by Don Moore, President, Semiconductor Equipment Corporation
|There is virtually nothing about flip chip attachment that is standard, due in large part to the bumps that make contact between the chip and the substrate. The bumps may consist of solder alloy, polymer, pure indium or gold alloy and range in size from 1 mil on 1 mil centers to 10 mil on 10 mil centers. They may be located on the periphery of the package or in an array or staggered array arrangement. Even the process used to deposit them on the wafer before it is cut up into individual die may vary.
Although assembling flip chips onto substrates can be achieved in different ways, its success depends on the use of versatile and flexible equipment during the assembly process. The following steps take place in a real-world application and describe the different processes of solder reflow (from the point of pick-up through reflow), inspection and rework.
2) Presentation and Pick-up
Pick-up from the waffle pack, alignment of the chip’s solder bumps with the substrate’s bond pads and placement are all accomplished using specialized equipment.
Operation of such equipment is relatively simple. In the case of the pick-up cycle, the chip must simply be targeted so that the operator can align it to the system´s pick-up tool (Figure 3). The system´s pick-up cycle is then initiated and the machine automatically picks up the flip chip. For chips presented with their bumps up, some bonders are outfitted with an optional inverter that picks up the chip, turns it over and passes it off to the system’s placement tool.
To prepare for chip placement, the operator aligns the bumps over the bond pads on the substrate, the latter having been previously placed on the system´s workstage to the right of the pick-up head. The operator then moves the substrate into position under the pick-up head, at which point the system´s cube beam splitter extends out to a position between the pick-up head and the substrate and presents a view of the chip’s bumps and substrate bond pads (Figure 5). Adequate lighting of both the flip chip bumps and substrate is attained with separate, adjustable illuminators – one for the bumps and one for the bond pads.
With a multichip device reflow is performed in a controlled atmosphere oven. The flow-through design oven2 used to reflow some of the flip-chipped substrates in this application (Figure 8) combines bottom-up conduction and top-down convection heating with precise calibration of the temperature and purity of the atmosphere within each of its four heat zones and two liquid cool zones. The temperature of the bottom conduction platen and upper convection platen in each zone can be individually calibrated and controlled on a repeatable basis to a setpoint, and the inert atmosphere on each zone can be purged to 10 ppm oxygen.
When reflowing miniature designs, hot gas cannot be used because the velocity from the bonder’s hot gas nozzles is too high (even after throttling down) for the lightweight small chips. Instead, either a Xenon lamp system (with the visible light being delivered through a fiber optic bundle) or an infrared laser system is used on the bonder to reflow the gold contacts. There are certain laser diode applications, for example, that call for accuracies within 1 µm, which is beyond the capability of available flip chip bonders. In such situations, etching, lithography, and vacuum deposition technologies are used. The bonder is then used to place the chip and the self alignment phenomenon maintains the 1 µm precision achieved in the bumping process.
When flip chips use polymer bumping instead of solder, the bonder holds the chip on the substrate using some amount of bond load, and then heats to achieve a snap cure or full cure.
7) Inspection and Rework
Should the inspection indicate that a flip chip needs to be replaced on the substrate, the rework can than be accomplished with spot heating via a flip chip bonder. Using an oven is not an option, since it would reflow everything on the substrate. After aligning the system´s pick-up head over the defective flip chip, the two hot gas nozzles are lowered over the rework site (Figure 10) just as they would be for reflow after initial placement. During rework, however, the temperature of the solder is elevated beyond reflow. The nozzles then retract and the defective chip is lifted off of the substrate. Finally, the site on the substrate is cleaned in preparation for pick-up, alignment, placement, and attachment of the replacement chip.
|Reprinted from Advanced Packaging, July/August 1997|