The squeezing process of complex epoxy patterns in the die-attaching of large IC-chip

During the die bonding process of large IC-chips (say 300mil or above), the application of complex "epoxy writing pattern" are usually needed as to enhance the evenness of epoxy distribution (or the so-called epoxy "spread-out") along the peripheral of the die. In order to obtain a nice epoxy "build-up" or "fillet-height" along the peripheral of the die, and to avoid trapping air bubble (i.e. making voids) inside the epoxy below the die, a suitable epoxy pattern (such as a "cross", a "snow-star", or a "double cross") must be selected as to guarantee that the epoxy dispensed on the lead- frame/substrate can evolve properly to the final shape of the chip that we wanted, after the squeezing process in die attaching. In this study, the thin-film assumption is used to reveal clearly how a particular type of epoxy pattern will evolve when it is subjected to the compression/squeezing action of the die. A simple 2D simulation software-program was developed to solve this viscous thin-film or so-called Hele- Shaw flow problems. The simulation results for the evolution of various epoxy patterns under the die-attach squeezing process are thus obtained and discussed in details. Through investigating the evolution of some simple epoxy patterns (such as a single-line, and a single cross), some general rules governing the evolution of epoxy patterns are derived; and subsequently certain guidelines for the selection of a suitable epoxy pattern in the die bonding process for a particular chip size are discussed. The reasons why it is necessary to use different epoxy patterns for different die-size and epoxy bond- line-thickness (BLT) are briefly illustrated. And finally, the various factors leading to the variation of the epoxy final- shape are also examined.