If the material of the connector terminals is changed from the original phosphor bronze to beryllium copper, the cost of the raw materials will increase from approximately 52 yuan/kg to approximately 254 yuan/kg. This price increase is nearly five times. This figure comes from the average purchase price disclosed by a listed company. What did this additional investment yield? The answer is not balanced: In terms of conductivity, the 22% IACS of beryllium copper is only about a quarter higher than the 14% - 18% IACS of phosphor bronze; in terms of elastic modulus, 128 GPa is only about 16% higher than 110 GPa.
What really sets them apart are the other two properties: yield strength and the ability to maintain stress at high temperatures. If one selects beryllium copper solely for conductivity, then this investment and return are not commensurate.
Conductivity: The gap between the two is less than expected.
On the beryllium copper side, the commonly cited one is UNS C17200, which is also known as QBe2 in the national standard. The as-honed conductivity is 22% IACS.
On the phosphor bronze side, the range is larger: UNS C51000 generally provides 14% - 18% IACS, JIS system's C5191 often writes 13%, and C5210 writes 12%. This difference comes from the tin content and supply status, and neither side is wrong.
This difference is not noticeable on the terminals. For a regular-sized conductive path, the difference in body resistance between the two materials is in the milliohm range, while the contact resistance at the contact point is at the same level. What is more important to note is that the increase in contact resistance caused by contact force attenuation is usually much larger than this difference caused by the material's conductivity. Therefore, the contribution of beryllium copper to conductivity is indirect: The contact force is maintained, and the contact resistance does not rise.
The difference in elastic modulus is limited. The difference lies in the amount of deformation allowed.
For this item, beryllium copper has an elastic modulus of 128-131 GPa, while phosphor bronze has 110-115 GPa. The difference is approximately 16%, which reflects the same magnitude of stiffness.
So where does the advantage of beryllium copper lie? This depends on what determines the maximum contact force that the elastic component can provide. This quantity is determined by the allowable stress, and the elastic modulus does not participate in it. That is to say, under the same cross-sectional size, the advantage of beryllium copper does not lie in higher stiffness, but in allowing a larger working deformation. Under the same size, it is approximately 1.6 times that of phosphor bronze, and the allowable contact force is approximately 1.9 times.
The engineering implications are two: under the same contact force requirement, beryllium copper terminals can be made thinner and shorter; under the same size, it can provide a larger portion of contact force or leave a bit more working stroke.
Another difference that is easily overlooked is that the ratio of beryllium copper's yield strength to its elastic modulus is higher, about 60% higher than that of phosphor bronze. And the springback of the stamping is positively correlated with this ratio, so the springback of beryllium copper parts is greater. The compensation amount on the mold needs to be recalculated and cannot be used the same parameters as those of phosphor bronze.
Temperature is the decisive boundary
The strengthening mechanisms of the two materials are different, which determines their performance in the face of temperature. Phosphor bronze is hardened by cold rolling, with tin solidified in the copper matrix. Cold rolling increases the dislocation density, and the strength rises accordingly. It does not have the precipitation strengthening layer; when the temperature rises, dislocations will rearrange and return, and the part of the strength brought by cold processing gradually recedes, and the clamping force drops accordingly.
Bechtol copper is different. After solidification, it forms an oversaturated solid solution, and during aging, the CuBe phase precipitates. These dispersed precipitated particles hold the dislocations, and they remain basically stable below 200°C. Its strength is not supported by cold processing, and there is no part that can return; this type of test is carried out in accordance with ASTM E328 (Method for Stress Relaxation Testing of Materials and Structures).
There is no unified public value for the phosphor bronze side. The brand and supply status of each manufacturer vary greatly. The reliable approach is to request the curve of the corresponding state from the strip supplier; the commonly used acceptance criterion in the industry is that the relaxation at 105°C for 1000 hours does not exceed 10%, and this number belongs to an industry reference value, not a standard clause.
There is a type of statement that phosphor bronze cannot be used at high temperatures. This statement requires qualification. The long-term working range given by the strip supplier is -40 to 120°C, and short-term peak 150°C. Within this range, the clamping force attenuation in normal and medium-temperature conditions is controllable. The majority of in-use terminals are in this state.
What needs to be calculated separately are two places: one is the position of the part close to the engine, power device, or where it is constantly powered and heated, with the continuous temperature entering above 120°C; the other is the part repeatedly experiencing reflow soldering peak temperature under stress conditions. The first one belongs to the inherent nature of the material, and the second one belongs to the superposition of processes.
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Process path difference: Beryllium copper adds a heat treatment process
The process for phosphor bronze is shorter: Cold-rolled hardened sheet materials are delivered to the factory, then stamped, deburred, electroplated, and the process is completed.
Beryllium copper has additional two processes: solution treatment and aging. The sequence is important and the conventional practice is to perform solution treatment (i.e., soft state) before stamping and forming, and then do aging hardening after the forming is completed. The aging temperature is within the range of 300-345°C, and the common temperature range is 320-340°C with a holding time of 2-3 hours; the solution temperature is 780-800°C for QBe2, generally 780-790°C.
Why must it be done in this sequence? The forming performance is clear. For a 90° bending with the minimum relative bending radius R/t, beryllium copper in the un-aged supply state, along the rolling direction is about 1, and in the vertical direction is about 2.9. After being tempered in the factory's aged state, it becomes 4 and 5. The aged material can still be bent, but it cannot be bent so sharply, and there is no more space to use aging to increase strength. For complex forming parts such as springs, the only way is to go through the process of forming first and then aging.
This step of aging itself also has a cost. While precipitated phases form, internal stress is released, and the part deforms accordingly. Therefore, fixtures are needed for aging, and the design and clamping method of the fixtures directly affect the final size and elasticity.
Heating must be carried out in a vacuum or protective atmosphere. Heating in air will generate an oxide film on the surface, which contains beryllium oxide. The pre-treatment before electroplating should add a film removal or activation process to avoid affecting the adhesion of the coating and welding. These processes are costs beyond the material unit price and are easily overlooked when quoting.
The cost difference by volume is approximately 4.5 times, and there are also compliance costs.
For this item, the average price disclosed by listed companies can be used as a reference: from 2018 to 2020, phosphor bronze sheet was 52-56 yuan/kg, while beryllium copper sheet was 254-315 yuan/kg. The same company's brass sheet was 40-42 yuan/kg, and copper-nickel-silicon C7025 sheet was 59.29 yuan/kg. On average, the multiple of beryllium copper compared to phosphor bronze was between 4.8 and 5.8.
This is slightly overestimated by weight ratio because of the different densities. Beryllium copper has a density of 8.25-8.36 g/cm³, while phosphor bronze has a density of 8.86 g/cm³. In the same volume, beryllium copper parts are about 7% lighter, and the actual multiple is approximately 4.5 times. This difference accumulates to a material cost difference of about 2,000 yuan for a million terminals, and this item is more sensitive for large-scale parts.
In terms of compliance, the additional management work for beryllium copper is not insignificant. The US OSHA (Occupational Safety and Health Administration) in 29CFR1910.1024 includes alloys with a beryllium content of no less than 0.1% in its management, with an 8-hour time-weighted average permissible exposure limit of 0.2 μg/m³, a short-term exposure limit of 2.0 μg/m³, and an action level of 0.1 μg/m³.
The EU REACH (Regulation on the Registration, Evaluation, Authorization and Restriction of Chemicals) has included beryllium and its compounds in the list of substances of very high concern.
It should be noted that the risks lie in the processes that produce dust and smoke, namely smelting, heat treatment, cutting, polishing, and welding; the beryllium in the finished parts is dissolved in the matrix, and there is no such risk in the assembly and usage stages.