Wafer Overcoat Simplifies Bumping
John Baliga, Associate Editor -- Semiconductor International, 2/1/1999
HD MicroSystems (Parlin, N.J.), a joint venture of Hitachi Chemical Company Ltd. and DuPont, recently introduced an i-line photodefinable polyimide material, HD-4000, for wafer overcoat and flip-chip applications. The two companies combined expertise to produce a polyimide overcoat material that can stand up to solder reflow temperatures, while having photosensitive properties stable enough for full flow manufacturing.
Overcoat materials protect the chip's surface for shipping, encapsulation and underfill operations. Encapsulants and underfills have filler materials that can damage the top film layers on the chip. In flip-chip applications, this overcoat can be used as the dielectric for redistribution, as well as a solder mask. In both flip-chip and peripheral lead applications, openings in the overcoat are required to expose contact pads. Typically, the overcoat goes on at the wafer level.
Cured polyimide materials provide thermal and mechanical stability. They also provide a reasonable amount of compliance, though not necessarily enough to act as an underfill. A photosensitive polyimide is imaged and developed directly, using typical lithography equipment. Image resolution is higher and processing simpler for the photosensitive material than for a non-photosensitive material, which requires photoresist and wet processing to pattern. After development, the polyimide acts as a single mask for subsequent operations.
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| An i-line stepper imaged this 10 µm via directly onto the polyimide material. |
Materials such as this provide more applications for i-line equipment, not
just in fabs, but also in packaging facilities. 
TAC Lines Include Packaging
Two kinds of chip packaging will be added to the Technology Advancement Center (TAC) manufacturing demonstration lines at NEPCON West '99. The TAC lines run in the exhibit hall in a section separate from the company exhibits and have historically demonstrated the state-of-the-art in board assembly. This year, one line will demonstrate a new Direct RAMBUS DRAM µBGA packaging process, and another will demonstrate a tape ball grid array (TBGA) strip assembly.
These packaging technologies are examples of how packaging and board assembly
technologies are borrowing from each other and merging. Steve Sytsma of RRA
Technical Services (Grand Rapids, Mich.), project director for the packaging
lines, said, "Both the µBGA CSP and the TBGA assembly processes show an
overlap between the semiconductor industry and the circuit board assembly industry.
These industries are sharing machine and material technologies to find the best
solution for a variety of processes."
