|
Copper Alloy
Metallurgy Limitations
By Dr.
Robert S. Mroczkowski, Bishop & Associates Inc.
This article will cover
limited aspects of copper alloy metallurgy as they apply to electronic
connectors. The majority of connectors use a copper alloy strip for both
plug and receptacle contacts. As mentioned in the
previous article,
this is because of the balance of strength, formability, and
conductivity that can be realized with copper alloys. Now, let’s look at
alloy types, strengthening mechanisms, and electrical conductivity.
Alloy types:
Compositionally, the
copper alloys used in connectors are predominantly copper, from 70 to
99+ percent copper, with the balance consisting of a relatively small
number of alloying agents. The most common additions are tin (phosphor
bronze), zinc (brass), and nickel (sometimes coupled with tin, zinc, or
cobalt). Specialty alloying agents, usually in small percentages,
include beryllium (beryllium-copper), silicon, cobalt, magnesium, iron,
and zirconium.
There are two basic types of copper alloys: Solution hardened and
precipitation/dispersion (P/D) hardened. Whether an alloy is solution-
or P/D-hardenable depends on the alloying additions. All the alloys are
also work-hardenable.
Alloys with tin and zinc are generally solution-hardened alloys.
Solution hardening is a
result of the solution of a few to several percent of the alloying agent
into the base copper.
For example,
phosphor bronzes generally contain 4 to 8 percent tin, and the most
commonly used brass alloy has the composition of copper (70 percent) and
zinc (30 percent). These alloys retain the crystal structure of pure
copper, face-centered cubic.
The specialty elements mentioned are often used to facilitate
precipitation (beryllium) or dispersion (the others) hardening. The
strengthening mechanisms of P/D alloys will be discussed in the next
section.
In North America, copper alloys are identified by composition according
to the Unified Numbering System (UNS) using CXXXXX, where C identifies
copper and the Xs are the alloying system. The most common designations
for the alloying agents mentioned above include C1XXXX (beryllium,
silicon, magnesium, and zirconium), C2XXXX (zinc-brasses), C5XXXX
(tin-phosphorous-bronzes), and C7XXXX (nickel, sometimes with cobalt,
silicon, tin, and iron). For more information concerning other alloys
and the UNS system, visit the Copper Development Association website,
www.copper.org.
Strengthening Mechanisms:
As mentioned, all
copper alloys are work-hardenable, meaning, when they are deformed—or
worked—they increase in hardness and strength. The strengthening
mechanism is basically distortion and creation of defects within the
crystalline copper lattice, which results in an increased resistance to
further slip and distortion. It should be noted that work-hardening
characteristics vary with alloy composition. If you recall, connectors
are typically made from strip material. The processing of bulk ingots
into strips consists of a series of rolling and annealing operations.
The rolling work hardens the strip and the annealing “softens” it so
that it can be subjected to additional rolling without cracking. The
rolling/annealing schedules that produce strip for connectors in
thicknesses of a millimeter to less than a tenth of a millimeter are
proprietary to the copper alloy strip manufacturers. For this reason,
different suppliers may have a different combination of yield strength
and ductility/formability for a given material strength level or temper.
Next, consider phosphor-bronzes as an example of a solid
solution-strengthened alloy system. As tin is incorporated into the base
copper crystal lattice, substitutionally (on the copper lattice sites),
the lattice is distorted locally due to the size difference between tin
and copper atoms. This distortion increases the resistance of the
lattice to slip mechanisms under applied stresses, in effect,
strengthening the material. Solid solution strengthening depends on the
alloying agent and the alloy composition. The strengthening effect of
tin in copper, per percent addition, is higher than that of nickel or
zinc, for example. But, zinc and nickel can be incorporated at higher
percentages than tin, while maintaining the copper lattice structure.
Finally, we come to precipitation/dispersion hardening mechanisms.
Precipitation- and dispersion-hardening mechanisms are similar in that
they both rely on a fine distribution of particles of a second phase
compound within the base copper lattice. P/D alloys of interest in this
discussion are primarily in the C1XXXX UNS category, and are either
coppers (minimum 99.3 percent copper) or high coppers (minimum 96
percent copper). Despite these low alloy concentrations, second-phase
structures are nucleated and controlled to create the desired
distribution of fine particles in the copper matrix.
The creation and functional role of the particles, however, is different
for precipitation and dispersion hardening. Beryllium-copper, C17200, is
an example of a precipitation-hardening, sometimes called age-hardening,
alloy. C17200 contains two percent of alloying agents, typically 1.8
percent beryllium and 0.2 percent cobalt. Copper cannot hold 1.8 percent
of beryllium in solution at room temperature. This fact is the key to
precipitation hardening. Briefly, the Be-Cu alloy is heated to a
solutionizing temperature to take all the beryllium into solution; it is
then quenched to room temperature. The beryllium is retained in a
metastable solution due to the rapid quenching. Next, the alloy is aged
at a lower temperature to allow the beryllium to precipitate out as a
controlled distribution of beryllide particles in the copper matrix. The
distribution of beryllide particles provides an additional strengthening
mechanism by preventing the motion of defects, called dislocations,
under applied stress. This dislocation pinning mechanism is stable at
elevated temperatures, where the work-hardening mechanism dissipates.
Thus, Be-Cu alloys have superior stress relaxation performance compared
to copper alloys that can only be work-hardened.
The manufacturing process for dispersion-strengthened alloys is more
complex than that for beryllium-copper. For example, C19400, a high
copper alloy containing 2.3 percent iron exceeds the solubility of iron
in copper at room temperature and the particle dispersion is nucleated
and controlled during annealing/rolling processing. The strengthening
mechanism is also more complex than in Be-Cu. as dispersion-hardened
alloys essentially enhance the response of the material to cold working
processes, providing higher strength for lower amounts of cold working.
As with Be-Cu, the dispersed phase provides enhanced thermal stability
compared to standard cold worked materials and, therefore, improved
stress relaxation resistance.
Conductivity:
The original
International Annealed Copper Standard (IACS) copper standard was
defined over a century ago, and since then, copper processing technology
has improved, so the IACS conductivity of C11000, pure copper, is now
101 percent (min) IACS instead of the “standard” 100 percent. For
reference, some minimum IACS conductivities for copper alloys used in
connectors follow: C26000 (brass), 28 percent; C51000 (phos-bronze), 15
percent; C17200 (Be-Cu), 18 percent; and C19400, 60 percent. The
conductivity of copper alloys is strongly dependent on the alloying
additions. As with solid solution strengthening, tin and nickel show
stronger effects on conductivity than does zinc. Other factors affecting
conductivity include grain size and work hardening, both of which have
small effects compared to that of alloying additions.
Now that we’ve covered how copper alloys act, we’ll discuss some
specific copper alloys and their areas of application in connectors in
the next article in this series.
Contact Dr. Bob at
connNtextassoc@aol.com.
 |
Dr. Robert S. Mroczkowski
Director Technology, Bishop and Associates Inc.
IIn 1998, Dr. Mroczkowski founded connNtext associates, a firm
providing consulting services in connector applications to the
electronics industry. Dr. Mroczkowski has more than 30 years
experience in the electronics industry. He joined AMP Inc. in
1971. While at AMP, his responsibilities included consulting on
connector design, materials, and reliability concerns, and he
provided an interface to AMP customers on these issues. In 1990
he joined the AMP Advanced Development Laboratories, where he
developed microstrip cable connectors and a new microcoaxial
connector for medical ultrasound diagnostic equipment. Dr.
Mroczkowski retired in 1998 as an AMP principal. He is the
author of the
McGraw Hill Electronics Connector Handbook, has contributed
chapters on connectors and interconnections to a number of
packaging handbooks, and written more than 20 technical papers.
He holds seven patents. In 1997, Dr. Mroczkowski received the
Lifetime Achievement Award of the International Institute of
Connector and Interconnection Technology.
He holds a bachelor’s, master’s, and doctorate of science
degrees in physical metallurgy from the Massachusetts Institute
of Technology.
|
|
