7.1 Preamble
Copper is the most popular metal amongst the commercially available highly conductive materials and used extensively for wide range of applications in transferring electricity and heat. Since copper has the face centered cubic (FCC) crystal structure, its atoms have the ability to roll over each other into new positions without breaking the metallic bonds. As a result, there is some effect of cold work on the conductivity values.
At the same time, thermal stability is also affected by the alloying as well as cold-rolled work hardening. But what happens on the conductivity and thermal capacity levels after the inclusion of solder elements in copper is not known. As such, this chapter addresses the effects of tin or lead inclusion in copper during maintenance or repair as soldering material and cold-rolling as well as thermal ageing in the process of manufacturing and operational conditions on conductivity/thermal capacity.
7.2 Experimental Details
Group-II samples prepared as described in chapter 2 were taken to investigate the electrical conductivity values of all four sample materials, i.e., material-I (pure Cu), material-II (Cu-Sn alloy), material-III (Cu-Pb alloy) and material-IV (SnPb-solder affected Cu). The conductivity values were measured using digital conductivity meter (Model: Technofour 979) with an accuracy of ±0.1% IACS (International Annealed Copper Standard). Basically, conductivity values were measured for all categories of samples whether untreated or thermally treated under each temperature of thermal ageing conditions at each cold-rolled work hardening level. Therefore, samples were divided into two groups of which the first group were of as-cast and cold rolled but free from heat treatment to observe the individual effect of work hardening for the cold work level of 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%
and 75% of the samples after mechanically ground with 300, 600, 900, 1200 and 1500 grits of SiC emery paper successively.
Few samples from the second group were heat treated isochronally at various temperatures, such as, 25oC, 100oC, 150oC, 200oC, 250oC, 300oC, 350oC, 400oC, 450oC and 500oC for a period of one hour and their conductivity values were measured.
Thereafter, the remaining samples of the second group were heat treated isothermally at 150oC, 200oC and 250oC corresponding to the higher strength with the variation of ageing period from 15 minutes to 240 minutes and the conductivity values were measured to investigate the effect of ageing period. The measurements of electrical conductivity were repeated at least twenty times for each condition of a sample so that the data can be considered as reliable and consistent. The mean value and standard deviation was utilized for discussions.
The calorimetric measurements were carried out as an observance of thermo-chemical changes of work hardened (75% deformation level) sample materials from their as-cast condition using differential scanning calorimeter (DSC) of model TA-DCS 2920. The information on endothermic and exothermic process of the samples has been netted during physical transitions at a constant heating rate in a controlled ambience starting from room temperature (25oC) to 700oC. The specimen of 150 mg was placed in a platinum crucible into the DSC furnace to measure the heat flow in varied temperatures and changes in heat capacity with the heating rate of 20oC per minute.
7.3 Analysis of Conductivity results 7.3.1 Effect of alloying
The average electrical conductivity values of material-I, -II, -III and -IV under as- cast condition obtained through twenty measurement readings for each case, are found to be 57.56 MS.m-1, 33.93 MS.m-1, 53.13 MS.m-1 and 26.91 MS.m-1, respectively. Here, material-I, i.e., pure Cu has shown the highest conductivity amongst the four sample materials; which has been reduced by 41.06% while only 1.134% Sn is added to form material-II, and reduced by 6.94% while only 1.197% Pb is added to have material-III.
The conductivity is found to be the lowest for material-IV, i.e., SnPb-solder affected Cu, where only 1.257% Sn and 1.195% Pb are present with Cu. The result is a significant
reduction in electrical conductivity of material-IV, which is 53.25%, 20.68% and 49.34%
lower than that of material-I, -II and -III, respectively. Basically, Cu has high electrical conductivity due to a large number of free electrons that are excited through the lattice in the crystal. But the crystalline defects serve as scattering centers for conduction electrons and lower the conductivity. The alloying elements having atoms of different sizes cause a variation of lattice parameters, i.e., increase the lattice imperfections which increase the scattering of electrons. Moreover, alloying element having different electron concentration than the base element changes the position of Fermi energy. Therefore, the electrical conductivity of alloys is lesser than that of pure metal.
Since the atomic size of solute elements, such as, Sn and Pb are different from that of Cu solvent forming the solid solution of material-II, -III and -IV, elastically strained local regions are developed. Moreover, Sn and Pb atoms in Cu act as scattering centers in the solid solution. As such, the electrical conductivity values of material-II, -III and - IV are found to be reduced significantly. Again, Sn is found to be prominently influencing the values of electrical conductivity compared to Pb in the copper alloys due to complex formation of CuxSny intermetallic compounds [34-35]. As a whole, the electrical conductivity of copper is reduced very significantly because of tiny alloying components added to develop the solid solutions with copper.
7.3.2 Effect of cold work
Once the samples have undergone work hardening from zero to 75% deformation levels, the electrical conductivity values of material-I, -II, -III and –IV are found to deviate to different extents as presented in Fig. 7.1. It indicates that the electrical conductivity of material-I, i.e., pure Cu is found to be varied insignificantly (black line in Figure 5.1) and reduced by only 1.53% after 75% cold-rolled deformation from its conductivity value obtained before work hardening. But the electrical conductivity values of other three alloys, i.e., material-II, -III and -IV are of little rise at lower levels of cold-rolled deformation, then remain almost constant for a certain range, and finally follow a decreasing trend with the rise of deformation levels as designated by red, green and blue lines in Fig. 7.1, respectively. The maximum conductivity values are observed at the cold-rolled deformation level of about 35% for the alloys. At 75% deformation level, these three alloy materials show significant reductions in conductivity by 10.71%,
7.31% and 15.38% from their conductivity values obtained before work hardening, and 17.29%, 9.32% and 28.36% from their maximum attained values, respectively.
Fig. 7.1 Electrical conductivity variation against cold-rolled work hardening conditions at room temperature for copper and the solder affected copper alloys.
It is noticed that the solid-solution alloying together with cold working could improve the hardness at the expense of reduction in conductivity. It would be worth mentioning that the ~35% work-hardening deformation is realized to be the optimum level of cold working for maximum recovery of electrical conductivity loss due to inclusion of solder in copper. In addition to electrical conductivity, this particular level of deformation has however been identified to be of great importance in defining the incremental behavior of material hardness as a function of deformation (see fig. 3.3). In fact, the micro- hardness values of Cu-Pb and Cu-Sn-Pb alloys are found to cross over those of pure Cu and Cu-Sn alloys, respectively, when the deformation level reaches nearly at the same value (~35%). This particular phenomenon associated with Pb-based Cu alloys for their higher rate of increase of hardness with respect to cold-rolling deformation is rather identified to be the critical level of deformation, beyond which micro-hardness still continues to increase with a relatively slower rate, while the associated electrical
conductivity decreases. Therefore, cold-working deformation of 35% can be considered as the critical level of deformation for both hardness and conductivity.
When the cast samples are subjected to cold-rolled work hardening, adjustments of minor casting defects and closing up of the voids are happened through alloying particles, thereby allowing the cast material to become more compact and solid, which, in turn, gives rise to electrical conductivity. On the other hand, the plastic deformation of alloys usually gives rise to electron-scattering dislocations, which has the characteristic of lowering the electrical conductivity [93-94]. Since the electron scattering by dislocations is low at lower degrees of deformation and the dislocation annihilation process remains active to moderate the dislocation density, an increase in conductivity is encountered with the increase of work-hardening deformation, especially at the initial stages [20]. It is interesting to note here that, when the cold-rolling deformation level reaches to about 35%, the rate of increase of conductivity with respect to deformation is reduced to zero for all the alloy materials. When the deformation is further increased more than 35%, the electrical conductivity of the alloys shows a decreasing trend, which is, however, found to be in contrast with the behavior of hardness (see fig. 3.3). This lowering of electrical conductivity may be attributed to the phenomenon that, at very high level of deformation, the increase in dislocation density as well as the number of coarse precipitation scattered at grain boundaries is encountered together with the generation of micro-cracks at the highly localized stress concentrated places. This rising and lowering characteristic of electrical conductivity is found to be most sensitive to cold-working deformation for the case of alloy-IV and almost insensitive for pure Cu, which, in turn, reflects the effectiveness of using the optimum level of deformation, especially for the alloy materials. It is noted here that the alloy material-IV contains higher number of alloying elements of different atomic size, which eventually leads to higher disorder of the atomic arrangement as well as higher mismatch in the localized strains developed in the crystal structure through work hardening.
7.3.3 Effect of thermal ageing on conductivity
It is well known that conductivity is affected significantly with the change of thermal state of a material, but what happens if a certain degree of temperature is maintained for a particular duration and the associated conductivity values are measured after removal
of the imposed thermal conditions is dealt through isochronal thermal ageing. Figures 7.2 (a), (b), (c) and (d) are representing the isochronal ageing effects on the conductivity of work hardened samples obtained through 0%, 25%, 50% and 75% cold rolling, respectively, against the variation of ageing temperature from 25oC to 500oC over a period of one hour. From these figures it is seen that material-I, i.e., pure Cu has not shown any noticeable change in conductivity for the variation of work hardening level as well as the ageing temperature. But material-II, -III and -IV have displayed considerable variations of conductivity as a consequence of isochronal ageing. However, without work hardening, the samples have shown minimum effect of isochronal ageing on conductivity. Material-II shows a fluctuating type of variation and material-III shows a little rising trend with higher initial slope for all four work hardening conditions. In contrast, material-IV shows initial falling and then rising trends against the rise of ageing temperature for 25% and 50% cold-rolled work hardening levels, but a gradual rising trend is encountered throughout for 75% cold-rolled work hardening level. Therefore, in an overall sense, the electrical conductivity of material-IV is found to be increased, but not that significant with higher ageing period together with ageing temperature.
Since stable micro-hardness values have been observed for the isochronal ageing at the temperature up to 250oC for as-cast, 200oC for 25% cold-rolled and 150oC for 50%/75% cold-rolled conditions, isothermal ageing is done at the temperatures of 150oC, 200oC and 250oC to observe the variation of electrical conductivity, and the results obtained are presented in figures 7.3, 7.4 and 7.5. These figures indicate that the conductivity values of material-I and -II remain almost unaffected at different ageing period for all work hardening conditions. But the conductivity of material-III and -IV are found to be fluctuating at the initial stage like transient variation and after thirty minutes of ageing the conductivity values are observed to be in steady state.
In overall perspective, direct correlation between electrical conductivity and hardness or strength has not been found for all work hardening and thermal ageing conditions.
Nonetheless, at the initial cold-rolled work hardening levels, the conductivity values of material-I, -II, -III and -IV are found positively correlated with hardness and strength having correlation coefficients of 0.73, 0.94, 0.87 and 0.95, respectively. However, the patterns of conductivity and hardness variations are different after ~35% deformation level and the correlation coefficients become negative with the values of -0.70, -0.78,-
0.80 and -0.89, respectively. But it is not applicable for thermal ageing because the hardness/strength values of all four sample materials have been decreased significantly with the increase of ageing temperature especially at 250oC or higher and ageing period.
On the contrary, the changes in electrical conductivity values against both isothermal and isochronal ageing have not been found to be significant. So, the electrical conductivity cannot be considered correlated with the hardness in respect of thermal ageing.
Fig. 7.2 Conductivity variation of copper and solder-affected copper alloys against isochronal ageing temperature (aged for 1 hour): (a) 0%, (b) 25%, (c) 50%, and
(d) 75% cold-rolled work hardening.
Fig. 7.3 Conductivity variation of copper and solder-affected copper alloys against ageing period for isothermal ageing at 150oC: (a) 0%, (b) 25%, (c) 50%, and (d) 75%
cold-rolled work hardening.
Fig. 7.4 Conductivity variation of copper and solder-affected copper alloys against ageing period for isothermal ageing at 200oC: (a) 0%, (b) 25%, (c) 50%, and (d) 75%
cold-rolled work hardening.
Fig. 7.5 Conductivity variation of copper and solder-affected copper alloys against ageing period for isothermal ageing at 250oC: (a) 0%, (b) 25%, (c) 50%, and (d) 75%
cold-rolled work hardening.
7.4 Differential Scanning Calorimetric (DSC) Analysis
According to the phase diagrams of Cu-Sn [33,36] or Cu-Pb [37] there is no strong phase transition of high Cu-Sn or Cu-Pb alloys within the temperature range of 25oC to 700oC. However, the results of calorimetric measurements carried out as an observance of thermo-chemical changes, i.e., the DSC results displayed in fig. 5.6 demonstrate the presence of transitions like behavior through two endothermic peaks visible in each curve. The first endothermic peaks of all four sample materials have been observed within the temperature range of 120oC to 180oC, which seem to be the results of evaporation of the bonded water, i.e., the effect of dehydration in sample materials. The location of the first endothermic peak has not been moved in DSC curve after cold-rolled work hardening as presented in figures 7.6 (a), (b), (c) and (d) for four sample materials.
The second endothermic peak of each DSC curve has occurred in between 340oC and 360oC, which seems to be the effect of recrystallization. It has shown the changes in location of the second peaks after the cold-rolled work hardening level of 75%
deformation for all four sample materials.
Fig. 7.6 (a) indicates that the first endothermic peak for material-I, i.e., pure Cu has occurred in between the temperature of 120oC and 140oC with the approximate indication at ~130oC against the heat inflow value of about 9.0 µV. The second endothermic peak of material-I at its as-cast and after 75% cold rolled work hardening conditions have been found to be at temperatures of ~348oC and ~320oC with the corresponding heat inflow values of 3.75 µV and 3.79 µV, respectively. It indicates that the cold-rolled work hardening has reduced the recrystallization temperature of pure Cu to a significant level [62,95]. It is because the energy is mainly converted into heat and a portion causes crystalline defects (dislocations) during cold working [19]. The more the number of defects are, the greater is the tendency to recrystallize. So, increased level of cold- working implies more driving force for the recrystallization to occur at early temperature.
Fig. 7.6 (b) signposts little sharpness of the first endothermic peak of material-II, i.e., Cu-Sn alloy in comparison to that of material-I, and it is shifted towards higher temperature (~170oC) than that of material-I as shown in fig. 7.6(a) as an effect of alloying with the addition of 1.134% Sn in Cu. The same figure also indicates the shifting of the second endothermic peak of material-II from around 357oC to 335oC after work
hardening, which demonstrates the change in its recrystallization temperature. However, no further significant change in the variation of thermo-chemical behavior is observed with the addition of Sn in Cu.
Fig. 7.6(c) shows the flatten shape of the first endothermic peak for material-III, which indicates that the addition of 1.197% Pb in Cu has changed the pattern of thermal capacity to a great extent. It depicts that material-III has less variation of energy flow for exothermic and endothermic reactions in comparison to material-I and -II. However, from the flat shaped variation of the heat flow curve, the first endothermic peak may be identified at ~150oC with the corresponding heat inflow of 8.15 µV. The second endothermic peak of the DSC curve for Cu-Pb alloy has been found to be shifted from around 355oC to 325oC, which notices the shifting of the recrystallization temperature as an effect of cold rolled work hardening.
Fig. 7.6 (d) indicates that the endothermic peaks of solder affected copper at its as- cast condition are at the temperature of around 160oC and 352oC with the corresponding heat inflow values of 8.1 µV and 4.0 µV, respectively. There is no change in the location of first endothermic peak after cold-rolled work hardening. However, the second peak has been found to be shifted to ~328oC after 75% cold rolled work hardening. Here, the common finding for all four sample materials is the lowering of the second endothermic peak temperature after work hardening. Therefore, high deformation level in cold- working may be considered as a contributing factor to lower the recrystallization temperature of copper based materials.
Fig. 7.6 DSC observation for as-cast and cold-rolled (75% deformation) samples from room temperature (25 oC) to 700oC with heating rate of 20oC per minute:
(a) material-I, (b) material-II, (c) material-III and (d) material-IV.
7.5 Summary
The electrical conductivity copper is found to be significantly affected by the inclusion of trace amount of lead or/and tin. As a result, the conductivity values of material-II, -III and IV are found to be reduced by 41.06%, 6.94% and 53.25% compared to the conductivity of copper at its pure state. The conductivity values of material-IV, i.e., solder affected copper can be increased to some extent through cold-rolling up to
~35% deformation level which is also identified to be the optimum value for recovering a portion of the loss of conductivity due to inclusion of solder elements in Cu. It is because the conductivity falls with further increase in cold work level. A post hardening thermal treatment shows that the alloys can offer a higher stable range of hardness against ageing temperature than that of pure Cu, while the electrical conductivity of the alloys is increased to a little extent by the treatment with higher ageing period.
DSC results indicate that the first endothermic peak of Cu is shifted to higher temperature due to alloying through the inclusion of SnPb-solder in it. DSC curves also reveal that the second endothermic peaks of all four copper based sample materials are shifted toward lower temperature as a consequence of cold-rolled work hardening and thereby the corresponding recrystallization temperatures of the samples have been decreased by about 20oC. The OEM micrographs, SEM images and corresponding EDX spectra are found to conform to the observed changes in micro-hardness and electrical conductivity of the solder affected copper alloys due to formation of intermetallic compounds.