Spiral via structure in a BGA package to mitigate discontinuities in multi-gigabit SERDES system

Abstract

Multi-gigabit serial channel design has become more and more important since channel margins are becoming tighter as operating frequencies increase. Most engineers working on these channels have been plagued by several different types of signal integrity problems including impedance discontinuities, reflections, attenuation, undershoot and overshoot, and crosstalk. In reality, many signal integrity problems are caused by impedance discontinuities. These discontinuities can be caused by physical discontinuities, system parasitics, or design mistakes. We need to understand these discontinuity issues to improve full channel from package to PCB board. When most designers simulate a channel, they concatenate separate package and PCB models. Using these concatenated models they simulate the data path in a channel simulation test bench. This piecewise or concatenated channel model is incomplete because it does not carefully consider the interface between the BGA package and the PCB. The hidden factor which makes the concatenated channel model differ from real channel measurement will be disclosed in this paper. A more detailed solder ball analysis has also been done during this process. We learn from the above results that if the package is soldered down to the PCB and measured together, additional loss occurs due to the package to board transition. The combined channel is more capacitive than the concatenated channel in the package to board transition region. The excess capacitance due to BGA solder ball transition is due to the physics of the interface and can not be easily eliminated. It is shown that system performance degradation is very sensitive to the impedance discontinuity introduced at the BGA/PCB interface. This paper will introduce a spiral via structure which is shown to be a more effective way to compensate the transition discontinuity seen at the BGA/PCB interface. This paper also shows how the spiral via structure was implemented and optimized in the BGA package. The overall channel performance improvement by using spiral via structures in BGA package is proven by the simulation and measurement.