Printed circuit board material considerations

The ° T1, in the Th decay series, and 7 " Bi in the decay series are used as indicators of the amount of their parents in the materials. Other members of each chain will also be present. Apart from these primordial nuclides (those surviving from the formation of the solar system), traces of anthropogenic (human-made) species are seen, such as Co in steels and Cs in molecular sieves. There are obvious reasons for some of these radioactive contents aluminium always has traces of uranium and thorium within it, and it is not unreasonable to expect in molecular sieves. However, relatively large amounts (compared to other materials in the list) of uranium and thorium daughter nuclides in epoxies and printed circuit boards are unexpected. Clearly, for some materials, there is ample scope for reducing the activity the detector sees by selecting a material with a lower activity. In a similar table to that above, Dassie (in a private communication) reported 4.3 Bq of Cs kg of Csl and 33 Bq of kg of Nal, considerably more than in Table 13.6 this latter would be a problem were it to be used for low level Compton suppression systems. 

Design considerations for these applications must include thermally conductive parts (heat sinks) for removing heat from the circuitry involved. The circuitry may or may not be encapsulated. In confined circuitry, as on a printed-circuit board, nonencapsulated heat sinks bonded in place are one solution. In this case, aluminum is usually the preferred heat-sink material because of its lightweight and high thermal conductivity. If good dielectric properties are required, a high concentration of inorganic or mineral fillers can be used. 

Have the appropriate stencil or screen design and materials (e.g., thickness, aperture geometry, wall finish, etc.) for the circuit board product. Design considerations must also address the need to widen fine-pitch aperture openings slightly for printing Pb-free solder paste. 

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