Flip-Chip Processes Affect Size, Cost, and Reliability

Originally Published MDDI June 2003

Glucose Monitoring

by Albert P. Kretz and Donald Styblo

Flip-chip processes allow miniaturization of electronic circuitry by utilizing integrated circuit (IC) chips without the conventional plastic packaging that adds a great deal of bulk. Combined with tiny surface-mount passive components, flip-chips allow complex electronic assemblies in final packages that are extremely small. With some ingenuity, a wide range of package shapes and aspect ratios are possible.

Crucial factors in making flip-chip processes economically viable are the techniques used to bond chips to printed wiring substrates. The connection methods and substrate properties also affect the degree of miniaturization possible, the reliability of the resulting package, and its cost. If the flip-chips can be reliably bonded to the thinnest flexible substrates now available (about 25 µm in thickness), the finished circuit assembly can be rolled or folded to further reduce its surface area. Conversely, the assembly can be left flat if the objective is to achieve the thinnest possible package.

For the sensor system described in this article, a proprietary flip-chip connection process will be used in the CGMS electronics. In this process, a fast automatic gold ball–bonding machine applies stud-bumps (below) to the flip-chip, either on the wafer or as a single die. Next, a nonconductive paste (NCP) adhesive is deposited onto the flip-chip zone of the printed wiring with a screen-printing machine. Then the IC is connected to the substrate in an automatic flip-chip machine using a unique assembly process. The flip-chip machine aligns the die stud-bumps and printed wiring bonding pads, then applies pressure and heat to assure sound connections. The final height of each bump is automatically adapted to substrate height variations. Pressure and temperature are varied, depending on the number of I/O connections. No additional adhesive underfilling is needed.

With pressure applied, both sides of the package are heated to 200°C for less than 10 seconds to cure the NCP. With the NCP cured, a permanent force has been established that pulls the die and printed wiring substrate together. This results in highly reliable electrical contacts, because there is no long-term mechanical stress due to differences in the thermal coefficient of expansion for different types of materials. The entire process eliminates several steps used in other flip-chip processes, resulting in a cost-effective, sterile package.

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