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Flex Talk: Simplified Assembly of Aluminum Flexible Circuits

Flexible circuit designs that come across my desk are predominately constructed with copper and polyimide laminates. As I learn more about automotive applications, I am intrigued by the possibilities of using aluminum in place of copper and the potential to use polyester in place of polyimide. Both aluminum and polyester have traditionally been difficult to solder to. One very interesting development has been the Mina™ chemistry. This coating not only simplifies soldering to aluminum, but it also enables the ability to automate low temperature soldering to polyester. Having many questions about this process, I sat down to discuss this Mina with Divyakant Kadiwala, vice president of manufacturing for Averatek. He has been instrumental in the development of this assembly process.

Tara Dunn: Divyakant, before we jump into the conversation about Mina, could you share a brief introduction to both Averatek and your background?

Divyakant Kadiwala: Averatek is a high-tech company based in Silicon Valley. It was founded by SRI International and private investors. It has two primary products: LMI™, a catalytic ink that enables the fabrication of very high-density circuits with the patented A-SAP™ process; and Mina, a surface treatment that enables soldering to aluminum. I am VP of manufacturing and my role includes overseeing process engineering, quality control, facilities management, and business development. I am leading our efforts on productization of Mina.

Dunn: Thank you. Let’s start the conversation with aluminum. What are the benefits of aluminum over other metals?

Kadiwala: Aluminum is the most abundant metal in the earth’s crust. This makes it more easily available and less expensive compared to other metals. It has different benefits when compared to other metals based on the field of use. For example, in automobiles, its superior strength at lower weight is a significant benefit against iron and its common alloy—steel. But in the field of printed electronics, its main competitor would be copper.

Aluminum is more than three times lighter than copper. It has 68% of the conductivity of copper but has only 30% of the weight of copper. This means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. This makes it the metal of choice for high voltage transmission cables. Also, it is three times less expensive than copper on an equal weight basis and six times less expensive on an actual usage basis. This is the biggest advantage that aluminum has over copper.

Dunn: Interesting. Why do you think aluminum is attractive specifically for automotive applications?

Kadiwala: Aluminum has been a boon for the automobile industry. Like any industry, the automobile industry has evolved due to various geo and political reasons over its 100-plus-year history. Whether it’s the oil embargo of the 1970s or global warming due to climate change, it has been constantly under pressure to improve fuel efficiency standards.

The easiest way to improve fuel efficiency is to reduce the dead weight of the automobile and improve the efficiency of the internal combustion engine. Aluminum helps reduce an automobile’s body weight by providing superior strength at lower weight when compared to steel and other alloys of iron. It also helps improve engine efficiency by providing better performance at lower weight.

Since most automobiles have their body and chassis made of aluminum, the integration of onboard electronics to aluminum is critical. Averatek’s Mina can provide a soldered connection to aluminum without any plating or surface finish. It can help ease manufacturing and reduce costs.

Dunn: Aluminum has historically been difficult to assemble to. What are the challenges of soldering to aluminum?

Kadiwala: Aluminum PC boards, whether rigid or flex, are limited in use due to challenges associated with soldering components to aluminum pads. This is because all aluminum surface is covered with a layer of aluminum oxide. Although self-limiting, this oxide layer cannot be overcome by flux in existing solder systems during reflow. It thus prevents the formation of a metal-to-metal bond. Even if this oxide layer is removed using etchants and fluxes, a new layer forms in situ upon exposure of clean aluminum to the atmosphere. This prevents the use of conventional SMT methods for attaching SMDs to assemble PC boards.

Dunn: Can you expand on a couple of ways that Mina could be utilized in automotive applications?

Kadiwala: Integrating electronics with the aluminum body and chassis of the car is an integral part of its manufacturing and design. Since aluminum is not easy to solder to, mechanical “crimp” and “pigtail” connections are common options.

Mina™ can provide a soldered joint instead of these mechanical connectors. It can help attach aluminum wire or PCB to aluminum chassis for grounding or other such connections. It can also help with attaching copper to aluminum PCB where applicable.

Dunn: This is exciting. With any new process, reliability is always a concern. What type of reliability data has been gathered?

Kadiwala: Flexible PCBs were made using an Al 9-mm/PET 38-mm substrate and components were soldered using low temperature Sn/Bi/Ag solder paste. The fully assembled PCBs were then subjected to these tests:

• K-09 - “Damp Heat, Cyclic (with Frost)”
• M-04 - “Vibration”

Both these come under the LV-124 European automobile standards.

1. Characteristics of the Test K09 – “Damp Heat, Cyclic (with Frost)”

• Chamber temperature: without cold phase: 23°C to 65°C; with cold phase: -10°C to 65°C
• Chamber humidity: 95% RH
• Duration: 10 d = 10 cycles at 24 h
• Cycling: 5 cycles with cold phase, then 5 cycles without cold phase
• Operation of samples: here not operated (usually intermittent operation = 30 s on, then 30 s off)

2. Characteristics of the Test M-04 – “Vibration”

• Chamber temperature = 8 h, profile between -40 and 80°C
• Broadband random vibration according to vibration profile D in LV-124 ? 5-2000Hz ? 30.8 m/s² ? X-axis, Y-axis, Z-axis ? 8 h each axis

Dunn: What materials does Mina work well with? Rigid, flex materials, others?

Kadiwala: Mina works well with both—flex and rigid materials. The alloys of aluminum that we have proven for our customers include Al1235, Al6061 and Al5052.

Dunn: Understanding the benefits of aluminum over copper and understanding the benefits of flexible materials such as polyester, the combination seems like a natural fit for automotive applications and a wide range of other applications. Which markets do you see as the early adopters of Mina taking advantage of the simplification of the soldering process for these materials?

Kadiwala: In addition to automotive, we see significant interest from the LED, SmartTag and heat-sink and high-power industries.

The LED industry would not only benefit from lower costs due to cheaper Al-PET substrate but also it would be a better product. A soldered LED operates much cooler compared to one attached using conductive epoxy. Based on information from a large LED manufacturer, the lifetime of an LED doubles with every 10°C reduction in operating temperature. Thus, soldered LED panels will be more reliable.

SmartTags have more components and are larger in size compared to generic RFID tags. They use a lot more silver epoxy and thus are more expensive to make than RFID tags. Also, SmartTags are designed to be used multiple times while RFID tags are usually meant for single use. Hence, longevity is important for SmartTags. A tag made with its components soldered will be more reliable and cheaper compared to one made using silver epoxy.

Heat management is important for high-power devices. Aluminum is commonly used to build heat sinks. Soldering a high-power device to aluminum would require a plated surface finish like ENIG and ENEPIG. These add costs and so other means like thermal tape, etc., are used at the cost of performance and life of product. Mina addresses all these issues by enabling direct soldering to aluminum heat sinks.

Dunn: Divyakant, thank you so much for talking with me about the Mina process. Working with flexible circuits for many years, it is always exciting to learn about new applications that change the way we look at flex assembly and integration.

Kadiwala: Thank you, Tara.

This column originally appeared in the FLEX007 section of the March 2021 issue of Design007 Magazine.