Low-cost direct chip attach: comparison of SMD compatible FC soldering with anisotropically conductive adhesive FC bonding

Small modules based on laminate substrates are often used as a functional subunit in electronic applications. Currently, most are made by the chip and wire technique to fulfil size reduction requirements. Low cost flip-chip technology is one of the most promising approaches for further cost and size reduction. In this paper, a special car radio submodule is chosen as an example. We compare an SMD compatible FC soldering process using eutectic solder bumps and underfilling with an anisotropically conductive adhesive (ACA) FC bonding process using tape or paste materials. For FC soldering, an electroless, maskless Ni/Au plating for under bump metallization (UBM) was chosen. The solder deposition is by stencil printing, while other cost efficient deposition techniques have been observed. The FC assembly is integrated into a standard SMD line. Different underfill methods for quick underfilling are shown, and failure mechanisms and lifetime predictions of assembled flip chips are demonstrated. For ACA-FC bonding, electroless Ni/Au bumping is also used. An assembly process for ACAs using a semi-automatic FC bonder is developed. In order to reduce mounting time, the ACA has been precured. Aspects of different process flows, including ACA deposition techniques, tape and paste adhesives and filler materials, are discussed. The influence of high current, climate, and thermal cycling on the contact resistance and low frequency noise spectrum is shown. In summary, we describe the benefits and disadvantages of both techniques and discuss the potential for further developments and applications