Wafer Backside CoatingTM of electrically conductive die attach adhesives for small IC packaging

The complexity of electronic devices is increasing rapidly. Mobile phones, camcorders and MP3 players are examples of devices that continue to incorporate more functionality into increasingly smaller footprints. Package designers have been able to accommodate these new requirements in some cases by reducing the outline dimensions of the semiconductor package so that they are only slightly larger than the silicon chip they protect. This design methodology, however, poses challenges for the die attach adhesive. In a traditional die attach process, paste is dispensed onto the leadframe pad and the chip placed into the paste. The leadframe pad must be designed significantly larger than the chip, to allow for the flow of the paste and the formation of a fillet around the die edge. In addition, the die must have a minimum thickness to prevent the paste flowing onto the top surface of the chip. One way to overcome these difficulties is to use a no-flow die attach which does not form a fillet. This allows the size of the leadframe pad to be reduced to a dimension smaller than the chip itself and also enables the use of thinner die. Die attach adhesive supplied as a film is one approach to this, but there are issues with cost and some restrictions on the adhesive thickness that is available. To address these issues, Wafer Backside Coating™ (WBC™) was developed to apply a non-conductive die attach material. Following this, the use of a highly conductive (silver-filled) WBC paste was described to reduce costs in the packaging of discrete devices. This paper describes the use of a novel conductive Wafer Backside Coating (WBCTM) die attach adhesive that delivers good performance and reliability, with negligible fillet size, that can be used over a wide range of die sizes. This work explores the manufacturability of the WBC conductive material, especially in terms of stencil printability, sawing and die attach. Relevant manufacturability responses (such as stencil open time, die shear strength, die tilt, coverage, etc.) are discussed. Finally, data is presented to confirm that functional devices achieve MSL L1/260 and as well as long term reliability testing.