Page 319 - System on Package_ Miniaturization of the Entire System

Radio Fr equency System-on-Package (RF SOP) 293

FIGURE 5.32 Fabricated K a -band Wilkinson divider.

is placed between the branches of the output ports to isolate any return signals between
them. This differential placing makes the Wilkinson divider useful in creating baluns.
In the forward case, the divider operates by splitting the input signal with a simple T-
style junction. Each path then travels through a l/4 transformer with an impedance
designed to step the signal up to twice the system impedance. This signal is then placed
across a resistance of twice the system impedance through the superposition of odd and
even modes. This halves the signal to the original system impedance with half of the
original input power on each branch. A photograph of the fabricated K band circuit is
a
depicted in Figure 5.32. The measurement data are given in Figure 5.33. An insertion
loss of about 0.35 dB is achieved from 28 to 40 GHz.
TCR Properties
One of the most important barriers for embedded resistors is the use of a single materials
system to cover the entire range of resistance requirement (near zero to 200 kΩ). In the
absence of this single material, several materials are typically used, each with its own
advantages and limitations such as the temperature coefficient of resistance (TCR). For
example, Polymer Thick Film (PTF) offers low cost and a wide range of resistance values
but has problems in temperature and humidity, oxidation at the interface, CTE mismatch,
high-frequency stability, and high temperature coefficient of resistance. Resistors
predeposited on copper foil, discussed above, are limited to a lower range of resistances.
Electroless plated resistors offer cost advantages, but this technology is currently limited
to low-value resistors within a small deposited area.
The materials systems used for embedded resistors are generally categorized as
metal and alloys, semiconductors, cermets, and polymer thick films [56]. From a
processing standpoint, these are classified as thin film, printable, and plated. Among
commercial materials, Ohmega-Ply (electroplated NiP), DuPont Interra (screen-
printable LaB6), MacDermid M-Pass (electroless plated NiP), Ashai Chemical (polymer
thick films), Shipley Insite (doped Pt on Cu by CVD), and Gould Nichrome on copper