New Wire Bonding Loops for Stacked Die Packages

The market for advanced, stacked die packages continues to grow. In 2005, over 743 million cell phones were produced, each containing at least one stacked die package (996 million stacked die packages in 2005). With the drive for even more advanced features including the Internet, e-mail, music and television incorporated within the "cellular" system, today's stacked die packages have as many as five die levels, with packages having as many as 12 levels in the later development stages. Interconnection between these levels is predominantly performed by wire bonding.

Only wire bonding offers the manufacturing flexibility and low costs capable of filling this role. The advanced looping controls provided by today's automatic wire bonders allow for flexibility and adaptability that other technological processes cannot provide. The ability to shape a wire bond loop, with well-controlled bends and kinks, has been in continuous development for over 12 years. The first worked loop shape patents were granted in 1993. These shapes led to the development of CSP shapes, with the addition of bends in the wire near the second bond, which were designed to provide buss bar clearance.

Now, with the advent of multiple level stacked die packages with thin profiles, the industry is driving to new, even lower loop height levels. New loop shapes are being developed to accommodate package design requirements. Early entries into the ultra-low loop marketplace included the Stand-Off-Stitch Bond (SSB), where a coined-bump (flat topped bump) is bonded to the die, a new ball is formed and bonded to the substrate, and the second (stitch) bond of that wire is bonded on top of the initial ball-bump. This reverse-bonded SSB produces very low 50-75µm loop heights. However, the process is significantly slowed because of the additional bump.

Recently, new loop shapes have been introduced to address applications requiring loop heights of less than 75µm. They meet these requirements without sacrificing throughput. This paper will talk about the new loop shapes and their capabilities in advanced packages.