Wafer Backside Coating™ of Electrically Conductive Die Attach Adhesives for Packaging of Discrete Semiconductor Devices

During the last few years, there has been rapidly increasing interest in the use of die attach adhesive applied to the back of silicon wafers. Adhesives in the form of solid films arc already in widespread use - mainly in die stacking of large memory devices. However, there is also increasing use of Wafer Backside Coating (WBC (TM)) adhesives in cost-sensitive IC applications for attaching small die (< approx. 3mm x 3mm). The WBC technique involves screen printing a conductive or non-conductive paste onto a wafer, and then drying (or B-staging) the coating(1). See Fig 1. The coated wafer is then mounted onto conventional wafer mount tape, diced, and the singulated die attached to a leadframe with beat and pressure. Recently, advances in resin and filler technology have facilitated the development of an improved electrically conductive WBC adhesive. The adhesive retains high strength and high modulus at elevated temperature and high silver content provides good electrical and thermal conductivity (> 10W/m/k). This novel adhesive can be used to attach sinal I die at high speeds, and offers a cost-effective alternative to both eutectic die attach and conventional silver epoxy dispensed paste. This paper presents the economic advantages of WBC as compared to traditional die attach methods. This work also addresses the properties of the adhesive and outlines the development of a stencil printing method to apply the adhesive uniformly to wafers up to 200mm in diameter, using readily available deposition equipment. Methods of controlling and measuring the coating thickness by contact and non-contact arc explained. The paper then Outlines the semiconductor assembly process parameters, including dicing die pick-up, die attach and Cure. Details of reliability test results arc presented. Conclusions include a roadmap indicating how further improvements will continue to accelerate the implementation of WBC technology.