Detailed processes are involved in underfilling flip-chip assemblies, and a variety of fluid formulations and dispensing alternatives are available to today's flip-chip process engineer.
In the past few years, flip-chip technology has migrated from a specialized packaging application to mainstream production as a key method for improving circuit densities, reliability and cost in miniaturized electronic products. The use of underfill encapsulant beneath the flip-chip die is necessary to significantly increase reliability by reducing the strain on the solder bumps during thermal cycling imposed by coefficient of thermal expansion (CTE) differences between the die and the substrate. By adhering to all surfaces under the die, the underfill adhesive makes the die-adhesive-substrate system stiffer. The underfill adhesive also keeps the solder bump in hydrostatic compression, thereby increasing fatigue endurance and holding the bump intact under strain. Underfill encapsulant has been shown to provide an improved degree of environmental protection.
When flip chips began to be more widely used three to four years ago, one of the primary concerns was the then relatively slow process of the underfill dispensing and curing process. As a result, fluid formulators, dispensing equipment manufacturers and flip-chip process engineers invested significantly and improved throughput and process capabilities for capillary underfill.
In parallel, some degree of effort and promotion has been expended on developing "no-flow" underfill materials, which are intended to be applied before the die is placed. Although no-flow underfill materials reportedly have some practical usage in real-world flip-chip applications, the bigger picture has also significantly changed with vastly improved throughput and process robustness for dispensing of capillary flow underfill. This article examines the processes involved in underfilling flip-chip assemblies and explores the fluid formation and dispensing alternatives available to today's flip-chip process engineer.
Basically, the conventional capillary underfill dispensing process (Figure 1) fits into the production line after the reflow process, whereas no-flow underfill is dispensed before placement of the die and is designed to cure during the reflow process. The capillary underfill process requires an additional cure oven and a means to flux the area before reflow. Each process requires a dispenser.
Figure 1. Conventional underfill process.
In a no-flow process, the underfill is dispensed on the flip-chip placement site. The chip is then placed on top of the underfill under force to press the die into the fluid and the bumps into contact with the pads. It is necessary to hold the flip chip momentarily (100 ms) in place to allow the fluid to form a fillet around the edges of the die. The hold-down dwell time is required to stop the die from rebounding away from the surface of the printed circuit board.
The placement accuracy requirement of the flip chip relative to the substrate pads is more precise than with conventional underfill materials. The die self-alignment that is normally accomplished during reflow is impeded by having a viscous fluid in the interface at the time of reflow, hence the need for more accurate alignment with no-flow underfills, which slows down the die placement rate. The assembly is then heated to reflow the solder, cure the underfill and bond the embedded fluxing agents into the cured polymer. The no-flow underfill must be precisely cured during the same process that forms the solder connections, which can lead to some degree of process tweaking to simultaneously achieve both objectives. This also requires some reflow profiling changes depending on the type and number of components on the circuit board. Also, obtaining a formulation that undergoes complete curing during reflow has been a technical challenge, as uncured adhesives can have a significant effect on the thermal cycling reliability.
No-flow underfill adhesives are not highly filled because the primary purpose of the underfill is to increase system stiffness to reduce strain on the flip chip bump. The normal underfill encapsulants are highly filled with silica or other fillers to increase an epoxy's modulus, reduce creep sensitivity and decrease a material's CTE. Because the no-flow adhesive is present before reflow, fillers would be present between the flip-chip bump and the pad. Therefore, complete reflow connection would be inhibited by the presence of fillers. Consequently, reliability of no-flow flip-chip systems with non-filled adhesive is considered good if 1,000 to 2,000 cycles are achieved without failure. A traditional filled capillary underfill system will achieve more than 3,000 cycles.
In a conventional capillary underfill process, dispensing and curing can be optimized independently of the other production steps, thereby allowing more latitude for developing a robust and controllable process. In addition, because the reflow process is completed before underfill dispensing, manufacturers also have the option for inserting a pre-underfill functional test, if desired, to improve yields and allow for rework before underfilling the assemblies.
The latest underfills have made significant advances in flow-out and cure times. Underfills for die sizes in the range up to 12 mm square with 60 micron gaps can flow out in 6 seconds. Each flip chip on a board further masks the flow-out time so there is no throughput effect because of flow-out time. Snap cure systems allow curing in 5 minutes to full cure. The adhesives have the fillers that allow high reliability so no compromise is made in the end product's ultimate performance.
Precision selective jet dispensing of flux is one of the key developments that has made conventional capillary underfill processes more robust and simpler. As a rule of thumb, the optimum amount of flux is the least amount of flux. A no-clean process can be achieved by applying an ultra-thin flux film thickness of 1 mil or less; these thin-film requirements have already gone well beyond the limitations of traditional fluxing methods, such as brushing, screening or dipping. As a result, many production lines are turning to selective flux jetting to increase accuracy and consistency with ultra-tight tolerances while simultaneously boosting overall throughput and resulting in higher reliability.
Underfill Adhesive Fluid Formulator Issues
Fast-flow and snap-cure underfills can be used to boost production throughput in high-volume consumer electronics applications while specialized underfills can be used to withstand operating temperatures as high as 125°C. High Tg underfills can provide glass transition characteristics as high as 155°C to provide extra reliability in hybrid and MCM applications, and underfills with special wetting agents can accommodate under-die gaps to 1 mil. All of these fluid formulations have been refined for use in tightly controlled capillary underfill processes using third-generation dispensing platforms. This gives a manufacturer a wider range of adhesive alternatives.
On the other hand, because no-flow underfill is designed to be cured during existing reflow oven processes, the range of no-flow fluid formulations has been limited. Any variations in the reflow processes can significantly impact results by causing the underfill to gel prematurely.
For example, if the production process for a large ball grid array package requires a slightly higher reflow temperature to ensure consistent reflow of all solder balls, the result may be inconsistent with no-flow underfill curing. Because a core objective of the no-flow process is the cross-linking of the embedded flux into the cured underfill polymer, if the adhesive gels prematurely it can leave behind undesirable flux residues, such as carbolic acids, that can lead to corrosion and latent quality problems. For this reason, it is suggested that a post-test cure may be required to fully cross-link the no-flow underfills.
Although no-flow underfill can certainly produce acceptable yields in selected processes, the fact that it must fully cure during the standard reflow process can involve extensive experimentation and tweaking within a relatively narrow process window. On the other hand, because conventional underfill dispensing occurs after the reflow process, its control parameters are completely independent of the reflow oven settings and, therefore, are more easily set up within a wider process control window, resulting in consistently reliable results.
New Reworkable Capillary Underfills
Another major milestone in capillary underfill has been the development of new reworkable underfill adhesives that allow defective flip-chip packages to be removed by simply heating the package and the underfill for one minute at standard rework temperatures of 210 to 220°C. The raised temperature causes the reworkable underfill adhesive to partially decompose, which enables the component to be removed by applying torque. An uncomplicated clean-up procedure involving gentle high-speed brushing eliminates any remaining residue and prepares the site to receive the replacement component.
High-throughput Precision Dispensing Capabilities
Many high-volume and high-mix production environments are already achieving sustained throughput and consistency by using third-generation dispensing platforms that provide a combination of:
- Highly repeatable precision positioning and motion systems
- Linear positive displacement needle dispensing methods
- Precise volumetric control over the dispensing process
- Careful management of temperature levels throughout the dispensing, flow-out and curing processes
- Flexible and simple programming, set-up and run-time operating environments.
To achieve a fast, complete and consistent flow of underfill encapsulant, it is important to have highly repeatable part handling and location, the use of precision volumetric dispensing controls, and the careful management of temperature levels throughout the dispensing process.
Part handling is the first critical factor in achieving high dispensing throughput. The loading and X-Y positioning systems must accurately and consistently place parts in the same location within the work envelope so the dispensing can be efficiently initiated at the pre-specified locations. The highest throughput rates are achieved with systems that automatically establish the precise position of each die using the minimum required number of fiducials, while avoiding undue operator intervention.
In flip-chip applications, precision control is a key concern because the dispensing needle has to move extremely close to the chip throughout the process. The needle must be positioned far enough from the chip to avoid backside contamination but close enough to promote capillary flow of the fluid under the chip. Accurate volumetric control of underfill fluids can only be achieved through the use of a linear positive displacement (LPD) pump that uses a piston to displace the exact volumes required, whether in large or small shots. Unlike older rotary or auger pump technologies, which work adequately for stable viscosity fluids like solder paste, the short pot-life fluids used as underfill encapsulant demand more precise pumping action, such as LPD, where the flow-rate never varies with changes in the viscosity and needle diameter. Even before the actual dispensing process begins, process control above the substrate temperature can help to prepare it for optimal encapsulant flow-out.
The dispensing system should also incorporate needle-heating to further optimize the flow characteristics of the encapsulant formulation before it is even dispensed into the part. Finally, maintaining a precise post-dispensing profile over the parts can be used to effectively complete the flow-out after dispensing. The post-heating step can actually improve overall dispensing throughput because the dispensed parts can be moved to the post-heat area to complete their flow-out, while the next set of parts is simultaneously cycled into the dispensing work envelope. By treating the dispensing and curing operations as a single, integrated "process module," it is now becoming possible to optimize overall production throughput while simultaneously improving the quality and consistency of results.
ALEC BABIARZ, vice president of Nordson Corp. and senior vice president of Asymtek, and STEVEN J. ADAMSON, product manager of semiconductors at Asymtek, can be contacted at 2762 Loker Avenue West, Carlsbad, CA 92009; 760-431-1919; Fax: 760-431-2678; E-mail: firstname.lastname@example.org and email@example.com.