Multi-28Gbps serial-link FCBGA equipped DC-block capacitors on the package

High speed multi serial-link devices are rapidly applied in the field of network server and switch. To achieve high pin count (exceeding 2,000 pin) and high speed devices, we have been developing FC-BGA (Flip-chip BGA) package applied high-density organic substrate to achieve multi Gbps transmission [1]. The demand of signal speed approaches 28Gbps, and multi-lane serial links are also required. There are some reports over 28Gbps transmission by electrical signal transmission [2]. On the other hand, the DC-block (AC-coupling) technique for serdes transmission method is commonly used in the field of high speed communication. This DC-block method can cancel the offset voltage of the direct-current between input and output circuit, set the optimize voltage of the receiver circuit, control the level shift and reduce the common noise. There are many advantages by adopting DC-block functions. On the other hand, there are some challenges regarding the design of mounting parts and signal integrity. In general, DC-block functions are achieved by mounting chip capacitors in series into serdes signal lines, and there are commonly mounted on PCB (printed circuit board) or connectors, moreover there are some approaches to improve the signal integrity by the PCB design or the connector structure [3]-[4]. However, if multiple DC-Block capacitors are arranged near the devices, the mounted area and PCB signal routing area are restricted and the power supply planes are weakened. In addition, multiple vias must be arranged to connect between DC-block capacitor and inner signal trace, because the PCB trace is generally preferred strip-line structure to reduce crosstalk noise. Furthermore, significant impedance discontinuity is occurred at both DC-block capacitor and vias. These problems will be more critical to achieve multi-28Gbps serial-link with DC-block capacitors on PCB. In order to solve these problems we have developed advanced FCBGA equipped DC-block capacitors on the package.