| With the continued miniaturisation of electronics increasing reliance is placed on very small connectors and contacts. The materials from which these parts are manufactured must have the optimum balance of strength, wear resistance, formability and electrical conductance. Key to their reliable operation over extended service periods is the material’s stiffness, to ensure that the correct contact force is maintained even after repeated connections and disconnections. ERA’s microanalysis system is able to confirm the specification of alloys to ensure proper performance of electromechanical components.
Choosing your material and checking it’s specification
Two types of copper alloy are widely used in these applications, phosphor bronzes and copper-beryllium. Copper-beryllium alloys typically contain between 0.2 and 2wt% beryllium and phosphor bronze alloys typically contain 0.01 to 0.35wt% phosphorus. Both offer enhanced strengths and stiffness over other copper-based alloys. However, one difficulty is ensuring that, when these alloys are specified, the alloys supplied do in fact meet the specification; the amounts of beryllium and phosphorus are very difficult to measure at low concentrations.
These alloys can be analysed by wet chemical methods, but a sufficient quantity of material is required for accurate analysis and careful dissolution is necessary to ensure no loss of the low concentration elements. Standard x-ray fluorescence (XRF) analysis is difficult because of the low element concentrations and the easy absorption of beryllium and phosphorus X-rays if the analysis is not conducted in a vacuum.
| Microanalysis using a scanning electron microscope |
In applications where only individual or low numbers of contacts are available to confirm composition, the most widely used method has been scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX) where typically element concentrations as small as 0.1wt% can be measured. At lower concentrations and for very light elements, such as beryllium, accurate analysis by SEM-EDX alone is not possible. |
Figure 1. A typical contact, such as this high performance pin connector, comprises only a small quantity of material and is best analysed in situ |
ERA’s recently updated SEM facility includes an extended analysis capability with the addition of Wavelength Dispersive X-ray analysis (WDX) that enables elements to be measured at much lower concentrations (typically in the parts per million or 0.001wt%) and is also more sensitive for light element analysis. The system also allows EDX spectra to be synthesised for known element concentrations to determine whether detection and measurement of an element will be possible by SEM-EDX only or will require simultaneous EDX and WDX analysis.
| Difficulty of analysing only using EDX |
Consider a 93.55wt%Cu - 6.3Sn - 0.15P phosphor bronze alloy used to manufacture contacts. A synthesised EDX spectrum generated under standard analysis conditions shows that it would be very difficult to detect and accurately measure the phosphorus composition of the alloy; the 0.15wt% phosphorus peak (labelled in the pale blue oval in figure 2) is only fractionally higher than the background signal.
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Figure 2. Synthesised EDX spectrum (yellow) for alloy composition 93.55wt%Cu-6.30Sn-0.15P |
Benefits of WDX analysis
Contrast this with the EDX and WDX spectra acquired from a connector material containing only 0.1wt%P. The phosphorus peak can be clearly seen in the WDX (blue) spectrum, of figure 3.
The high resolution of WDX can be seen in figure 4. The phosphorus peak has been extracted from the spectrum in figure 3 to show its fine structure over a narrower energy range.
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Quick and easy analysis
By using the spectrum synthesis “virtual tool” and where necessary combined EDX-WDX analysis, it is possible to tailor the measurement to achieve accurate results very quickly. Copper beryllium and phosphor bronze alloys can be routinely analysed. Here are examples of analysis recently undertaken on two connector materials at ERA.
Connector 1 |
Wt% |
|
Connector 2 |
Wt% |
Cu |
96.21 |
|
Cu |
97.90 |
Sn |
3.70 |
|
Be |
1.72 |
P |
0.09 |
|
Co |
0.23 |
|
|
|
Pb |
0.15 |
In addition, WDX analysis also permits measurement of trace elements at the levels required for recent environmental legislation e.g. concentration of 0.01wt% Cd in electrical goods covered by the EU RoHS Directive.
Further information
The Reliability and Failure Analysis group carries out investigation of many contact and connector problems going beyond simple chemical analysis to consider the whole system and determine the root cause of failure. To find out more call us on +44 (0)1372 367444 or email.
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Composition - the key to connector and contact performance
