Routable Wettable Flanks for MEMS Devices

Wettable Flank Quad Flat no-Lead (QFN) packages have been in production for many years specifically to enable Automatic Optical Inspection (AOI) during post Surface Mount Technology (SMT) Printed Circuit Board (PCB) assembly [1], [2], utilizing either a partial saw cut or a partial etch dimple within the leadframe [2], [3]. There are several issues when applying such packages to Micromachined Electromechanical Systems (MEMS) devices: maintaining a small package footprint, the two dice (MEMS and Application-Specific Integrated Circuit (ASIC)) stacked structure, and the ASIC output pads location (they are all located on one side of the device). This necessitates the use of an organic substrate, which allows metal traces to be routed within the substrate to the required pads. Therefore, a routable Wettable Flank package suitable for MEMS devices that can reach a 0.1mm high wettable flank is required for reliable AOI. In this paper, two approaches will be outlined: one using an organic substrate and the other using a C2iM (Copper Connection in Material) or Moulded Interconnect (MIS) substrate.