Evaluation of fine-pitch chip-to-chip interconnects using ACF material with arrayed particles

As the demands for high density 3DIC stacking increase, a fine-pitch chip-to-chip interconnects is becoming imperative. In conventional flip-chip technology, anisotropic conductive film (ACF) has been used in place of solder and underfill for chip attachment to substrates in some applications, because it provides many advantages. Generally speaking, ACF consists of an adhesive polymer matrix with randomly dispersed conductive particles, which establish the electrical contacts between the interconnections. In this paper, a new type of ACF with Ni/Au-coated polymer arrayed particles in the bottom layer of the film was prepared. The 30µm-pitch chip-to-chip (C2C) bonding technology was presented and the chip with 3264 I/Os was bonded onto the substrate using the ACF material. After ACF lamination process, conductive particles capture rate and attach status were calculated and observed from the optical microscope (OM). Therefore, the ACF lamination condition was determined. After bonding process, the effects of bonding pressure and bonding temperature were discussed by considering particle deformation, electrical performance and adhesive flow status. After optimizing the lamination and bonding parameters, the reliability of C2C was evaluated by precondition test (125°C/24hrs backing, 30°C/60%RH/178hrs soaking, and 3times/265°C reflow). Cross-sectioned inspection of ACF joints by Scanning electron microscopy (SEM), the interface between adhesive and silicon observed by Scanning Acoustic Tomography (SAT), and adhesion strength test of ACF film after bonding and precondition were conducted to determine the failure modes. Moreover, after precondition test, the variation of near 70% of the C2C daisy chain resistance was found less than 15%. The stable electrical resistance and high reliability of ACF joints could be obtained. On the other hand, no any obvious delamination or failed joints could be observed within failure samples by SEM or SAT. Otherwise, the adhesion strength measured by shear test did not show any relations after the precondition test. We assumed that during the reflow process the entrapped adhesive matrix expanded much more than the conductive particles because of its higher coefficient of thermal expansion (CTE). Thus, induced thermal stress would separate the ACF joints and decrease the contact area of the conductive path.