Gallium Alloy

This year’s Society of Vacuum Coaters Technical Conference was certainly as focused on vacuum deposition as it has been over the years, but we were delighted by the new emphasis placed on solar cell fabrication. As I mentioned before , one of our key topics was thermal evaporation material - although we also presented on the topics of nano-bonding sputtering targets and the availability of indium and gallium. Along with these topics, the audience was treated to themes of cell fabrication, increasing solar cell efficiency, and roll-to-roll processing.

At the show, we had a chance to discuss new ideas with many of our existing and potential customers. Improved throughput and eliminating alloy segregation were hot topics at the Indium booth. Many customers wanted to learn more about CIGS materials like indium forms for evaporation, or full-size Cu-Ga, CIG, & In rotary targets for magnetron sputtering.

2011 raised the bar for the Society of Vacuum Coaters Tech Convention, I can’t wait for the 2012 event to top it!

I’ve received numerous questions about using gallium liquid metal alloys, so thought I’d present some of my answers for all.  The customer questions are in black, and my responses in red.     

1. Indium’s product data sheet, "Indalloy Metals Liquid at Room Temperature" mentions that "any liquid metal will wet another clean metal surface". Can you please elaborate on the conditions that make such wetting possible? 

As far as wetting, gallium alloys (Indalloy 46L, Indalloy 51E, Indalloy 60, etc.), coat nearly any organic, ceramic, or metal surface. It is difficult to come up with materials used in packaging and processing these alloys that come clean of the alloy after use.   The physics of this affinity is unknown, however the low melting point and surface tension are the source/consequence.

Many metals will form some alloy with the gallium, but the solubility in gallium is limited. The gallium wets the surface and forms a solid gallium-alloy layer, which then acts as a diffusion barrier. In the case of aluminum, gallium forms an amalgam which ends up consuming a large volume of aluminum before a stable solid layer.

2. Do you assume an oxygen atmosphere (gallium oxide does wet most surfaces)? Or, to the opposite, do you refer to the wettability of plasma-clean metal surfaces?

3. Can you let me know if Indium Corporation sells droplet dispensing equipment for liquid solders/metals?

Liquid metals are typically offered in a syringe and various needle gauges can be screwed to the tip to adjust the dispensed droplet size. For dispensing thin layers of liquid metal, such as for a thermal interface, we recommend PVA selective coating equipment.

4.  We are interfacing our thermoelectrics with both steel and aluminum and the hottest point will be around 600°C. Can liquid metals be used for this?

InGa alloys are selected typically because they remain liquid at all times, negating any contact resistance between substrates.  The typical application for these has temperatures up to approximately 100°C or slightly higher.  At these temperatures we have noted some aluminum corrosion by the gallium, which will only become worsened by your elevated temperature. 

There are other alloys which do not contain gallium and have only a slightly higher temperature, such as the Bi/In alloys. At 70°C, the eutectic composition of this alloy will also be liquid in phase, however is much less reactive with your bonding metals, and therefore may be better suited.  

We've had some questions about using liquid metal or indium alloys liquid at room temperature containing indium and gallium, and their reaction to stainless steel. 

Many scientists are actively searching for a stable, thermally conductive liquid material to replace NaK.  Our data sheet on the liquid indium/gallium metals indicates that these are corrosive to many metals.  Will the liquid metals which are indium and gallium (Indalloy 51E) react with stainless steel?  

Here was the answer by our metallurgist, Bob Jarret:

Short Answer:

I assure you, the following is not some strange alien test of foreign materials. Even though it resembles a substance from the spaceship in E.T. – this is definitely earthly matter. This video shows the following 3 low-temperature liquid metal alloys wetting to glass:

• 61Ga/25In/13Sn/1Zn (Indalloy #46L)
• 66.5Ga/20.5In/13Sn
• 68.5Ga/21.5In/10Sn

Get Flash Player to view our TIM Wetting Video. Click here.

These are variations of Gallium, Indium, and Tin (with Zinc in #46L) that have amazing abilities – including incredibly low thermal resistance in heat transfer applications. These alloys also wet to other non-metallic substrates like silicon, quartz, ceramics, and diamond.

Call us @ (315) 853-4900, or email us if you'd like to learn more about metals that are liquid at room temperature.

A drop of liquid metal.

Gallium is corrosive to aluminum. This picture was taken after gallium reacted with a sheet of aluminum foil. Image source: http://sci-toys.com/scitoys/scitoys/thermo/liquid_metal/liquid_metal.html

Liquid thermal interface materials are available in two forms:

1. metals liquid at room temperature
2. phase change metals

The major difference between the two is in the temperature which these alloys become molten.

Liquid metals remain molten at room temperatures. The following three liquid metal alloys become liquid at temperatures below 30 °C.

– Indalloy 51 (62.5Ga, 21.5In, 16.0Sn)
– Indalloy 46L (61.0Ga, 25.0In, 13.0Sn, 1.0Zn)
– Pure Gallium

Phase Change Metals are applied in solid form and melt when exposed to heated junction temperatures. The most popular phase change alloy melts at 60°C.

– Indalloy 19 (51In, 32.5Bi, 16.5Sn)

The advantages to using these liquid metal thermal interface materials are many and include:

• Extremely Low Thermal Resistance
  • The resistance obtained with the Indalloy 51 was tested to be less than 0.015 cm2-°C/W.
  • Metal in its liquid form has virtually no contact resistance
• High Thermal Conductivity
  • As a completely metal thermal interface material, bulk thermal conductivity is premium
• Ability to Withstand dramatic Thermal Expansion Mismatch
  • The low flow stress of liquid metals allow them to maintain surface contact while substrates pump during power or temperature cycling
• Ultra Low Bondline Thicknesses
  • Liquid metals can be compressed to thicknesses below 0.001"

One difficulty encountered when using these alloys is the ability to contain them. All of the alloys which are liquid at room temperature contain gallium. Gallium is corrosive to various metals, especially when hot. As the temperature of the gallium is raised, it becomes increasingly corrosive, reacting through thicker layers in a short amount of time. One metal which gallium is very reactive with is aluminum. It will corrode through .002" thick aluminum foil within hours at room temperature, and at 500°- 1000°C, this reaction becomes much faster.

Gallium is non-reactive with other metals however such as molybdenum, tungsten, and nickel.

In addition to reacting with metals, gallium with also stick to non-metallic materials, making it difficult to package. Stay tuned for my next blog posting which will discuss packaging options for liquid metals.

23rd European Photovoltaic Solar Energy Conference and Exhibition

The European Photovoltaic Solar Energy Conference and Exhibition is a premier venue for learning and networking in the Solar Industry. It has been one of the early shows dedicated to the solar industry.

This year the event will be held at Feria Valencia in Valencia, Spain. The event starts on Sept 1st and ends on Sept 5th.

Below is the link to find more information,

23rd European Photovoltaic Solar Energy Conference and Exhibition

Indium Corporation will be exhibiting at the Hall 3 / B-2. Indium Corporation will also host a press conference at 2:00 pm on Tuesday, Sept. 2nd. The press conference is focused at discussing our ability to support the industry with > 5 GW / year of solar PV production with a wide variety of materials including indium and gallium.

Some of the products we would be showcasing at the show are,

• OnSpec® LTTF-6363 Metallization Paste
• OnSpec® Tabbing Ribbon
• OnSpec® CIG Alloy Sputtering Target
• OnSpec® Solar Grade Fluxes

I will be glad to meet and host you at our exhibit booth, please plan on stopping by.

Have you seen the statement on Indium's product data sheet for liquid metals that says:

"Care should be taken in reheating the alloy in the original packaging provided. Temperatures should not exceed 65.6ºC"?

Recently there has been some confusion regarding this statement, so I'd like to make a clarification the current and potential liquid metal users out there. This statement does not mean that these alloys cannot be used above 65C. The reason for the statement is to address the fact that the gallium alloys increase in corrosiveness above this temperature and the bottles that the alloys are supplied in are not rated to withstand that corrosiveness.

It is suggested that you avoid heating the gallium alloys above 65.6 ºC while they are in their original packaging, but once they are dispensed on your corrosion-resistant substrate, they should not cause an issue.

The typical extreme device needs an advanced thermal interface material to keep the interface cool while the device generates extreme amounts of heat. As discussed in my previous posting, the interface material chosen for extreme conditions often melts at a very high temperature so that it remains solid and stable during high temperature operation.

Other types of extreme devices have an opposite issue, a low reflow temperature requirement. A significant consideration when choosing a solder TIM is whether the entire package will withstand soldering temperatures. As indium supplies TIM materials with high temperatures, we have also studied and understand low temperature metals.

At the very lowest temperatures, there are metal alloys which are liquid at room temperature. These alloys contain indium, gallium, and tin. These materials can be deposited and remain molten during the entire life of the product. They do not require any elevated temperatures for their application.

For those thermal designers who are not ready to take the liquid metal leap, there are also alloys available for use in thermal interfaces which melt at approximately 60C. These alloys will wet a substrate, forming an interface with very low resistance.

Unfortunately, without a flux, these low temperature alloys are unlikely to form any intermetallic with the heat sink or spreader. If the interface material alone is to provide mechanical stability, an intermetallic is needed. The flux is used to remove oxides from the substrates, allowing for intermatallic formation. Most fluxes do not activate until temperatures or 125C. There are some fluxes available which activate as low as 100C however. For more information on these, please read Jim Hisert's solder blog on this topic.