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Effects of Surface Finish on HF Signal Loss Using Various Substrate Materials
The amount of information transferred on wireless networks has increased dramatically with the tremendous growth of
mobile phones, Internet access, and hand held devices. In order to build the infrastructure needed to handle
ever-increasing data transfer, manufacturers of electronic devices turn to high speed, high frequency electronic
signals. The need to render these electronic devices portable is another technology driver. The merge of
high-frequency signals with small geometry conductive traces means that the topic of signal loss has reached
a critical point in existing device production.
The integrity of a high-frequency signal can be affected by several factors during Printed Circuit Board (PCB)
construction. The geometry of the conductor, the type of metallization used in/on the conductor, the signal
frequency, the temperature of the substrate, and the type of dielectric material insulating the conductor all
play an important role.
Representatives of material supply, board fabrication, and chemical process supply commenced a project to study
some of the influential factors in more detail. A primary variable was the type of metallization used on the
circuit board surface. Organic solderability preservative (OSP), hot air solder leveling (HASL), electroless
nickel immersion gold (ENIG), and immersion silver were studied as PCB surface finish alternatives. Substrates
under investigation included woven glass PTFE, ceramic-filled woven glass PTFE, and FR4 epoxy as a control.
Specifically designed microstrip circuits were tested within a frequency range of 60 MHz to 26.0 GHz for signal
loss across a range of PCB operating temperatures. At this stage, data was used to 10.0 GHz. Continuing studies
will inspect data to 40 GHz.
The results from the investigation allow prediction of signal loss characteristics across a broad frequency
range when parts are re-designed to use alternative Pb-free surface finishes on PTFE substrates.
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