PCB007 Magazine

PCB-July2014

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48 The PCB Magazine • July 2014 This was a very popular innovation to the industry as now the surface footprint was dras- tically reduced and the topography on the sur- face of the PCB was now open to accommo- date 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 capabili- ties on the inner layers to provide not only an overall smaller PCB but a more powerful final product. These technologies reduced the footprint of the PCB, but by no means could we have the smartphones, tablets and many day-to-day de- vices we know and rely upon today. Further it would be impossible to use the older technolo- gies of yesteryear to power the mammoth Inter- net infrastructure that we take for granted today. Something still had to change. The next evolu- tion was to hide the passive components com- pletely from the surface of the PCB and allow the needed high power footprints of the BGAs, micro BGAs and HDI components. As of now these high speed components have challenges to be buried in the board design due to their size and the need for heat dissipation. However the passive components could be. The buried passive component allowed the ability to pro- vide the pull-up, terminating, current limiting and voltage dividing properties without taking needed real-estate from the surfaces of the PCB. There have been many options of the bur- ied resistor. This spans from the carbon ink screened resistor through to thin film resistor conductor material. This technology allows the ability to devel- op a buried resistor by the use of a resistive core laminated into the PCB itself. By etching away a "square" or a multiple of them, a specific re- sistance can be achieved. There is no longer any "hard" component. But this now provides a unique challenge to the electrical test arena as they need to test this product with the buried signature and also certify the board to the end-user specification, be it IPC Class I, II, III or 3/A. Many times these resistors are chained in series or used in paral- lel, which does not allow the conventional elec- trical test machine to provide a "pass." Trying ELECTRICAL TESTING OF PASSIvE COMPONENTS continues Figure 1: carbon resistors (as noted by their color rings). Figure 3: thin film embedded resistor. Figure 2: Surface mount resistors.

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