The Occam Files - No. 7

Substrate Options for Occam Assemblies

 Those readers who have been associated with electronics design and fabrication since the EU Parliament’s mandate that lead be eliminated from solder more than a decade ago will doubtless recall the many problems that both preceded and followed its implementation. The high temperatures associated with common SAC (Sn-Ag-Cu) alloys wreaked havoc on the industry. The high temperatures caused components to blister due to entrapped moisture out-gassing and the industry scrambled to improve materials and pre-processing by baking to remove moisture and help mitigate the problem.

Unfortunately, it was only the most visible of problems getting the attention; beneath the components, the substrates were also being damaged and new failure mechanisms were seen, including decomposition of the substrate polymers and a phenomenon called “cratering,” where component lands were actually being pulled out of the laminate, evidenced by a void seen beneath them. 

The industry responded in a couple of different but logical ways: they introduced new higher temperature (and more costly) laminates and newer and more exotic (and more costly) alloys. Today, new material solutions continue to be introduced to deal with the lingering problems facing electronic designers, fabricators and assemblers. These are problems that would largely disappear when designing and fabricating an Occam assembly. In fact, a wide range of materials could potentially be tapped when the high temperatures associated with solder assembly are bypassed. Let’s take a look at just a few. 

Metals

Numerous metals and alloys could serve as substrates for an Occam type assembly; however, aluminum may be, in my opinion, the most attractive. This prospective substrate material was referenced at the end of the last Occam File post (No. 6). Aluminum has many attractive features. It is at once, abundant, inexpensive ($0.85/lb at the time of this writing) and intrinsically dimensionally stable. It is, at 200WmK, a very good thermal conductor (but not a good attribute for soldering, unfortunately). It can be easily machined, punched and formed. It’s also chemically-millable. It has a unique quality in that it can be anodized, creating a relatively high dielectric constant, insulting [LL1] aluminum oxide coating of precise and predictable thickness (which offers potential for building a capacitor in situ). It is electrolytically coat-able with insulating polymers to exacting thickness (think about precision automobile painting). It has a coefficient of thermal expansion that roughly matches and tracks well with copper (22ppm/C versus 18ppm/C), which is the metal most often used for conductors on circuit boards. It is also among the least dense of elements allowing for the construction of lighter weight assemblies.

 There are some drawbacks to using metals as substrates related to certain traditional chemical processing steps but those may be moot depending on how one chooses to process the product. There are many ways to “skin a cat” as the old saying goes, and it is just a matter of choosing the one that works best. This topic will be discussed in more detail as this series continues. 

Thermoplastic and Thermoset Resins

Plastics are a mainstay of virtually all manufacturing today. Their properties can be engineered to nearly any type of need. Thermoset resins have long been used as a substrate material for PCB construction and so no surprise that it would work for an Occam assembly. The same holds true for thermoplastic polymers, which are perhaps the most commonly used and thus most familiar. Reinforcements of glass and other stabilizing materials help keep features where they are supposed to be. Because the Occam Process is not a high-temperature process, the list of potential polymers is long and full of promise. Consider for example, polyethylene. It is inexpensive, and many consumer products use it for disposable packaging (looking at the amount of disposable trash accumulating in the world’s oceans, I should add the word “unfortunately.”) It and other polymers unsuitable for standard solder processing could provide significant cost savings in high-performance application. 

I am reminded of an anecdote shared with me by a friend who worked for Raychem in the 1980s as they were rolling out a new product that was a self-resetting fuse comprised of polyethylene and carbon powder. During the presentation to management, the presenter described the product, which was expected to bring a premium price, as made of the two primary materials just mentioned, adding that “Polyethylene is cheaper than dirt… and carbon powder IS dirt.” 

Polyethylene happens to also have a dielectric constant near that of Teflon (2.3 for polyethylene vs 2.1 for Teflon) However, Teflon has a current price of about $4-5 per pound while polyethylene is $0.025 per pound or about 1/200th the price.  

Plastics can be used in concert with metals such as aluminum or others if the product warrants it. Molding of cavities to support the components can be produced very accurately and reliably with precision, to create the “component boards.” 

Ceramics and glass are also candidate materials for Occam assemblies. These are obviously very rigid materials, but they are also very dimensionally stable. Their application is really dependent on the problem addressed and the imagination of the designer.

In short, the prospective substrates are virtually unlimited. With the proper process selection (such as direct printing of circuits and insulation layers) and with the exercise of care, wood, paper and even cloth could potentially be used to make an Occam circuit. 

In subsequent parts of this series, such audacious suggestions will be reviewed. More than for immediate and practical applications but to open the mind of the reader to think beyond the here and now, which is should be coming increasingly clear to the reader, is an important objective of this series. 

 Next time: Package-Under-Package Occam Constructions 

PS I have been remiss in not thanking Lisa Lucke for her invaluable editorial acumen and ability to see the errors I am so often blind to. If you enjoy good writing with a sense of both humanity and humor, I recommend you visit her website and read her blog at the link below

https://lisalucke.com/category/the-surreal-housewife-blog/