Flip Chip Assembly of Thinned Silicon Die on Flex Substrates

Driven by a growing range of applications in the automotive, industrial, military, aerospace, computer, telecommunication, consumer electronics, and medical electronics industries, miniaturization and the use of flex circuits continue to be of prime interest to electronics manufacturers. The assembly of thinned silicon die (25-100 mum) onto flex substrates provides options for ultrathin, flexible electronics for applications ranging from smart cards to space-based radars. For high-density applications, 3-D modules can be fabricated by stacking and laminating preassembled and tested flex layers and then processing vertical interconnections. This paper describes a low cost, highly manufacturable process developed for flip chip assembly of thinned die to poly-imide flex substrates that eliminates the need for special handling tools and techniques. In this paper, solder bumped thinned die are reflow soldered to the patterned flex using a method that maintains the flex substrate flat during die placement and reflow. Reflow is followed by underfill dispense and cure. The underfill dispense process is critical to avoid underfill flowing onto the top of the thin silicon die and will be discussed. Parts assembled using these processes have undergone reliability testing, a high degree of reliability has been found, and those results are presented.