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High Frequency (77 GHz) MMIC Die Stabilization on Organic Substrates
Keywords: dispense-jetting, high-frequency, flip-chip
Delphi Electronics & Safety is currently manufacturing and developing high frequency radar products for automotive applications. These products require substrates with low-loss, and a relative permittivity or dielectric constant property suitable to insure high efficiencies of microwave signal propagation. A variety of material sets (ceramic to organic substrates, flexible to rigid, wire bonded or flip chip die) and assembly processes have been evaluated for creating high-value safety products with high mechanical reliability, all within a small package footprint. A recent evaluation utilizing corner-bond dispensing techniques of single-part epoxy glob tops or dam-forming materials for stabilizing both “thin” 1.37 x 1.69 x 0.10mm and “thick” 1.37 x 3.38 x 0.38mm GaAs MMIC flip chip devices attached by reflowed Pb-free solder to organic transceiver substrates will be presented. The copper-stud solder-bump design provides a very short (0.08mm) electrical pathway from board topography to active die elements. Although these die are relatively small, the thermal expansion differences of the material set requires die stabilization for extended thermal cycle test survival. A comparison of micro-jetting and traditional needle dispense techniques for generating the micro-corner bond features, which provide the necessary support and yet do not interfere with the die or substrate high frequency trace connectivity, is discussed. Dispense efficiencies and material capability are the most significant factors for success. The high frequency signal, either on the trace or the GaAs die circuitry, is adversely affected by the presence of adhesive material. Significant microwave signal losses are experienced when epoxy attachment material replaces atmospheric air between the active die and organic board surfaces. The feature size of the corner bond pillars must not exceed 0.8mm diameter and requires tight placement control. Test die cross-sections are reviewed that document solder fatigue, stud-structure damage results after 1000 cycles under -40/105 C thermal shock test conditions.
David Ihms, Senior Project Engineer
Delphi Electronics & Safety
Kokomo, Indiana

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