In an era of collapsing economies, waning affluence, and too much garbage amid scarcity, a characteristic that is part of the engineering character - resourcefulness - is due to come back in vogue, as we are challenged to optimize the use of limited resources. One method of such optimization is to increase utilization, the fraction of use that is derived from a given item. Common items thoughtlessly discarded as garbage are worth a second look and reappraisal. Some of these items are masterpieces of engineering and are deserving of continued use in an electronics context. They are just too well-designed to throw away.
Plastic Jars and Bottles
Packaging is purchased as a hidden cost of products. Aluminum or steel cans (sometimes erroneously referred to as “tin” cans for historical reasons) are marvels of technology. Cans with plastic lids are especially valuable in that the problem of closing the open end is solved. Plastic jars have screw lids and can similarly be used. They are especially good for storing larger parts that do not fit well into component drawers and can be seen in the jar. Jars used to enclose viscous substances, like peanut butter, usually have wide mouths because the substances contained in them are not poured. These are particularly valuable for parts storage because they do not shatter when dropped on tile floors or marble countertops, are often transparent, and access is not hindered with the wide mouth, allowing a few large parts to be stored in them. Some examples in use in my laboratory are shown below.
As electromagnetically-shielded enclosures, steel cans also serve as sturdy, outdoor drip-proof packaging into which a rectangular circuit-board can be slid along the diameter and holes drilled, punched, or nibbled anywhere in the can for access. These enclosures are optimized nowadays for strength, using ribbed sections that allow the amount and weight of steel to be minimized.
What most cans have going against them as electronic enclosures are their cylindrical shape. It is the same problem that geodesic-dome houses have in a rectilinear world of furniture and appliances. However, not all steel enclosures are round. Chinese tea is often packaged in square containers having an additional inner lid that seals the container. Kippered herring, sardines, and smoked oysters come in containers with pop-off lids. After lid and contents are removed and the container washed, a can opener removes the remaining lid material for a smooth opening. Then peen any rough spots with a sliding tool such as the shaft of a screwdriver or pliers handle. Examples are shown below.
A square can with a lid encloses a pocket-sized photovoltaic array for a backpack or other mobile use, shown below.
Sectors of round cans can also be flattened for mounting power devices. Steel has one-fifth the thermal conductivity of aluminum and a tenth that of copper. However, if the relatively thin can is used to conduct heat to a heat-sink, then this difference is functionally not so important. For power electronics, heat sources can be distributed around the circumference without variations in thermal resistance because of the package shape. The surface area (or perimeter, viewed on-end in two dimensions) to enclosed cross-sectional area is the same as a square shape. A square having a side length equal to the diameter of a circle, d, has a perimeter to area ratio of 4 x d/d 2 = 4/d. A circle has a circumference of
x d and area of
(d/2)2. The ratio is also 4/d. However, with the same major dimension, d, the circle has both a smaller area and perimeter by
/4 ≈ 0.785. The square shape maximizes both to advantage for a given major dimension. Yet the round shape has the same ratio of surface area for heat removal to interior area for a circuit board. The advantage of the square shape is that for the same major dimension, d, the square has about 21.5 % more surface and interior area.
The electrical conductivity of steel is also about one-fifth that of aluminum, with aluminum about 60 % that of copper. The lower conductivity of steel is also not desirable, yet quite adequate for shielding and grounding in many applications, making it more a dissipater than a reflector of incident EM radiation.
The higher yield strength of steel over aluminum or many plastics suggests that cans are best used for rugged environments: the outdoors, or mobile environments which might subject the enclosure to mechanical stress. Common stress events occur when electronic packaging is dropped or banged around in a vehicle or subjected to the exigencies of a mechanical shop bench. More rugged enclosures are an advantage in power electronics, where components such as chargers or inverters are often found among larger mechanical items and are often handled similarly.
The round shape of most cans also makes them compatible enclosures for electronics driving motors. A can that slips over the electrical end of the motor provides an enclosure for the drive electronics and integrates the drive with the electric machine in a single unit. The can also provides for some heat-sinking of the power transistors.
Small cans also provide shielding for sensitive circuits. A whole subsystem can be placed underneath a can such as those shown above, and it can be attached to a circuit-board by soldering. Aluminum does not solder easily but can form an acceptable joint at a high enough temperature. Plastic straps can also be used; tie wraps can hold a can in place on a board, passing through holes in the board. Pins can be used to mechanically and electrically attach to aluminum cans or sheets, and have been used for aluminum heat sinks and also ceramic-substrate circuits.
Aluminum Extrusions as Building Components
Aluminum L, U, H and more complicated bracket strips, used in construction of door and window frames or patio posts, make feasible prototype heat sinks and board mounts. Some examples are shown below.
The varying shapes of these aluminum extrusions appeal to creative thermal designers of power electronics. They are similar to, though not quite as efficient as extrusions designed as heat sinks, but are produced in much higher volume with a correspondingly reduced price. Thus, they offer a price-efficiency tradeoff for power-electronics packaging of low to medium-volume products. The rugged outdoor plastic covers that accompany patio extrusions are also possible enclosures for inverters or other devices. Though they might not be a good fit for products - especially in high volume - they can be appealing for prototypes, demonstrators, and low-volume or limited-edition products, and can also speed development while waiting for custom enclosures.
These ideas for recycling packaging material or finding alternative uses for low-cost items in the urban environment are probably not going to be accepted generally for electronics manufacturing, though engineers usually build prototypes and need quick methods for assembling electronics. Packaging material is plentiful and cheap (even free, once acquired as purchased-product packaging) and can be put to use in ways the original packagers never had in mind. For manufacturing, there are occasions where high-volume consumer-product packages might be adapted without modification for electronic products, thus reducing a significant fraction of product cost and development time for packaging. The next time you stroll through a hardware store, keep this idea in mind. It might make the visit more creatively appealing!