Flip chips are altering the face of electronic packaging

Technical Insights’ Sensor Technology Alert

Flip chip packaging delivers enhanced electrical performance, saves space, and provides high conduction speed, making it an ideal technology for use in handheld devices and medical electronics, among other applications.

“As integrated circuit fabrication advances rapidly and the market for faster, smaller, yet less expensive electronic products accelerates, flip chip packaging comes into play,” says Technical Insights Research Analyst Anand Subramanian.

In this method, the active side of the silicon chip faces down and is directly connected to the substrate or printed wire board. This allows a much higher input/output count than in conventional packaging, since the whole area under the chip can be used for interconnection. Moreover, the reverse side of the chip can be used for heat dissipation. Increased signal propagation speed owing to reduced inductance is yet another advantage.

“Flip chips offer the possibility of low cost electronic assembly for modern electronic products because interconnection on the chip can be made simultaneously in a single step,” says Subramanian.

One of the most important parts of the chip manufacturing process is the detection of flawed chips. Generally, the chips are closely examined by infrared laser imaging with 1064 nm excitation. In the “face down” mounting of flip chips, inspection techniques are more complicated since only the back of the chip is discernible.

Physicists at the Ultrafast Optics Group of Heriot-Watt University, Edinburgh, and at Schlumberger Test and Transactions, San Jose, seem to have found a different, more accurate method of detecting defective chips.

Using the two-photon optical beam induced current (TOBIC) effect, they have been able to get 3D profiles of integrated circuit components on a silicon flip chip, with resolutions of 1 micrometer or better in all three dimensions. A mode-locked femtosecond optical parametric oscillator at 1.275 micrometers provided two-photon excitation of the chip.

Two-photon fluorescence imaging has typically and successfully been used in biological microscopy, where it has had a significant impact. This technique now stands poised to revolutionize chip work as well.

The Ultrafast Optics Group has successfully demonstrated 2D two-photon imaging in semiconductor devices. The technique employed by his laboratory involves using a high peak-power infrared laser operating at a wavelength longer than the one-photon. This technique seems particularly useful for optical probing of silicon flip chips, such as those used in high-speed microprocessors.

A thinned silicon substrate was largely instrumental in achieving focal discrimination in TOBIC imaging. Having discovered this, the physicists decided to produce a depth profile of the features across a component on the chip. By recording images at 20 different focal positions across a range of 9 micrometers, they were able to build a map of the peak photocurrent focal depth across the component.

With suitable processing of the TOBIC data, the ability to detect and image the interfaces between epilayers in light-emitting diode structures, or within vertical-cavity surface-emitting lasers, looks a distinct possibility.

Analysis by Technical Insights, a business unit of Frost & Sullivan (www.Technical-Insights.frost.com), featured in its Sensor Technology subscription, highlights the impact of flip chips in electronic packaging.

Frost & Sullivan is a global leader in strategic growth consulting. Acquired by Frost & Sullivan, Technical Insights is an international technology analysis business that produces a variety of technical news alerts, newsletters, and reports. This ongoing growth opportunity analysis of breakthrough sensor technologies is covered in Sensor Technology Alert, a Technical Insights subscription service, and in Sensor Sourcebook, a Frost & Sullivan Technical Insights technology report. Technical Insights and Frost & Sullivan also offer custom growth consulting to a variety of national and international companies. Interviews are available to the press.

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