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Posted on 05 July 2019

Cu Bonds and Chip-to Substrate Joints Beyond Silver Sintering

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A new set of interconnection methods is the enabler for an extended module’s lifetime

Since power electronics employs power modules the reliability of bonded semiconductor dies inside a package was always a concern. By optimising the standard aluminium wire bond process a constant power cycling reliability improvement has been observed over last years. Nevertheless, looking at the mechanical and electrical limitations of the Al bonding, we now seem to have reached the limits of this technology [1], [2]. With copper bond wire bonding and diffusion soldering Infineon Technologies opens a gate for the next module power density and reliability level [3]. This new set of technologies is ready for operating at junction temperatures up to 200°C.

By Piotr Luniewski, Karsten Guth, Dirk Siepe, Infineon Technologies AG

Every power electronic system is designed for a certain failure free operational time where the time is often taken as a design criterion and depends on the reliability of system ingredients. The second design criterion is very often a high power density which can be associated directly with the semiconductor operation temperature. The combination of both criteria is a major challenge for power module design and calls for new reliability curves. Therefore, this requests for new connection technologies to be implemented [1-4].

Copper wire bonding

The lifetime of today's aluminium (Al) bond wire interconnect is not limited by the bond interface anymore but by the wire material itself. As the coefficient of thermal expansion (CTE) of the semiconductor die (Si) and the aluminium bond wire (Al) do not match, a periodic stress is introduced during temperature cycling [5]. The thermomechanical stress results in the wire lift-off additionally accelerated by increasing semiconductor’s forward voltage drop. It is important to note that the wire-to-die contact degradation does not happen at the die surface, but within the wire itself. The initial bond crack is always formed near the semiconductor interface and propagates within the Al matrix along the Al-Al grain boundaries. In order to limit the degradation of the bond interface copper (Cu) seems to be a good candidate as replacement material for an aluminium wedge bond. In addition to its superior mechanical properties, copper also offers better electrical and thermal characteristics compared to aluminium. A comprehensive collation of thermo-mechanical parameters is shown in table 1. A lower electrical resistivity and increased thermal conductivity can directly be converted into higher current densities for IGBT modules.

Comparison of material properties

Up to now, the main obstacle for ultrasonic Cu bonding of heavy wires was the mismatch in the mechanical properties of Cu and the semiconductor's topside metallisation. For the standard Al topside metallisation the Cu wire simply sinks into the soft Al matrix, leading to chip damage and weak bond interfaces. Consequently for new chip generations a new metallisation stack with Cu as the final front side layer has been developed. Figure 1 shows a DCB substrate with die copper metallisation and 400µm copper wedge bonds.

DCB substrate with 400µm Cu wire bonds on Cu metalized IGBTs.

Diffusion soldering versus silver sintering

Nowadays, the most common method of attaching semiconductor dies to substrates is a soft soldering process. Nevertheless, this technology limits further semiconductors’ operational temperature increase by the low melting point of today’s soft solder materials. Therefore, in addition to a new bonding process, an overall power module reliability improvement requires a change in the die-to-substrate interconnect as well.

One alternative technology invented in 1986 [4] is sintering, often called as low temperature joining technique (LTJ). The technique is productively used in manufacturing of large area bipolar semiconductors. Recently, LTJ was also implemented in IGBT module production [6]. During the LTJ process Ag powder and chemical additives are sintered under moderate temperatures (approx. 230°C) and high mechanical loads (20-30 MPa) to form a porous interconnection layer between substrate and die. The process time depends on temperature and pressure but needs some minutes. Finally, a very strong and homogenous connection between die and substrate is created.

Due to high material costs, non-compatibility with today’s soldering technologies, extreme process parameters, long time process, need of noble materials and complex tools (machines) the rollout of this technology is not really seen in mass production.

Based on above considerations Infineon has developed a diffusion soldering process for power semiconductors to form a high melting bond between chip and substrate [1]. Depending on the choice of chip metallisation and the soft solder material in standard soldering usually Cu-Sn or Ni-Sn intermetallics are formed as thin interfacial layers. All these intermetallic compounds have a much higher melting point than the Sn-based solder from which they were formed. For example, depending on the process parameters in the Cu-Sn system either Cu3Sn with Tm=676°C or Cu6Sn5 with Tm=415°C is formed during the soldering process. In diffusion soldering this solidification process is exploited to form pure intermetallic joints with a re-melting temperature Tm>415°C from Sn-Ag solder. Figure 2 shows a schematic comparison between a standard and a diffusion solder joint.

Schematic comparison of a diffusion soldered joint and a standard solder joint

While both joints are formed from a Sn-rich solder, in the standard joint only a fraction of the Sn is transferred into a high melting intermetallic phases. By contrast, in the diffusion soldered joint, the whole volume of low melting solder is consumed by the solidification process. The result is a high melting bond between chip and substrate. Depending on the ratio between the two different intermetallic phases, that are formed in the Cu-Sn system, the homologous temperature for these joints ranges from Thom=0,52-0,65.

Optimised process parameters yield a controlled solidification of the joint within seconds. The complete conversion of the solder into high melting intermetallics can be ensured by the parallelisation of process steps. Diffusion soldering finally creates a high melting chipto- substrate bond (Tm>415°C) with joint thickness d ≤10 µm where the cross-section is shown in figure 3.

Cross section of a diffusion soldered sample

During technology development, this new diffusions soldering technique has been transferred to a fast pick and place process, realising high throughput and a high degree of automation.

.XT Technology

A set of new Infineon technologies: copper bond wires, diffusion soldering (both described in the article) and improved system soldering, called .XT technology, result in a new power cycling curve presented in figure 4 [3].

PC diagram - standard IGBT 4 Curve and IGBT4.XT power cycling

The power cycling curve of the .XT technology reports a higher power cycling capability compared to the standard IGBT4 power cycling curve. Besides this the target curve is already valid for an operation junction temperature up to 175°C. The increase of power cycling gives additional freedom in the inverter design, for example:
- increased lifetime for same output power and cooling conditions
- increased output power for same cooling conditions and lifetime
- decreased cooling conditions for same output power and lifetime.

The first commercially available power module with the .XT technology will be the FF900R12IP4LD representing PrimePACK™ modules family.

References:

[1] K. Guth, at. all, New assembly and interconnections beyond sintering methods, PCIM2010.
[2] D. Siepe, at. all, The Future of Wire Bonding is? Wire Bonding1, CIPS2010.
[3] A. Ciliox, at. all, New module generation for higher lifetime, PCIM2010.
[4] T Licht at. all, Sintering technology used for interconnection of large areas: potential and limitation for power modules, CIPS2010.
[5] J. Goehre. at. all, Degradation of Heavy Wire Bond Interfaces, Bodo’s Power, June 2010.
[6] U. Scheuermann, P. Beckedahl, The Road to the Next Generation Power Module – 100% Solder Free Design, CIPS2008.

 

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