Design of High-Density Interconnects for High-Speed Transmission Wanlin Fu 1 , Makoto Kimura 1 , Kenichi Okada 1 , Jun Sakai 2 and Kazuya Masu 1 1 Integrated Research Institute, Tokyo Institute of Technology 4259-R2-17 Nagatsuta, Midori-ku, Yokohama, 226-8503 Japan Tel: +81-45-924-5031, Fax: +81-45-924-5166, E-mail: paper@lsi.pi.titech.ac.jp 2 NEC Corporation, Jisso and Production Technologies Research Laboratories Abstract In this paper, we discuss about low-loss and crosstalk- robust line structure on the build-up printed-circuit boards. The equivalent attenuation is proposed as an evaluating figure of the line characteristics using attenuation and crosstalk coefficient. The characteristics of co-planar and diagonal-pair lines are compared by using the proposed equivalent attenuation. We demonstrate the diagonal-pair line is low-loss and crosstalk-robust structure as compared with the co-planar line on high-density build-up boards. 1. Introduction High-speed interconnection between LSIs is essential to realize the high performance computer server and network equipment. Recently, over 10 Gbps signal transmission has been required for printed-circuit boards. The attenuation, reflection, and characteristic impedance are important issues for design of high-speed printed circuits. High-density integration is also required due to increase of input-output terminals of LSI. [1] It is difficult to keep signal integrity in high-density printed circuits because reduction of cross- sectional wire area poses increase of line resistance. Crosstalk is also a serious problem in high-density printed circuits because reduction of line-to-line space causes increase of coupling between lines. Therefore, low-loss and high crosstalk robustness are required for a high performance printed-circuit board. In this paper, we evaluate the suitable line structure for the high-density build-up printed-circuit board, which has low- loss and crosstalk-robustness. The co-planar line has two differential signal lines arranged parallelly in a layer, and the diagonal-pair line has two parallel signal lines obliquely- arranged in two layers, which can be used as differential signal lines on the build-up board. [2] The attenuation and crosstalk characteristics of the co-planar and diagonal-pair lines are simulated by two-dimensional electromagnetic simulator. We propose the equivalent attenuation as an evaluating figure of the line characteristics using attenuation and crosstalk coefficient. The co-planar and diagonal-pair lines are compared by using the proposed equivalent attenuation. We demonstrate the diagonal-pair line is low-loss and crosstalk-robust structure as compared with the co-planar line on high-density build-up board. 2. Equivalent Attenuation In designing line structure, attenuation, crosstalk characteristics and characteristic impedance are the most important considerations. In this work, the characteristic impedance is determined by the transceiving and receiving circuits as 100 Ω. The propagation constant of transmission line with loss is defined as ) )( ( C j G L j R ZY j ω ω β α γ + + = = + = (1) where α is attenuation constant, and it is defined by the following expression when the signal is transmitted in differential mode. 2 ) ( ) )( ( 2 2 2 2 2 2 2 diff diff diff diff diff diff diff diff diff C L G R C G L R ω ω ω α − + + + = (2) From Eq. (2), the attenuation can be obtained by the following expression, where l is the length of line. l diff e Att α − = (3) Crosstalk can be evaluated using the crosstalk coefficient. [3][4] The backward crosstalk coefficient K b and the forward crosstalk coefficient K f ’ are defined by the following expressions. ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ + = jj ii ij jj ii ij b L L L C C C K | | | | 4 1 (4) ] sec/m [ | | | | 2 1 ' jj jj ii ii jj ii ij jj ii ij f C L C L L L L C C C K × ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − = (5) The backward crosstalk affects when the signals are transmitted in the opposite direction, and the forward crosstalk affects when the signals are transmitted in the same direction. The forward crosstalk noise is proportional to the line length l, inversely proportional to the pulse rise time t r , so we use the following expression as a forward crosstalk coefficient. r f f t l K K × = ' (6) We can obtain the crosstalk noise of victim line, approximately, by multiplying the crosstalk coefficient and the voltage of aggressor line. [5] 1-4244-0985-3/07/$25.00 ©2007 IEEE 352 2007 Electronic Components and Technology Conference