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3D flexible microwave polyimid T-line assembly for system in package  

 Timoshenkov V.P.
 Rodionov D.V.
 Khlybov A.I.
 Musatkin A.S.
 Vertianov D.V.
Date of publication

 Major trends of microelectronics are associated with a reduction in weight and size parameters, increasing of frequency response and decreasing of power consumption. The three-dimensional (3D) technology integration is important in improving the manufacturing parameters of the electronic equipment. The key techniques of 3D integration is the using of ultra-thin (20-35 micron) crystals stacked (stack) with Si interposer boards using through-silicon vias (TSV). This technique works well for relatively low frequency signals. In the case of microwave signals, there are limitations associated with the manufacturing of transmission lines (T-lines) on conductive silicon base (crystal chip, interposer). It is possible successfully to resolve this issue by using a flexible flat polyimide T-lines, including microwave paths, as well as the low-frequency signal lines, power bus and ground plate. Moreover, the use of polyimide with RF T-lines may for compound of IC and PCB in 3D package collected.
On the polyimide film base may be formed 50 - ohm and 75 - ohm microstrip or coplanar lines using microwave technology. Microstrip advantage is its simplicity of calculation and implementation. The disadvantage is an additional inductance in the ground plane. This "parasitic" inductance appears as a result of connecting the respective pads on the integrated circuit and a ground bus on the back side of the microstrip structure.
Coplanar structure is devoided of this disadvantages, since ground and signal lines are arranged in one plane. On the polyimide carrier expedient design with coplanar upper and lower location of ground plane which are interconnected by sets of metallic through plate windows. Contribution of lateral side ground to impedance is considerably smaller than the lower, since the dielectric thickness is usually much smaller than the distance from the central strip of the line to the ground side.
The simulation of the impedance of the microwave structure, depending on the width of the central strip and the dielectric constant of the material. The magnitude of the standard 50-ohm impedance corresponds to the width of 90 to 117 microns of the strip. It plays an important role measuring the actual value of the permittivity of polyimide. Direct measurements of capacity at microwave frequencies, is almost impossible, so the determination of the dielectric constant in the specified frequency range was done by measuring the propagation speed of the strip line by Time -domain reflectometry (TDR) method.
Mesuremets shows significant reduction of dielectric constant from 3.4 to 2.25 with increasing frequency. Low-frequency experimental measurements of dielectric constant capacitive method give values of 3.6-3.7, which corresponds to the specific capacitance of 0.43 pF/mm for 50 micron thick polyimide.
A study of S parameters of quasi-coplanar structure to determine the approval of the 50 - ohm impedance, as well as to measure the frequency bandwidth were done. The results of S-parameter measurements show that the S11=—11.3 dB, bandwidth is10,7 GHz.
Three dimension (3D) assembly of microwave polyimid transmission line (T-line) for system in package was investigated. Flexible T-lines may be used for 3D RF connection of integrated circuits or RF PCBs. Transmission lines were created by using of polyimid film with metallic lines deposited on it. Thickness of metal and polyimide film are 39 μ and 50 μ. Theoretical calculations and computer simulations of impedance for microstrip and coplanar lines were done. Wave impedance versus geometric dimensions and dielectric constant for transmission line were presented. Experimental research of flexible T-lines were done in frequency range up to 12 GHz. The direct transmission loss, including RF connectors are not exceed 0.0833 dB / mm..
 Three dimension (3D) assembly; integrated circuit; S – parameters; polyimid, flexible transmission line.
Library reference
 Timoshenkov V.P., Rodionov D.V., Khlybov A.I., Musatkin A.S., Vertianov D.V. 3D flexible microwave polyimid T-line assembly for system in package // Problems of Perspective Micro- and Nanoelectronic Systems Development - 2016. Proceedings / edited by A. Stempkovsky, Moscow, IPPM RAS, 2016. Part 3. P. 8-13.
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