Copper.  It’s abundance and amazing properties have made it universal in our modern devices.  But not long ago, its unstable price made it one of the most-stolen materials in high technology.

Copper is a transition metal and is one of the noble metals.  It is one of only a few elemental metals that isn’t grey in its metallic form.  Its name an atomic symbol of “Cu” comes from the Latin “cuprum,” meaning “from the Island of Cyprus”.  Copper alloys such as bronze have been known for many thousands of years.

On the periodic table of elements, copper shares group 11 with silver and gold.  All three of these metals have been used in coinage and jewelry for millennia.  Copper, silver, and gold also share similar physical properties that make them especially useful in modern technology.  At standard conditions, copper is one of the best conducting elemental metals; being second only to silver. 

From printed circuits boards to bulk wiring in power substations, it’s hard to find any type of electronic device that hasn’t benefited from coppers’ outstanding properties.  Its ubiquity in the electronics industry is owed to its high electrical conductivity and thermal properties.

Electrical-grade wires are refined, and often heat-treated to improve its performance and durability.  The highest-quality copper wires are labeled Oxygen-Free High Thermal Conductivity (OFHC). 

Copper in Early Semiconductors

Copper compounds have had a long history through the 20^th century in early rectifier diodes.  Before silicon manufacturing economized, rectifiers of the early 20^th century were being made of other elements like germanium, selenium and copper oxides.

The copper oxide rectifier consists of a copper disc.  On the surface, manufacturers would apply heat treatment to grow a layer of copper (I) oxide (i.e.: cuprous oxide).  The diode rectifier effect would take place at the interface where the oxide meets the metallic copper with wires present for connecting to each material.  This arrangement is a type of metal barrier diode, like the type studied by Walter H. Schottky.  To deal with high voltages, these rectifiers were chained together in stacks that were separated by spacers and cooling fins.  In the 1940s, they were well noted for their reliability and long life. 

Diodes like this were important in both low-voltage instrumentations and in converting high voltages from alternating currents (AC) to direct currents (DC).  While silicon technology would replace copper oxide decades later, the copper oxide diodes managed to outperform silicon in a few small areas, like forward voltage.

In application, copper compounds like red copper (I) oxide, green verdigris, or blue copper sulfate can form as the result of unwanted corrosion.  These materials do not have the original properties of metallic copper, sometimes the color of the corroded material can give away clues about type of contaminant you are dealing with.

Copper in Electrical Wire

Copper and aluminum are two very important conducting materials.  Copper itself is malleable and ductile enough to be drawn into long strands without breaking.  These strands can be bundled or braided into wires generating or delivering electricity.

While copper conducts electricity better than most materials, this property can be diminished by even a small presence of oxygen and other contaminants in the material.  Commercial grades of copper inside the United States are specified by ASTM International.

The metal is also used to form the winding coils for generators, electric motors, inductors, and transformers.  You have probably seen this already if you’ve inspected a motorized device closely.  The wire inside the motor is often magnet wire—a copper wire coated with thin insulation.  In coils, this insulation forces the moving electric charges to travel the entire length of the coil in a helical path, even though the windings may be in direct contact between turns.  This spiral motion of charges will generate a magnetic field that can perform work.

Copper In Circuit Boards

Although professional-grade printed circuit board assemblies (PCBA) will often add a barrier that makes circuit boards appear green, these are devices where copper traces are used instead of copper wires.  Eliminating wires has numerous benefits including miniaturization. 

Well before the intricate circuit patterns are in place: the copper exists only as a simple, flat sheet of copper foil.  This thin material is created using the process of electroplating.  A rotating metallic drum called the cathode will be placed in a copper solution, such as blue copper (II) sulfate.  An electric power source will apply a negative electric charge to the rotating drum, and the dissolved copper ions (with their positive charge) will be attracted to the negatively-charged surface.  When they make contact, the copper ion has its positive charge reduced, and it solidifies to the surface.  This is the process of electrolytic reduction.  As a side note: I recently mentioned the process of reduction in my article on Lithium batteries, so be sure to check that out as well.

The large drum will continue to rotate slowly enough to build up a layer of metallic copper, which can be removed and sold as a copper foil to PCBA manufacturers. 

When the copper foil is attached to an insulative sheet, it is ready for etching.  As trivial as it may sound, the smoothness of the copper foil surface has become increasingly important in the 21^st century.  For circuits that operate at ultra-high or extremely high frequencies (UHF and EHF), a rough copper surface can cause unwanted signal loss and signal dispersion.  Consequentially, applications involving certain types of radar high-bandwidth data have become more selective about the smoothness of their copper surfaces.

Next, highly accurate and precise patterns in the copper foil need to be selectively removed through another process called etching.  Today, this is a subtractive process: where the entire board is copper-clad at the start, and then an etching process removes copper from where it is not wanted.  However, as advancements in 3D printing mature, additive board printing may become mainstream.

The result is the iconic field of traces, pads, and vias that circuit boards are known for.  The copper lines (traces) form signal and power-carrying traces.  Round features (vias) operate like tunnels that interconnect different copper layers.  PCBAs can have dozens of copper layers with insulation sandwiched between them.  Because of copper’s excellent thermal conductivity, and copper features in contact with an electric component will also conduct and radiate heat from hot areas to cold.

Copper In Electric Components

Above the circuit board, copper is still just about everywhere, especially inside the wire-like leads of resistors, diodes, and connectors. 

Copper can also be wound into a coil to form inductors, electromagnets, solenoids, and radio antennas.  Two or more coils can be wound around a common core material to form a transformer.

Going down to the micro and nanoscale: copper (and more recently, its nanoparticles) form the internal or external electrodes for some multi-layer ceramic capacitors (MLCCs).  Using copper in these components helps side-step of the higher cost of other precious metals like palladium and silver.

In integrated circuits, copper forms the leadframe that helps bond the silicon die to the conducting leads outside of the IC packaging. 

On the silicon itself, there are countless microscopic interconnects made of copper put in place to join different circuits throughout the chip.  Aluminum had been used for this purpose since the 1960s.  But at the submicron level, aluminum atoms begin to drift out of their original position as electric current passes through it.  This process is electromigration, which is a common failure mechanism for micro-scale devices.  Additionally, aluminum is nearly 40 % less conductive than copper.  In logic circuits, its higher resistance acts at the resistance (R) in an RC filter: causing sluggish voltage level transitions compared to a copper feature of the same size.

Eventually, the semiconductor industry would switch from aluminum interconnects to copper for these reasons.  These copper interconnects (introduced in 1993) were added as part of the Damascene and Dual Damascene process.

Unfortunately, copper brings with it a few problems for semiconductors.  Copper atoms are able to rapidly diffuse into surrounding silicon.  Once the copper has contaminated the silicon, it creates deep-level defects in the silicon crystal structure; resulting in degraded performance.  To counteract, other metals such as tungsten, titanium, or tantalum must be added as a diffusion barrier between copper and other materials such as silicon dioxide.  In spite of these complications, copper not only avoided the electromigration problem but also resulted in faster chips by reducing the RC filtering effect.

Copper In Electrical Solder

Copper is used as a minor ingredient for the most popular class of lead-free electrical solders: called SAC (for Stannic Argentum Cuprum).  Copper is common on components dues to its high conductivity and tin is common because of its low melting point.

Common electrical-grade solders though the late 1990s typically were alloys of tin and lead.  Lead has mechanical and chemical properties that are highly valuable for the soldering process.  Alloys that were 37 % lead, and 63 % tin had the added bonus being eutectic.  This means the material would transition quickly from being liquid to solid or vice versa.  Unfortunately, lead is also a neurotoxin, and an arguable threat to potable groundwater when lead-based electronics wind up in landfills.  The growing public interest in environmentally friendly products eventually would drive directives such as the restriction of hazardous substances (RoHS) in the European Union and the Consumer Product Safety Improvement Act (CPSIA) in the United States.  Those in common use include (in no particular order)…

• SAC305
• SAC405
• SAC387

The transition period to lead-free alternatives was done very quickly prior to any widespread study about their reliability.  Soldering involves heating metals to join them at a boundary.  At these high temperatures, the two metals tend to diffuse into each other at different rates and have different reactions to contaminants like oxides.  Many of the SAC formulations saw material problems such as copper dissolution, surface roughness and related phenomena.  The near-eutectic SAC materials also suffered problems related to poor mechanical shock performance where silver content was high. 

As a result of the uncertainty, electronics intended for high-reliability end-uses such as national defense and aerospace were granted temporary exemptions to continue using tin-lead materials.

Still, the transition to lead-free materials was necessary to avoid the toxicity problem.  Industry knowledge and standards for using SAC alloys to their full benefit gradually improved.  And the market use of new alloys (including eutectic lead-free materials) accelerated as well.

Future Implications

The upcoming demand for electric vehicles and similar copper applications is expected to drive demand for copper massively through the 2020s.  With its many uses, copper is an increasingly valuable mineral resource.  The refining of copper is an energy-intensive process subject to the cost of energy.  Moreover, most copper ore production is concentrated in South America, where unforeseeable geopolitical events or natural disasters may drive prices upward.

Near the start of this century, copper price surges drew the attention of criminal scavengers; who were able to poach unguarded copper conductors for cash.  In the future, this could pose a major challenge for public safety—as substantial service interruptions have resulted from some of these thefts.  Luckily, many countries have already enacted countermeasures to combat metal theft.  For legitimate recyclers: copper holds much of its original value even when scrapped.  Therefore, corporations that manage their electronic waste responsibly can often sell these materials: thus saving money, while reducing global energy use.

References

[1]	U.S. Department of Energy Office of Electricity Delivery and Energy Reliability, “An Updated Assessment of Copper Wire Thefts from Electric Utilities,” Oct. 2010. [Online]. Available: https://www.energy.gov/sites/prod/files/Updated%20Assessment-Copper-Final%20October%202010.pdf. [Accessed 3 Nov 2019].
[2]	C. R. Hammond, “The Elements,” in CRC Handbook of Chemistry and Physics, 88th ed., D. R. Lide, Ed., Boca Raton, FL: CRC Press, 2008, pp. 1-44.
[3]	P. O. Huss, “An analysis of copper-oxide rectifier circuits,” Electrical Engineering, vol. 56, no. 3, pp. 354-360, 1937.
[4]	R. N. Harmon, “Copper-oxide rectifiers in standard broadcast transmitters,” Proceedings of the IRE, vol. 30, no. 12, pp. 534-535, 1942.
[5]	X. Guo, D. R. Jackson and J. Chen, “An analysis of copper surface roughness effects on signal propagation in PCB traces,” in Texas Symposium on Wireless and Microwave Circuits and Systems (WMCS), Waco, Tx, 2013.
[6]	W. Songping, “Termination of BME–MLCC Using Copper–Nickel Bimetallic Powder as Electrode Material,” IEEE Transactions on Components and Packaging Technologies, vol. 29, no. 4, pp. 827 – 832, 2006.
[7]	S. Sze and M.-K. Lee, “Film formation,” in Semiconductor Devices Physics and Technology, 3rd ed., John Wiley and Sons, 2013, p. 424.
[8]	R. J. L. Guevara, J. A. M. Plomantes and R. A. D. Mamangun, “Enabling Pb-Free Flip Chip QFN Technology: Understanding Kirkendall Voiding and Factors Affecting Its Formation During Bump Process,” in IEEE 20th Electronics Packaging Technology Conference (EPTC), Singapore, Singapore, 2018.
[9]	J. Bath, J. Nguyen and S. Sethuraman, “Lead-free surface mount technology,” in Lead-Free Solder Process Development, G. Henshall, J. Bath and C. A. Handwerker, Eds., John Wiley and Sons, 2011, pp. 33-35.
[10]	G. Henshall, “Lead-free alloys for BGA/CSP components,” in Lead-Free Solder Process Development, John Wiley and Sons, 2011, pp. 95-121.
[11]	Office of Electricity Delivery and Energy Reliability, U.S. Department of Energy, “Infrastructure Security and Energy Restoration,” Oct. 2010. [Online]. Available: https://www.energy.gov/sites/prod/files/Updated%20Assessment-Copper-Final%20October%202010.pdf. [Accessed 6 Nov. 2019].