Shuang Yang, Ji Wu, and Michael Pecht
University of Maryland
College Park, MD 20742
Land grid array sockets are used as separable interconnects between printed circuit boards and high I/O integrated circuit components. Metal-in-elastomer land grid array sockets consist of an array of elastomer contacts, a polyimide insulator and a thermoplastic frame. Silver particles are embedded in the silicone elastomer as contact medium to provide electrical paths between a printed circuit board and a component.
In order to assess the reliability of this type of socket, biased, highly accelerated stress testing (HAST) was conducted. The surface insulation resistance (resistance between two adjacent contacts) was monitored, and was found to decrease significantly. Silver electrochemical migration (ECM) was identified as the cause. Both dendrites and film were observed on the polyimide insulator surface as a result of ECM. The dendrite grows from cathode contact to anode on the polyimide insulator surface. The ECM products were analyzed using both energy dispersive spectroscopy and X-ray photoelectron spectroscopy. Silver and silver oxides were detected as ECM products on the polyimide surface after HAST exposure. The potential chemical mechanism of silver electrochemical migration was described based upon the analysis of the ECM products.
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