| dc.description.abstract | A new lightweight package material for Radio Frequency (RF) hybrid circuits were investigated to complement the currently used FeNiCo (kovar) package material. Kovar is extensively used as a package material for hybrids in space applications due ease of manufacturing and its Coefficient of Thermal Expansion (CTE) of 5.9 ppm/°C matching commonly used glass seals with a CTE of 5.8 ppm/°C for pin sealing and internal components such as Al2O3 (alumina) substrates with a CTE of 5.4 ppm/The main drawbacks of kovar are its high material density and poor heat conductivity. New materials with better heat conductivity and lower density were researched and compared. Ceramic packaging materials were deemed unsuitable due to the additional process steps and increased cost of enabling the possibility of hermetic sealing by parallel seam sealing, as a metal seal ring must be attached.
Of the metals and metal alloys, two materials were chosen as possible candidates. Aluminium 6082-T6 (Al 6082-T6) and Aluminium silicon CE-11 (AlSi CE-11) because of low density and high heat conductivity. The drawback of Al 6082-T6 is its high CTE, which will cause a greater amount of thermal stress to be generated due to CTE mismatch between the Al package and the internal components. AlSi CE-11 is an Al alloy with 50 wt% Si, which halves the CTE compared to Al 6082 and keeps a high heat conductivity. Resistance welding of Al and Al alloys to hermetically seal a package is difficult due to the high heat and electrical conductivity of the material and should ideally be laser welded or solder sealed.
Two package designs were modelled in Solid Edge. One package with down-stepped walls, with a package to lid interface called butt joint, designed for hermetic sealing by laser welding or solder sealing. Package design two had identical dimensions, but with flat walls and a down-stepped lid to attempt hermetic sealing by parallel seam sealing. The packages were machined in the material Al 6082-T6 and plated with Gold (Au) over Nickel (Ni) according to the applicable drawings and specifications. AlSi CE-11 as a package material was deemed not relevant mainly due to long production lead-time and high cost.
The heat dissipation of commonly used active dies at Kongsberg Space Electronics (KSE) in each package material was analytically analysed and simulated in COMSOL. The thermal stresses generated in each system due to CTE mismatch was also analysed mathematically and modelled using COMSOL.
Several hybrids were produced using standard assembly methods employed at KSE and applicable relevant MIL and ESA standards. The qualification test program was split in two, one focusing on hermetic sealing and the other focusing on thermomechanical testing. In the hermetic sealing branch, five Ni plated Al packages were hermetically sealed using tin-lead (SnPb) solder and five Au over Ni plated Al packages were hermetically sealed with a parallel seam sealer and a kovar lid. All packages were tested for hermeticity using Cumulative Helium Leak Detection (CHLD) or a water bath to verify the hermetic seal quality. As an additional quality control test, two cross-section samples were inspected in a SEM to locate any seal defects, such as voids or cracks. For the thermomechanical branch, one substrate, five ceramic capacitors and three monolithic microwave integrated circuits (MMIC) were adhered to the package using conductive silver epoxy. Five Ni plated Al packages were solder sealed with SnPb solder, and two Au over Ni kovar packages were parallel seam sealed as a reference. Five Au over Ni plated Al packages was supposed to be parallel seam sealed but was not performed due to the worry of wear and tear of the parallel seam sealer having an impact on normal hybrid production at KSE. All packages were hermetically tested before and after thermal and mechanical stress testing to verify the hermetic seal quality. Destructive bond pull, ball shear and die shear were performed after stress testing to verify the quality of the mechanical attach of wire bonds and adhered components. | |