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|Evaluation of die to organic laminate to PCB interconnects up to 50GHz|
|Keywords: RF, organic BGA, flip chip|
|There is interest in using organic packages for high frequency applications as the cost of organic laminates is lower than the cost of ceramic substrates, which have been traditionally used for RF and microwave applications (1). In previous work, design of organic packages for 10GHz was presented in (2) and practical characterization of plastic ball grid array package was presented in (3). Characterization methods and results for various wire bond transitions for RF applications were presented in (4) (5). In this work, evaluate the interconnect from die to organic laminate to printed circuit board (PCB) up to 50GHz. We describe a test vehicle designed to characterize the interconnects at multiple levels, and present results for both wire bond and flip chip interconnections. This paper will discuss in detail the test vehicle design, assembly, modeling, testing, characterization and model to hardware correlation results. The test vehicle used in this study consisted of a 10 metal layer organic laminate BGA package, 15mm x 22.5mm in size, with 364 Pb-free ball grid array interconnections, soldered on to a printed circuit board. The laminate was designed with bond fingers for wire bond interconnects and flip chip pads for C4 interconnections. The flip chip pads were connected through differential strip-line transmission lines to the BGA. On the printed circuit board, the BGAs were connected to 2.5mm long, micro-strip transmission lines. Probe pads placed at flip chip pads and probe pads placed at the end of the micro-strip lines were used for characterization of the connection from flip chip pads, through laminate and solder balls to printed circuit board. A test vehicle chip with bond pads and probe pads was designed, fabricated, attached to laminate with die attach adhesive and was wire bonded to characterize the die to laminate interconnects. Different wirebond lengths and bonding schemes were evaluated to improve die to laminate interconnect performance. In the laminate, the wire bond fingers were connected to BGA solder balls through differential strip-line transmission lines. The BGA solder balls were connected to micro-strip transmission lines on printed circuit board. Similar to flip chip interconnects, placing probes on probe pads on die and probe pads on printed circuit board allowed characterization of the die to wire bond, laminate, solder ball to printed circuit board interconnections. The test vehicle was placed on a probe station (Cascade Microtech) and S parameters were acquired (Keysight Vector Network Analyzer) from 10MHz – 50GHz frequency range. In addition, the interconnects were modeled in ANSYS HFSS software and S parameters were obtained. The results of the testing, characterization and model to hardware correlation will be discussed in detail.|
|Selaka Bulumulla, PMTS