Process and Product Development

Flip Chip Adhesive Bond Technologies #

SEM picture of a flip chip ICA-interconnect, ICA dip transfer with Au-Stud-Bumps
SEM picture of a flip chip ICA-interconnect, ICA dip transfer with Au-Stud-Bumps
X-Ray / CT image of an ultrathin flip chip-in-flex stack with 4 IC levels, 170 µm thickness in total, realized with ACA technology
X-Ray / CT image of an ultrathin flip chip-in-flex stack with 4 IC levels, 170 µm thickness in total, realized with ACA technology
Cross-section of an ultrathin ACA flip chip-in flex stack with 4 IC levels
Cross-section of an ultrathin ACA flip chip-in flex stack with 4 IC levels

Among the various joining technologies, such as soldering and thermo-compression, adhesive bond technologies have made the greatest advances due to the development of new materials and process technologies. This has lead to an expansion of possible application fields for adhesive bond technologies ranging from low cost consumer products up to high-end applications. For each of those fields with ambition to flip chip technology, solutions based upon the major three flip chip adhesive bond technologies such as ACA (Anisotropic Conductive Adhesive), NCA (Non Conductive Adhesive) and ICA (Isotropic Conductive Adhesive) are developed. Main benefits include low process temperatures, bonding of non-solderable surfaces and fine pitch capabilities. To achieve the optimum results, the entire process chain, from adhesive material selection, -characterization, -modification, parameter optimization up to testing and qualification is taken into account.

In ACAs (Anisotropic Conductive Adhesives), the electrical conductivity between chip and substrate is reached by conductive particles (metal, metal coated polymer) that are randomly dispersed in the adhesive matrix and trapped between the electrodes after bonding. Properties like an easy, large-area application of ACA in film or paste form, fine-pitch capability (≤ 40µm) and snap-cure are especially beneficial for Chip-On-Flex applications (COF) as for example driver-IC, smart card and RFID-tag assemblies. Its high compatibility with different bump and substrate metallizations widens the possible field of applications.

Non conductive adhesives (NCAs) are often used in combination with Au-Stud Bumps or other compliant chip or/and substrate metallizations. Its flexibility makes it ideal for the assembly of single chips (prototyping) or other applications, as MEMS-devices (Micro-Electro-Mechanical Systems) for example, where stud-bumping often is the only possible bumping technology. In this case, the contact resistances are very low and stable, and therefore comparable with solder or thermo-compression (Au/Au) flip chip interconnects.

Both technologies are known as thermode-bond processes, requiring the application of a bond force and temperature during the process. In addition, ICA (Isotropic Conductive Adhesive) flip chip technology is compatible with standard pick&place equipment for a flip chip solder technology. Local deposition of ICA follows the principle of either solder paste printing or a dip transfer. A wide choice of underfiller materials is available to meet the material requirements. Typical applications are pressure or temperature-sensitive devices, such as medical devices or pixel detectors.