ACA interconnects

To achieve line pitch connections, a metal sphere or metal-coated plastic sphere coated with an insulating resin fillers were developed. The insulating resin layer is broken only under pressure to expose the underlying conductive surfaces, referred to as a microcapsule filler- (MCF). A higher filler loading can be achieved with MCFs for fine pitch applications, which avoid creating electrical short circuit conditions between printed circuit features (7,12). A typical cross section of an ACA interconnection with MCF material is illustrated in Figure 2. 

ACA Bumped Flip Chips for High Frequency Applications. The high frequency behavior of ACA interconnections has attracted much attention in the past several years. Sihlbom and co-workers demonstrated that ACA-bonded flip chips can provide performance equivalent to solder flip chips in the frequency range of 45 MHz to 2 GHz on FR4 chip carriers and 1 to 21 GHz on a high frequency Telfon-based chip carrier (Fig. 5). The different particle sizes and materials in the conductive adhesives gave little difference in high frequency behavior of ACA joints (22,23). 

Compared with solder interconnection, ACA interconnection offers benefits in terms of pro-... 

ACA contains insufficient particles, there is of course a certain probability that no particle exists in the joint and an open will result. On the other hand, bridging is possible due to too many particles in a short spacing, causing a short circuit between neighboring pads. Accurate probability estimates of open and bridging are needed to define the limiting pitch of ACA interconnects. An analytical method to estimate the open probability was proposed (Ref 47). Assume that the number of particles on a pad obeys Poisson s distribution ... 

From the viewpoint of their conduction and mechanical joining, ACAs are similar to ICAs, except that they have lower concentrations of conductive particles. This lower concentration provides unidirectional conductivity in the vertical or z-direction (perpendicular to the plane of the substrate), which is why they are called anisotropic conductive adhesives. In the same way, ACA materials are prepared by dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. The concentration of particles is controlled, so that sufficient particles are present to ensure reliable electrical conductivity between the assembled parts in the z-direction, while insufficient particles are present to achieve percolation conduction in the x-y plane (Kim et al. 2008b). O Figure 50.6 shows a schematic description of an ACA interconnect, showing the electrical conductivity in the... 

Using this derivation, the connection resistance of the ACA interconnect can be easily calculated by the simple investigation of the physical shape of the connected pathway. This derivation is based solely on the physical contact of the two unmelted metals, so it could be applied to the other two types of adhesives, i.e., ICAs and NCAs. In the case of ICAs, the shape of the current flowing path is more complex, which means that the contact resistance might also be more complex than that in the ACA case. However, the resistance components of an ICA interconnect are completely identical to those of an ACA interconnect, so the derivation and calculation of the components are also easy to perform. In the case of an NCA, there are no conductive fillers and, therefore, Rqk and Ri can be excluded from the derivation. This means that the connection resistance for an NCA joint consists only of Rf, so that the resistance should also be lower than that of an ACA joint. 

Failure mode for ACA interconnect after thermal shock testing (Kim et al. 2008c)... 

Isotropic adhesives conduct current equally in all directions and are the most common and widely used in industry. The anisotropic types, also referred to as z-direction adhesives or anisotropic-conductive adhesives (ACA), although filled with metal particles, are filled at much lower levels (0.5%-5% by volume) than isotropic types (filled 25%-30% by volume). The volume Iraction of filler is well below the percolation threshold at which the adhesive becomes highly conductive in all directions. Because of the low volume Iraction of metal particles, there are no continuous electrically conductive paths in the x-y plane. During the connection process, the anisotropic adhesive, either as a film or paste, is positioned between a flip-chip bumped die or a tape-automated bonded (TAB) die and the corresponding pads of an interconnect substrate. Pressure and heat are... 

Fig. 2. Schematic depicting the cross section of an interconnection using an MCF-filled ACA.

Fig. 4. Schematic illustration of the formation of electrical interconnects between a bumped chip and a mating carrier using a Bi-filled ACA. (a) The chip is aligned and placed on a chip carrier, (b) The Bi particle is deformed between a chip bump and a carrier pad when a bonding pressure is applied, (c) The Bi particle dissolves into the liquid lentils upon exposure of heat, (d) Bi diffuses into the Sn-Pb matrix and forms fine solid precipitates.

Since the adhesive matrix is a nonconductive material, interconnection joints rely to some extent on pressure to assure contact for conventional ACAs. Adhesive interconnections therefore exhibit different failure mechanisms compared to soldered connections, where the formation of intermetallic compoimds and coarsening of grains are associated with the main mechanisms. Basically there are two main failure mechanisms that can affect the contacts. The first is the formation of an insulating film on either the contact areas or conductive particle surfaces. The second is the loss of mechanical contact between the conductive elements due to either a loss of adherence or relaxation of the compressive force. 

Metal-Bumped Flip Chip Joints. ICAs can also be used to form electrical interconnections with chips that have metal bumps. ICA materials utilize much high filler loading than ACAs to provide electrical conduction isotropically (ie in all directions) throughout the material. In order for these materials to be used for flip chip applications, they must be selectively applied to only those areas that are to be electrically interconnected. Also, the materials are not to spread during placement or curing to avoid creating electrical shorts between circuit features. Screen or stencil printing is most commonly used to precisely deposit the ICA pastes. However, to satisfy the scale and accuracy required for flip chip... 

Although dating as far back as the 1960s, anisotropic adhesives have only recently been rediscovered and advanced to a state where they are being used in place of solder to interconnect fine-pitch, high I/O-count flip-chip devices, TAB devices, and high-density interconnect substrates. Anisotropic adhesives are also used to interconnect edge connectors and flex cable to PWBs and flat-panel displays (FPDs). ACAs are commercially available in either paste or film form and as either thermoplastic or thermoset types. 

The bonding process is very critical to the ACA joint performance and reliability, since both mechanical integration and electrical interconnection are established in this process. Bonding pressure and temperature are the two most important parameters. To achieve reliable ACA joints, adequate bonding pressure should be applied uniformly, and suitable bonding temperature should be kept for sufficient time (Ref 30). 

Vanfleteren, J., De Baets, J., Van Calster, A., Dravet, A., Deckelmann, K., Wiese, J., Schmitt, W., Allaert, K., Vetter, P., Schols, G., Cort, E. (1994). Anisotropic conductive adhesives (ACAs) for high density interconnection in liquid crystal displays (LCDs). Adhesives in Electronics 94, International Conference on Adhesives Joining Technology Electronics Manufacture, VDWDE-IT. 

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