Assessment of high temperature, lead free paste alternatives for semiconductor packaging

Electronic manufacturing has always been on a rapid progression with the never-ending introduction of new techniques, machines and materials fuelled mainly by product demands, cost and market legislations. The recent Pb-free legislation for example has sparked off many new material developments for semiconductor packaging mainly for solder paste and mold compound. Solder pastes are used not only to mount components in System-in-Package (SIP) but also to attach dies in high power products that require maximized thermal and electrical conductivities. In short, solder pastes are still regarded as the best interconnection medium, a role which is not easily replaced by other options. The drive towards meeting the requirements of Pb-free legislation results in the replacement of the commonly use Sn/Pb solder paste to SnAgCu (SAC) solder paste. With the removal of Pb from the solder paste system, the peak temperature in the reflow profile has now increased from 240C to 260C which is now the Moisture Sensitivity Level (MSL) reflow requirement adopted by JEDEC. However, as most SAC-based solder pastes have melting temperature less than 260 C, re-melting during 3 times MSL test reflow or during board assembly is inevitable. For an SIP or multi-die type of package where the finished module will be encapsulated by mold compound, the ability of re-melting is not desirable as its liquid pressure may cause mold compound delamination. In many incidences, re-melted solder will extrude into the delaminated areas and cause electrical failures. Several other Pb-free options such as gold-based, bismuth-based pastes and even electrical conductive adhesives have been introduced in the market. This paper looks into each of these alternatives and discusses its advantages and limitations in several aspects such as materials compatibility, assembly process requirements and last but not least its reliability performances.