Study on signal transmission performance of microwave multi-chip modules interconnect via hole structure

Abstract

This paper taking the microstrip-via hole-microstrip interconnect structure of microwave multi-chip modules (MMCM) as the research object establishes the simulation model of the via hole structure and using Ansoft HFSS simulation software analyzes the effects of the structure parameters of the connecting angle θ between microstrip and via hole, the via hole aperture, the via hole length and the size of pad on the scattering parameters of the signals. The influence law of the structure parameters of the number of uniform coaxial shield via hole and the distance from the shield via hole to the interconnect via hole on the parameters of signal scattering properties is studied by establishing the model of coaxial line structure and a low pass filter circuit. The research shows that: the impedance mismatch generated by reflection could be reduced and signal transmission performance will be improved by appropriate expanding the circular pad at joints between interconnect via hole and microwave transmission line. In 0~16.5GHz frequency channel, the signal transmission has a good performance when θ close to 0° and get worse when θ equals to 30°. In 16.5 to 20GHz band, the signal transmission performance will become better and better with the angle θ increases, and the best transmission performance is obtained when θ close to 180° with the relatively low reflection. Therefore, joint angle θ should be controlled within 30° in the microstrip line wiring. The effect magnitude order of Interconnect via hole structure parameters on the signal transmission performance is, the via hole diameter D > the via hole length H > the pad diameter D. When other parameters remains constant, the via hole diameter is directly proportionate to S11 and inversely proportional to S21, that is to say the performance of signal transmission will become worse with the via hole diameter increasing. The larger the pad diameter, the greater is the parameter S11 and the reflected energy. The larger the size of the pad, the smaller is the parameter S21 and the signal transmission coefficient. The increasing of the via hole length will lead to deterioration of impedance discontinuities to cause more serious reflection, meanwhile, the insertion loss will increase, the return loss will decrease, the frequency of the first resonance point will decrease, the performance of the signal transmission will degrade. Therefore, comprehensive consideration of the curve that the insertion loss varies with TDR, the length of via hole should be controlled less than 1mm. The addition of the coaxial shield via hole around the interconnected via hole can improve the scattering properties of the signal. In the range of X-band of 8~12GHz, when the shield via hole radius is 0.1mm, the via hole number is 6, the equivalent coaxial outer conductor radius is 0.8mm, the via hole height is 0.8mm, the signal transmission loss with the shielded via hole is nearly less 0.45dB than without it and the signal transmission has a better performance.