C51000 Phosphor Bronze Selection Guide

C51000 is a 5% tin phosphor bronze — the default choice for electronic spring contacts, connectors, and switch components. It sits between pure copper (high conductivity, low strength) and beryllium copper (extreme strength, higher cost).

What Is C51000 Phosphor Bronze?

At the core of C51000 (ASTM phosphor bronze Grade A): 5% tin for solid-solution strengthening and work-hardening capacity, plus a small phosphorus addition that acts as a deoxidizer and grain refiner — the latter directly improves fatigue life.

In strip and sheet form, C51000 appears in stamped connector terminals, relay springs, switch contacts, fuse clips, and EMI shielding fingers. Wire grades are used for springs, fasteners, and woven cloth. The alloy is sometimes called “phosphor bronze A” to distinguish it from C52100 (~8% tin).

C51000 Chemical Composition

Copper (Cu)	Remainder
Tin (Sn)	4.2–5.8
Phosphorus (P)	0.03–0.35
Lead (Pb)	≤ 0.05
Iron (Fe)	≤ 0.10
Zinc (Zn)	≤ 0.30

Tin below 4% noticeably drops in strength. Above 6%, cold formability degrades — that’s C52100 territory. Phosphorus at 0.10–0.35% provides meaningful grain refinement. If fatigue life is critical, specifying phosphorus at the higher end of the range is worth negotiating.

Lead is kept below 0.05%, making C51000 RoHS-compatible.

C51000 Mechanical Properties by Temper

C51000 is supplied in cold-worked tempers. The alloy cannot be strengthened by heat treatment — it’s cold-work only.

Temper	Tensile Strength (MPa)	Yield Strength(MPa)	Elongation (%)	Hardness
Annealed (O61)	310–380	105–170	40–50	30–55
H02 (Half Hard)	420–520	310–415	20–35	60–75
H04 (Hard)	510–620	450–550	10–20	75–85
H06 (Extra Hard)	590–690	550–640	5–12	84–90
H08 (Spring)	650–760	600–700	3–8	88–94
H10 (Extra Spring)	690–830	640–760	1–4	91–97

Temper selection guide:

O61 — Deep draw and complex forming. Not for parts seeing repeated loading without post-forming cold work.
H02–H04 — The sweet spot for stamped connector terminals. Enough formability for 90° bends, enough strength for reliable contact force.
H06–H08 — Flat springs and simple bends. At H08, limit bend radii to 2–4× material thickness and expect 10–15° springback.
H10 — Maximum strength, minimal ductility. Flat parts only. Bends tighter than 4× thickness will crack.

If your design needs a post-forming strength boost, C51000 is the wrong alloy — switch to precipitation-hardening C17200 or C70250.

C51000 Electrical Conductivity and Thermal Properties

Electrical Conductivity	15% IACS (min), typically 15–17%
Electrical Resistivity	~11.5 µΩ·cm at 20°C
Thermal Conductivity	~70 W/m·K

Conductivity-strength context:

Grade	Electrical Conductivity (% IACS)	Typical Tensile Strength (MPa)
C11000 (Pure Cu)	100	~250
C26000 (Brass)	28	~350
C51000	15	450–550
C52100	13	550–650
C17200 (BeCu)	22	1000+

C51000 provides conductivity adequate for signal and low-to-moderate-power contacts, paired with spring properties that hold up over thousands of cycles. For power contacts above 5 A continuous current, switch to a higher-conductivity alloy or increase the cross-section.

Conductivity drops roughly 10% per 100°C rise. Above 100°C operating, derate accordingly.

Formability and Fabrication

90° bend limits by temper and grain direction:

Temper	Min. 90° Bend Radius Across Grain	Min. 90° Bend Radius With Grain	Typical Springback	Recommended Fabrication Use
H02–H04	1T	2T	5–8°	Stamped connector terminals
H06	1.5T	3T	10–12°	Simple-formed contacts
H08	2–4T	Avoid	12–15°	Flat springs only
H10	4T minimum	Cracking risk	15°+	Flat parts only

Springback varies ±3–5% across heats. Confirming with a trial strip from the production lot cuts tooling rework risk. At H06+, over-bend compensation in the progressive die is required.

Deep drawing. Only annealed temper (O61). C51000 work-hardens faster than cartridge brass — intermediate annealing may be needed for reductions above 40%. For stamped parts stored before assembly, VCI paper or sealed-bag packaging prevents surface oxidation that degrades solderability.

Joining and plating:

Solder: compatible with standard Sn-Pb and lead-free solders
Resistance welding: feasible, needs a higher current than copper. At higher temperatures, copper-tungsten electrodes are preferred.
Plating: tin, silver, gold — all compatible. Low lead content simplifies bath management vs leaded brasses.
Nickel underplate recommended for corrosive environments

Corrosion Resistance and Environmental Limits

C51000 has better atmospheric corrosion resistance than brass, forming a protective patina rather than dezincifying.

Safe conditions: indoor electronics, mild outdoor exposure, freshwater contact, most organic atmospheres.

Caution required:

Ammonia/amines — Stress corrosion cracking risk, especially in H06+. Check proximity to cleaning chemicals or industrial atmospheres.
Sulfide atmospheres — Accelerated tarnishing. Gold plating or seals may be necessary.
Salt spray — Better than brass, worse than marine bronzes. Not for continuous marine exposure.
pH below 4 — Tin oxide layer breaks down. Nickel barrier underplate recommended for battery compartment contacts.

Galvanic compatibility:

Mating Material	C51000 Galvanic Role	Design / Corrosion Risk
Aluminum, steel	Cathodic	Mating part corrodes — avoid in wet conditions without coating
Gold, stainless steel	Anodic	C51000 self-sacrifices — gold plating recommended
Tin-plated brass	Near-neutral	Common, benign combination. Gold-plated C51000 + tin-plated brass pin works reliably

Standards, Specifications, and Procurement Checklist

Standard	Form
ASTM B103	Phosphor bronze strip (primary for connector stock)
ASTM B139	Sheet and plate
ASTM B159	Wire

Purchase order essentials:

Alloy: C51000 (not just “phosphor bronze”)
Temper: O61 / H02 / H04 / H06 / H08 / H10
Thickness and tolerance (per ASTM B103)
Width and edge condition
Surface finish
Quantity and coil/sheet dimensions
MTR/MTC: per order, per heat, or per coil

A standard MTR covers chemical analysis (Cu, Sn, P, Pb, Fe, Zn), tensile properties (UTS, YS, elongation per ASTM E8), hardness, and grain size (for annealed temper). Conductivity testing, stress relaxation data, or fatigue S-N curves are supplementary — request these during quotation, not after the order is placed. For fatigue-critical or relaxation-critical applications, agree on test method, sample count, and acceptance criteria up front.

When C51000 Isn’t the Right Choice

Conductivity above 25% IACS needed → C26000 brass (28%) or pure copper (100%). I²R heating in C51000 at sustained currents above 5 A typically pushes the design toward higher-conductivity alloys.
Post-forming heat treatment required for final strength → C17200 (BeCu) or C70250 (Cu-Ni-Si). C51000 has no precipitation-hardening response — all strength comes from cold work, which is lost if the part is annealed after forming.
Stress retention > 80% after 1,000 hrs at 105°C → Beryllium copper or mill-hardened specialty alloys. C51000 at this temperature drops below 70% retention — normal force erodes to the point of unreliable contact.
Maximum formability in as-supplied condition → C26000 cartridge brass, cheaper and with 50%+ elongation in annealed temper vs C51000’s ~40%.
Continuous salt spray or acidic immersion → Marine bronze (C46400) or aluminum bronze. C51000’s tin oxide patina does not withstand continuous chloride attack.
Cost is dominant, and performance margins are generous → C26000 brass at roughly half the material cost. Only pay for phosphor bronze when you need what phosphor bronze does.

Conclusion

C51000 earns its place by conducting electricity adequately and resisting fatigue and stress relaxation better than anything in its price range. Getting the temper wrong costs more in tooling rework than the material itself — H04 for stamped contacts, H08 for flat springs, annealed for deep drawing.

If your design requires C51000 in a specific temper, thickness tolerance, or slit width — with MTR documentation — send your specification for a technical quotation.