BGA backplane connector for high speed differential signals

Abstract

The effort to achieve data-rates of 5 Gigabits/second and beyond is compromised by the relatively large plated-through holes (PTHs) required for conventional through hole connectors. The PTHs introduce impedance discontinuities that result in signal reflections. Additionally, the PTHs restrict the widths of the traces resulting in higher conductor losses. Increased bandwidth can be achieved by replacing conventional through-hole connectors with surface mount connectors using ball grid array attachment techniques. The elimination of the PTHs and substitution of smaller vias result in lowered impedance discontinuities and the ability to use wider traces. However, the ball grid array method of attachment has not, up to now, been used in backpanel systems. This paper discusses the approach to developing a BGA header for high-speed backpanel applications. Using a high volume BGA connector as a test vehicle, testing was initiated that simulated conditions found in backpanel applications. The test vehicle was incrementally loaded with weights that simulated the loading of a backpanel style connector until the limits of ball support were reached. The importance of level loading of the Ball Grid Array structure was established. Alternative methods of mechanical attachments were also explored. Finally, based on these findings, a prototype design was tooled that successfully passed accelerated life testing including thermal cycling and high temperature life.