Development of Nano-solder for Low Temperature Pb-free Electronic Interconnect Applications Using a Modified Polyol Process


In recently years, two trends have emerged in the electronics industry, the first one is Pb-free soldering in electronic packaging and assembly, and the second one is the miniaturization of electronic products. Both trends have led to new challenges for electronic materials and for manufacturing. The conventional Pb-free Sn3.0Ag0.5Cu solder, with the solder alloy particle size in the micrometer range (10-100 μm), possesses a relatively high melting point (217℃), which results in defects and builds up stresses during the soldering process, and has limited application for low temperature processes, high density and very fine pitch (< 100 μm) electronic applications. Decreasing the size of the solder alloy particles to the nanometer range, can both decrease the melting point of the solder alloy and enable such solders to be used in very fine pitch (< 100 μm) applications. Chemical reduction is the most suitable method to synthesize nano-solder for industrial manufacture as it is a low temperature process with relatively low cost and high yield. The existing chemical reduction process to synthesize nano-solder in aqueous solution with NaBH4 as reducing agent is of low productivity, as precipitates must be centrifuged after the chemical reaction. It is also very difficult to obtain fine particles smaller than 10 nm due to the fast reaction rate. The aim of this project is develop a modified polyol process to synthesize Sn3.0Ag0.5Cu nano-solder with diameter smaller than 10 nm for low temperature Pb-free process and high density interconnect (HDI) packaging applications. No protective agent is needed in the reduction process. No centrifugal process is necessary after the reaction which results in high-throughput. The manufacturability of the nano-solder and the reliability of the nano-solder prepared interconnects will be evaluated.

R&D methodology

This project emphasizes the development of a modified polyol process to synthesize fine Sn3.0Ag0.5Cu nanoparticles for low temperature Pb-free solder interconnect applications with high-throughput.

(1) To develop a modified polyol process to synthesize Sn3.0Ag0.5Cu nano-solder.
The primary reaction involved in the reduction of silver nitrate with ethylene glycol (EG) at 120°C is called polyol process. EG serves both as a reducing agent and as a solvent. In the past decade, the polyol process was widely used to synthesize monodisperse metal nanoparticles, such as Fe, Co, Ni, Cu, Pd, Ag, Sn, Pt, Au, and their alloys. The advantage of the polyol process compared with aqueous process is that it can produce ultra-finer metal nanoparticles with uniform size distribution on a large scale. In this study, a modified polyol process with ethanolamine (EA) and EG used as reducing agents, will be developed for the sythensis of the nano-solder. As EA and EG are vehicles of flux, if we use metal organic compounds of Sn, Ag and Cu as precursors, and EA and EG used as reducing agents, the by products will be organic acid, so filtering or centrifugal separation of the precipitates is not necessary after the reaction. Vaporizing the solvent can result in suitable solder paste. This may improve the productivity and produce nanoparticles with a diameter smaller than 10 nm. To benchmark the modified polyol process, the conventional process using NaBH4 as reducing agent to synthesize Sn3.0Ag0.5Cu nano-solder in aqueous solution will be adopted as a reference.

(2) To demonstrate the manufacturability of the nano-solder.
As nano-solder is promising for use in the low temperature assembly processing of critical components, rework process, and HDI packaging, so the manufacturability of the developed nano-solder is very critical for these applications. A conventional surface mount technology (SMT) procedure, including solder paste printing, component placement, and reflow soldering will be adopted to evaluate the manufacturability of the nano-solder that will be developed.

(3) To evaluate the reliability of solder interconnects produced by the nano-solder.
Few results on the reliability of solder interconnects prepared by nano-solder have been reported in the published literature. In this project, a ball shear test will be used to study the mechanical strength of the solder interconnects which are prepared using the nano-solder. Thermal cycling tests, HAST (highly accelerated stress testing) and humidity tests will be performed at package-level to study the long term reliability of the newly developed interconnections.


The objectives of this project are:

  1. To develop a modified polyol process to synthesize fine nanoparticle-based solder with high productivity;
  2. To explore and demonstrate the manufacturability of Pb-free solder interconnects based on a nano-solder system;
  3. To investigate the reliability of Pb-free nano-solder formed interconnects.

Deliverable(s)

  1. Development of a modified polyol process to synthesize Sn3.0Ag0.5Cu nano-solder;
  2. Demonstration of the manufacturability of a solder joint based on a nano-solder system, substrate level ball grid array (BGA). Package-level bonding samples will be prepared by applying the nano-solder;
  3. Reliability evaluation of interconnects prepared using the nano-solder. Also, a database of development and collection of processing parameters will be established.

Project Commencement Date:
October, 2010

Project Completion Date:
September, 2011

Principal Investigator:
Dr. Winco K.C. Yung
Tel (852) 2766-6599

Project Team Member:
1.Dr. Winco K.C. Yung
2.Prof T.M. Yue
3.Dr. Terence Law
4.Mr. CP Lee
5.Mr. Brian Cheung
6. Ms. Joanne Wong