ICA interconnects

After the adhesive is cured, the fillers are randomly distributed and form a network within the polymer matrix. By this network, electrons can flow from one adherent to the other across the filler contact points. The overall result is to create numerous electron pathways, but with each path made up of a large number of mechanical contacts. Any factors affecting intimate contacts among fillers will surely influence the performance and reliability of ICA interconnects. 

Besides die attachment, ICAs are utilized in surface mount and flip-chip packages as alternatives to traditional solders. Due to their low surface tensions, ICAs are not suitable for wave soldering (Ref 6). Despite the advantages of ICA interconnection, the wide use of this technology has not been adopted by the electronics industry. The main concern is long-term reliability. 

Cracking. Due to the temperature fluctuation caused by the circuit power on/off cycles, ICA interconnects have to sustain cyclic stresses from thermal expansion mismatch between the substrate and component, and thermo-mechanical fatigue cracking is considered as one of the primary failure mechanisms. Based on temperature cycling tests and cross-section observations, the fatigue cracking behavior of ICA joints of... 

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. 

Underfill. An underfill is then injected into the gap between the chip and chip carrier and then cured to complete the flip chip process. The function of the underfill or encapsulation as it is sometimes referred to is to provide mechanical integrity and environmental protection to a flip chip assembly. Studies have demonstrated that both thermoset and thermoplastic ICAs can offer low initial joint resistances of less than 5 mS2 and stable joint resistances (Au-to-Au flip chip bonding) during all the accelerated reliability testing listed in Table 1. The reliability results have indicated that there is no substantial difference in the performance of thermoset and thermoplastic bumps and both types of polymers apparently offer reliable flip chip electrical interconnections (53). 

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... 

Electrical conductivity of ICAs is inferior to solders (76). Even though the conductivity of ICAs is adequate for most applications, a higher electrical condnctivity of ICAs is still needed. To develop a novel ICA for modern electronic interconnect applications, a thorongh imderstanding of the materials is required. 

Schematics of a micro-nano-micro substructure in ICA and the potential energy profile at the interconnect betv een the micro- and nanoparticles... 

Encapsulated ICA Flip-chip Interconnection. Stress analysis of encapsulated ICA flip-chip interconnechon was also executed with 2D FEM calculahons (Ref 45). Corresponding to experimental tests, four chip dimensions, 3 by 3 mm, 6 by 6 mm, 9 by 9 mm and 15 by 15 mm, were modeled. Only one joint was considered in each model, and the distance to neutral point (DNP) is half the diagnostic length of the test chip. To consider its anisotropic properties, the PCB substrate was modeled as a sandwich structure consishng of hve epoxy layers and four hber layers. The mesh for the whole package with hne structure details is presented in Fig. 27. Since the real joints were peripherally located on the test chip, axisymmetric boundary condition was employed. 

Fig. 27 2D model of an ICA fllp-chip interconnection, showing the mesh for (a) global and (b) local structures... 

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... 

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