High-speed Placement of Wafer-level Devices

Estimated reading time: 6 minutes

Advanced chip packages leverage the design advantages of bare die. Suppliers and manufacturers facing the challenge of placing more die at higher speeds, eliminating intermediate packaging, and feeding flip-chip devices at high speeds from wafer format, will meet the demands by combining SMT and semiconductor placement processes into one high-speed workflow.

By Gheorghe Pascariu

Wafer-level packages based on flip-chip technology may contain just flip-chip devices, but often are combined with surface mount devices (SMDs). In any case, assembly-process schemes generally use a dedicated flip-chip assembly line with placement machines that tend to be relatively expensive and slow - 2,000 to 4,000 real components per hour (cph). When the packages include SMDs, a second dedicated assembly line is also required. Such assembly-process schemes are costly, complex, and require excessive floor space.

Packaging and SMT Processing

Recent advancements in the accuracy of SMT placement machines permit processing of semiconductor advanced packages as an integral part of the SMT workflow. Wafer-level packages processed on SMT placement machines yield a high-speed/low-cost solution suited for high-volume manufacturing. Several semiconductor OEMs and subcontractors are implementing this approach; this technology is giving them a competitive advantage in the complex world of advanced packaging. This combined packaging and SMT workflow is being used in the high-volume production of multi-chip modules, advanced memory devices, MEMS, System-in-Package (SiP), and Chip-on-Board (CoB) applications.

Direct-die feeding is a modular feeding concept that meets many die-placement tasks within the SMT process. A direct die feeder* can be mounted on a variety of pick-and-place systems using a bayonet that provides a universal mounting interface. The bayonet, combined with a simple communication protocol, ensures seamless integration into a system that enables high-speed delivery of components directly from a diced wafer (Figure 1).

Figure 1. Multiple direct-die feeder installed on a high-speed pick-and-place system.

The die feeder can be moved easily between various platforms, enabling a flexible production flow that meets the most stringent technological demands and short product cycles. The concept pertains to flip-chip and direct-die components, both of which are processed without the need for conversion or special tooling. The feeding system is equipped to handle both chip types. Chip type and process are chosen on-demand using a touch-screen interface.

With this universal feeding system, wafers up to 200 mm can be handled, and components from 0.5 mm² to 5 mm² can be processed. Wafers are loaded and processed in a vertical position, which provides a footprint savings.

No More Back-end Processing

The wafer is mounted onto a metal ring (wafer frame) using adhesive tape. This enables the wafer to be diced into individual dies. To attach the components to the PCB substrate, they must be taken from the wafer. In the past, this required a separate process step, and an entire industry - the back-end semiconductor industry - was dedicated to it. Specialized equipment such as die and flip-chip bonders were used to pre-package the components, prior to placing them in tape-and-reel to suit SMT pick-and-place equipment.

No More Pre-packaging

Flip-chip technology eliminated the need for pre-packaging individual components (except in special applications). This has led to the development of a universal direct-die feeder that gives SMT pick-and-place systems access to semiconductor feeding technology to feed components supplied in a wafer format. Savings are attained by eliminating the back-end semiconductor process.

Using this feeder process, boards are loaded onto a feeder at the beginning of the line. This step feeds them one-by-one as needed. The boards go through the screen printer(s), where flux and adhesive (solder paste or epoxy) are applied. From there, they enter the SMT pick-and-place system, where the boards are locked in place and the vision system locates the reference marks to enable accurate component placement. Once vision processing is finished, the pick heads move to the dedicated feeders to pick up the components, locate them on the pick nozzles, and accurately place them onto the board (Figure 2). When all components are attached, the boards progress to a reflow oven for curing.

Figure 2. Pick-and-place nozzle picking from the feeder.

Board Processing in a Single Pass

With the direct-die feeder fully integrated into a SMT pick-and-place line, entire boards can be processed in a single pass. Once the wafer is loaded, the vision system locates the components on the wafer using pattern-matching algorithms. Ink-dot detection or electronic wafer mapping are used to define the known good components to be picked from the wafer and transferred to the placement machine.

The pick process is an important element of this cycle. The ejection needle is synchronized with a low-force pick tip, enabling the handling of very delicate components. This advanced process control can handle a variety of components - from silicon die and solder-bump flip-chip die to delicate Gallium Arsenide (GaAs) MEMS devices.

Flip-chip Mode

Flip-chip components are presented on the wafer with bumps facing up. The system must pick the component and invert the chip so that the pick-and-place machine can place the flip chips onto the board bumped-side down. A synchronized combination of vacuums on the pick tip will grip the bumped side (front) of the component, while an ejection needle on the backside releases the tape from the back. This synchronized movement removes the component from the wafer. With the component safely on the tip, the pick head will rotate 90° up and place the component on a flipper. The flipper holds the components with their bumps facing down and transfers them to a shuttle that, in turn, transfers the components to the presentation point, where they are ready to be processed by the pick-and-place system (Figure 3). The flipper is designed to hold up to six components. This enables parallel processing because components are picked continuously from the wafer, while the pick-and-place nozzles remove processed components from the shuttle.

Figure 3. Flipper moving flip chips onto the shuttle for transfer to the pick-and-place machine.

Direct-die Mode

In direct-die mode, components are presented on the wafer with their active surface (bond pads) up. Components must be picked and presented to the placement machine with their active surface facing upwards - enabling the machine to place them onto the board accurately. The synchronized movement of the pick tip and eject system is achieved using a combination of vacuums on the pick tip attaching to the active surface (front) of the component and the ejection needle on the back. Both routines combine to gently release the component from the tape.

With the component safely on the pick tip, the pick head will rotate 90° down and place the component on the shuttle, which holds the component with the bonding pads facing up, and transfers it to the presentation point for processing. After one component is placed onto the shuttle, the pick head will select the next one and index to place it on the next location on the shuttle.

Since the shuttle holds up to six components, it enables pick-and-place systems with multiple nozzles on one head to maximize productivity and achieve high cph rates. The components on the shuttle are replenished while the placement head places the components onto the board.

Conclusion

The concept of universal direct-die feeding may change the way next-generation electronics will be assembled. As a result, throughput is going up and costs are coming down, while productivity and yields rise. Higher profits are giving savvy electronics manufacturers a sustainable competitive edge. In an extremely competitive global context, direct-die feeding may offer an advantage over traditional flip-chip bonders, using high-speed SMT equipment.

*Universal Direct-die feeder (DDF Ultra), Hover-Davis, Rochester, N.Y.

Gheorghe Pascariu, product manager, Hover-Davis, may be contacted at (585) 617-2165; gpascariu@hoverdavis.com.