Grantee Research Project Results
Final Report: Green Nanosolder Paste for Next Generation Electronics Assembly and Manufacturing
EPA Grant Number: SU835938Title: Green Nanosolder Paste for Next Generation Electronics Assembly and Manufacturing
Investigators: Gu, Zhiyong , Gao, Fan , Wernicki, Evan , Shu, Yang , Fratto, Edward , Wang, Jirui , Kepner, Robert , Essigmann, Mikayla
Institution: University of Massachusetts - Lowell
EPA Project Officer: Page, Angela
Phase: II
Project Period: October 1, 2015 through September 30, 2017 (Extended to August 31, 2018)
Project Amount: $75,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2015) Recipients Lists
Research Category: P3 Challenge Area - Chemical Safety , Sustainable and Healthy Communities , P3 Awards , Pollution Prevention/Sustainable Development
Objective:
Soldering is widely used in electronics assembly and packaging processes, and also in plumbing. Eutectic tin-lead (Sn/Pb) solders have excellent mechanical and electrical properties that make them the primary solder choice for a variety of soldering applications. With a melting temperature of183 °C, PCB/component material warpage issues due to thermal stresses during reflow processes did not pose a large issue. Beginning in the 1990s, investigation into the life cycle of tin-lead solder has revealed significant environmental impacts, causing many future designs to use lead-free solder materials. Usage of lead-free solders as electrical interconnections has placed significant thermal and technological burdens on the industry. The solder transition to lead-free materials continues to take place while ambition to miniaturize and increase performance in electronic design increases.
Solder pastes are very common materials for joining surface mount assemblies to printed circuit boards (PCBs). The use of paste form is currently favored in surface mount electronics industries for its ability to be applied in both low and high volume PCB designs. Applied via automated screen- printing processes, stencils are designed to match the conductive pad layout on the PCB where a passing squeegee will fill the stencil apertures with solder paste. For lead-free solder based pastes, even though they are more widely adopted in the electronics manufacturing processes, there still several significant issues which need to be solved, e.g., relatively high melting temperatures, thermal stresses, tin whisker formation, and reliability issues. Nanosolders, solder materials in the form of nanoparticles or nanowires, create an opportunity as a new solder material with improved material and thermal properties. It has been shown that decreasing nanoparticle radius is correlated with an exponential decrease in melting point, referred to as a melting temperature depression. Increased mechanical properties may also result, as gold nanoparticles have been shown to have increased hardness and elastic modulus compared to the bulk gold. There has been limited research in the area of nanosolder pastes, and further study is needed to fully understand nanosolders in the paste form. Use of nanosolders will very likely allow smaller pitch devices where conventional microsolder powders are simply too large. Overall, lead-free nanosolders are intended to provide a non-toxic alternative to lead-based solders while offing smaller scale implementation capability.
The objective of this project is to develop a new type of environmentally friendly solder paste, green lead‐free and halogen‐free nanosolder pastes. These new nanosolder pastes have the potential to replace the conventional solder paste with micron sized solder balls, which contains toxic and hazardous materials, including lead or halogen or both in a typical solder paste formulation in current electronics industry. As electronics and devices are getting smaller, lighter and more powerful, new assembling materials and methods are required to accommodate all electronic parts in a much smaller single device. The nanosolder pastes, composed of solder nanoparticles, can be an enabling material for next generation electronics assembly and manufacturing. This project is involved in the synthesis of new nanosolder particles, preparation of these nanosolders into a solder paste with halogen-free flux, and their application toward Cu-Cu wire soldering and even nanowire joining as well.
Summary/Accomplishments (Outputs/Outcomes):
Nanosolder Synthesis and Nano-Paste Preparation
Lead-free alloy nanoparticles have been synthesized using a surfactant-assisted chemical reduction method in aqueous solution. All alloys produced were near-eutectic alloy compositions due to the desire of having a low melting temperature or temperature range. Resulting nanoparticle size, shape, and composition were examined using a JEOL JSM 7401F SEM equipped with X-ray energy dispersive spectroscopy (EDS) and a Philips EM400T TEM. Thus far, the nanosolder alloys that have been synthesized include Sn/In, Sn/Ag, and Sn/Ag/Cu with the Sn/In and Sn/Ag systems being prioritized due to the simpler nature of a binary alloy system. The overall resulting size of the synthesized Sn/In nanoparticles averaged 60 nm in diameter with an Sn/In ratio of approximately 50/50. The other alloys were confirmed via EDS to have the compositions of Sn-4Ag and Sn-4Ag- 0.5Cu, with both having average particle diameters between 20 and 25 nm from a typical synthesis. The melting temperatures of synthesized nanoparticles were measured to be slightly depressed (1- 5 °C) compared with their bulk counterparts, due the size-effect of nanoparticles with small diameter.
Nanosolder Melting and Cu-Cu Wire Soldering
To melt the nanosolders, different reflow methods were employed to test various parameters: (1) a hotplate, (2) a tubular furnace with programmable temperature profile, and (3) a benchtop surface mount reflow oven. For Sn/In samples, solder pastes were heated to a surface temperature of ~140 °C to allow for complete melting of the material. Sn/Ag based nanosolder systems are still currently being optimized and therefore reflow methods (2) and (3) are primarily used due to greater temperature control and the ability to reflow in inert environments (N2 or Ar). Typical processing temperatures of the discussed non-Sn/In alloys range up to ~265 °C due to the large difference in alloy melting temperatures.
Cu-Cu wire soldering is the model system that is used to demonstrate the feasibility of the Sn/In: nanosolder paste while allowing electrical measurements post reflow. Electrical measurements utilized a four-point method to produce the more accurate solder resistance values while neglecting contact resistance of the probes used. Measurements were taken five times for each solder joint and the resistance values were averaged for the 500, 250, 100 and 25µm diameter Cu wires, respectively. Experimental results have shown that increased resistance values have been observed when decreasing Cu wire diameter. This has been confirmed for both the SAC solder joints and Sn/In solder joints. The results showed that the electrical resistance of Sn/In solder joints are very comparable to those of SAC solder joints.
The synthesized nanoparticles were mixed with a flux to form the nanosolder pastes with similar rheological properties as the traditional type 4 solder pastes. The flux materials used are commercially available and currently being used in microsolder systems. Due to the desire for similar rheological properties as current pastes, the nanosolder loading is lower, with typical loading ranging from 20-60 wt% nanosolder particles. Commercial pastes are commonly in the range of 87-90 wt% with type 4 micropowders. A Sn/4Ag nanosolder paste containing ~45 wt% of nanosolders was printed though an uncoated, stainless-steel stencil. The stencil had a thickness of 6 mil (152 µm) with an aperture diameter of 28 mil (711 µm). The Sn/4Ag nanosolder paste released from the stencil walls and remained on the Cu substrate that was used. This demonstrated that it is possible to screen print nanosolder pastes.
Nanosolders on Nanowires and Infrared (IR) Heating and Melting
Tin (Sn) nanowires were synthesized using a template-assisted electrodeposition method that utilized a polycarbonate (PC) template and commercial tin plating solution. Nanowire size and morphology were characterized by the FE-SEM. The final length of the synthesized nanowires was in the range of 4 to 6 μm and the measured diameters ranged from 100 - 150 nm. Tin was selected as the best element to study, as many lead-free solder alloys are largely tin based. The melting temperature of tin (232 °C bulk) differs from the lead-free alloys by only 12 - 15 °C, so this was deemed to be the best choice for a single element soldering system.
After nanowire synthesis, the nanowire samples were prepared for infrared (IR) heating and melting. A suspension of Sn nanowires was deposited on to a silicon (Si) wafer, dropwise, and allowed to evaporate inside a fume hood. A flux was placed on a separate Si wafer, with both wafers being placed together on the IR preheating station. A thermocouple was attached to the surface of the substrate to monitor the temperature during the IR heating process. Prior to IR heating, there was a 1- 2 min preheat step that caused the flux to evaporate. The heating time recorded when the temperature rose to above 230 °C. The time above liquidus is referred as TAL, which can have a strong influence on the resulting solder joint properties. The peak temperature for the melting process was ~250 °C. When the melting occurs in the samples, it is expected to come with a large change in morphology,
i.e. from rod to spherical shape. Without introducing flux to the IR heating process, very little melting can be observed. Introducing a flux to the system showed a significant effect, as melting is observed with a soldering time of 30 seconds. Using flux helped remove surface oxide of tin solder and ensured a complete melting of solder.
Overall, most of the objectives listed in Section 2 have been achieved, with the solder joint reliability tests still ongoing; this is largely due to the lab renovation and lab moving during the project period and delayed purchasing of the temperature chamber for thermal cycling measurements. Solder nanowire melting and nanowire-nanowire joining are beyond what was originally proposed and the results showed the feasibility for even smaller scale soldering by using lead-free solders.
Conclusions:
In summary, nanosolder pastes were prepared using synthesized nanosolders of various alloys, from low melting temperature (Sn/In) to high melting temperature (Sn/Ag and Sn/Ag/Cu). Our synthesis method is a more environmentally friendly approach, using aqueous phase instead of toxic organic solvents like other methods. The nanosolder pastes prepared contains no lead and halogen flux, and thus are more environmentally benign. This ensures that future electronic devices manufactured with this material can be more easily recycled and, in the case of improper waste management handling, will pose a reduced threat if exposed to weathering in a landfill. The nanosolder pastes using Sn/In nanoparticles have been used for Cu-Cu wire joining, where joining down to 25 µm Cu wire has shown to require less effort compared with traditional micropowders. Nanosolder pastes prepared with Sn/Ag have been screen printed and observed to melt on pure Cu substrates with a peak melting temperature of 265 °C. Melting is observed to be more difficult to
achieve, due to the increased amount of surface oxides present on the smaller 25 nm nanosolders. Electrical measurements have shown Sn/In nanosolder joints to have similar resistance when compared with joints prepared with SAC commercial solder paste. Finally, tin nanowires have been prepared and characterized for use in IR heating processes. The nanowires have been shown to melt under IR heating conditions, with flux playing a critical role in the melting.
Recommendations: (1) Surface oxidation is a significant issue for smaller nanosolder particles; effectively removing the surface oxide and ensuring complete solder melting is critical in nanoscale solder joining. (2) The printing performance through nanosolder dispersion and paste homogeneity could be improved. As previously mentioned, the aperture walls were completely uncoated, generally representing the most difficult scenario for paste release to occur. Various coatings could be studied that help improve paste release for various geometries, which may further help nanosolder paste printing in the future. (3) A combination of a more active flux and higher solder paste loading will very likely enable even smaller features to be produced from a lead-free nanosolder paste.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 16 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Wernicki E, Fratto E, Shu Y, Gao F, Gu Z. Micro-scale solder joints between Cu-Cu wires formed by nanoparticle enabled lead-free solder pastes. In2016 IEEE 66th Electronic Components and Technology Conference (ECTC) 2016;1203-1208. |
SU835938 (Final) |
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Supplemental Keywords:
Lead-free, Nanosolder, Cu-Cu joining, solder paste, electronics assembly and packagingProgress and Final Reports:
Original AbstractP3 Phase I:
Green Nanosolder Paste for Next-Generation Electronics Assembly and Manufacturing | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.