Ag-Sintering as an Enabler for Thermally Demanding Electronic and Semiconductor Applications


Marco Koelink, Business Development Manager and Commercial Manager at Alpha Assembly Solutions
Michiel de Monchy, European Applications Manager Die Attach and Preforms

Silver sintering (Ag sintering) has become a reliable industrial bonding technology with superior thermal and electrical performance while meeting automotive grade quality standards.

 

What is Ag sintering?

Silver (Ag) sintering or low-temperature diffusion bonding is receiving an increasing interest, mainly because of excellent electrical and thermal conductivities compared to other metals. In combination with some very interesting optical properties, the potential applications range from power electronics to printable electronics and (optical) biosensing (Peng, 2015).

The use of Ag sintering is currently mainly driven by either the replacement of lead-containing bonding materials (environmental or sustainability considerations) or the application in power electronics, specifically in applications which are sensitive to energy efficiency due to limited power availability such as is the case for instance with electrical vehicles (EVs).

The main consideration in transitioning from traditional bonding materials to Ag-sintering are cost and reliability (Scola, 2015). As early adopters of this technology are now pioneering the application of this technology on an industrial scale, more and more reliable data is becoming available.

As the technology is maturing and the number of applications is growing, it is to be expected that also prices will eventually go down to become more comparable with more traditional bonding materials. This will open the market for more widespread applications.

This paper discusses some of the background of Ag sintering, as well as some of the industrialization and reliability aspects of this technology. The companies APC, Boschman Technologies and Alpha Assembly Solutions provide respectively development, equipment (industrialization) and material services in this field.

Many developments in solder technologies have over the last 10 years been driven by international legislation to achieve lead-free solder materials and improve the reliability of the joints. Recently also, the introduction of electric and hybrid automotive vehicles spurred the demand for efficient high-power electronics, mainly to improve the driving range (most important benchmark in competition with traditional vehicles). Several technologies have been introduced meanwhile to achieve high-performance power modules with high reliability. Some examples of such technology include for instance gold base, high-cost solders such as AuGe and AuSn, SnSb alloys, as well as silver sintering.

The silver sintering technology has been pioneered by several solder material companies for several years (Siow, 2014). Using the proprietary knowledge of APC and Boschman Technologies, specifically the dynamic insert technology in which the technology has recently and successfully been industrialized.

 

The Ag Sintering Process

The Ag sintering  process is based on solid state diffusion, where silver particles are fused together and to the metallization of dies and substrates. One of the major drivers for this process is the change in free energy within the silver sintering product. Smaller particles will have more free energy and need less external energy to initiate the fusion process. Argomax®, a product group developed by Alpha Assembly Solutions (see Figure 1) that contains agglomerates of particles of about 20 nm, thus allowing sintering parameters at temperatures comparable or lower to those of lead-free solder reflow.

This temperature together with the relatively low pressures of maximally 10 MPa allows for a wide range of products to be sintered. The unique sintering inhibitor of the Argomax® allows the material to be delivered in either paste or pre-dried film. This will again increase the processing possibilities within industrialized processes.

 

Figure 1: Alpha offers a range of sintering materials in different formats suited for different applications

Figure 1: Alpha offers a range of sintering materials in different formats suited for different applications

 

The fusion of the silver will only succeed if the interface materials are pure metallic. Also, both the metallization of the dies and the substrates need to be relatively free of oxides. The easiest surface to bond to is Ag itself. If Ag cannot be used, noble materials such, as Au, Pd, or Pt are the next useable materials. The thicknesses of the metallization does not need to be more than 1 µm because the diffusion of the silver does not penetrate beyond 25 to 75 nm. When another specific Argomax® material is used, sintering to Copper in an ambient atmosphere can also be used. Unfortunately, it is not possible to bond to surfaces that have got dense oxide structures such as Nickel and Aluminum, nor can bare Si be used.

 

Figure 2: Sinterstar Innovate-F-XL; Universal Sinter System using Film-Assist and Dynamic Insert Technology; Ag Sintering Temperature up to 320 °C; Real time controlled pressure (0.2-40 MPa); Large sintering area 350 x 270 mm; Protecting gas supply optional and Suitable for all kind of carriers, such as: - Lead frames – Substrates - Ceramics – Wafers

Figure 2: Sinterstar Innovate-F-XL;

 

Universal Sinter System using Film-Assist and Dynamic Insert Technology; Ag Sintering Temperature up to 320 °C; Real-time controlled pressure (0.2-40 MPa); Large sintering area 350 x 270 mm; Protecting gas supply optional and Suitable for all kind of carriers, such as: - Lead frames – Substrates - Ceramics – Wafers

Meanwhile, silver sintering offers a new die attach technology with a void-free and strong bond with very high thermal and electrical conductivity (upt to 200-300 W/mK and 2-2.5 µΩcm). The Ag sintering process is defined either by temperature and time or temperature, time, and pressure. Whereas the process defined by temperature and time (“pressure-less”) is relatively easily industrialized (via reflow ovens or comparable), the process defined by temperature, time, and pressure require accurate and independent control of all three variables.

It is for the process that requires pressure that Boschman was able to develop and create both semi-automated (see figure 2) and full-automated equipment using their unique high precision dynamic insert pressure control in combination with sophisticated and precise temperature control.

The systems provide automated control of the sintering process with programmable temperatures up to 320 ⁰C, pressures dynamically variable between at least 10-30 MPa and a maximum sinter area of 350 x 270 mm. Boschman offers specific tool solutions according to customer wishes and application specific requirements. A roll to roll film protects the devices during the sintering operation and keeps the die clean. The system can also run without film in case direct hard sintering is needed.

 

Silver Sintering Applications and Reliability

Although silver sintering has attracted considerable attention, fundamental understanding of this technology is still limited. Recently, several studies have been published aimed at gaining in-depth insights into the physics of material and processes related to silver sintering (e.g. Yan 2015, Peng 2015). Increased fundamental understanding leads to better understanding of reliability and failure mechanisms.

Although with work is far from complete, sufficient information has become available to suggest that Ag sintering offers basically good shear strength performance and thermo-mechanical reliability under various conditions (Khazaka, 2014, Yan, 2016, Henaff, 2016 and Greca, 2016). The pressurised version shows better performance and better process control compared to the pressure-less version (Khazaka, 2014).

 

Figure 3: an example of a lead frame based power device using silver sintering

Figure 3: an example of a lead frame based power device using silver sintering

 

Silver sintering can be applied in many high-power or high-power-density applications. This includes solid state lighting, high-power (semiconductor) lasers, solar application (e.g. concentrated photovoltaics), power electronics for wind turbine systems and more. Figure 3 shows an example of a power device. Design studies for LED packages have shown to yield excellent performance and extremely stable against further assembly processes and harsh operating conditions (Jordan et al, 2015, see also figure 4). But also applications that are sensitive to energy losses are utilizing silver sintering for instance in energy harvesting in thermoelectric devices (piezoelectric) or printed electronics.

Specifically power applications, however (IGBT’s, RF-power, power MOSFET’s, thyristors,  and more) benefit from this technology (Yu, 2016). In particular application areas that are primarily driven by (ultimate) performance justify the use of this technology over the (initial) cost adder that is associated with this.

A prime example is the industry of electric and hybrid vehicles where efficient power electronics is a necessity to compete with traditional combustion engine vehicles to achieve sufficient driving range. Studies have shown that also automotive applications high performance can be combined with high reliability (Steger, 2012).

 

Figure 4: Relative comparison of different die-attach technologies suitable for LED applications. Courtesy of Courtesy Gyan Dutt, ALPHA, LED A.R.T Conference, Nov 17-19 2015, Atlanta USA

Figure 4: Relative comparison of different die-attach technologies suitable for LED applications. Courtesy of Courtesy Gyan Dutt, ALPHA, LED A.R.T Conference, Nov 17-19 2015, Atlanta USA

 

Another advantage of silver sintering is that, after processing, the melting temperature of the layer will be equal to the melting temperature of Silver (962°C). This entails that the maximum junction temperature Tj of a device can be significantly higher compared to the conventional die attach materials. (Khazaka, 2014).

Materials can as a rule of thumb only be reliably operated to 0.8 x the melting temperature in degrees C. (Knoer 2010). This means that a high lead solder can be operated up to 180°C, whereas a silver sintered bond can, in theory, be operated up to 760°C. In practice, the silver bond has been tested up to 500°C. This facilitates the application of silver sintering in a combination of wide band-gap semiconductor materials (SiC, GaN) which can operate at much higher temperatures compared to silicon-based materials.

Meanwhile, other application areas are being investigated, ranging from surface mount applications, interconnect fabrication, substrate bonding, printable electronics and more (Natsuki, 2015). Siow et al (Siow, 2016) published an overview on the development state of silver sintering as a function of patent applications, processes, materials and industries and companies that are commercializing this technology.

 

Ag Sintering Vs. Traditional Bonding Technologies

Ag sintering is emerging as a proven and reliable bonding technology for high-power or high-power-density applications providing superior electrical and thermal conductivity compared to traditional bonding technologies. The technology is particularly suited for high power electronics such as IGBTs and MOSFETs, applications with wide band-gap materials (SiC and GaN), and applications that require lead-free bonding materials or high performance (notably high power electronics for electric and hybrid automotive vehicles).

The sintering technology is mostly categorized in pressure-less and pressurized applications. Companies like Alpha Assembly Solutions and Boschman provide industrial services and solutions for the industrialized use of materials, processes, and production equipment for the pressurized applications that yield the best performance and reliability.

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More information: Advanced Packaging Center (APC)  •  Alpha Assembly Solutions    Source: Bodo's Power Systems, February 2017