​Challenges in Liquid Metal

Taking on the challenge of liquid metal applications - Developing connectors using liquid metal -

Do you know about liquid metal?

An alloy made of gallium, indium, and tin has an extremely low melting point of -19°C for a metal. In other words, it is a metal that is liquid at room temperature. It is used in thermometers as a substitute for mercury, the use of which is restricted due to its toxicity, and various other applications are being researched.

At IRISO, we thought that by applying the characteristics of liquid metal to connector technology, we could create completely new products. To date, we have been considering various concepts and continuing our research towards practical application. In this article, we would like to introduce some of our findings.

Possibilities and challenges arising from the characteristics of liquid metals

When considering the application of liquid metals, the most distinctive feature is, of course,

• It is liquid at room temperature
• Less toxic than other metals that are liquid at room temperature

That's the point.

Their low toxicity leads to product safety, and they have the advantage of being able to be formed into any shape compared to solids.

Connectors contain metal parts with complex shapes that enable their mechanisms to function.

Furthermore, it is possible that liquids may be able to overcome the limitations on miniaturization that exist from the standpoint of processability.

On the other hand, because it is a liquid it will flow away if left as is, so finding some way to retain it becomes a challenge.

The characteristics that give hints for retention are

• It has a low vapor pressure and does not evaporate even when exposed to high heat.
• High wettability and adhesion to many materials, including glass

This feature can be utilized in commercialization.

On the other hand, when using it as a connector, there are concerns about its relationship with the following solid metals.

• When in contact with other solid metals, it seeps in, embrittling the metal and reducing its strength.
• Easy to form intermetallic compounds

To commercialize it as a connector, it will be necessary to use solid and liquid metal parts according to the requirements of each part. In other words, the challenge will be to combine liquid metal with other solid metals, i.e., to match them in the form of contact.

To summarise briefly, the following possibilities and challenges become clear:

Challenge 1: Development of film-type connector

Concept Overview

From the perspective of miniaturization, one of the advantages mentioned in the previous section, we have conceived and are currently developing a film-type connector that is configured on an FPC as a zero-height, low-contact pressure connector.

Specifically, the male side of the film has bumps made of solid metal, while the female side is made of liquid metal filled in a VIA cup and maintained by surface tension, creating a connector connection similar to that of a bandage.

Film type connector formed on FPC

Preliminary verification example: Vaporization of liquid metal and its compatibility with other solid metals

We will introduce an example of pre-verification that was conducted prior to development.

The following is a copper plate that has been plated in various ways and left at high temperatures.

• Mass loss due to vaporization of liquid metal
• Corrosion of other metals and intermetallic compounds that occur

This is an overview of the confirmation carried out.

These results eliminate concerns about vaporization of liquid metal and confirm the possibility of connecting to specific metals.

Concerns, checklists and results

Surface treatment of copper plate - Confirmation of reaction with liquid metal

Towards prototyping: Prototype of film-type connector completed

After carrying out the above-mentioned verification tests, we became convinced that the concept was feasible and created an actual prototype.

A VIA cup like the one shown below was formed on the female side film, which is filled with liquid metal, and a bump was formed on the male side film.

We prototyped a mating connector using two rectangular films arranged in a 6x6 pattern with the upper structure as the contact points.

Below are some images and diagrams of the actual prototype.

Film connector prototype

It has been confirmed that the prototype connector can Mating with a contact resistance value that is sufficient for the connector to function properly.

We are also currently continuing to evaluate our environmental dependency and reliability. Below are some examples.

Confirmation of contact resistance of film connectors / Initial value and environmental impact confirmation example

We are currently investigating more specific application targets. We are also looking for partners who will consider applications together with us, so if you are interested, please contact us.

Challenge 2: Application to high current Floating Connector

Heat generation in Floating springs at high currents: a problem to be solved

Our company, which specializes in Floating Connector, also releases high current compatible products as variations of the product. A representative product is the 10122B/S series, which is compact in size but has a high current capacity of 15A.

10122 series

https://www.irisoele.com/jp/series/view/10122B/

https://www.irisoele.com/jp/series/view/10122S/

One of the key points for allowing a connector to pass a large current is to reduce Joule heat caused by resistance loss, in other words, to reduce resistance at each part of the current path.

The 10122B/S series is a high-heat resistant (125°C rated) connector that achieves low resistance without interfering with Floating structure movement by branching the Floating part, and also improves reliability with a four-point contact structure.It has been evaluated as an attractive option for power transmission in board-to-board connections that require Floating, and has been adopted by many customers.

Our high current Floating Connector 10122B/S (compatible with 15A)

Not satisfied with the current situation, we are considering ways to handle even higher currents.

However, even in the 10122B/S series, which has this ingenious spring structure, Floating spring is the largest heat source when a high current is applied. In Floating Connector, which is characterized by absorbing misalignment, improving assembly, and relieving stress in the board mounting area, the spring strength of Floating part cannot be made extremely strong.

This requires that the spring section be narrowed or tapered, or that it be routed long, which inevitably results in a certain degree of high electrical resistance.

We are currently working on various ideas for the terminal structure design to achieve both high current compatibility and Floating, but at the same time, we considered whether there was a way to make a breakthrough from a completely different approach.

Possibility of achieving both high current capability and Floating

The idea of liquid metal Floating terminals

It occurred to me that the liquid metals I had been researching could be applied to this problem.

• Because it is a liquid, it can move freely without relying on springs, even with a simple structure: reducing the heat generation area
• Although the specific resistance is high (6.27×10-7Ω・m), it is possible to lower the resistance by adjusting the filling volume: reducing the amount of heat generated

Therefore, we considered a structure based on the 10122B/S terminal, which could be separated into two pieces, had a bold Floating spring, and reconnected with a joint part filled with and sealed with liquid metal. We actually built a prototype and carried out evaluations.

Concept of changing Floating spring to liquid metal

Confirming the effect through prototypes and further improving performance

After creating and evaluating samples, the first prototype, which used the 10122 terminals, was able to reduce the amount of heat generated by nearly 40% when a high current was passed through it.

As a next step, we are completing the design of the terminal structure that will make the liquid metal terminal joint more effective.

Preliminary simulation results show that the amount of heat generated is expected to be reduced by 65% compared to current products.

Reducing heat generation during current flow in high-current Floating Connector by using liquid metal

Our findings to date have confirmed the significant benefits of applying liquid metal to high current Floating Connector. There is still a long way to go before we can commercialize the product, from optimizing the design to building the production process and optimizing costs. We will continue to develop the product further, aiming to be of service to our customers.

IRISO welcomes requests for other liquid metal applications and welcomes joint development proposals.

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