Grantee Research Project Results

2016 Progress Report: Green Nanosolder Paste for Next Generation Electronics Assembly and Manufacturing

EPA Grant Number: SU835938
Title: Green Nanosolder Paste for Next Generation Electronics Assembly and Manufacturing
Investigators: Gu, Zhiyong
Current 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 Period Covered by this Report: October 1, 2015 through September 30,2016
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: Pollution Prevention/Sustainable Development , P3 Awards , P3 Challenge Area - Chemical Safety , Sustainable and Healthy Communities

Objective:

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.

Progress Summary:

Lead-free Nanosolders

Lead-free alloy nanoparticles were synthesized using a surfactant-assisted chemical reduction method [8] that utilized water as a solvent, making for a greener nanosolder production process. Nanoparticle size, shape, and composition were examined using a JEOL JSM 7401F SEM equipped with X-Ray Energy Dispersive spectroscopy (EDS). A Philips EM400T TEM was also used for further observation of size and shape of the synthesized nanosolders. The overall resulting size of the Sn/In nanoparticles that were synthesized averaged 60 nm in diameter. Composition analysis via EDS was determined to have a Sn/In ratio of approximately (50/50).

Nanosolder Paste Preparation

To create the nanosolder pastes, the nanoparticles were mixed with a flux to result in a solder paste with appropriate rheological properties that are comparable to industrial grade micro-solder material, which are typical Type 3 or Type 4 solder pastes. On a ceramic hotplate, the solder pastes were heated to a surface temperature of ~140°C to allow for complete melting of the material. The nonwetting substrate the nanosolder pastes were deposited on allowed for spherical solder features to form once solidified. Complete melting of the nanosolder paste was able to be achieved. The same paste was able to form many different sized solder features. The smallest reflowed solder bump shown (31 μm) is the approximate size of a single micropowder that can be found in Type 3 and 4 solder pastes. This showed the paste was able to melt and form different sized solder features.

Cu-Cu Wire Joining

The demonstrated capability to melt the nanosolder paste moved the focus to introducing Cu surfaces via the use of wires with varying diameters. Use of Cu wires allowed the interaction of common electronic joining surface with a nanosolder paste to be studied. Figure 2 shows 250 µm wires and 100 µm wires joined with the lead-free nanosolder paste under SEM. The visual evidence suggests the nanosolder pastes were able to wet the surfaces of both the Cu wires samples and form a joint upon solidification. Wires with a diameter as small as 25 µm were also able to be prepared via the lead-free nanosolder pastes. For comparison purposes, Cu-Cu wire joints were prepared using a type 3 Sn-3.0Ag-0.5Cu (SAC 305) solder paste. Due to the wire diameter being smaller than the SAC metal powders, the preparation of the 25 µm wire system was much more difficult. This was a notable difference from an application point of view with different joining scales. The solder features without Cu wires were spherical (Figure 1), however, after melting with wires they were observed to wet the copper surfaces and take on an elongated shape for all wire diameters. Due to copper being a wetting surface for tin-based solder materials, this suggested good wetting properties of the lead-free nanosolder materials. As a result, this material was proved to be capable of joining small wires with reliable and consistent wetting. With industry ambitions to join smaller feature sizes, the initial nanosolder paste joining results suggested the same phenomena may occur using with using smaller Cu materials. Future work will focus on decreasing the size of solder joints within small wire (<30 µm) joining using developed preparation and manipulation techniques.

Interface Study

After melting and joining of the Cu wire samples, further characterization was needed to confirm that a joint between the melted nanosolder paste and Cu wire surface has occurred. The visual indication of melting and wetting is a good initial indication of joining, however it must be confirmed that an interfacial reaction has occurred between the nanosolders and Cu wire surface. In it observed an intermetallic compound (IMC) has formed along the interface of the reflowed nanosolder and Cu. Energy dispersive spectroscopy (EDS) measurements suggested the formation of a Cu6Sn5 compound. This compound is often found in Cu joining systems using tin-based solders [9-11]. Formation of a small continuous IMC layer has been observed to form along the interface. The layer becomes discontinuous as it moves away from the interface. The IMC formation at the interface is due to the diffusion that occurred during the melting of the solder material.

Electrical Resistance Measurements

The Cu/nanosolder joint structures demonstrated the capability of joining Cu surfaces of varying size scales via the use of nanosolder pastes. Additionally, the resulting solder joints were used for further characterization of the nanosolder material via electrical resistance measurements. Four-point resistance measurements were used to determine the resistivity of the solder joint after deducting the resistance from the Cu wire. As the diameter of the wire decreased, this resistance became a very large contribution of the overall measurement. Measurements were taken 5 times for each known inner electrode distance.

Journal Articles:

No journal articles submitted with this report: View all 16 publications for this project

Supplemental Keywords:

Lead-free, nanosolder, Cu-Cu joining, solder paste, electronics assembly and packaging

Progress and Final Reports:

Original Abstract

2017 Progress Report

Final Report

P3 Phase I:

Green Nanosolder Paste for Next-Generation Electronics Assembly and Manufacturing  | Final Report