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Authored by: SzePei Lim, Jason Chou, Maria Durham, Dr. Hyoryoon Jo, and Dr. Andy Mackie.

Ultralow Residue (ULR) Semiconductor Grade Fluxes for Copper Pillar Flip-Chip

AbstractCopper pillars topped with solder microbumps are emerging as a standard flip-chip solderbump replacement in the semiconductor assembly industry. The relentless drive towards finer pitch, combined with reduced copper pillar height, makes aqueous cleaning of flip-chip flux residues more difficult. An emergent failure mode is joint damage during aqueous jet impingement. The move towards Semiconductor Grade ultralow residue no-clean fluxes and away from cleaning processes is therefore inevitable, and this paper discusses the testing of assembly materials for this purpose.

SZE PEI LIM

email: splim@indium.com Full biography: www.indium.com/ biographies

Sze Pei Lim is the Technical Manager for Southeast Asia and is based in Malaysia. She manages the technical support team in the Southeast Asia region. Working closely with customers in the semiconductor and SMT industries, she helps them to optimize their processes, troubleshoot, and offer solutions. Sze Pei has over 15 years of experience, mainly in the areas of PCB assembly and surface mount technology. Sze Pei is a SMTA-certified Process Engineer and has earned her Six-Sigma Green Belt.

Form No. 99031 R0

IntroductionThe most recent edition of the International Technology Roadmap for Semiconductors (ITRS) [1] shows that package pitches, cost per I/O, die thickness and overall package thickness will all see significant reductions in the remainder of the millenium (Figure 1).The stress caused by the CTE mismatch between the substrate and die is exacerbated by reduced die/substrate clearances, and for this reason, as well as avoidance of electromigration issues, electroplated copper/pillar microbumps are becoming standard, replacing the more common solder bumps [2].Flip-chip assembly onto substrates using solder has typically been carried out by either spraying oxide-removing flux onto the substrate; dipping the die bumps into flux; or occasionally, both (Figure 2). In most instances, the flux used has been able to be cleaned away using water or an aqueous-based solvent system. However, the combination of large die, finer pitch, and reduced clearances has led to increased complexity in ensuring adequate cleaning [3].

Figure 1. Package changes 2014-2020 from ITRS.

Characteristics of Ultralow Residue FluxesUltralow residue (ULR) no-clean fluxes have several characteristics that are critical to their functionality, such as rheology, residue level, solderability, the ability to retain die in place, and compatibility of the final residue with molded underfill (MUF) and capillary underfill (CUF).

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As with most fluxes, the purpose of the flux is to promote good solderability (wetting) between two surfaces. It does this by reacting with metal oxides, making a salt that then dissolves into the remainder of the flux residues (away from the joint), leaving clean metal surfaces, and allowing the formation of intermetallics that drives the formation of a strong bond between the cooled solder and the underlying metal. If the solderability of flux is insufficient, it will lead to weak joint formation and/or voiding, whereas, if the flux solderability is excessive it may, in certain instances, lead to bridging. Hence the solderability of flux must be optimized to promote good wetting without causing solder bridging between adjacent I/Os.It is important for the flux to be able to retain the flipped die in place during reflow, to minimize die-skew. A test method (the MDR test) designed to study the movement of solder on flux during the reflow process is studied in this paper. Underfill compatibility is becoming a growing concern as clearances and the pitch of flip-chips is getting tighter, as discussed previously.

Experiment and Discussion Rheology: A Brookfield Cone and Plate Viscometer was used to measure the viscosity of the flux as a function of time. The spindle used was a CP-51 and measurements were taken at 25°C at 20RPM. The results for a few versions of the ULR no-clean fluxes are shown in Figure 4.

Figure 2. Typical flip-chip flux application methods. Figure 3. Flux rheology for fine-pitch dipping.

Rheology of the flux is important in several different aspects of the flux behavior, such as flux-reservoir height (that is, the actual measured flux height); ability to remove die from the flux dipping reservoir; consistent dipping performance; and ability to hold the die in place during placement and reflow (so-called “MDR” – movement during reflow). If the viscosity is too low, flux may wick up the bump/copper pillar and contaminate the die surface. If viscosity is too high, it may lead to bridging (Figure 3). If the tack (extensional viscosity) is too high, it may even make it impossible to remove the device from the flux dipping reservoir. Please note that viscosity is only one of the rheological characteristics of a flux, and only for a Newtonian material is the viscosity independent of the shear rate; fluxes are mostly non-Newtonian. Therefore the use of this single point measurement, although common, is not recommended for complete characterization, but may be used as a “shorthand” for many different rheological parameters [4].All fluxes leave behind a certain amount of material after reflow, called “flux residue”. ULR no-clean flux is designed to provide the same functionality as both standard (high-residue) no-clean fluxes and watersoluble types, and leaves almost undetectable amounts of flux residue. Because the level is so low, free flow of the CUF/MUF is enabled, minimizing the risk of void formation and delamination. If the residue chemistry is compatible with the underfills, then delamination during subsequent thermal cycling and other stress testing is avoiding.

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For this study three different versions of the ULR no-clean fluxes were tested and have consistent viscosity over a period of 10 hours, which is more than a single shift. After each shift, the user is advised to clean the flux tray and replenish with fresh flux, this is to ensure consistent flux dipping performance is achieved. The rheology (viscosity or tack) of a flux can be fine-tuned to suit the specific application, package configuration and equipment capability.Residue Level: The ULR no-clean flux was compared to a standard flux residue before and after reflow was explored. Figure 5 shows the results.Thermal Gravimetric Analysis (TGA) (equipment: TA Instruments SDT Q600) is used to measure the amount of flux residue after reflow. This test is done by using a ramp rate of 10°C/min, with 10mg±1mg sample size, and nitrogen

Figure 4. Flux stability shown by Viscosity versus Time.

flow of 100ml/min. The results are shown in Figure 6. Two different ULR no-clean fluxes (NC-826 and NC-26-A) and one standard residue flux (TACFlux®007) are used to show the differences in residue levels. As shown in Figure 5, there was little or no residue visible from the ULR flux, whereas with the standard residue material there was clearly still some flux present. Figure 6 confirms this: at a standard reflow peak temperature range of 235-250°C, the ULR no-clean fluxes have about 10% or less residue in comparison to about 50% for the standard residue flux. However, the TGA test is unrepresentative of real situations for flip-chip flux. The exposed surface area is much smaller in TGA than for most real flip-chip applications. In most reflow conditions, the flip-chip fluxes should therefore leave lower residue levels than shown here.Solderability: Testing for solderability was done by printing flux onto a metallized coupon using a stencil, then 96.5Sn/3Ag/0.5Cu (SAC305) 28mil solder spheres were placed onto the flux using an automated pick & place machine. The metallized coupon with the printed flux and sphere was then reflowed in a BTU oven in a nitrogen-purged environment at <500ppm O2.After reflow, solder wetting is calculated from measurement of the height of the solder bump, and the solder spread ratio (%) is calculated using the below equation:S = [(D-H)/H] * X100 Where: S = Spread factor D = Initial sphere diameter H = Post-reflow solder height (1)The results of the solderability test are show in Figure 7.Figure 5. Visual comparison of flux residue types.

Figure 6.Results of TGA .

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Three different ULR no-clean fluxes (NC-26-A, NC-510, and NC-826), one standard residue no-clean flux (TacFlux007), and one water-soluble flux (WS-575-A) were used in this test. Typically the standard residue no-clean flux and water-soluble flux will have better wetting. As shown in Figure 7, NC-826 ULR flux wetting behavior is close to the WS-575-A water soluble flux. Depending on the application and the solderability requirement, a suitable ULR no-clean flux can be chosen.Movement During Reflow (MDR): The ability of the flux to hold die in place during reflow was studied by using a proprietary “movement during reflow” (MDR) test. This used the same test setup as the solderability test. The results are shown in Figure 8.Same as the solderability testing, three different ULR no-clean fluxes (NC-26-A, NC-510, and NC-826), one standard residue no-clean flux (TACFlux®007), and one water-soluble flux (WS-575-A) were again used. The results shown in Figure 8 show that TACFlux®007 has the least movement during reflow, as is typical for a standard residue flux. However a standard flux will have a high residue, partially-filling the area under the chip, and so making it impossible to get a void-free underfilling process.

Two of the ULR no-clean fluxes, NC-26-A and NC-826 show a similar amount of movement to that seen when using the water soluble flux WS-575-A, which is generally acceptable for most applications.Compatibility with MUF/CUF: In this study for underfill compatibility, two different test methods were used. The first test (Test 1) is used as a more visual aid with a copper lead frame and glass slide, whereas the second test (Test 2) was used to evaluate the shear strength.Test 1:Test 1a: Tape Test (No T/C): To test the compatibility of underfill using an adhesive tape test method, a thin film of flux was spread onto a glass microscope slide. Flux was then placed onto a copper leadframe and heated on a hot plate at 240°C for 10-15 seconds. Two leadframes were used: one with a ULR no-clean flux (NC-26-A) and one as a control (no flux). After being removed from the hot plate the leadframes were then placed on a microscope slide. CUF is then deposited and cured under the recommended conditions. A visual check (eye and optical microscope) was used to check the sample surface. A peel/adhesion test was then performed by placing adhesive tape on the sample surface then tearing it off to check the adhesion of the interface.This test showed no voiding or delamination between the interface for both the ULR no-clean flux sample, and the control sample. Test 1b: Tape Test (with Thermal Cycle): The samples were initially prepared in the same way as in Test 1a. The glass slide was then treated with either no bake (control), baking at 180°C for 1 hour, or baking at 180°C for 3 hours. After the bake treatment the underfill was deposited and cured under the recommended curing conditions.After cure, the underfill was again checked by eye and OM. An adhesive tape test was performed, tearing it off the sample surface to check the adhesion of the interface. SEM cross-sections are then used to check the interface.After cure, the underfill was again checked by eye and OM. An adhesive tape test was performed, tearing it off the sample surface to check the adhesion of the interface. SEM cross-sections are then used to check the interface.Checking by eye and optical microscope showed no void or delamination between the interface for flux on glass slide sample. The adhesive tape test also showed before and after the test a smooth surface with no film delamination.

Figure 7. Results of solderability test.

Figure 8. Results of the MDR test.

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Test 2: Shear Strength Test. The interfacial shear strength test of the ULR no-clean flux NC-26-A was also used to evaluate the compatibility between flux residue with the MUF and CUF by using a MTS Alliance RT/10. The compatibility is related to the chemistry between flux residue and the underfill. Therefore the degree of compatibility can be varied by applying different materials. For each shear strength measurement, one combination of flux and underfill (five samples of each) was tested.For this test there were three test parameters: open surface of flux, sandwich shape, and a blank substrate for the control. Open: For the open surface of flux test, flux was applied on the top of the substrate, then reflowed under a typical lead-free reflow profile using a BTU reflow oven. Underfill was then applied between the substrate and cured under the recommended conditions.Sandwich: As in the previous tests, flux was applied on each of two substrates. These substrates were then plcaed together in a sandwich, then treated at 180°C for 3 hours. After treatment, underfill was applied between the substrates and then cured.

Figure 9. SEM cross sections: before and after thermal cycling.

Figure 10. Underfill shear strength.

Test 1b: SEM Inspection: The SEM cross section on the three post-reflow treatment condition samples: the control (no baking), baking at 180°C for 1 hour, and baking at 180°C for 3 hours the results are shown in Figure 9. The interface between underfill and glass slide shows no voids or delamination. Post thermal cycle also shows the good compatibility result with no voids or delamination issue.

Blank: For the blank substrate (control), no flux was applied to the substrate, but underfill was applied between the substrates, then cured. With this test method, ULR no-clean flux NC-26-A was evaluated using two underfills: one from Hitachi and one from Shin-Etsu (Figure 10). There was almost no influence from the residue of the ULR no-clean flip-chip flux NC-26-A. The open surface sample allows easy escape of flux and shows high strength, but even with the sandwich samples (two flux/underfill interfaces) good compatibility with the underfills was also seen.

ConclusionsSimple test methods for flux materials alone (that is, tests outside the auspices of system-level tests like those from JEDEC [5]) are, in many instances, inadequate to quantify the critical to functionality parameters of ULR no-clean fluxes. The authors have worked together to develop a suite of novel test methods and associated data that have given customers confidence that the materials are suitable for use in emerging copper-pillar flip-chip applications at the sub-100micron pitch node.The successful implementation of flip-chip fluxes such as the NC-26-A and the NC-826 at customer sites in China, Taiwan, and Korea underlines their utility in the high-volume package, and high-yield assembly processes demanded by today’s manufacturers of portable electronic devices.

References1. http://www.itrs.net/ 2. Gerber et al., “Next generation fine pitch Cu Pillar

technology”, ECTC 20113. Lee et al., “Flux study for ultra fine pitch flip chip packages”,

Microsystems, Packaging, Assembly and Circuits Technology Conference, 2009. IMPACT 2009.

4. S. P. Lim et al., “Ultralow Residue Semiconductor Grade Fluxes for Copper Flip-Chip”, SEMICON Taiwan 2014

5. http://www.jedec.org

First published at the 36th International Electronic Manufacturing Technology Conference, Malaysia, 2014.