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Substrates have become more critical with regard to pitch and density in today’s designs with challenges for passive components in terms of surface placement. This negates the opportunity for high speed, high cost components to be placed on the surfaces of the PCB. With this the capacitance and resistive components have to be embedded into the design. This has been accomplished with the advent of buried capacitance cores and buried resistors. Unfortunately, this has caused some challenges to the ET test centers/labs in the ability to effectively test these buried passive components. Processes have had to change and adapt to these new technologies. The paper will discuss what these new technologies are and how the electrical test arena has adapted to provide accurate testing of the buried resistors and accommodate the buried capacitive cores to not receive false errors from the grid testers and flying probes.
In the past, pull-up, terminating and voltage dividing resistors have been placed on the surface of the PCB. Early applications were standard carbon resistors placed on the board utilizing plated through-holes.
As can be seen in Figure 1, the standard carbon resistor took up a lot of space on the PCB. You will also notice in the photo that capacitors are also stealing valuable space from the surface topography. As time progressed, SMT technology was introduced and the older, bulky standard carbon resistor was replaced by the newer SMT packages (Figure 2).
This was a very popular innovation to the industry as now the surface footprint was drastically reduced and the topography on the surface of the PCB was now open to accommodate more active components which reduced the overall size of the PCB, but also allowed the complexity of the designs to grow. No longer was a PTH as needed for the resistor, which allowed the multilayer to expand its capabilities on the inner layers to provide not only an overall smaller PCB, but a more powerful final product.
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Editor's Note: This article originally appeared in the July 2014 issue of The PCB Magazine.