Advanced Sintering Materials
Enable Next Generation Power Electronics

Thermal performance demands on advanced power electronics are increasing while traditional interconnection materials are reaching their thermal performance limits. Developing viable alternatives is essential to meeting the higher power densities, switching frequencies, and heat tolerance requirements of rapidly evolving power technologies. Some notable examples include:

  • Electric Vehicle (EV) Batteries & Charging Stations,
  • Solar & Wind Energy Production,
  • Big Data & Cloud Computing,
  • Advanced 3D Semiconductor Packages,
  • 5G Telecommunications, and
  • Electronics for Satellites and Space Exploration.

These technologies are pushing operating temperatures beyond 150°C, targeting 300°C in the near future.  High operating temperatures can lead to thermal stress induced failures, a significant issue that must be overcome to meet the increasing performance demands of next generation power electronics.

Limitations of Solder Alloys

In 2006, RoHS regulations were adopted by the European Union creating a demand for lead-free solder alternatives. Formulating lead-free solder alloys for high operating temperatures has been problematic due to melting temperature limitations. The melting points of most lead-free solder alloys are too low compared to traditional high-lead solders. As operating temperatures approach their melting point, strength decreases and the risk of solder joint deformation and failure increases.

Eutectic Gold Tin (AuSn) solder with a composition of 80% Au is a lead-free solder solution currently available. With a 283°C melting point, AuSn is well-suited for applications up to about 200°C. But gold is very expensive ($60-$120/g) and has a highly volatile cost structure. Currently, there are no other viable lead-free solder alternatives with melting points high enough to meet the high temperature demands of next generation power electronics.

To push beyond these limitations, a paradigm shift away from classic solder alloys was needed.

Nanostructured Sintering Metals

Nanostructured sintering materials consist of metallic nanoparticles with organic binder, dispersant and fillers added to create a paste with application specific properties. During sintering, heat is applied, organic materials are eliminated, and the paste is converted to a bulk metal state with its corresponding bulk metal melting point. 

Nanostructured sintering metals like silver (Ag) and copper (Cu) have been shown to be good alternatives to lead-free solders since they are known to exhibit a markedly reduced sintering temperature (about 80% below their bulk metal values) in the sub 50nm size range. At these small particle sizes, sintering temperatures drop to around 200°C, offering the advantages of both low processing temperature and high operating temperature.

Silver Sintering

Nanostructured silver was first to be investigated and commercialized for sintering due to the relative ease of synthesizing nanosilver particles, silver’s excellent thermal and electrical conductivity, and high bulk metal melting point that easily overcomes the operating temperature restrictions of lead-free solders. Silver sintering materials are also RoHS compliant.

Although silver sintering is a promising technology, its widespread adoption has been hindered by cost and limited availability. There are other drawbacks, the most prominent of which are electromigration and high pressure processing needed to reduce porosity.

At high temperatures and when subjected to an electric field in the presence of oxygen (or moisture), silver ions tends to migrate and form conductive filaments. This phenomenon, called “electromigration,” can cause short circuit failures.

Porosity during sintering can significantly reduce sintered silver’s thermal and electrical conductivity, requiring sintering to be done under high pressure, a process that is not conducive to high volume production.

Copper Sintering

Copper (Cu) has been used in the electronics industry for decades and is well known for its superior electrical and thermal characteristics. Copper is more abundant than silver, is about 100X cheaper, has a high melting point and is RoHS compliant. So, it is understandable that nanostructured copper sintering materials have been the focus of studies in the search for alternative materials that can meet the high-performance requirements of next generation power electronics.

However, commercialization of copper sintering materials has faced a couple of challenges. First, it is difficult to synthesize nanocopper, and second, it is susceptible to oxidation in air before sintering.

Kuprion’s ActiveCopper™ Sintering Materials

Kuprion has developed and patented a family of ActiveCopper™ sintering pastes, adhesives, inks and gaskets that overcome both the challenges of nanocopper synthesis and the oxidation dilemma. ActiveCopper™ outperforms silver sintering by eliminating the silver migration issue and has all the benefits of bulk copper without the drawbacks, fusing to form solid copper interconnects and strong bonds with a pressure-less, low temperature sintering process.

ActiveCopper™ materials can be stored at room temperature and handled in air and still fuse readily under a nitrogen atmosphere using standard manufacturing reflow equipment and reflow profiles. And because ActiveCopper™ converts to bulk Cu after the initial fusion step, it can be subsequently processed in unlimited reflow cycles, making it a good alternative to high temperature solder and sintered silver for reliable high-volume manufacturing.

After sintering, ActiveCopper™ exhibits high thermal and electrical conductivity with outstanding shear strength at room temperature and at 260°C, easily surviving over 1000 thermal shock cycles. And it demonstrates durability at 150°C for 1000 hours with a marked increase in strength over time, as ActiveCopper™ continues to densify and increase in strength. Characterized by a very fine, uniform nanoporosity (4-12%) that prevents hot spots, ActiveCopper™ materials distribute heat evenly across an interface.

ActiveCopper™ materials have been engineered to be resistant to oxidation at room temperature yet active enough to not require aggressive reducing environments during sintering. In addition, ActiveCopper™ has the unique capability of being adjusted to coefficient of thermal expansion (CTE) values ranging from 3 to 17 ppm. This unique CTE tuning capability enables CTE matching to ceramics, FR4, silicon, silicon carbide (SiC), gallium nitride (GaN) and more, allowing engineers to optimize heat dissipation while minimizing stresses caused by CTE mismatch.

Performance Characteristics of High-Temperature Interconnect Materials

Interconnect Materials
Properties ActiveCopper™ Gold Tin (AuSn) Sintered Silver Lead Solder
Thermal conductivity (W/mK) 250-393 57 140-200 25-50
Electrical conductivity (% IACS) 35-75% 8% 10-50% 10%
Melting Temperature (°C) 1084 280 960 183
CTE range (ppm) 3-17 16-17 17-18 24-25
Processing Temp (°C) 200-235 300-330 250-300 215-225
Shipping/Storage Room Temp Freezer Freezer Refrigerate
ROHS Approved Yes Yes Yes No: Banned in most Applications
Processing Time (min) <8 8 – 10 30-90 <8
Migration Negligible Negligible Yes No