Advances in Conductive Adhesives at Ambient Temperature

Fig. 1. Various applications of metallic glue: (a) A central processing unit (CPU) on a printed circuit board (PCB) connected to a heat sink, (b) a surface mount device attached to a PCB, (c) a press-fit pipe fitting for environments where welding is dangerous or impossible, and (d) a glass plate attached to metal with a different coefficient of thermal expansion to cover a cavity with a hermetic seal.

Fig. 1. Various applications of metallic glue: (a) A central processing unit (CPU) on a printed circuit board (PCB) connected to a heat sink, (b) a surface mount device attached to a PCB, (c) a press-fit pipe fitting for environments where welding is dangerous or impossible, and (d) a glass plate attached to metal with a different coefficient of thermal expansion to cover a cavity with a hermetic seal.

Metallic glues can serve as excellent conductors for heat dissipation and electrical current in electronic devices and also as leak-resistant seals for vacuum environments. The potential market for these applications is extensive and growing rapidly.

Technological Relevance

It is common practice to join two solids together using a third substance for gluing or soldering. Gluing usually refers to the joining process that is made in ambient conditions—at room temperature, in air, and without pressure, or with a small amount of mechanical pressure.1 Sealing an envelope with polymer glue is a good example. Despite this process being easy and inexpensive, it often produces properties that make it unsuitable for use in high-tech environments. For example, polymer glue—unlike metallic solder—is permeable to air and moisture, degrades fast in ambient temperature or environment, has low mechanical strength, does not effectively conduct electricity or heat, and does not retain its function at high temperatures.2,3

Gluing usually refers to the joining process that is made in ambient conditions—at room temperature, in air, and without pressure, or with a small amount of mechanical pressure.

Fig. 2. Low-temperature metallic gluing enabled by well separated metallic nanorods: (a) Two sets of well separated nanorods, which have metallic cores and shell elements that form a eutectic alloy, are brought together, (b) they interpenetrate under fingertip pressure, (c) shell elements meet and form a eutectic alloy, which is liquid at room temperature, and (d) mixing of eutectic liquid with a metallic core leads to formation of three-component alloys that are solid at room temperature.

Fig. 2. Low-temperature metallic gluing enabled by well separated metallic nanorods:
(a) Two sets of well separated nanorods, which have metallic cores and shell elements that form a eutectic alloy, are brought together, (b) they interpenetrate under fingertip pressure,
(c) shell elements meet and form a eutectic alloy, which is liquid at room temperature, and
(d) mixing of eutectic liquid with a metallic core leads to formation of three-component alloys that are solid at room temperature.

In contrast, soldering usually refers to the joining process that uses added molten metal at increased temperatures, generally much higher than room temperature.1 Similarly, welding and brazing also involve high-temperature melting, where brazing refers to joining through added molten metal at even higher temperatures than soldering, and welding involves melting or fusing the members to be joined, often under an inert environment.1 The joining from such high temperature processes, as compared to polymer glue, is mechanically strong, effectively conducts electricity and heat, and degrades slowly (if at all) in ambient environments. Further, its leak resistance to air and moisture goes from good to better with time due to oxidation.1

Metallic gluing refers to the process of joining two solids with metal as the connecting party, which operates at room temperature, in air, and under low pressure. Metallic glues feature the combined advantages of the ambient condition of gluing and the superior properties of the joint from high-temperature soldering (or welding and brazing), making them beneficial to many advanced technologies.

As an example, consider desktop and laptop computers. The core of computing is the central processing unit (CPU), and connecting the CPU to external components for heat dissipation or electrical conduction is necessary. The process of making the connection, if it requires high temperature, can damage the CPU by exceeding the thermal budget.4 For heat dissipation,5 an ideal connection conducts heat efficiently, which makes metals with high thermal conductivity desirable. However, if solder is used, the temperatures necessary to create a good bond can damage the CPU. Also, solder bonds can be relatively thick, resulting in reduced heat transfer. Further, the thermal conductivity of most solders is low, conducting roughly 5%-20% as effectively as a pure metal such as copper.6,7

Fig. 3. Scanning electron microscope image of well separated Cu nanorods. Cour- tesy of X. Niu, et al., Phys. Rev. Lett., Vol 110, 136102, 2013.

Fig. 3. Scanning electron microscope image of well separated Cu nanorods. Cour- tesy of X. Niu, et al., Phys. Rev. Lett., Vol 110, 136102, 2013.

Thermal grease is often used as an interface material, filling the space between the heat sink and CPU. However, the thermal conductivity of this grease is only a fraction that of copper—a mere 1%-2%.7,8 This low conductivity limits the amount of heat that can be dissipated from the CPU and is a significant barrier to further miniaturization and reliability of devices such as tablets and computers. Thermal greases also suffer from problems such as pump out, where grease is forced out of the interface during thermal cycling, and dry out.5 Figure 1a shows the configuration of a CPU with a heat sink in a laptop computer, for simplicity. Desktop computers often contain an additional protective and heat transferring plate between the CPU and heat sink with two separate interfaces requiring conventional technologies. Now, advancements in both science and technology have made this sought-after ability a reality.11 Figure 2 outlines a new process that uses nanostructures and eutectic alloys to produce a room temperature metallic glue with the desirable properties of solder. In Fig. 2a, two surfaces to be bonded together are shown facing one another. Each surface is covered with core-shell nanorods. When the mating surfaces are brought together, the large spacing of the nanorods allows them to slide between those on the opposing surface and to interpenetrate (Fig. 2b). When the shell materials from opposing sides come into contact, which together form an alloy with a eutectic temperature at or below room temperature, a liquid alloy is quickly formed (Fig. 2c). Interdiffusion between the liquid alloy and the nanorod cores leads to solidification as thermal grease, effectively doubling the thermal barrier problem.

In CPUs, and also in many throughhole and surface-mount devices, it is necessary to connect the electrical component to other components, generally through a printed circuit board (PCB). The components experience heating when they are soldered to a PCB or require very precise wire bonding or flip chip equipment, which often demands a thermosonic bonding method. In some cases, temporary heat sinks must be attached to the component during soldering to prevent damage.9 Also, as component size decreases, soldering or wire bonding becomes more challenging and voids can lead to joint failure.10 A metallic glue bond eliminates the possibility of heat damage during attachment and simplifies the soldering process to merely pressing parts together to attach (Fig. 1b).

Metallic glues feature the combined advantages of the ambient condition of gluing and the superior properties of the joint from hightemperature soldering (or welding and brazing), making them beneficial to many advanced technologies.

A third example involves connecting pipes or construction parts together, which highlights the benefits of the metal bond’s strength (Fig. 1c). With metallic glue, no gases, electricity, or heat is required. This facilitates a process that poses zero risk of asphyxiation, electric shock, or burns, and occurs in safe environments where welding may not be safe or possible, such as hot work in confined spaces. In addition, no welding skill is required.

Fig. 4. Metallic glue formed in air and under a small pressure of 9 MPa (a) at room tempera- ture, and (b) at 100°C. Reprinted with permission from Scientific Reports[15].

Fig. 4. Metallic glue formed in air and under a small pressure of 9 MPa (a) at room tempera- ture, and (b) at 100°C. Reprinted with permission from Scientific Reports[15].

As a fourth example, the hermetic sealing of materials with much different coefficients of thermal expansion (CTE) benefits greatly from a room temperature bonding method. Generally, when sealing metal to ceramic or glass, materials must be carefully selected to have a similar CTE. If the CTE difference is too large, parts may separate due to geometric mismatch when cooled. When selection of similar CTE materials is not possible, part geometry must be carefully designed so that thermally induced stresses do not become too large to cause warping or material failure. Application examples include compact fluorescent bulbs, glass encapsulated diodes, and windows for inspection and diagnostics in industrial processes and vacuum chambers (Fig. 1d).

Nanoscience-Enabled Technology

Combining the ambient conditions of gluing with the desirable properties of soldering would be possible if one could use metal as solder at room temperature. Until recently, this remained wishful thinking based on the composition deviates from that of eutectic alloys of low melting temperature (Fig. 2d).

Development of this emerging technology is based on efforts to understand how and why nanostructures grow at a fundamental level. One important subject of investigation in nanoscience has been nanorod growth using glancing angle physical vapor deposition.12 A recent breakthrough in this field involves the development of a theory for both the diameter and separation of nanorods.13,14 Guided by this theory, the smallest, well separated metallic nanorods came to light (Fig. 3).

Developing the ability to produce well separated nanorods is an important step in realizing this technology, due to the necessity of interpenetration of the nanorods. If they are not sufficiently well separated, the rods will contact one another head-on and act like a porous film. Consequently, bonding will not be successful at low temperatures.15 At this small scale, if the separation is sufficient, a small shear stress will align the nanorods for inter-digitation, even if they are not well aligned upon initial contact. Further, at the small diameter, a new mechanism of surface diffusion becomes active, moderate load with forced air cooling, the metallic glue reduces the CPU temperature by 8°C ±3°C compared to the widely used thermal grease, Arctic Silver 5, operating at 61°C. This is significant, as keeping the CPU 10°-15°C cooler can double its lifespan.5 The leak rate of the metallic glue shown in Fig. 4a is three orders of magnitude lower than that of polymeric glue. This leak resistance meets the standard for organic solar cell and organic light emitting diode technologies.15 allowing them to survive long-term, which may lead to a new generation of inexpensive solar and lighting technology. Further, as demonstrated in Fig. 1d, metallic glues are also useful as a vacuum seal. Capitalizing on the superior leak resistance so diffusion on the nanorod surface is much faster than on flat surfaces.16 Contact of the sides of the nanorods through interpenetration provides high surface area contact, maximizing the effects of the fast surface diffusion.

While the use of eutectic materials as shells shows preliminary results of a room temperature bond at very low pressure, it is possible to use simpler, single element nanorods in place of the core-shell structure required in the eutectic. Silver was successfully used to create such a bond, but requires higher pressure for sealing.15

Technological Impacts

The impact on technology is clear, even using only well separated silver metallic nanorods without a shell. Following the processes in Figs. 2a and 2b, the fast surface diffusion of nanorods without the liquid formation of eutectic alloys, gluing also occurs, although with some voids (Fig. 4a).15 To reduce void concentration, a higher processing temperature is needed. As shown in Fig. 4b, performing the gluing process at 100°C largely eliminates voids. Using core-shell nanorods, and therefore the assistance of liquid from the eutectic alloy, it is expected that the room temperature gluing process will produce a bond that is void free, as seen in Fig. 4b.

Even with voids, the metallic glue shown in Fig. 4a has superior thermal conductivity and leak resistance. In tests running a simulated CPU at of the metallic glue, MPF Manufacturing is investigating using the technology and licensing the patent.17

Looking forward, the core-shell nanorod glue is expected to perform even better. First, the use of eutectic alloys through the core-shell nanorods will reduce or completely eliminate the voids. As a result, leak resistance will further increase, and heat conduction will become even more effective. Second, the presence of liquid alloys instead of solids will likely reduce the processing pressure from a few megapascals to a fraction of one megapascal, equivalent to fingertip pressure.

FOR MORE INFORMATION

Hanchen Huang is professor and chair, department of mechanical and industrial engineering, Northeastern University, 360 Huntington Ave., Boston, MA 02115, 617.373.5558, h.huang@neu.edu, ilab.coe.neu.edu.

ACKNOWLEDGMENTS

The authors acknowledge financial support from the Department of Energy Office of Basic Energy Sciences (DE-SC0014035), and Hanchen Huang also thanks Jianmin Qu for suggesting applications involving heat sinks.

This article was originally published in Advanced Materials and Processes, January 2016 pages 22–24.

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