The packaging of chips—the electrical connection and mechanical mounting of an integrated circuit onto a substrate—is becoming increasingly challenging be cause the number of connections to an individual component is growing while the distances between adjacent chips is decreasing. Current flip-chip bonding technology connects bumps on the outside of the chip to copper leads that have been deposited on flexible polycarbonate tapes—"flex." For soldering, heat from a thermode is applied to the rear side of the flex at the point where the lead and bump cross. This process—windowless tape-automated bonding—suffers, however, from lack of heat resistance of the flex material.
A new technique that overcomes this problem takes advantage of the high transmission of flex at the Nd:YAG laser wavelength of 1063 nm. Energy at this wavelength can be passed directly to the lead-bump solder spot without damaging the material. The process was developed at the Technische Universität of Berlin and the Fraunhofer Einrichtung in Berlin.
Pulsed laser radiation at 1063 nm is directed into an optical fiber, the tip of which is placed above the spot to be soldered. The transmission of the flex at 1063 nm is about 88% so that the radiation impinges onto the copper lead positioned over the bumps on the chip from the rear. The chip bumps are gold-plated. The absorption of the copper-plated surfaces is about 7%—more than for carbon dioxide laser output but is still low enough to minimize heating of the solder spot. The researchers found that pretreatment of the leads by blackening them considerably improved absorption.
During the laser-pulse heating, the temperature rise at the solder spot must be carefully monitored. For this reason, the thermal radiation emitted from the spot is fed via the same optical fiber to an infrared detector. The detector output signal is used to control the laser energy applied to the solder spot. Temperature monitoring and laser-pulse-energy control are the keys to reproducible results.
To provide optimal thermal contact between lead and bump, the fiber tip is slightly pressed onto the flex. Typically, using a 5-W laser beam, a pulse of about 10 ms—that is, 50 mJ—is sufficient for an almost perfect soldering result. Pull-off tests have confirmed perfect adhesion of the solder joint—that is, any break that occurred was elsewhere. Hence, the process works reliably and is expected to be cost-effective.
The technique has been successfully applied to chip packaging and has resulted in the formation of a new company, Pac Tech Packaging Technologies GmbH (Falkensee, Germany). The company specializes in the preparation of chip bumps and in laser soldering including pack aging and has already licensed its technology in the USA and elsewhere. Pac Tech Technologies plans to expand its services for chip manufacturers and will manufacture laser-soldering stations based on the new technique.