Alternative solder materials for electronic devices working at low/cryogenic temperatures

Leuven - PhD
More than two weeks ago

Which solder will work for the packaging of quantum computers? The goal of this PhD thesis is to select and study the metals that can be used as solder and UBM for packaging and 3D integration of devices for low temperature applications particularly quantum computers.


For some applications including quantum computers, aerospace, medical devices like MRI, cyclotrons for high energy physics research and cryogenic oxygen generators, electronic circuits and devices must be operated at very low temperatures. For instance, in quantum computers, to prevent unwanted thermal excitation of the states, qubits must work at temperatures around 20-50mK. These applications strongly push for the development of future (3-5 years from now) microelectronics devices and packages which can work reliable at these low temperatures.

At this moment, Sn-based solder bumps are used typically to electrically connect silicon chips to a substrate for packaging, or to each other for 3D-technology (stacking and interconnecting of many thinned Si-chips). These bumps are plated between Cu-pads, called under-bump metal (UBM).

However, at such low temperatures, these conventionally used Sn based solders will undergo a ductile to brittle transition due to a phase change from the white (β) to the gray (α) phase, resulting in powder-like material and thus catastrophic failure of electronic components.   

The goal of this PhD thesis is to select and study alternative metals that can be used as solder and UBM for packaging and 3D integration of devices for low temperature applications.

Indium and bismuth based solders are two potential candidates.

The properties to be studied include:

  • processing related properties, such as wettability and assessment of electroplating
  • material properties such as IMC (intermetallic compound) phase formation, grain size and orientation, and the thermal expansion coefficient. And how these properties change at lower temperatures.
  • mechanical properties such as tensile strength and fracture location, also as a function of (lower) temperature
  • electrical properties such as the resistance in function of (low) temperature
  • reliability properties, such as electromigration and corrosion resistance, at low temperatures

To understand the IMC phase evolution and wettability, phase field and molecular dynamic simulations might be needed as well.

The PhD candidate should be familiar with material science and different analyses techniques. Practical know-how of techniques such as SEM, XPS, Auger, contact angle measurements, TEM, AFM, EBSD is an asset. Experience with electrical measurements to study the electrical properties of different UBM and solder materials is a plus-point.

Required background: material scientist, chemistry, electrical engineering, physics

Type of work: 10% Literature, 70% experimental,20% modeling/simulation

Supervisor: Ingrid De Wolf

Daily advisor: Jaber Derakhshandeh

The reference code for this position is 1812-84. Mention this reference code on your application form.


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