High frequency signal integrity in high-density assemblies

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The demand for faster, portable and reliable electronic devices is increasing the pressure on the development of assembly techniques for signal integrity (SI). The advance of integrated circuits toward a large number of Input/Output (I/Os), a high number of operations and up to microwave communication frequencies, is behind the drive for the search for new packaging solutions. The materials and assembly techniques have an important impact on the propagation of high speed signals. Signal integrity issues emerge due to the electrical losses of materials, reflections from impedance discontinuities in the signal path and fast transitions of the signals. For these reasons, signal integrity in lead-free connections of WLCSP, flip chip (FC) and Integrated Module Board (IMB) assemblies were investigated up to 50 GHz. The increase of conductor loss resulting from the presence of thick oxide layers on the surface of solder bumps of hot running components was experimentally studied for the first time. Utilizing theoretical calculations, a design rule was developed to account for the 40 % increase in losses due to the presence of oxide layers at high frequencies. The research into the influence of solder microstructure on signal quality showed that it did not negatively affect the wave propagation. Experimental results proved that the presence of underfills and high density routing on printed wiring boards (PWBs) under the WLCSP components, detuned the components and the connections. The effects of three different underfills on signal propagation were studied. It was proven that the changes resulting from the rheology and parameters of curing process influence the losses and reflections of circuits. The analysis of microwave performances of flip chip (FC) and Integrated Module Board (IMB) assembly techniques demonstrated that they are well suited to Radio frequency (RF) and high speed applications. Comparison showed that IMB performed better as the wave encountered smaller discontinuities and had an optimized propagation path. Full wave simulations of IMB assemblies were performed considering finite ground coplanar waveguide (FGCPW), microstrip and stripline connections with stack-ups that included high dielectric constant materials and four connection possibilities. The research was carried out in the domains of both frequency and time to rigorously determine the sources of signal reflections. The results emphasized that in the design for match impedance and optimal current return path, discontinuities and reference planes had significant impact on signal integrity.
lead-free, electrical losses, integrated module board, underfill
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