Electrically Conductive Adhesives: The New Solders

Electrically conductive adhesives (ECAs) claim to possess many superior properties than widely used conventional adhesives. With the increasing demand for electronic goods, this decade may see ECAs gradually establish themselves as the new welder.

The correct placement of electronic components, such as integrated circuits and LEDs, on substrates is the essence of developing an operational circuit board for an electronic device.

While conventional lead-free solder excels in its task for rigid PCBs, it is not well suited for emerging applications that require mounting components on flexible substrates or conformal surfaces.

The most commonly used solders are based on bullets, despite the low cost and good strength to hold the electronic components firmly, but it does cause health problems after prolonged use. It has been banned in the European Union and restricted to commercial use in other regions of the world. Although lead-free solders are a good alternative, and mitigate health risks, they fail to match the potency of their lead-based counterparts.

The alternative electrically conductive adhesive (ECA) is not only high strength but also completely safe. ECA comes in two types – isotropic conductive adhesive (ICA) and anisotropic conductive adhesive (ACA) – which correspond to different methods of conducting electricity through the joint. ACA is further divided into anisotropic conductive paste (ACP) and anisotropic conductive film (ACF).

Figure 1 compares a typical welding interconnection to an ECA. In ICA, electricity is conducted in all directions, due to which only a small amount is needed between the coupling and the metallic pad. ECA consists of a polymer matrix that holds the conductive filler in place. The conductive filler is usually a metal powder that acts as a bridge across the joint in such a way that the connection is made. In terms of placement for proper flow of charge, the ICA is very comparable to that of a solder joint.

Unlike ICA, ACA conducts shipments in only one direction. Therefore, even if applied between the entire component and the substrate, it will only conduct electricity across the contact – without causing a short circuit.

Some other conductive conduction technologies include silver sintering, transient liquid phase sintering (TLPS), and lead-free solders. Silver sintering requires very high temperatures, making it unsuitable for large commercial applications, similar to TLPS, which is largely on the emerging side of a conductive conduction technology. As mentioned earlier, non-lead soldiers are incompatible with lead-based soldiers for strength and are not suitable for large-scale applications.

Comparison of different attaching techniques

It is clear from Figure 2 that lead-based (Pb) welding is an ideal technology in all respects, except that it is highly toxic – which makes it highly unsuitable. With a good mix of green and yellow indicators, the ECA appears to be a pioneer. Although silver (Ag) and TLPS sintering also provide excellent performance, they are not practical in terms of cost and workability.

Color-coded comparison chart (green: preferred, yellow: medium, red: unfavorable)
Figure 2: Color-coded comparison chart (green: preferred, yellow: medium, red: unfavorable) (Credit: IDTechEx)

Replacement soldiers fail to provide the required performance and reliability, even if they stand out as an easy successor to traditional lead-based soldiers at low costs. Amidst all the characteristics of different mold bonding technologies against each other, ECA can be said to have the best potential in this market.

Characteristics of ECAs

Silver is the most preferred conductive filler material in ECAs due to its high conductivity and reliability. But due to its very high cost, other fillers can be considered where cost plays a big decisive role.

Figure 3 shows that silver meets all aspects, regardless of density and cost. On the other hand, copper oxidizes very easily, which makes it unsuitable. Nickel broadly shortens in conductivity but makes up for in a good filler density.

Color-coded comparison chart (green: preferred, light green: slightly favored, yellow: medium, red: unfavorable) (Credit: IDTechEx)
Figure 3: Color-coded comparison chart (green: preferred, light green: slightly preferred, yellow: medium, red: unfavorable) (Credit: IDTechEx)

It can be seen that gold and silver are the only two materials that have the required conductivity and lack reactivity. However, gold is a more expensive alternative to silver. Regarding polymer resin materials, most epoxy-based materials are used although other types are also useful, depending on the exact application and desired properties.

Silicone works best as the oscillations must be absorbed by the adhesive. Polyimide has a high functional temperature but at the cost of a higher processing temperature, which makes it expensive. Acrylics also work well, but unfortunately, they do not have a high functional temperature tolerable.

Color-coded comparison chart (green: preferred, light green: slightly favored, yellow: medium, red: unfavorable) (Credit: IDTechEx)
Figure 4: Color-coded comparison chart (green: preferred, light green: slightly favourable, yellow: moderate, red: unfavorable) (Credit: IDTechEx)

Advantages and disadvantages of ECA

Positives.

1. Low environmental impact due to the absence of heavy or toxic metals. This is useful in all applications, particularly healthcare or consumer electronics.

2. The ability to associate very subtle tones. More components can be connected in a small circuit area because the minimum distance required to make a successful connection is low, which is beneficial to the miniaturization of electronic components.

3. Mechanically soft due to the large amount of polymer filler that holds everything together. Because ECAs are not structurally based around a brittle metal, they can deform under stress. This property may be useful for electronics with brittle components where stress needs to be adjusted during processing, such as oscillating components, flexible electronics, or in-molds.

4. Lower temperatures required than welding. Instead of melting, the ECA just needs to cure at a high temperature for a long time so that the polymer can react with itself and stabilize to hold everything in place. This is useful for temperature sensitive components, such as flexible electronics where low-temperature polymer substrates are used.

Negatives. 1. Although the temperature required for processing is low, the process is longer than that of traditional welding.

2. Silver, a conductive filler used in ECAs, is very expensive and presents a major barrier to the widespread use of ECAs.

3. Compared to lead-free soldering, ECAs require very different equipment to replace lead-based and many other similar technologies. Therefore, this may require an upfront investment to change all devices for better compatibility with ECAs.

4. Self-biased. Although solders allow for perfect placement of components, even when improperly positioned, more precision is required with ECAs.

Global ECA Market Outlook by ECA Type, 2022-2032 (Credit: IDTechEx)
Figure 5: Global ECA market forecast by ECA type, 2022-2032 (Credit: IDTechEx)

Applications

Electronic consumers. ECA is ideal for surface mount device placement. Since more electronic goods are expected to be produced each year, ECAs have a lot of potential to grow in this large market. However, the speed at which they hack may not be fast.
Currently, they share the market with other widely used soldiers. But this may change as it is seen as a much better option for certain applications. Due to the high cost, they are unlikely to replace resellers soon in the consumer electronics market, which uses only ICAs so far.

The main things to aim for when choosing or developing an ECA are low cost and ease of processing. High cost and fragile connections are features to avoid as many of them cause problems. Consumer electronics are bulky goods, so it is best to make most communications as cheap and fast as possible.

Auto electronics. Being a very well established market, there is a lot of room for growth of ECAs as vehicle electronic systems become more advanced with the goal of increasing sales by 2035. Currently, almost all ECAs used in automotive electronics are ICAs. ACAs have better miniaturization capabilities than ICAs but are expected to grow at a very slow rate.

Desirable properties are ease of processing and high chemical resistance. Features to avoid are fragile connections and low reliability. Cars go through a lot of vibration and stress in their daily use, requiring highly reliable connections that can work overtime. Thus, high chemical resistance to withstand rugged environments and ease of processing are beneficial for large-scale production of automotive electronics.

display technology. The market for ECAs in display technologies is expected to grow at a steady rate, albeit not as fast as other technologies. ACAs, and more specifically, ACFs are the ECAs used in displays. There is not a lot of potential for ICAs in this market. Desirable properties are low temperature, fast curing, accurate pitch amplitude, and thin film thickness. Poor adhesion can be a critical failure point for display applications due to screen peeling.

Some other applications include EMI protection, RFID tags, photovoltaics, and aerospace. Printed electronics, in-mold electronics, and wearable electronics are those emerging markets in which ECAs hold a lot of hope.

ECAs, successor to welding?

After taking a peek at what ECAs are all about, and analyzing their advantages and disadvantages, it appears poised to become a major conductive accession technology. Their application predictions are seen in emerging markets, but with rapid advances in electronics technology, such as component miniaturization, they can be conveniently used to connect circuits across a wide range of applications, providing both mechanically strong and electrically conductive connections.


This article was prepared by Vinay Prabhakar Ming, technical journalist at EFY, based on a webinar titled “Electrically Conductive Adhesives (ECAs): Is it the Successor to Welding?” Organized by IDTechEx.

THOMAS LYNCH is a Technical Analyst at IDTechEx

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