Precise, High-Throughput Underfill Dispense In Chip-On-Wafer Packaging
BY HANZHUANG LIANG, NORDSON ASYMTEK
In today’s microelectronic packaging, components are continuously designed smaller and assembled more densely to allow more functions to fit into compact portable devices. To enable this trend, more manufacturers are using Flip chips that have more I/O’s and smaller bumps sizes. This has introduced underfill dispensing that fills the gap between the Flip chip and the substrate with polymer epoxy to help reduce thermal and mechanical stress at the bonding interface. In device packaging, the demands for cost reduction and miniaturization encourage the use of wafer-level packaging, such as the chip-on-wafer process. As a result, the challenges to this process have grown exponentially, and so have the challenges to underfill dispensing. For example, to package a device with a chip-last process, the keep-out-zone (KOZ) for underfill epoxy placement to nearby components is shrinking, e.g. from 700μm to 300-500μm within one year.
A high-precision, high-throughput underfill dispensing process has been developed to conquer these challenges. This underfill process is being used in production for chip-on-wafer packaging. In one example, underfill must be dispensed within 300-500μm KOZ at 4000 UPH. New equipment and new dispensing techniques are under development to further push the limit on higher throughput and precision.
The usage of organic Flip chip has introduced an important step of underfill into the assembly processes. Underfill encapsulation uses a thermoset polymer epoxy that fills the gap between the Flip chip and the substrate. Underfill helps reduce stress on the bumps caused by a thermal mismatch between the die and the substrate, limits creep movement of solder joint, and reduces cracks that are initiated at the bounding interface. It largely enhances the solder fatigue resistance and provides mechanical support to the Flip chip interconnect assembly.
The challenge of this work came from the small keep-out-zone and high throughput request, both of which are much more restrictive than industry standards.
The technology to underfill Flip chips was introduced in the 1980’s and was broadly accepted as an industry breakthrough. It has been the electronic packaging standard since the 1990’s. There are reviews and technical papers about capillary underfill in literature and the mechanism underneath is well known.
Polymer epoxy is dispensed via jetting along the die edge and the surface tension pulls the fluid into the gap between the Flip chip and the substrate (Figure 1a). Jetting here means that the fluid droplet or stream has enough momentum to break from the dispenser nozzle and to shoot on the substrate. The underfill flow-out time, T, is proportional to the fluid viscosity and the square of the chip length, and inversely proportional to the gap height, surface tension, and cosine of the contact angle (Figure 1b). After the fluid flow-out, the epoxy cures at room temperature or higher. To speed up the underfill process, a substrate can be heated (typically 70°C-90°C) to reduce fluid viscosity. Underfill should fully fill the entire space under the chip.
The industry request for smaller and denser components in a Flip chip (e.g. 30-75μm gap and 100-200μm pitch) constantly pushes underfill application engineers and scientists to the frontier of their fields. To redefine technical limits, advanced dispensing solutions appear to target denser packaging at high throughput.
II. New challenge
In device packaging, the demands for cost reduction and miniaturization encourage the use of wafer-level packaging, such as the chip-on-wafer process. As a result, the challenges to this process have grown exponentially, and so have the challenges to underfill dispensing.
Recently, work has been done to develop a dispensing application for a chip-last process used in packaging a portable device. Dies and components were densely populated on the substrate and final underfill epoxy had to be applied beside the die without contaminating the die surface and other components (Figure 2). The keep-out-zone (KOZ) requirement for underfill epoxy placement to nearby components actually shrunk over time, going from 1-2mm in 2015 to 700μm in early 2016, then 300-500μm from late 2016 to early 2017.
The challenge of this work came from the small keep-out-zone and high throughput request, both of which are much more restrictive than industry standards. For example, the 300-700μm KOZ is much smaller compared to 3mm for most other underfill applications, and the throughput request for 4000-5000 UPH is much higher than the 500-1000 UPH demanded for most high-precision process designs.
This new chip-on-wafer underfill application created opportunities for automatic high-speed precision dispensing. At the same time, it brought new challenges to the dispensing industry for high UPH, high precision, and flexibility. It requires (1) a throughput that could fit into the production lines that run high-speed pick-and-place packaging processes; (2) dimensions as small as 0.5-2mm, with placement accuracy of 0.025-0.05mm; and (3) smooth and precise coverage along rectangular die. To meet these new demands, it had to be demonstrated that (4) the system was flexible in programming and dispensing capabilities and could adapt to new designs quickly and at any stage of the design or assembly process.
III. New approach
A high-precision, high-throughput underfill dispensing process was developed to conquer these challenges. The approach is to move the dispense applicator to its position quickly and precisely, and then jet a thin stream of fluid at a high firing frequency.
The original approach was to develop a new underfill dispensing process to fulfill the electronic assembly industry’s demand for precision and high volume. The dispensing system that was developed (Figure 3) delivers cutting-edge reliability and micro dot dispensing for manufacturing advanced semiconductor and mobile electronics packages, and has been used in laboratories and production lines. The platform provides the latest technology in closed-loop process controls, vision targeting, and jetting. The hardware design, dispense control parameters and software features were optimized to achieve high throughput, high precision, and process flexibility. The advanced valve technology can achieve high frequency jetting cycles, high precision in dispense dimension, and adaptability to different applications. The jetting system enhances productivity –dispensing up to 5000 dots over a 10-second cycle time at up to frequencies of 1000 Hz. In applications for wafer-level packaging, this dispensing platform and jetting system significantly increased productivity and yield compared to prior solutions.
This underfill process is being used in production for chip-on-wafer packaging and maintains 400-700μm KOZ at 5000-6000 UPH. New equipment and new dispensing techniques are under development to further push the limit on higher throughput and precision.
This new dispensing configuration has been successfully applied to a wafer-level underfill dispensing application that runs on a high-throughput production line, e.g. 6000 UPH for chip packaging on smart phones.
(1) PRODUCTION ACHIEVED 700μm KOZ AT 6000 UPH
With released hardware and software design, the dispense process was successfully applied to a production line in 2015. Since then, the specifications of KOZ 700μm along the dispense side with a wet-out distance of 300μm along the other three sides have been maintained at a high throughput of 6000 UPH.
(2) PROTOTYPE DESIGN ALLOWS 400μm KOZ AT 5000 UPH
Within one year, a new specification of 400μm KOZ was proposed for a next-generation product. While pushing the limit on precision, the goal was to keep high throughput as much as the underfill quality would allow.
A dispensing configuration was developed to solve this quality and throughput conflict. Prototype hardware was designed and produced for a laboratory trial. Aided by further optimization on process design, this application succeeded during multiple trials of high-volume mini-production. The new specification is defined as 400μm KOZ at 5000 UPH for production in 2017.
(3) NEXT GENERATION DESIGN WILL ALLOW 300μm KOZ AT 5000 UPH
Internal demand at the dispensing company to continuously improve, raises the standard for similar applications and sets the goal of 300μm KOZ at 5000 UPH. Multiple projects are being proposed, designed, and some are now in early trials to push limits for deliverables in the near future.
V. Conclusions and discussion
This paper presents one application that uses new dispensing systems and technologies to solve problems associated with underfill dispensing for chip-last wafer packaging. During this continuously improving process, throughput increased and accuracy and process flexibility improved. Even though underfill dispensing has been used for a large range of dimensions, from 20mm to 0.5mm, new product research and development focused on microdevice packaging applications with small end dimensions of 0.5-2mm and accuracies of 25-50μm in x, y, and 25-50μm in z. Attention was also focused on jetting capability in microgram weight precision and in 500-1000 Hz high-firing frequency. The high throughput, high precision, and flexibility of these underfill dispensing systems have enabled manufacturers to meet their challenges and requirements of microdevice packaging. The combination of the dispensing platform, valves, features, and software, were all optimized for this application on different products. Dispensing speeds and accuracy have been significantly improved and have cleared the path for future packaging breakthroughs.
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