High Temperature Reliability of Copper Wire - Bonded Packages Encapsulated with Mold Compounds Containing Sulfur compounds

Abstract

Copper wire-bonded (CuWB) packaging is more susceptible to corrosion than traditional inert gold wires. CuWB reliability greatly depends on the compatibility of Cu wire with the surrounding encapsulating mold compound as this matrix can provide a corrosive environment leading to reliability issues. Many mold compounds contain specific components which are sulfur-based compounds. Since the reliability testing of an encapsulated packaged device involves thermal treatments such as high temperature storage life (HTSL) test, there is a concern that corrosive sulfur compounds can be produced at high temperatures, e.g. 150 °C and 175 °C, endangering CuWB reliability. This paper describes methods of detection of sulfur compounds produced from mold compounds, if any, at high temperatures such as 175 °C and CuWB die package reliability with mold compounds containing sulfur compounds. Dynamic Headspace Concentration-Gas Chromatography–Mass Spectroscopy (DHC-GC-MS) analysis technique was used to test liberation of gaseous and volatile sulfur compounds from mold compounds at temperatures 25 °C, 150 °C, 175 °C, and 200 °C. No gaseous sulfur compounds were detected by chromatographic methods within the time period of the experiments. In order to determine sulfur containing anionic species present in the mold compound matrix, such as sulfide, sulfite, sulfate and thiosulfate, ionic compounds were extracted to water and analyzed by Ion Chromatography. Upon analysis, the only sulfur bearing anion found in the samples was sulfate. Thermally treated mold compounds for 2000 hours at 150 °C and 1000 hours at 175 °C were also extracted and analyzed to determine possible decomposition of sulfur compounds due to the thermal aging process. Corrosion due to sulfur compounds and reliability of CuWB (bare Cu-25 μm wire diameter) were evaluated by HTSL for 2000 hours at temperatures of 150 °C and 175 °C with devices packaged with mold compounds containing sulfur compounds. CuWB ball bond – Al interface and Cu stitch bond integrity were evaluated by FIB (Focused Ion Beam) - SEM (Scanning Electron Microscope) with EDX (energy dispersive X-ray spectrometer) analysis and wire pull and ball shear testing of CuWB ball bonds. No reliability issues due to sulfur compounds were found with mold compounds containing sulfates up to about 45 ppm.