Performance of metal ball grid array (metal BGA) package

For the transmission of high frequency signals, it is important to adopt the microstrip line structure in BGA packages. Metal BGA packages consist of a Cu/polyimide/Cu substrate system without an epoxy adhesive layer, such that its electrical characteristics are superior to other plastic package which include an epoxy resin insulator. It is easy to fabricate the 90 /spl mu/m pitch fine pattern circuit due to the polyimide surface flatness. The polyimide layer in the chip mounting area is removed by chemical etching and a cavity down structure is formed by the mechanical stamping process. The reliability of the metal BGA was investigated. Good results were obtained for pressure cooker testing over 288 hrs. The dielectric constant (/spl epsiv//sub r/) of polyimide is 3.2 and tan-/spl delta/ is 0.002 at a frequency of 11.3 GHz. These values are very good for the construction of microstrip line structures in BGAs. The pulse response of a 10 mm long 50 /spl Omega/ line in the metal BGA was measured, and a rise time of 0.35 nsec was observed. This result indicates the possibility of a maximum operating frequency of more than 500 MHz.