Low Pressure and No Pressure Sintering Approaches for Large Surface Areas

In this article we will present the latest results in the area of low pressure/ no pressure silver sintering on using solvent free silver sintering pastes. With these materials sintering with low or no pressure at 250°C results in very dense silver interconnects between electronic components.

 

Silver Sintering

Silver sintering is on the rise. The need of more reliable and better performing interconnects in power- and optoelectronics pushes the limits of the state-of-the-art interconnection materials. Soldering based on the tin is becoming the bottleneck in terms of heat dissipation, electrical resistance, re-melting, and reliability. Furthermore, due to the trend of miniaturization, growing power densities, and switching to SiC and GaN challenges tin-based soldering up to its physical restrictions.

Silver sintering solves all of the above-mentioned problems with Rel. = 1.6 μΩ*cm, RTh. = 459 W/m*K and a melting point of Tmp = 961°C the performance of a silver interconnect is superior to the state of the art! Of course, there is a downside. Generating reliable interconnects with a simple process as soldering is, challenges the manufactures of silver pastes.

 

Sintering with Pressure

Here, sintering with pressure is an established process, which yields good interconnects based on silver. But, setting up pressure of 22 MPa is only applicable for some components and surface areas. In addition, manufacturers tend to avoid the high costs of new product lines that are capable of pressure-assisted and wait for solutions that are easy to implement in their already existing infrastructure: pressure-less sintering.

 

Figure 1: Modes of failure of silver sintered interconnects, via pressure-less sintering. Top left: crack through the layer, with delamination of the substrate.; Top right: High porosity of sintered silver layer.; Bottom: Drying channels, large voids, delamination

 

The drawback of current solutions of silver pastes for pressure-less sintering is that they only work reliably for surface areas below 20 mm². The modes of failure are common and always can be assigned to the used silver material (Figure 1). All of the modes show n figure 1 are not favorable for a reliable interconnect and will not out-perform tin-based soldering solutions.

 

Two Main Factors Contributing to a Negative Outcome: 

1) Wrong sintering process parameters and

2) Poor quality of silver pastes. Finding the right process parameters for several geometries and surface finishes can be really simple, but only if the quality of the chosen silver sinter paste is matched with the scope of the application. Usually, too high a content of organics and the metal particles and additives are not perfectly matched.

 

Figure 2: Approach of Nano-Join pastes.

 

Products of Nano-Join

The products of Nano-Join distinguish themselves from its competition through a high metal loading (minimum of 92 wt% pure silver) without the addition of any solvents. To make our paste production independent from material suppliers we invented a different approach compared to our competition. We only use two main components for our paste, a highly designed silver salt and our in-house produced silver micron particles, such an innovative process allows us to form a material with industry-standard workability (figure 2).

This unique approach leads to a paste-system which is not only capable of sintering surface areas of up to 500 mm² in a pressure-less process with very dense silver layers for highest conductivity but also offers the possibility to sinter surface areas of up to 3000 mm² with a pressure of only 5 MPa. In both cases an RTh. = 300 W/m*K and a Rel. = 3.0 μΩ*cm are possible, in comparison standard soldering (SAC or Au/Sn) can only offer a maximum RTh. = 100 W/m*K.

Furthermore, there is no limitation in terms of surface finish, silver, gold, copper or even copper alloys can be interconnected. Paired with a long shelf life of at least 6 months at room temperature, the avoidance of silver nanoparticles in the paste and a large variety of scenarios of applications gives manufacturers a powerful tool to improve existing products and processes! Customers, with the desire to switch from soldering to the next generation of interconnection materials in a pressure-less process, do not have to change the already existing infrastructure.

Nano-Join materials can be applied through common techniques: dispensing, pin-transfer, screen or stencil printing. The sintering can be carried out in a standard reflow or convection oven under air or any inert gas. If customers already have a sintering process in line, where pressure is used, switching to our products, will also have a positive effect on the processes. Currently, for the above-mentioned application scenarios, we have four products available (figure 2): NJ-One, NJ-Dispense, NJ-Surface and NJ-Force. The latter one for pressure-assisted sintering, the other materials for pressure-less processes.

 

Figure 3: Performance of silver sintered copper-to-copper interconnects. Top left: Demonstrator with a total surface area of 3000 mm².; Top right: X-ray scan of one DBC to Cu-plate interconnect.; Bottom: Cross section through two copper plates sintered in a pressure-less process with a surface area of 500 mm².

 

In several projects, we have been able to prove the performance of the above-mentioned materials. With industry partners, we have been able to demonstrate the capabilities of our silver pastes to interconnect three copper DBCs (surface area above 3000 mm2) with a copper baseplate (figure 3). Sintering was carried out for 5 min and a pressure of 5 MPa was applied.

The resulting C-SAM, X-Ray, and Lock-in-Thermography measurements revealed a good interconnection between the substrates, no voids or delamination of the substrates was observed. To evaluate the mechanical performance of the resulting interconnects, small copper shear bodies have been sintered under the same conditions. On average, shear values of 73 MPa have been observed.

This approach offers the chance to sinter not only a dye on a DBC and still interconnect the copper ground plate to the DBC via soldering, but also manufacturers can now sinter both interconnects in two or even in one step! The limitations of surface area and pressure are no longer a challenge in the assembly of modern power modules. To stretch the limits of pressureless sintering even further, we have shown in previous results that with our materials the interconnection of surface areas of up to 500 mm² is already feasible (figure 3).

 

The usage of non-toxic and REACH/ ROHS compliant Nano-Join silver pastes offers our customers new possibilities to,

1) finally make the switch from soldering to pressure-less sintering without the need of new investments in infrastructure,

2) assemble power modules in a one-step approach by using only low pressure to sinter dyes and DBC together,

3) a superior performance compared to the competition, and 4) the potential for manufacturers to lower the overall system costs.

Please contact [email protected] for further information.

 

About the Author

Battist Rabay received his Diploma in the Field of Chemistry and Doctor of Philosophy(PhD) Field of Study Inorganic Chemistry at the Humboldt University of Berlin. He is the Managing Director and Founder of Nano Join.

Michael Doktor received his degree in the Field of Information System at Kiel University of Applied Sciences, MBA in the field of Activities and Societies: IADIS, Publication in the field of Knowledge Mangement and E-Learning, and Master of Arts in Business Informations Systems. He is the CCO and Founder of Foxy Power.