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John O | November 2018

Researchers develop method for attaching heat sink without need for thermal interface materials

By Josh Perry, Editor


Researchers at the University of Binghamton (N.Y.) have developed a process for laser metal printing a heat sink directly onto silicon, removing the need for thermal interface materials (TIM) between a heat sink and the component and reducing the component temperature by as much as 10 degrees.


One way that the researchers tested their technique was by printing the Binghamton University logo onto silicon with the 3D metal laser printer. (University of Binghamton)


According to a report from the university, the researchers were looking for a method to reduce the thermal impedance of the TIM. While it is better to have the TIM than to leave air gaps between a heat sink and the chip, the researchers still saw room for improvement.


The team used a laser to melt and bond a tin-silver-titanium alloy directly onto silicon. It formed a thin bonding layer (titanium silicide) that provided the glue between the silicon and the alloy. It solidified at low temperatures to reduce thermal stress during contraction and the bonding process took only microseconds, which was fast enough for additive manufacturing to print the metal directly on the silicon.


By printing the alloy onto the chip, researchers can create microchannels for coolant to travel through directly and removes the need for a TIM to connect the metal heat sink to the chip. The technique was tested from 130°C to -40°C for a week to ensure its success over constant use.


“This solution removes both the lid and two thermal interface materials by printing the heat sink directly onto the silicon, giving heat a shortcut and lowering chip temperatures,” the article explained.


The research was recently published in Additive Manufacturing. The abstract stated:


“By employing selective laser melting (SLM), we demonstrate how Sn3Ag4Ti alloy can robustly bond to silicon via additive manufacturing. With this technology, heat removal devices (e.g., vapor chamber evaporators, heat pipes, micro-channels) can be directly printed onto the electronic package without using thermal interface materials.


“This has the advantage of keeping the current microprocessor about 10 °C cooler by eliminating two thermal interface materials. This reduces operating temperature, saving power and reducing electronic-waste. The bonding of common metal alloys used in additive manufacturing onto silicon is relatively weak and generally possesses high contact angles (poor wetting and interfacial strength).


“By using the proper interlayer material, wettability and reactivity with the silicon substrate increase drastically. Unlike conventional dissimilar material brazing that can take tens of minutes to form a strong bond, this study demonstrates how this kinetic limitation can be overcome to form a good bond in sub-milliseconds via intense laser heating.


“The mechanism for rapid bonding lies in using an alloy that can form a strong intermetallic bond to the substrate at a low temperature, and exposing the sample multiple times to give sufficient diffusion time for a strong bond. Bonding of Sn3Ag4Ti to silicon occurs through the formation of a thin (∼μm) titanium-silicide interfacial layer that makes the silicon wettable to the Sn3Ag4Ti.


“These printed parts are mechanically resistant to thermal cycling, with no mechanical failures visible after over a week of continuous thermal cycling (−40?°C and 130?°C).”

Learn more about the process in the video below:

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