Electrically Conductive Adhesives: The New Solders

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

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

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

The most widely used solders are lead based which, in spite of having low cost and good strength to firmly hold electronic components, create health issues after a prolonged use. They have been outlawed in the European Union and restricted to commercial use in other areas of the world. Although lead-free solders are a good replacement, mitigating health risks, they fail to match the strength of their lead based counterparts.

The alternative electrically conductive adhesive (ECA) not only has high strength but is quite safe as well. ECA comes in two types—isotropic conductive adhesive (ICA) and anisotropic conductive adhesives (ACA)—which correspond to different ways of conducting electricity across a joint. ACA is further divided into anisotropic conductive paste (ACP) and anisotropic conductive film (ACF).

Fig. 1: Schematic of interconnects (Credit: IDTechEx)

Fig. 1 compares the interconnect of a typical solder to an ECA. In an ICA, electricity is conducted in all directions, due to which only a small amount is needed between the connection and the metal pad. An ECA consists of a polymer matrix that holds the conductive filler material in place. The conductive filler material is typically a metal powder that acts as a bridge across the joint in such a way that a connection is made. With respect to placement for proper flow of charges, the ICA is very comparable to the solder interconnect.

Unlike ICA, an ACA conducts charges in only one direction. So, even if it is applied between the entire component and substrate, it will only conduct electricity across the connection—without causing a short circuit.

Some other conductive joining technologies include silver sintering, transient liquid phase sintering (TLPS), and non-lead based solders. Silver sintering requires very high temperatures, making it unsuitable for large commercial applications, similar to TLPS, which is very much on the emerging side of the conductive joining technology. As mentioned earlier, non-lead based solders do not match the lead based solders in strength and are unsuitable for large-scale applications.

Comparison of different die-attach techniques

From Fig. 2 it is evident that lead (Pb) based solder is an ideal technology in every way, except for being extremely toxic—making it highly unsuitable. With a good combination of green and yellow indicators, ECA seems to be an allrounder. Although silver (Ag) sintering and TLPS also give excellent performance, they are not practical in terms of cost and workability.

Fig. 2: Colour-coded comparison chart (Green: favourable, yellow: moderate, red: unfavourable) (Credit: IDTechEx)

Alternate Solders fail to give the required performance and reliability, even if they excel as an easy successor to traditional lead based solders at low costs. Amidst all the properties of different die-attach techniques against each other, ECA arguably stands out to have the best potential in this market.

Properties of ECAs

Silver is the most preferred material for conductive filler in ECAs owing to its very high conductivity and reliability. But since it has an extremely high cost, other filler materials can be considered where cost plays a huge decisive role.

Fig. 3 shows that silver meets all aspects, apart from density and cost. Copper, on the other hand, simply oxidises too easily, making it unsuitable. Nickel falls widely short on conductivity but makes up with good filler material density.

Fig. 3: Colour-coded comparison chart (Green: favourable, Light green: slightly favourable, yellow: moderate, red: unfavourable) (Credit: IDTechEx)

It can be noted that gold and silver are the only two materials that have the required conductivity and lack in reactivity. However, gold is an even more expensive alternative to silver. With respect to polymer resin materials, most epoxy based ones are used although other varieties are also useful, depending on the exact application and properties required.

Silicone performs better where oscillations need to be accommodated by the adhesive. Polyimides have a high functional temperature but at the cost of more curing temperature, making it expensive. Acrylics also perform well but, unfortunately, do not have a tolerable high functional temperature.

Fig. 4: Colour-coded comparison chart (Green: favourable, Light green: slightly favourable, yellow: moderate, red: unfavourable) (Credit: IDTechEx)

Advantages and disadvantages of ECA

Pros.

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

2. Ability to connect highly fine pitches. More components can be connected in a small circuit area because the minimum distance required for making a successful connection is low, which is beneficial for the miniaturisation of electronic components.

3. Mechanically soft due to the large amount of polymer filler that holds everything together. Since ECAs are not structurally based around a brittle metal, they can deform under stress. This characteristic might be beneficial for electronics having fragile components where stress needs to be accommodated during processing, such as with oscillating components, flexible, or in-mould electronics.

4. Lower temperatures are required than solder. Instead of melting, an ECA only needs to be cured at a high temperature for an extended time so that the polymer can react with itself and firm up for holding everything in place. This is beneficial for temperature-sensitive components, like flexible electronics where low-temperature polymer substrates are used.

Cons. 1. Although the temperature required for curing is low, the process is longer than conventional soldering.

2. Silver, a conductive filler used in an ECA, is extremely expensive and a significant drawback for using ECAs on a large scale.

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

4. Lack of self-alignment. Although solders allow perfect component placement, even when they are placed incorrectly, with ECAs more precision is required.

Fig. 5: Global ECA market forecast by type of ECA, 2022-2032 (Credit: IDTechEx)

Applications

Consumer electronics. ECAs are perfect for placing a surface mount device. Since more electronics goods are expected to be produced each year, ECAs have a lot of potential to grow in this large market. However, the speed with which they penetrate may not be fast.
Currently, they share the market with other widely used solders. But this may change as they are seen to be a much more superior option for certain applications. Due to the high cost, they are unlikely to replace solders soon in the consumer electronics market, which uses only ICAs as of now.

The key things to aim for when choosing or developing an ECA are low cost and easy processability. High cost and brittle connections are features to avoid as too many of them cause problems. Consumer electronics are high-volume goods and hence it is best to make most connections as cheap and quick as possible.

Automotive electronics. Being a highly established market, there is a lot of room for ECAs to grow as car electronic systems are becoming more advanced with an aim to increase sales by 2035. Currently, almost all ECAs used in automotive electronics are ICAs. ACAs have better miniaturisation capabilities than ICAs but are forecast to grow at a very slow rate.

Desirable properties are easy processability and high chemical resistance. Features to avoid are brittle connections and low reliability. Cars go through a lot of vibrations and stress in their daily use, needing highly reliable connections that can work overtime. Thus, high chemical resistance to withstand rugged environments and easy processability are beneficial for large-scale production of car electronics.

Display technology. The market for ECAs in display technologies is expected to grow at a steady rate, although not as rapid as other technologies. ACAs and, more specifically, ACFs are the ECAs that are used in displays. There is not much potential for ICAs in this market. Desirable properties are low temperature, rapid curing, fine pitch capacity, and thin film thickness. Poor adhesion can be a critical failure point for display applications because of which peeling from the screen can occur.

Some other applications include EMI shielding, RFID tags, photovoltaics, and aerospace. Printed electronics, in-mould electronics, and wearable electronics are those emerging markets where ECAs hold a lot of promise.

ECAs, the successor to solder?

After getting a glimpse into what ECAs are all about, and analysing their benefits and shortcomings, it does seem that they are set to become a key conductive joining technology. A prediction of their application in emerging markets is seen, but with the rapid advancement in electronics technology, such as miniaturisation of components, they can be conveniently used for connecting circuitry across a wide range of uses, providing joins that are mechanically strong and electrically conductive.


This article has been prepared by Vinay Prabhakar Minj, Technology Journalist at EFY, based on webinar titled ‘Electrically Conductive Adhesives (ECAs): The Successor to Solder?’ organised by IDTechEx.

THOMAS LYNCH is a Technology Analyst at IDTechEx

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