Electrical characterization of anisotropic conductive adhesive based flip chip for a direct access sensor

Anisotropic conductive adhesives (ACA) are ideal interconnect materials for flip chip assembly due to their low cost and low processing temperature. They also provide an advantage for fine pitch assemblies, high speed interconnection packages and sensor assemblies. They are often used as interconnect material in mainstream applications such as flat panel displays, camera modules, mobile phones, semiconductor packages and RFID tags. In a direct access sensor the ACA seals the chip and substrate area and thus comes in direct contact with fluid environment. Moisture is known to affect the dielectric constant of the ACA. The objective of this paper is to study the effect of moisture on the impedance parameter of ACA during a frequency sweep from 50Hz - 50MHz. In order to observe the effect of moisture on the ACA and thus its frequency response, a flip chip interconnect structure is made whereby the ACA is exposed to the fluid environment. A HP4294A Impedance Analyzer was used to measure the impedance parameters of the samples before and after soak testing. The preliminary results after soak show that the real part and the imaginary part of the impedance have varied during the frequency sweep. The resistance increased from around 0.2ω to around 6ω while a small shift in resonant frequency was attributed to a slight increase in capacitance.