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|Enabling MSL-1 Capability for QFN and Other Design Leadframe Packages|
|Keywords: MSL-1, Leadframe, QFN|
|As the conversion of the electronics industry to lead free soldering materials continues, some unexpected negative side effects of higher lead free reflow temperatures have occurred. Defects such as delamination or "popcorning" in surface mount components have increased significantly since lead free soldering has become mainstream. Popcorning is a defect that manifests itself as a fracture between the epoxy based encapsulant and the metal, usually copper alloy, leadframe components used to form a surface mount component. This fracture occurs when moisture in the package volatilizes during reflow processes and forces its way through the encapsulation material and leadframe interface. Peak reflow temperatures for leaded solder typically run around 215° - 225° C but due to the higher melting point of lead free solders, they require peak reflow temperatures of 240° to 260°C range. This 30° increase in reflow temperatures can have a significant effect on the electronic devices and any resident moisture in the component The keys to popcorning, or delamination, defect reduction is twofold. The first objective is to enhance the bond between the encapsulant and the copper leadframe materials to form a stronger bond that can resist the vapor pressures induced during reflow. The other objective is to provide a superior bond between the leadframe and encapsulant thus minimizing moisture ingress. New chemical treatment processes have been developed that pre-treat the copper surfaces of the leadframe and significantly enhance the bond between the encapsulant material and the metal leadframe. The chemical treatment process results in micro-roughening of the copper surfaces and at the same time depositing a thermally robust film that enhances the chemical bond between the epoxy encapsulant material and the copper. This paper examines the possible issues and the real life successes when comparing standard component manufacturing methods to those that incorporate the aforementioned chemical adhesion promotion process. Components are assembled using both processes and final performance is tested using MLS-1 conditioning protocols, acoustic microscopy analysis (SAM), and final yield improvement.|
|Dan Hart, Associate Research Fellow