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82 DESIGN007 MAGAZINE I MAY 2022 dimensionally stable. It also expands isotropi- cally (unlike composite materials which typi- cally expand at different rates in X, Y, and Z dimensions). Aluminum can be anodized, which converts its skin to alumina (Al 2 O 3 ), which is ceramic, and thus offers a potential to create an in situ distributed capacitance layer. e metal can also be electrophoretically coated uniformly with polymer and enamel insulating materials (think automobile paint uniformity). It also has a CTE which is close to that of copper (22 ppm/°C for aluminum vs. 18 ppm/°C for copper) and is non-toxic (not a RoHS target). It is historically low cost at about $1 per pound. ose in PCB manufac- turing will know that aluminum has for many years been a process consumable as drill entry material when drilling printed circuits to miti- gate copper burr formation. Typical laminates, exclusive of copper foil, can be two to three times more costly on a by-weight basis and much more expensive to create. With such a long list of benefits, it would seem aluminum might be an ideal candidate for making printed circuits (including rigid-flex circuits), except for one hitch. Being a great thermal conductor makes aluminum a lousy choice for soldering components in place due to the high risk of creating cold solder joints, and/or if one is to avoid thermally damaging components by using excessive heat during the soldering process. What If Soldering Could Be Avoided? Following is a "blue sky" thought experi- ment/description of one possible way to build a rigid-flex assembly with an aluminum base and no solder. Rigid-flex circuits were origi- nally designed to be used in rugged military applications and thus are perhaps ideal as a candidate for being adapted to the use of an aluminum core. While such an assembly could be comprised of any suitable metal carrier (brass, bronze, steel, etc.), aluminum, for the many reasons cited at the outset, is arguably preferred. e process that will be described is basi- cally the reverse of traditional manufacturing, in that, rather than building a rigid-flex board and soldering components to the finished product, the aluminum cores which support the components are processed in a manner to accept the chosen components so that their terminations face outward from the body of the aluminum sections on one or both sides of the metal cores and later plated with copper to interconnect their terminations. It is advanta- geous, aer creating cavities which hold the components in place, that the assembly be pro- cessed by anodizing or electrophoretic coating with a suitable dielectric to make the surfaces nonconductive. e "component board" can then be coated with a flexible dielectric. e components are preferably of a common thick- ness, but it is not imperative as there are ways to deal with height differences. More impor- tantly, components should be fully tested and burned in, though tested bare die are a possible option. At this point, a layer of flexible insulation is to be applied to one or both sides of the core (Fig- ure 1). e assembly can now be processed as if it were a rigid printed circuit using a build-up process to make the circuits. Side-to-side con- nections can be made by drilling and plating through-holes but will require a process step to coat the exposed aluminum and a second drill through the dielectric to accept the electroless and electrolytic copper plating. Typical laminates, exclusive of copper foil, can be two to three times more costly on a by-weight basis and much more expensive to create.