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Flexible Thinking: Blue Skying It With Aluminum Rigid-Flex

Aluminum is an amazingly versatile metal and has found its way into countless products since its discovery during the reign of Napoleon III of France. At the time, it was more valuable than gold, and at hosted dinners, the emperor and his honored guests dined using aluminum cutlery while the others had to make do with gold utensils.

It took some time for scientists to calculate that it was by far the most abundant metal found in the earth’s crust at 8.3%, ranking third among all elements found in the crust, ranking behind oxygen (46%) and silicon (26%).

Today, aluminum is one of the most affordable metals and arguably, also one of the most useful. Aluminum has a long list of desirable properties. It is lightweight, and an excellent electrical and thermal conductor. It can be easily machined, stamped, chemically milled, and formed. Being a metal it is, by nature, dimensionally stable. It also expands isotropically (unlike composite materials which typically expand at different rates in X, Y, and Z dimensions). Aluminum can be anodized, which converts its skin to alumina (Al₂O₃), which is ceramic, and thus offers a potential to create an in situ distributed capacitance layer.

The 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. Those in PCB manufacturing will know that aluminum has for many years been a process consumable as drill entry material when drilling printed circuits to mitigate 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 experiment/description of one possible way to build a rigid-flex assembly with an aluminum base and no solder. Rigid-flex circuits were originally 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.

The process that will be described is basically 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 advantageous, after creating cavities which hold the components in place, that the assembly be processed by anodizing or electrophoretic coating with a suitable dielectric to make the surfaces nonconductive. The “component board” can then be coated with a flexible dielectric. The components are preferably of a common thickness, but it is not imperative as there are ways to deal with height differences. More importantly, 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 (Figure 1). The 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 connections 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.   

An advantage of not soldering to make connections is that the component lands can be treated as microvias and the free space can be used for increased routing. When all the required interconnections and circuits are added, the assembly can be partially machined in areas where bending is performed to thin the metal carrier sufficiently to allow the residual metal to easily bend. If left, the residual metal can serve as a ground layer for the assembly. If isolation is required or desired, the residual metal can be etched using chemistry appropriate for the metal used as a carrier. Alkaline chemistries such as dilute sodium hydroxide and sodium gluconate solutions are suitable for aluminum, but other chemistries such as ferric chloride are more universal in their potential application. There are many proprietary chemistries which can be used as well. The patented process concept is illustrated in Figure 1. 

Accepted flex circuit design practices suggest that the number of circuit layers through the areas where flexing or bending is required should ideally be limited to one or two metal layers. That aside, the processes described offer significant potential benefits for increased reliability and reduced overall cost.

Conclusion
The continuing challenge of making lead-free solder work in electronic assembly has given rise to new ways of looking at circuit assembly and fabrication, using methods that could eliminate solder and its myriad issues altogether. What has been offered for the reader to ponder is just the tip of the iceberg. My mind (and perhaps now yours) lights up daily with ideas and ways to integrate packaged chips with the interconnections they require to carry out their mission. Improved approaches to assembly such as those that become possible using the Occam process offer both the independent designer and OEM a new option for products that are highly reliable and cost-effective. Such products will skirt the numerous concerns that have vexed manufacturers for the last decade.    

Currently, solderless flexible circuit assembly technology is making headway in several ways using printed inks and conductive adhesives in place of copper traces and solder. These are important first steps, but it is still uncertain if the inks will ever compete with copper metal. Still those efforts hold out the promise of significant potential for the future. They are helping to broaden the range of material options and significantly expand the horizon of electronic interconnection design. Once interconnection technologists and product designers begin to open-mindedly climb this new “Occam technology tree,” they will find branches that take them to many new types of processes, methods, and structures that they would not have anticipated or seen before they began the climb. The specific structure and process example provided here has not yet been reduced to practice, but it is possible. All that is missing is a willingness to try something, and unlike building a new semiconductor fab, it won’t cost billions of dollars to try it out. It just requires a few more people to take up Steve Jobs’ challenge to the industry issued at Apple some 20 years ago to “think different.”

This column originally appeared in the May 2022 issue of Design007 Magazine.

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