By Christian Pichler, Datacon Technology GmbH
Originally targeted to high-end applications, flip chip assembly technology is rapidly finding its way into low-end applications for high-volume products with multiple chips, due to cheaper substrates and cost-effective bumping. This requires high speed and precision, which can be achieved cost-effectively by processing directly from the wafer using tried-and-tested, high-throughput flip-chip bonder platforms.
Regardless of the overall economic development, the demand for handheld electronics continues unabated the enduring mantra being smaller, cheaper, faster. Advances in the front- and back-end segments of semiconductor manufacturing give reason to hope that this trend will continue. In the back-end production steps, flip-chip assembly, system-in-package (SiP), and stacked dies in particular are contributing to this Figure 1. In the meantime, flip chip interconnections can be found not only in array components, such as BGAs, FBGAs and LGAs, but also in QFN, DFN, stacked packages and SiP.
The latter two methods in particular, which combine multiple dies and other components in one package, are increasingly being used for cell phones. In SiP technology, active and passive components are combined in one circuit for system integration; this allows a combination of different technologies, and the relatively low design overhead means short time-to-market. There are, however, limits to the miniaturization and cost saving. Modules from Skyworks, Epcos and RFMD, for example, are already on the market, which cover the front end of the phones antenna switch, power amplifier, receiving filter (SAW, BAW).
Figure 1: Flip-chip applications are increasing by more than 30% a year.
By comparison, system-on-chip (SoC) technology implements the majority of functions on a single chip. However, because of the complex design, this approach to the lowest possible costs causes longer development times, and analog functions can only be integrated to a limited extent. Examples of this technology include connectivity (WLAN, Bluetooth) and RF transceiver/power manager modules.
A third miniaturization technology is 3D packages, such as stacked-die or package-on-package (PoP), used, for example, to integrate processors or ASICs and memory.
But these technologies are now becoming feasible for manufacturers of other high-volume and price-sensitive consumer applications, which require components with a low pin count in addition to low installation height, for example, MP3 players or camcorders.
Figure 2 shows a production line for existing high-end, single-chip products that are typically assembled with a single large semiconductor chip and multiple surface-mounted SMD components. It comprises a screen printer for the soldering paste of the discrete components, an SMD placement machine, a flip-chip bonder, and the re-flow furnace, from which the heat treatment permanently fixes all components.
Figure 2: Typical production line comprising screen printer, SMD placement machine, flip-chip bonder and re-flow furnace.
For designing a production line for high-throughput panelized production of more complex PCBs, which can each contain multiple flip-chip-assembled circuits, or multi-chip modules (MCM), as well as a variety of passive components, two alternatives suggest themselves Figure 3; assembling all components with the SMD placement machine, or processing the bare die in a multi-chip bonder. Bare-die processing directly in the SMD bonder is not yet perfected, especially when handling of thin or brittle die and/or placement accuracy is concerned.
Figure 3: Two solution approaches for multiple-chip assembly:
Placement machine takes over the attachment of previously taped dies. Multi-flip-chip bonder handles the dies directly from the wafers.
For the first option, the semiconductor components must be detached from the wafer and sorted into tapes in a die sorter before being processed by the placement machine. However, with a multi-chip bonder, die can be processed directly from the wafer, which reduces the total overhead and therefore the price, not to mention the lower handling stress, which improves reliability and yield, especially with the increasingly popular ultra-thin dies. From a logistical point of view, the first method is more involved, as it requires key steps to be performed outside of the actual production line.
The crucial requirement of a multi-chip bonder for high-volume applications is the throughput, but this should not be accommodated at the cost of reliability. Reliability is a direct consequence of precision and process stability. To allow these three requirements throughput, precision and process reliability to be satisfied without compromise for multi-chip assembly, a new tool has been developed.
High Throughput and Precision
The new machine's high throughput is not due to "tuning" of the dynamic processes, which would reduce the process stability, but rather because the key components in the machine are duplicated: two independent bond heads, flip units, slide fluxers and upward-looking cameras. These work temporally in parallel and independently of each other, which considerably increases the throughput. Added to this is the strategy of short paths for architecture and flow control.
Optimize Short Paths
From the same ejection position of a wafer taken from the magazine, which is fitted with mixed wafers Figure 4, the two flip units alternately select one die. Immediately next to the substrate, the flip arm passes the die to its assigned bond head. The bond head dips the die into the slide fluxer to coat bumps with soldering flux, then presents it to the upward-looking camera, which inspects the chip from below and positions it with high precision on the substrate.
To avoid collisions or unproductive downtimes with two independently working units on the same work surface, an anticipatory "anti-collision arbiter" ensures appropriate, temporally optimized evasive movements. Furthermore, the flow control independently optimizes the allocation of the assembly positions to the two heads according to the criterion of the shortest overall path to further reduce processing times over and above the geometric arrangement of the flip/place handover point and the fluxer.
Fill Magazines Individually
Because this machine is designed to bond several different die to each of a number of substrates in one pass for as long as possible without operator intervention, the wafer magazine must be loaded accordingly. The magazine occupancy here depends on three factors: the number of same-type die-per-substrate, the size of the die, and the number of known good die (KDG) per wafer. Therefore, the adjusted magazine occupancy considerably reduces intervention or immobilization times.
Quick Assembly Sequences
The practical assembly sequence has a considerable effect on the throughput of the multi-chip bonder. Once the machine has been loaded with a new substrate, the two bond units begin processing the die from the currently loaded wafer Figure 4. When all substrates have been fitted with these die, the following occur simultaneously: fully-automatic changeover of the placement tool and the ejector tool, the used wafer is deposited in the magazine, and another wafer is withdrawn from the magazine. Once all die have been bonded on all substrate strips in the machine, the final wafer remains in the working position and the assembly sequence is reversed to reduce the overall time for the wafer switching.
Figure 4: The wafer magazine is loaded with different wafers, which are processed in parallel by both flip/flux/place units in one pass.
Through the dual-head architecture with the additional topological and organizational measures described, and based on a high-precision flip chip bonder platform, a multi-chip bonder has been developed that is characterized by high throughput with high process reliability and at the same time allows the complete production of a multi-chip product (MCM) in one production cycle. This is further proof of how innovative and cost-effective assembly solutions arise in close cooperation with customers: machines of today for the demands of tomorrow.