new system modules nor any adjustments to the die bond equipment Development of Lead Free Solder Wire: Based on legal situation and customer expectations following criteria were listed for the development of this leadfree solder wire for die-attach: 1.non toxicity and melting range between 260°C and 450°C 2.alloy wettability on copper, nickel and silver substrates 3.wire manufacturability