Electrically Conductive Adhesives in Microelectronics Packaging

Electronic packaging is the science behind designing protective enclosures and placing electronic devices in them while maintaining electrical continuity using interconnections [1]. It is essential to protect the system from damage due to various factors like handling, environment, heat, etc., dissipate heat, transmit electrical signals and even power. It is critical for ensuring high performance of an electronic device along with preserving reliable functioning for extended periods of time. Micro-electronics packaging has become a vital industry for the world economy, and it is currently booming. Light emitting diode packaging alone, a subset of micro-electronics packaging, is projected to be worth 19.4 billion USD by 2029 [2].

There can be many important components in a package with each serving a specific purpose [3]. For example, the package can contain die attach material, encapsulant, underfill, thermal interface material, etc. Complex systems like integrated circuits (ICs) consist of many components that need to be electrically connected in a mechanically reliable manner. So, a die attach material is one of the most crucial components of a package. Traditionally, Sn–Pb solders have been used for this purpose because of their low melting point (T_m = 183 °C) [4]. Most common interconnection technologies like surface mount technology, where the components are mounted on a printed circuit board (PCB), ball grid array, where a grid of interconnect is used to conduct electrical signals between wafer and the board, and chip scale package [5], where a die is mounted on the interposer, have historically been using Sn–Pb solders. But Pb has been shown to be environmentally hazardous as it can disrupt soil functions, food chains, soil, and water organisms. It is also harmful for human beings as it can cause kidney damage, brain, and nervous system damage [6]. Additionally, there is not an economical and safe way of recycling Pb without affecting the biosphere or any water sources. Since, electronic devices have a short life, the issue of recycling compounds the threats Pb poses to human beings and the environment [7].

Alternative solutions explored in the past have been Pb free metallic solders and electrically conductive adhesives (ECAs). The most popular Pb free metallic solders are the SnAgCu (SAC) systems. These systems have a melting point >217 °C. The reflow ovens need to maintain a temperature at least 25 °C above liquidus to ensure uniform melting of solder and good wetting of the copper circuit pads [8]. Higher temperatures would induce higher thermal strains, leading to higher thermal stresses in various components of the system [9,10]. High thermal stresses could increase the probability of yielding or even failure. Warpage of the package has also been reported to be a significant issue for manufacturing. The overall reliability and integrity of the device can also be compromised. Additionally, generating higher temperature needs higher energy. So, the cost basis for production must be justified. Alloys like In/Sn, Bi/Sn/Ag, Sn/Zn/Al, etc., having melting temperatures <183 °C are gaining traction [11]. These materials are still in developmental stages and their reliability is of concern. But they are slowly gaining popularity with Intel recently adopting Sn/Bi alloy [12].

Electrically conductive adhesives are basically composite materials made up of conductive particles, which are generally metallic, dispersed in a polymer binder. The reflow temperature required to bond using ECAs can be much lower than that of SAC systems. Unlike SAC systems, they do not require to be cleaned with a flux. So, the number of processing steps are lowered, and the whole process is simplified. The polymer binder component plays a big role in determining the mechanical properties of the package. It can offer higher flexibility and creep resistance [13]. All metallic solders suffer from electromigration (the transport of atoms due to a current), whereas in certain types of ECAs, electromigration is minimal due to the presence of a large amount of organic component. These unique features of ECAs, summarized in Table 1, make them ideal candidates for replacing solders, especially in low power devices like flexible electronics. Flexible electronics has been a major driver for research in ECAs as they have a substantially lower processing temperature requirement than SAC reflow temperatures. ECAs can be divided into two types—isotropic conductive adhesives (ICAs) and anisotropic conductive adhesives (ACAs) based on their filler concentrations as shown in Fig. 1. ICAs conduct electricity in all directions due to high filler content exceeding the percolation threshold. ACAs only conduct vertically because their lower filler content prevents connections in the horizontal plane.

The purpose of this paper is to review the working of the two types of ECAs, identify some of the major issues, and discuss some of the promising recent developments in ECAs and their applications.

Isotropic conductive adhesives, as the name suggests, conduct electricity isotopically in all directions as shown schematically in Fig. 2. The polymer matrix is heavily loaded with conductive fillers above the percolation threshold. Below the percolation threshold, the material is almost like an insulator. At the percolation threshold, a three-dimensional interconnect network of conductive particles is formed resulting in a sudden lowering of resistivity [16]. Further addition of fillers continues to drop resistance due to formation of multiple conducting pathways. The fillers are typically Ag flakes because of its high conductivity and high energy for formation of silver oxide [17]. Even if the oxide forms, silver oxide has one of the highest electrical conductivities among metal oxides [18]. Several additives are added to control the rheological and viscoelastic properties of the ICAs. For example, surfactants like stearic acid used to prevent agglomeration of polar filler particles in the polymer matrix. Contact printing/screen printing is a popular method to deposit the ICA to bond with flip-chips due to the ease of manufacturing and low cost.

Even though the filler concentration is above the percolation threshold, particles are not completely touching each other. There is a thin layer of polymer binder covering the particle surfaces. To improve conductivity, some, or all the lubricant needs to be removed during ICA curing. A good way to achieve this is using a short-chain dicarboxylic acid. This acid has a strong attraction to silver and effectively removes the lubricant, allowing for better contact between the silver flakes and ultimately leading to higher conductivity in the ICA.

So, the contact resistance (R_cr) at each linkage is high. Also, the electron tunneling resistance (R_t) is high due to the presence of the nonconducting polymer layer. Upon the application of heat, the polymer resin cures and shrinks. This shrinkage brings the filler particles closer together and increases the contact area as shown in Fig. 3 [19]. This increase in contact area reduces the contact resistance at the interface between the particles. The reduction of the thickness of the nonconducting polymer layer between the conducting particles decreases the electron tunneling resistance. The combination of these two effects results in a dramatic increase in the overall conductivity of the ICA.

Isotropic conductive adhesives' electrical conductivity is generally low (of the order of 10⁻⁴ (Ω cm)⁻¹). One approach focuses on maximizing the shrinkage of the polymer matrix during the curing process. Uncured ICA pastes lack conductivity, but as the polymer binder cures and shrinks, the silver flakes within the adhesive are pushed closer together. This tighter packing creates more effective electrical pathways, ultimately enhancing conductivity. Studies have shown that ECAs formulated with a higher crosslinking density experience greater shrinkage during curing, leading to a noticeable decrease in electrical resistivity. For instance, incorporating small amounts of multifunctional epoxy resins into epoxy-based ICAs can increase crosslinking density and shrinkage, thereby improving conductivity [20,21].

Another strategy involves modifying the lubricant layer on the silver flakes. Most ECAs utilize a thin layer of organic lubricant, typically a fatty acid like stearic acid, on the silver flake surface. This lubricant plays a crucial role in dispersing the flakes within the adhesive and influencing its overall rheology. However, the lubricant also acts as an insulator, hindering conductivity [22]. Researchers have investigated replacing this lubricant “in situ” with short-chain dicarboxylic acids. These acids possess a stronger affinity for silver compared to the traditional lubricants. Additionally, their shorter molecular length allows for closer contact between the silver flakes. This improved intimacy between flakes facilitates electron tunneling, significantly enhancing conductivity.

A third method leverages reducing agents, with aldehydes being a prime example. Silver flakes are the most common conductive filler in ECAs due to the superior conductivity of silver oxide compared to most other metal oxides. However, silver oxide itself is not as conductive as pure silver. By incorporating reducing agents like aldehydes into the ICA formulation, a chemical reaction occurs between the silver oxide and the aldehydes during the curing process. This reaction helps to remove some of the insulating lubricant layer on the silver flakes and improve overall conductivity [23].

Finally, transient liquid phase fillers offer another avenue for boosting conductivity [24]. These fillers are a composite material containing a mixture of high-melting-point metal powder (like copper) and a low-melting-point alloy powder (like a tin–lead or tin–indium alloy). During curing, the low-melting-point alloy melts, temporarily dissolving the high-melting-point metal particles. This molten phase creates a network of conductive pathways within the adhesive. Once the curing process is complete, the alloy solidifies, locking the conductive network in place as shown in Fig. 4.

Generally used polymer matrices in ICAs like epoxies have a high natural affinity toward moisture absorption. This absorption of moisture results in swelling. Swelling of die attach material. The difference in coefficient of moisture absorption between the ICA and the nonpolymeric components induces a hygroscopic swelling stress. As the mismatch in moisture absorption increases, the stresses induced are higher. So, the probability of die separation is higher. Additionally, the hydrolysis of polymer chains decreases adhesion strength and increases the possibility of cracking and delamination. At high temperatures (∼240 °C), the vapor pressure of the absorbed moisture can be high enough to cause of internal delamination. As the swelling increases, the pressure applied by the bumps on the pads decreases. So, the contact resistances at the interface of particle/bump and particle/pad can increase.

Almost all filler conducting particles—Au, Ag, Cu, Ni, Ni plated polymer spheres can be prone to oxidation from the absorbed moisture as water molecules can penetrate through the polymer relatively easily [25–29]. So, moisture absorption can also result in formation of a thick oxide layer on the surface of the metallic filler particle. This can further reduce the electrical contact area in the joint and thereby increase the contact resistance. If the difference between the galvanic potentials of the different metals present is large, then they can act as electrodes. The absorbed water may condense and act as an electrolyte. The electrodes along with the electrolyte can collectively a galvanic cell. The larger the gap between the galvanic potentials of the two metals, the larger is the rate of galvanic corrosion. So, the use of a noble metal may accelerate corrosion. In fact, the only effective way to mitigate corrosion is to minimize moisture absorption. Coating the PCB/substrate with parylene seems to be efficacious in lowering moisture absorption.

The reliability of a package interconnect is heavily influenced by its ability to withstand thermomechanical cycling. This is especially true for soldered flip-chip designs, where there is a wealth of data available on the performance of ICAs in the literature. Voiding in the resin due to outgassing of solvents or dissolved moisture from the resin is another major drawback as they can act as regions of high stress concentrations and make the die attach inhomogeneous. Voids, if present in large fractions, can even become a major reliability issue.

Compared to solder reflow temperatures, polymer curing temperatures in ICAs are generally lower. This translates to a lower initial thermomechanical stress on the interconnect at room temperature. Additionally, ICAs boast superior creep properties compared to solders due to their polymer base [30]. This allows them to relax stress more effectively, potentially reaching a zero-stress state. Consequently, it is unsurprising that ICAs outperform solders in mechanical cycling tests by a significant margin [31].

However, the very characteristic that gives ICAs an edge in cycling tests—their thermoplastic nature—presents a challenge in surface mount technology applications. The ongoing cycling can lead to the accumulation of plastic strain within the polymer, eventually initiating cracks and compromising reliability.

Anisotropic conductive adhesives conduct only along the vertical direction (Z axis). They are also composite materials with conductive metallic fillers dispersed in a nonconducting polymer matrix like the ICAs. However, the concentration of the metallic filler particles is lower than the percolation threshold. This lowers the contact between successive particles and prevents the in-plane conduction (X–Y plane). The conductive particles can also be coated with a nonconducting polymer layer to prevent short-circuiting. Ag and Au are the most used material as the conductive filler. The filler particles are typically spherical or elliptical in shape. Since, Ag and Au are expensive, in some cases, the core of the particle is made polymeric to minimize the cost. The ACA can be easily laminated on to the PCB after lining up the ACA with the circuitry using a laminator.

The bumps on chips are lined up with the circuitry before pressing onto the PCB. Temperature and pressure are used to line up the conductive particles along the Z axis and trap them between the bumps and the circuit pads [32]. The pressure and temperature are critical in ensuring the bridging of the bumps and circuit pads to generate the conducting pathway as shown in Fig. 5. Since, generally, the metallic fillers are coated with polymers, it essential to eliminate the insulating layer and expose the metallic surface by inducing deformation in the particle. The curing/hardening of the polymer holds the particles together and prevents any slip between the particles and conductive surfaces. Several models have been proposed to model the electrical behavior or ACAs. However, many inconsistencies were noticed between the proposed models and the experimentally observed electrical resistance [33]. These inconsistencies were attributed to inaccurate calculation of contact resistance.

Anisotropic conductive adhesives offer certain unique advantages over ICAs. The application of pressure along with temperature can make the polymer flow and fill any voids. So, ACAs can be reliable. Additionally, low concentration of electrically conducting filler particles allows the polymer matrix to be loaded with particles having high thermal conductivity and makes the heat dissipation more efficient [34].

A major function of the die attach material is to transmit signal and power. To do this, the interconnection must deal with electric fields. Conductive particles may carry charge. Under a strong enough applied electric field, the force due to the electric field on the charged conductive particles can overcome the adhesive forces of the ACA [35]. This causes movement of the conductive particles at a macroscopic level. Conductive particles from adjacent joints can gravitate toward each other and form conductive interconnected chains resulting. If the chains get trapped in between the conductive surfaces, short-circuiting will occur. When the conductive particles are coated with an electrically insulating layer, due to the friction between particles or particle and bump, there can be wear while squeezing out of the clearance between bump and pad to the bump gap region. This wear can erode the insulation layer. So, short-circuiting cannot be effectively prevented by coating the conductive particles with a nonconducting layer. Degree of curing has shown to be a major influencer as under cured polymer matrix may not be strong enough to hold the conductive particles together against the force due to the applied electric field. This issue often puts a limitation on fine-pitch capability of ACAs. Moreover, their effectiveness can be limited by the way conductive fillers are distributed within the adhesive. When these fillers are randomly scattered (as opposed to being aligned), they may not make enough contact with each other to form a robust conductive network. This can create voids and hinder overall conductivity, making such ACAs less suitable for high-end electronics applications. Additionally, unlike ICAs and solders, ACAs require both heat and pressure to cure, adding complexity to the assembly process.

Researchers have explored the use of organic monolayers to improve the electrical performance of ACAs. These monolayers are single-molecule-thick films applied at the interface between the metal filler and the metal-finished bond pad within the ACA. By adhering to the metal surface, these organic molecules create a special bond that allows easier electron flow, reducing electrical resistance and enabling higher currents. A key advantage of organic monolayers is their ability to modify the metal's work function, influencing its electrical properties. By selecting appropriate organic monolayer coatings, researchers can achieve lower work functions on the metal surfaces. However, the effectiveness of these monolayers depends on their compatibility with specific metal finishes. Table 2 provides examples of suitable organic monolayers for different metals.

Interestingly, introducing dicarboxylic acid or dithiol at the interface of nanosilver filled ACAs led to significant electrical property improvements even in high-temperature curable systems. This suggests that the organic monolayers remained stable on the silver nanoparticles at the curing temperature (150 °C) [36,37]. The enhanced bonding is attributed to the larger surface area and higher surface energy of the nanoparticles, which allows for easier and more thermally stable coating with the organic monolayers.

Nanofibers have a high surface area to volume ratio. So, they can be used to get in between the conductive particles and restrict their movement. There are mainly two ways to introduce nonconductive polymer nanofibers in the ACAs. First, ACAs can be laminated onto nanofibers [38,39]. Second, conductive filler particles dispersed in a nonconductive polymer's solution can be electrospun to form nanocomposite nanofibers [40]. These materials can then be laminated onto a nonconducting adhesive or the polymeric matrix of the ACA. In the former case, particles could move into the spaces not defined by the nanofibers, thereby only partially increasing the electrical performance. Whereas in the latter case, the nanofibers could not only greatly suppress the movement of conductive particles as evidenced by the stable contact resistance but also demonstrate good insulating property and very small wear-out. So, this can potentially enable joining of chips with pitches <30 μm.

An interesting application of ferromagnetism resulted in self-aligning ACAs. In ferromagnetic materials, dipoles can be induced by externally applied magnetic fields. The induced dipoles can generate forces internal to the material. These forces can physically transport the particles and place them on top of each other. Integration of several such particles can result in formation of rods/columns and behave like an ACA as shown in Fig. 6 [41]. The conducting particles do not have to be ferromagnetic. They can simply be coated with a ferromagnetic material like Ni to make them magnetically self-aligning. The joining process is extremely simple as the need for pressure and/or patterning is eliminated completely. However, heat is required to cure the resin and arrest the particles in their position. ACAs generally perform poorly at high frequencies. But this material has demonstrated to work at high frequency of 90 GHz. It is versatile as it can be used for Si to Si, bump to Si, or bump to pad interconnects. They have even been commercially launched by Sunray Scientific (Eatontown, NJ) [42]. However, curing process can deform the particles and degrade the overall performance of the material. Also, elaborate, and complicated coplanar waveguides are required to ensure proper alignment of particles. The waveguides need to be changed with any major change in chip or substrate.

Nanometals, generally, have a lower melting point than their bulk counterparts. Bulk Ag has a high melting point of around 960 °C. But nano-Ag can sinter at low temperatures like 150–200 °C. This has been attributed to the exceptionally high interdiffusivity of the nanometal atoms. So, it has been used as a filler in ACAs [43–45]. Interfaces between Ag particles contribute highly toward overall joint resistance. Decreasing the number of interfaces would decrease the joint resistance. After sintering, the number of interfaces reduces significantly. Electrical contact area of conductive filler per bond pad increases. Thus, the joint resistance decreases remarkably. However, the joint has a small but finite resistance which is decided majorly by the interparticle resistance. Significant increase in electrical conductivity is complemented by significant increase in mechanical strength of the joints as well. Figure 7 illustrates an example of using nano-Ag sintering for a die attachment application. Recently, using Ag nanoparticles, conductivity as high as 3.2 × 10⁴ S/cm [44] has been reported, which is several orders higher magnitude of conductivity can be achieved than a typical ICA.

Manufacturing copper nanoparticles proves cost-effective compared to silver nanoparticles, making Cu nanoparticles a viable alternative [46–48]. Additionally, copper boasts low resistivity and excellent electromigration characteristics. However, copper is prone to oxidation in corrosive environments, such as those containing oxygen and moisture, leading to degradation in adhesive electrical properties. Unlike silver, which can be sintered at lower temperatures (<300 °C), copper lacks this capability. To address this, a chemical plating method coats the surface of microcopper with nanosilver particles, facilitating sintering joints between adjacent particles and reducing electrical resistance. These modified nanosilver-coated copper particles are integrated into epoxy and phenolic resins used in ECAs as fillers. The resulting adhesive, with silver-coated copper filler, exhibits significantly lower electrical resistance compared to pure copper-filled adhesive, regardless of the shape of the silver-coated copper particles (whether spinous or spherical). Under high temperature exposure for 100 h, pure copper particle-filled adhesive forms copper oxide near the surface, impacting electrical resistance, whereas silver-coated copper particle-filled adhesive maintains stable conductivity as oxygen concentration near the surface remains minimal.

Die attach adhesives play a crucial role in securely bonding ICs to substrates, necessitating materials with mechanical, thermal, and electrical properties suited to the desired functionality. While traditional inorganic adhesives like silver-filled glass or gold/tin eutectic have been used, they present challenges such as low throughput and difficulty in controlling processes, especially with larger die sizes. To mitigate these issues, polymer-based adhesives have gained traction due to their lower material moduli, ease of use, and cost-effectiveness. In cases requiring electrical contact to the backside of the die, conductive die attach adhesives are employed, with their electrical reliability crucial to certain IC devices' operation. However, challenges such as outgassing and die attach delamination persist, particularly with larger die sizes and thinner packages. Cyanate-ester-based adhesives offer advantages such as high heat resistance and low outgassing, making them popular for various applications, including hermetic and plastic molded IC packages. Modifications to cyanate-ester-based adhesives aim to address reliability issues, such as popcorn cracking, by reducing moisture absorption and offering a combination of properties suitable for diverse package configurations.

For flip-chip applications, ICAs must be selectively applied to areas requiring electrical interconnection while preventing spreading that could cause electrical shorts. ICAs are typically supplied in paste form and are often deposited precisely using screen or stencil printing. However, for flip-chip bonding, accurate pattern alignment is crucial, and the transfer method may be employed. This technique involves using raised studs or pillars on the die or substrate to selectively transfer the ICA to these areas, ensuring controlled thickness and preventing spreading between pathways. In high-volume environments, precise screen printing directly onto the substrate's I/O pads can be employed, potentially eliminating the need for stud pillars, or bumping of flip-chip pads. Despite the potential for speed and ease of processing, substantial improvements in bond strength are necessary for ICA flip-chip bonding to compete effectively with ACA methods. Unlike ACA flip-chip bonding, ICA bonding requires a separate underfilling step to enhance long-term reliability. While reliability is generally good with ICA flip-chip joining on rigid substrates, challenges remain regarding poor processibility and a narrow process window directly after assembly.

As the complexity of driver ICs for high-resolution LCD modules increases, the density of bumps on the IC rises, leading to smaller bump sizes and pitches. For fine-pitch chip-on-glass (COG) connections using ACA, a high density of conductive particles is necessary to ensure sufficient contact between the bump and substrate pad. However, the increased density also raises the risk of electrical shorts between adjacent bumps, primarily due to the accumulation of conductive particles during the COG bonding process. To address this issue, double-layer ACA structures have been developed, comprising an ACA layer and a nonconductive filler layer, to provide both electrical conductivity between the bump and electrode and insulation between adjacent bumps (Fig. 8).

As bump sizes and pitches decrease further, insulating layer-coated conductive particles have been introduced in anisotropic conductive film (ACF) layers, along with nonconductive fillers, to enhance electrical insulation. Studies have shown that double-layer ACF with insulating coated conductive fillers effectively reduces electrical shorts, achieving insulation capability even at a 10 μm gap level.

In chip-on-flex (COF) bonding, which is relatively newer compared to COG, several materials and processes, such as Au–Sn joining, stud bump bonding, and ACA joining, have been explored. ACA joining is particularly prominent, similar to COG technology. However, COF substrates typically exhibit weak adhesion properties with ACF materials due to their two-layer structure lacking an adhesive layer. Therefore, efforts have been made to improve adhesion between the IC chip, ACF, and the two-layer flex substrate to meet increasing reliability requirements. Triple-layered ACF structures have been developed to enhance interface adhesion and control bonding properties for fine-pitch applications during thermocompression bonding, leading to improved reliability in COF module assembly.

Electronic packaging plays a critical role in safeguarding the functionality and performance of electronic devices. It protects systems from various environmental and operational hazards while facilitating heat dissipation, signal transmission, and power delivery. As the micro-electronics industry thrives on miniaturization, higher performance, and sustainability, ECAs are poised to play a pivotal role.

Electrically conductive adhesives offer compelling advantages over traditional solders, including lower processing temperatures, environment-friendly nature, and compatibility with diverse substrates. However, both ICAs and ACAs present distinct benefits and challenges. Ongoing research focuses on overcoming limitations related to conductivity, moisture sensitivity, and particle behavior in ECAs.

Recent advancements in nanofiber integration, magnetic self-assembly, and low-temperature sintering techniques hold promise for enhanced performance and broader applicability of ECAs. Applications of ECAs encompass various electronic packaging scenarios, including die attach adhesives for IC bonding, flip-chip applications, and chip-on-flex packaging for LCDs.

The future of electronic packaging is expected to witness further innovations in ECAs and related technologies, driven by the relentless pursuit of improved performance, reliability, and environmental compatibility in electronic devices.

This work was supported by Cornell University.

No data, models, or code were generated or used for this paper.