Staying Connected - November 2013

The Use of Copper Conductors in Medical Cable Assemblies



Native copper, approximately 4 cm (1.6”)
diameter – Image by Jonathan Zander

Copper wire is by far the most common electrical conductor used in the manufacture of medical cables.  Today, nearly half of all copper that is mined is used to manufacture copper wire.  Copper is one of the few metals that is more widely used in its pure form rather than as an alloy.

Copper wire has a number of beneficial properties when used as the conductor material for medical cable assemblies including:

  • Excellent electrical conductivity
  • High tensile strength
  • Good ductility
  • High fatigue resistance
  • Excellent corrosion resistance
  • Low coefficient of thermal expansion
  • Excellent solderability


Published in 1914 by the Bureau of Standards,
Copper Wire Tables documented the standard
of conductivity for annealed copper conductors


Electrical conductivity

Medical cable assemblies conduct electrical signals and copper has the highest electrical conductivity of all non-precious metals.   Electrical conductivity is a measure of how well a material transports an electric signal.  Because copper has excellent conductivity, the conductivity of other materials is stated as a percentage of the conductivity of copper.

The International Annealed Copper Standard (IACS) was established in 1913 by the International Electrotechnical Commission (IEC).  The commission established the standard for the conductivity of commercially pure annealed copper which is still used today.

Electrolytic Tough Pitch Copper

The standard grade of copper used in electrical wire is 99.95% Copper (Cu), 0.02 to 0.05 % oxygen, and less than 50 parts per million other metals. It has very high electrical conductivity, in excess of 100% of the IACS.  In the cast form it is called Electrolytic Tough Pitch (ETP) copper.  This grade of copper is often referred to as CA-110 and meets ASTM B152 specifications.  While CA-110 is one of the purest forms of copper it is also referred to as “110 Alloy.”  The electrical conductors of most medical cables are CA-110 copper.

Silver is the only metal with a higher electrical conductivity than copper. The electrical conductivity of silver is 106% of that of annealed copper on the IACS scale.  But, because of the high cost of silver and its low tensile strength its use is limited to special applications.

Work hardening

As common metals, including copper, are repeatedly bent, the underlying crystal structure of the material deforms into defects known as dislocations.  These defects build up with repeated flexing with the result of the material becoming less flexible.  This characteristic is called “work hardening.”  For flexible conductors this is an undesirable trait and can lead to conductors breaking under the stress of repeated flexing.

Annealed copper

Annealing is the process of heating a metal to make it softer and therefore more flexible.  Annealing is particularly effective for copper.  Annealed copper is much softer than un-annealed copper and resists work hardening.  All copper used for electrical conductors in medical cables, is annealed copper.

Tensile strength

Tensile strength is a measure of the maximum stress a material will sustain with uniform elongation.  Annealed copper’s tensile strength is 200-250 N/mm2.  For copper wire, tensile strength is generally considered the amount of axial force required to pull bare wire until it breaks.

For wire used in medical cables, tensile strength is often referred to as nominal break strength and is given in pounds or kilograms.  As an example, 28 gauge annealed copper wire made up of 19 strands of 40 gauge wire has nominal break strength of 2.3 kilograms or 5 pounds.  If a higher tensile strength conductor is required, the options are to use a larger diameter (lower gauge) wire or use a higher strength copper alloy.



Copper’s high ductility allows it to be
easily drawn down into individual conductors

Ductility

Ductility is a material's ability to deform under tensile stress.  Copper has a higher ductility than other metals used as conductors with the exception of gold and silver.  Because of copper’s high ductility, it is easy to draw down copper during the manufacturing process to the small diameters needed for the manufacture electrical wire.

Normally, the stronger a metal is, the less pliable it is.  However, this is not the case with copper.  Copper’s combination of high strength and high ductility make it an excellent material for electrical wire.


Structure showing the difference between
copper refurbished in 2010 and the
original copper installed in 1894


Corrosion resistance

Copper naturally resists corrosion from moisture, humidity, and other atmospheric influences.  Electrolytic tough pitch (ETP) copper is not subject to galvanic corrosion when connected to other, metals and alloys.  This characteristic allows copper to function well as an electrical conductor over a long period of time.

Materials that corrode typically loose flexibility.  Because copper is corrosion resistant, the flexibility of cables and wire used in medical applications is not reduced due to corrosion.

A good example of copper’s corrosion resistance is the East Tower of the Royal Observatory in Edinburgh, Scotland.  The structure is cloaked in copper and stood exposed to the elements for 116 years before being refurbished.  When it was refurbished, oxidation on the copper skin was measured at less than .127mm (.005”).



Copper’s low coefficient of thermal
expansion makes reliable and long
lasting wire terminations possible

Coefficient of thermal expansion

Most materials expand upon heating and contract upon cooling.  This is an undesirable characteristic in most electronic assemblies.  Copper has a low coefficient of thermal expansion when compared to other electrical conductors.  The low coefficient of thermal expansion of copper is beneficial because electrical terminations, such as solder joints, could easily be compromised by cycles of expansion and contraction.

Solderability


Copper conductors are readily soldered
forming a long-lasting electrical connection


Soldering is the process of joining two metals by the use of a material that has a lower melt point than the material being joined.  Because of the high melting point of copper, 1,083˚ C, it can be soldered with a wide variety of alloys.

A characteristic of copper is that it easily “wets out.”  This allows solder to flow and form a smooth, uniform and unbroken coating on copper wire or traces.  Because surface oxidation of copper is minimal, common varieties of flux are sufficient to remove that oxidation which, in turn, allows the surface to wet-out and facilitates an excellent solder connection.

Stranding copper conductors for increased flexibility

While annealed copper is ductile and is easily flexed, flexibility may be increased by combining a number of smaller conductors into stranded wire.  Stranded copper wire is a group of smaller copper wires that are braided or twisted together.  Stranded copper wire is more flexible than a single-strand of copper wire of the same diameter.


Diagram showing two 28 gauge conductors –
one with 7 strands and one with 19 strands


The smaller the diameter
of the individual strand,
the smaller the allowable
bend radius

Stranding improves the flex life of copper conductors because the bend radius of each of the individual conductors is less than that of a single larger conductor.

An additional failure mode of electrical conductors is exceeding the minimum bend radius.  The minimum bend radius for copper conductors is typically eight to twelve the times the diameter of the conductor.  For 28 gauge wire made up of 7 strands of 36 gauge conductors, the minimum bend radius would be approximately .060” (diameter of single strand of 36 gauge = .005” X 12 = .060”)

This potential mode of failure can be reduced by using strands of copper with a smaller diameter.  The smaller the diameter of each conductor, the less stress individual conductors are subject to when bent around a radius.  The less stress the conductors absorb, the longer the conductor is likely to last.

Coating copper conductors

While copper resists corrosion, the surface of bare copper slowly combines with oxygen to form a thin layer of copper oxide.  At higher temperatures, such as required for soldering, this reaction is accelerated.  The oxide that forms on the surface of copper is a poor conductor and it is best to prevent it from forming.

One way to prevent surface oxidation of copper conductors is to coat the copper with another metal which oxidizes more slowly than copper.  The two most common coatings are tin and silver.

Tin oxidizes much more slowly than copper, is readily available and is a relatively inexpensive material.  It is commonly used as a protective coating for copper conductors.  And, because most solder is tin based, achieving a high quality soldered termination is easier using tinned copper as compared to bare copper.

Because silver has a much higher melting point than tin, it is used as a coating where wire terminations will be subjected to higher temperatures.  Tin-coated annealed copper wire is normally manufactured to conform to ASTM B965 and silver coated annealed copper wire to ASTM B298-07.

Copper alloys

Where the required flex life cannot be achieved by using smaller individual conductors, or where a higher tensile strength is required, conductors made of copper alloys can be specified.  Before RoHS and REACH requirements were nearly universal, the most common materials to alloy with copper to improve tensile strength and flex life were cadmium and chromium.  Cadmium/chromium/copper alloys typically offered over 25 times the flex life of pure copper with only about a 10% increase in resistance.

Cadmium-free copper alloys are becoming more commonly available and offer flex life and tensile performance near that of copper alloys that contain cadmium.



Copper wire is at the heart of all
medical cable assemblies

Summary

The Affinity engineering team has a great deal of experience designing medical cable assemblies.  The most appropriate raw cable designs are generally a collaborative effort between our OEM customers, the Affinity engineering team and one or more of our cable extrusion partners.

If you would like to take advantage of Affinity’s experience and expertise in providing robust, long lasting cable assemblies, we welcome you to contact us at +1 949-477-9495 or via email to CustomerCare2@Affinitymedical.com.

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Meet Jack Sowin – Affinity Quality Assurance and Regulatory Affairs Manager


Affinity QA/RA Manager Jack Sowin


Jack joined the Affinity team in late September filling the open Quality Assurance and Regulatory Affairs Manager position.

Jack came to Affinity with more than 25 years of experience in Quality Assurance and Regulatory Affairs compliance.   During his career, he has been involved with quality engineering, new product development, design controls, risk management and quality systems improvement.  He is a Certified Quality Manager with the American Society for Quality (ASQ) and is also Regulatory Affairs Certified with the Regulatory Affairs Professionals Society (RAPS).

“Jack brings a wealth of experience and expertise in Quality Assurance and Regulatory Affairs to Affinity,” said General Manager, Bob Frank.  “We were fortunate to find him and are very enthusiastic about his future with the company.  Having such a qualified individual as part of our team will help Affinity to continue to grow and add capabilities.”

Before joining Affinity, Jack was Director of Quality Assurance & Regulatory Affairs for Philips Respironics in Carlsbad California and prior to that was Director of Quality Assurance for Biolase Technologies in Irvine California.  Jack also brings experience from Johnson & Johnson’s Advanced Sterilization Products where he was a Quality Group Leader for six years.

When asked what attracted him to Affinity Medical, Jack replied, “Affinity is a company that has a great family feel to it and it is very customer focused.  Employees work well with each other to identify issues and resolve them quickly but thoroughly.  Every aspect of our business, quality system and daily operations provides the best quality and service possible.”

Jack received his Bachelor of Science degree in Electronic & Computer Engineering from California State Polytechnic University, Pomona.  More recently he completed his MBA from University of Phoenix in San Diego and also earned a Bachelor of Arts degree in Psychology from California State University in Fullerton.

Jack is a native southern Californian and has lived most of his life in Orange County.  He and his wife live in Irvine with their Chocolate Lab “Jackson”.  Jack is an active member of Saddleback Church in Lake Forest.  He spends a lot of time at the gym and also enjoys camping, swimming, traveling and the beach.

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Visit Affinity Medicalâ„¢ at COMPAMED 2013 in Dusseldorf

Affinity Medical™ will be part of the Molex group exhibiting at COMPAMED in Dusseldorf Germany, November 20th to 22nd.

This will be Affinity’s first year exhibiting at COMPAMED, leaving its Hall-9 location at Medica where the company exhibited for many years.

“Exhibiting at Medica and COMPAMED is important for Affinity,” said General Manager, Bob Frank.  “We are able to meet with many of our OEM partners from around the world as well as meet new, potential customers.  We typically visit with more people in the three days of the exhibition than we could in a month of traveling.”



MediSpec™ silicone overmolded
cable assembly

Introducing MediSpec™ Silicone Cable Assemblies

Affinity will be using COMPAMED to formally introduce MediSpec™ silicone overmolded cable assemblies.  “Being part of Molex allowed us to acquire the additional space, personnel and equipment to add silicone molding capability,” said Affinity Business Development Manager, Jim Itkin.  “We’re excited to introduce this new capability that many of our OEM partners have been asking for.”

Representing Affinity at the show will be: Bob Frank, General Manager, Hank Mancini, Marketing Manager and Jim Itkin, Business Development Manager.  In addition Wayne Shockloss, Molex Director of Medical Connectors & Cable Assemblies and Roberto Henker, Medical Account Manager will be at the Molex stand in Hall 8b.

If you are attending Medica/COMPAMED, please stop by our stand in Hall 8B, Stand N03.  If you would like to schedule an appointment, please contact your local Molex Sales Engineer or Jim Itkin at jim.itkin@molex.com.

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Announcements, Information and Trivia



Roast turkey is part
of the traditional
Thanksgiving meal in
many western countries

Thanksgiving Holiday

Affinity Medical will be close Thursday and Friday, November 28th and 29th to allow our team members to celebrate and enjoy the Thanksgiving holiday.

The entire Affinity Medical team is grateful for the ongoing friendship and support from our OEM partners.  Affinity exists because of you.  We wish you and your family a wonderful, healthy and happy Thanksgiving.

November Trivia

The 9th Month? The name 'November' is believed to be derived from “novem” which is Latin for the number nine.  In the ancient Roman calendar November was the ninth month after March.  In the Gregorian calendar, November is the eleventh month of the year and one of four months with 30 days.


 


Armistice Day
celebrated on Wall
Street in New York
City, 1918 – Public
Domain photo from
The New York Times

Meteor Showers – In 2013, the Leonids meteor shower reaches their peak on the evening of November 16th.   This year, the full moon will interfere with viewing the Leonid meteor shower.

Armistice Day – Armistice Day is celebrated November 11th to commemorate the end of hostilities on the Western Front of World War I.   The armistice was signed on November 11th, 1918 at 11:00 AM – the eleventh hour of the eleventh day of the eleventh month!

Thanksgiving – Thanksgiving is celebrated in the United States on the fourth Thursday in November and is a national holiday.

Observances known as Thanksgiving are celebrated on other countries and include Emtedankfest (The Harvest Thanksgiving Festival) in Germany and Kinro Kansha no Hi (Labor Thanksgiving Day) in Japan.