Conductive adhesives

Conductive adhesives are normal adhesives that are enriched with conductive fillers. The fillers are metallic, carbon or metallized nano or micro particles [16] [32]. The conductive adhesives can be made hard or soft, which is an advantage compared to conventional solders. In addition, the adhesives require low temperatures to form conductive joint, which makes them well compatible with plastic substrates. Based on the amount of con-ductive fillers, there are isotropic concon-ductive adhesives (ICA) and anisotropic concon-ductive adhesives (ACA). [32]

3.3.1 Isotropic conductive adhesives

Isotropic conductive adhesive (ICA) consists of conventional adhesive body that includes conductive fillers. The amount of filler particles in ICA is high, which enables continuous filler network and further the isotropic conductivity. The filler network is a requirement for high conductivity and it is formed after amount of fillers increases over the percolation threshold. The percolation threshold is the point when the conductivity of ICA increase

considerably and the filler network is created. Below the percolation threshold, there is no continuous filler network and conductivity is poor. However, wide filler network de-crease the cross-linking potential of adhesive polymers, which dede-crease the strength of adhesive. The amount of fillers should be limited for a durable adhesive but still high enough over the percolation threshold. In commercial ICAs, the amount of conductive fillers is between 70 and 82 percent by weight. [32] [52]

As in conductive inks, conductive fillers such as gold, silver and CNTs are used in ICAs.

[38] [41]. Currently, the most used ICA type includes epoxy adhesive and one-size micro silver flakes [32] [52] [53]. The epoxy has great heat resistance and is compatible with various fillers, which makes it good resin for ICA [38]. The micro silver flakes have good electrical and thermal conductivity and are chemically stabile [53]. There are also options to use silver nanoparticles or CNTs with silver flakes, which increase conductivity of adhesives [53] [54]. However, they are not yet commercially used.

Underfill is non-conductive epoxy adhesive that is commonly used in surface mount de-vice (SMD) and flip-chip technologies. The main purpose of the underfill is mechanically support ICA or solder connections in electronics structure. The underfill must be heat treated like ICA, which can be done after establishment of conductive connections. [31]

[32] Thermal and chemical properties of underfill and ICA should be compatible to en-sure stabile joint. Mismatch of thermal or chemical shrinkage of the underfill and the ICA cause cracking, delamination or other premature joint failure. [55] Figure 6 shows the forming of ICA contacts after the ICA and underfill are added between substrates:

Figure 6. The concept of an ICA joint.

Originally ICAs are developed to miniaturize electronics and substitute traditional lead containing solders that are categorized as environmentally hazard problems. The ICAs have low curing temperatures that makes them compatible with more sensitive substrates.

Furthermore, they have simple process steps and are non-conductive until the heat treat-ment. [31] [53] [56]

3.3.2 Anisotropic conductive adhesives

Anisotropic adhesives (ACAs) are adhesives that are selectively conductive. ACAs have an adhesive body and conductive fillers like the ICAs but the amount of conductive fillers is considerably lower. The filler quantity is below the percolation threshold (1-30 percent by weight), which obstructs mutual conductivity of fillers. However, by using uniformly dispersed big size fillers in thin adhesive layer, it is possible to create conductivity to normal direction of adhesive layer (z-direction). The z-direction conductivity is strength-ened by firm pressure and heat of the bonding process, which squeezes and locks conduc-tive particles of ACA between contact pads. Figure 7 shows an example about the forming of anisotropic adhesive film joint. [32] [57]

Figure 7. The concept of an ACF joint.

ACAs are provided in two different forms. There are anisotropic conductive pastes (ACPs) that are in liquid form and anisotropic conductive films (ACFs) that are in solid film form. Generally, ACPs consist epoxy based resin like ICAs and the ACFs are made from thermoplastic polymer or rubber. Conductive fillers in ACAs are generally fabri-cated from nickel coated polymer particles that are further coated with gold or silver. The highly conductive coating and polymer core increase plasticity of the filler particles and decrease costs of ACA. [32] [57]

ACAs have many advantages. Joints made with ACA require low bonding temperature and simple equipment. ACA joints are thin and, especially with ACF, the adhesive thick-ness is well defined. Because of low conductive filler content, they are a low costs appli-cation and are used, for example, in electronics industry. [57] In Table 8 is presented ACF tape tesa HAF 8412 that is used as the conductive adhesive in the tests:

Table 8. The tesa HAF 8412 ACF tape properties [58]

tesa HAF 8412

Adhesive type Phenolic resin and nitrile rubber

Thickness (µm) 45

Lamination temperature (°C) 180 – 220 Lamination pressure (N) 80 – 130 Lamination time (s) 1,5

The tesa HAF 8412 is designed for one step bonding process to form reliable mechanical and electrical bonds. The main applications of the ACF tape are to embed chip-modules to smart cards, where PCV, ABS and PC based cards are especially suitable. The ACF has support film on the other side, which makes application of the ACF easier. [58]