Copper-based metal alloy of improved type, particularly for the construction of electronic components

A new copper-based alloy is described, the principle characteristic of which lies in having two different age-hardening temperature intervals corresponding to which significantly different electrical and mechanical characteristics are obtained from an alloy of the same composition; the alloy is composed, in parts by weight, of from 0.05 to 1% Mg, from 0.03 to 0.9% P and from 0.002 to 0.04% Ca, the remainder being Cu with possible very small additions of other alloying elements such as Sn, Zr, Mn and Li.

BACKGROUND OF THE INVENTION 
The present invention relates to a new copper-based alloy, or rather one 
containing more than 90% by weight of copper, particularly adapted for the 
construction of components for the electronics industry thanks to its 
mechanical and electrical characteristics. It is known that numerous 
electronic components which are heavily stressed both mechanically and 
thermally, such as parts of switches, "lead frames" (that is the frames 
which support the semi-conductor plates constituting microprocessors 
and/or memory elements) serial bus terminal support plates, thermostat 
contacts and the like have to be made with alloys having, simultaneously, 
high ductility, high durability and mechanical strength, and high thermal 
and electrical conductivity; today there exist on the market very many 
copper-based alloys which, however, all have the inconvenience of being 
adapted only to a specific application for which they have been 
appropriately developed, and consequently each is only suitable for the 
construction of one or a few of the above-listed components, which is 
entirely unsatisfactory. Moreover, a large number of such alloys contain 
cadmium so that their manufacture involves heavy environmental pollution; 
moreover, the majority of such alloys are expensive, either because of the 
particularly rare elements used or, above all, because of the difficult 
processes for obtaining these, which require an accurate deoxidation 
preferably effected by means of accurate proportioning of particular 
deoxidising components. It is in fact known that very small percentages of 
oxygen drastically lower the thermal and electrical conductivity of such 
alloys and, above all, make soldering them impossible because of reactions 
which lead to hydrogen embrittlement; it is also known that, on the other 
hand, the addition of deoxidising elements having a high affinity for 
oxygen, such as phosphorus, involves the problem of accurately 
proportioning the content of these in dependence on the anticipated oxygen 
content if a drastic reduction in the conductivity by formation of solid 
solutions and/or phosphates is to be avoided. U.S. Pat. No. 3,677,745 
resolves this latter problem in an economic manner by means of the 
addition of small percentages of magnesium to the alloy; this element 
combines with the excess phosphorus forming an intermetallic compound; 
this drastically limits the quantity of free P and/or Mg in the matrix and 
therefore avoids a drop in the conductivity even in the presence of 
imprecise proportions of P; moreover the intermetallic compound which 
forms renders the alloy subject to age-hardening by precipitation which 
improves its mechanical characteristics. However, the alloy the subject of 
the said U.S. Patent simply shifts the problem of the correct proportions 
from the P to the Mg, with the single advantage that the limits between 
which the proportion of magnesium can vary with respect to the 
stoichiometric proportion without detrimentally affecting the conductivity 
are very much wider than those of the P and can be further widened by also 
adding to the alloy silver (up to 0.2%) or cadmium (up to 2 %). These 
further additions, always present in alloys produced commercially on the 
basis of the Patent, evidently involve the disadvantages of high cost of 
primary materials and the above-mentioned risk of pollution. Moreover, 
alloys according to U.S. Pat. No. 3,677,745 do not resolve the technical 
problem of making available an alloy adapted to different uses in the 
electronic components field; for this reason users of alloys known today 
must, for each type of component to be produced (lead frame, contact, 
etc.) arrange to store an alloy of particular chemical composition, 
different from that of the alloys utilised for other components. This 
evidently involves the impossibility of effecting economies of scale and 
complicates the management of production and supplies. 
SUMMARY OF THE INVENTION 
The object of the present invention is just that of providing a new 
copper-based metal alloy which has characteristics of conductivity and 
mechanical strength which are variable according to the requirements of 
the user, with the same composition, within limits sufficiently high to 
satisfy requirements which today are satisfied only by alloys of different 
composition, and at the same time present maximum values of mechanical 
strength and conductivity satisfactory for the electronic applications, 
high ductility and solderability, reduced cost, great ease of production, 
and which does not make use of cadmium. 
This object is achieved by the invention in that it relates to a 
copper-based metal alloy, particularly for the construction of electronic 
components, characterised by the fact that it contains, in parts by 
weight, from 0.05 to 1% magnesium, from 0.03 to 0.9% phosphorus and from 
0.002 to 0.04% calcium, the remainder being copper, including possible 
impurities, the ratio by weight between magnesium and phosphorus contained 
in the alloy lying between 1 and 5 and, in combination, the ratio by 
weight between magnesium and calcium contained in the alloy lying between 
5 and 50. 
An alloy having a composition lying within these limits in fact has, as has 
been found experimentally by the applicant, high values of thermal and 
electrical conductivity, high mechanical strength imparted by optimum 
combinations of resistance to breakage and yield under tension and 
hardness, high deformability, excellent behaviour when hot, absence of 
brittleness, immunity to stress corrosion and hydrogen embrittlement, good 
solderability and ability to be subject to heat treatments for producing 
segregation at the edge of the grains of finely sub-divided intermetallic 
compounds such that the alloy is subject to hardening by age-hardening; 
surprisingly, moreover, such an alloy possesses the unusual characteristic 
of having two different precipitation temperature intervals corresponding 
to which the alloy has, with absolutely identical chemical composition of 
the alloying elements, completely different mechanical and conductivity 
characteristics; with substantially the same conductivity (that is within 
narrow intervals of variation thereof), moreover, the alloy according to 
the invention, in both the different physical states following the 
age-hardening treatment in correspondence with one or other of the 
precipitation temperature intervals respectively, has the capacity of 
varying its mechanical characteristics over a wide range in dependence on 
its state of work-hardening consequent on rolling or cold drawing with 
different degrees of percentage reduction of the section.

DETAILED DESCRIPTION OF THE INVENTION 
The alloy according to the invention is substantially a metal alloy having 
a copper-based matrix which is present in the alloy in percentages by 
weight greater than 99%, and containing a new combination of alloying 
elements constituted by magnesium (Mg), phosphorus (P) and calcium (Ca) in 
special proportions able to make them interact in such a way as to form 
between them and with the copper, binary, tertiary and quaternary 
intermetallic compounds, the possibility of the existence of these latter 
being brought to light for the first time by the present invention; the 
alloy advantageously also contains tin, in percentages by weight variable 
between about 0.03% and 0.15%, preferably close to the upper limit, and 
can moreover contain, as well as the inevitable traces of various 
elements, in particular iron, which constitute, however, non-dangerous 
impurities, small quantities of silver and/or zirconium, respectively in 
percentages of the order of 0.01-0.05 and 0.01-0.06% by weight, for the 
purpose of increasing the firing temperature, and small quantities (not 
greater than 0.01% by weight) of lithium and/or manganese utilised as 
desulphurising elements. The alloy according to the invention thus has a 
nominal composition by weight constituted by 0.22% Mg, 0.20% P, 0.01% Ca 
and 0.10% Sn, the remainder being Cu, including possible impurities; these 
nominal percentages of the said alloying elements can vary within 
relatively wide limits without altering the above-described novel 
characteristics of the alloy, and more particularly the magnesium can vary 
between 0.05 and 1% by weight, the phosphorus can vary between 0.03 and 
0.90% by weight, and the calcium can vary between 0.002 and 0.040% by 
weight, whilst the tin can vary between the limits already explained, but 
preferably never less than 0.08% by weight. Although the previously 
described new and appreciable characteristics of the alloy according to 
the invention are also obtainable without the introduction of the tin, so 
that the invention refers essentially to a quaternary alloy Cu-Mg-P-Ca, 
penternary alloys Cu-Mg-P-Ca-Sn must also be considered as part of the 
invention it being surprisingly found that the tin not only considerably 
increases the hot flowability and castability of the alloy of the 
invention, but can also directly participate in the formation of the 
intermetallic compounds on which the superior characteristics thereof 
depend; these latter are improved by the tin, and the range of possible 
variation in the proportions of the alloying elements, in particular the 
deoxidising phosphorus and dephosphorising calcium are increased with 
respect to the basic quaternary alloy free of tin. 
The alloy according to the invention arises from the research conducted by 
the Applicant starting from U.S. Pat. No. 3,677,745, from the tertiary 
state diagrams of Cu-Mg-Sn and Cu-Mg-Ca alloys developed on the basis of 
the studies of Bruzzone (Less-Common Metals, 1971, 25, 361) and of 
Venturello and Fornaseri (Met. Ital., 1937, 29, 213) and on the studies of 
W THURY (Metall, 1961, Vol. 15, Nov. PP. 1079-1081) which have 
demonstrated how copper can be deoxidised by additions of phosphorus 
without influencing the conductivity by means of the elimination of the 
excess phosphorus with additions of calcium, which combines with the 
phosphorus to give calcium phosphate which does not reduce the 
conductivity. On the basis of this state of the art the applicant's 
technicians, encouraged by the theoretical possibility for Ca and Sn of 
forming intermetallic compounds with Mg and Cu, sought to produce copper 
alloys having a high strength and conductivity and good solderability by 
means of the addition to copper, preliminarily deoxidised according to the 
method of THURY with the addition of P and Ca, of Mg and/or Sn in the hope 
that one or both of these alloying elements would be capable of bonding 
with the possible excess of calcium to form intermetallic compounds with 
this or with the copper of the matrix; in this way it was hoped to make 
the resulting alloy susceptible of hardening by age-hardening, thus 
obtaining an increase in the mechanical strength, and simultaneously it 
was hoped to resolve, without recourse to precious alloying elements such 
as silver, the problem of the proportioning of the deoxidising elements. 
Limited to this latter aspect, in fact, the deoxidising mechanism effected 
in U.S. Pat. No. 3,677,745 by means of P and Mg was not satisfactory in 
that, as already emphasised, it did not overcome the problem of monitoring 
the proportioning of the deoxidising agents, but simply rendered them less 
serious, especially in the presence of Ag in the alloy. On the other hand, 
the use of Ca in place of Mg as dephosphorising agent with respect to 
residual P after the deoxidation already on its own appeared more 
advantageous in relation to the conservation of a high conductivity, and 
in any case offered the further theoretical possibility of combining the 
two methods by means of the elimination of the residues with an addition 
of Mg, which could offer the same advantages offered in the said U.S. 
Patent by the addition of silver or cadmium. Experimental tests made by 
the applicant have, on the other hand demonstrated that not only have the 
expected results been obtained, but that the interaction between the 
alloying elements was very much more than expected and involved, before 
the precipitation treatment, or rather already upon solidification of the 
alloy after fusion, provided that certain proportions between the 
ingredients of the alloy was respected, the formation of entirely 
unexpected and completely unforeseeable intermetallic compounds such as a 
quaternary CuMgPCa compound which has been detected by electron microscope 
in transmission and which has dimensions of the order of 0.4-0.5 microns; 
such compounds were also accompanied by the presence of sub-microscopic 
particles of CuP, CuPMg, PCa and CuMg detected in the metal matrix with a 
scanning electron microscope having an enlargement of 6-9000. Accompanying 
the presence of the said intermetallic compounds before age-hardening 
treatment it was found that there was a surprising behaviour of the alloy, 
which is entirely new and unexpected, that is to say this had two 
age-hardening temperatures, or rather temperature intervals, which were 
different from one another. In substance the applicant has brought to 
light that, in the presence of such unexpected compounds, due to the 
particular composition of the alloy, this became susceptible of being 
subject not to one, but to two different age-hardening treatments at 
different temperatures, following which the alloy assumed completely 
different final characteristics whilst having entirely the same initial 
composition. Such entirely new and surprising behaviour in a copper-based 
alloy makes it possible to effect great economies of scale, in particular 
in the electronic component industry; in fact, the alloy of the invention, 
thanks to the said characteristic, is able, on its own, to satisfy 
requirements which are even very different from one another simply by 
subjecting it to a different heat treatment, a treatment which because of 
its simplicity can be performed even by the final user who, therefore, can 
store raw elements which have not been age-hardened and, in dependence on 
the variable requirements, effect on these an artificial age-hardening at 
different temperatures and a subsequent cold, more or less forceful 
deformation working in such a way as to obtain a final product having the 
characteristics desired from time to time, something which has been 
obtainable until now only by using different alloys of different chemical 
composition which were absolutely not interchangeable as to the final use. 
This fundamental result of the invention is obtained not only by realising 
a copper alloy having the above-described content of Mg, P and Ca, but 
also by taking care that the ratios between these alloying elements 
remains within certain limits, beyond which the alloy loses its particular 
characteristics; in particular the ratio by weight between the magnesium 
and phosphorus content in the alloy must lie between 1 and 5 and, 
simultaneously as well as respecting this primary ratio the ratio by 
weight between the magnesium and calcium content in the alloy must lie 
between 5 and 50. The improved results are obtained with a content of 
calcium in the alloy lying between 0.002 and 0.02% by weight and with an 
Mg/P ratio by weight lying between 1 and 3 in combination with a ratio by 
weight of Mg/Ca lying between 10 and 20. It is supposed that these 
limitations correspond to the necessity of determining within the alloy 
particular stoichiometric ratios between the components in correspondence 
with which, and only with which, the first discussed quaternary 
intermetallic compounds are formed which, it is believed, determines 
whether the alloy has imparted to it the capacity of assuming different 
mechanical characteristics in correspondence with different age-hardening 
temperatures; the presence of CaP, CuMg and CuP before the precipitation 
is, in fact, normal, whilst the presence of CuMgP and CuCaMgP is entirely 
unexpected and can be considered to be due to a partial precipitation 
which has already taken place during the hot working. Consequently it is 
justified to think that during the precipitation which takes place upon 
age-hardening the CaP reacts with CuMg to give CuCaMgP finely dispersed at 
the edges of the grains. For the rest, the copper alloy according to the 
invention is produced in a conventional manner by means of fusion and 
subsequent casting, then working the solidified alloy by means of rolling 
or hot extrusion at a temperature lying between 860.degree. and 
890.degree. C. and subsequent working the alloy by means of rolling or 
cold drawing to obtain a reduction in section lying between 50% and 80%; 
then artificial age-hardening of the alloy is effected by means of a 
precipitation heat treatment which, as opposed to the methods of 
production used for conventional alloys, consists in maintaining the 
alloy, for a sufficient time (1 or 2 hours) at a temperature lying within 
a selected interval either between 365.degree.-380.degree. C. or between 
415.degree.-425.degree. C. depending on whether it is desired to obtain 
improved mechanical or electrical characteristics respectively. 
EXAMPLE I 
In a gas crucible furnace having a crucible of the silicon carbide type, of 
a capacity of about 100 Kg experimental melts were made with loads of 70 
Kg of 99.9 ETP copper melted under a covering flux of borax with 
successive casting in water cooled ingot moulds having a diameter of 220 
mm; subsequently they are deoxidised by the addition of 1.1 Kg of copper 
phosphate (85% by weight Cu and 15% by weight P) positioned by means of a 
tool onto the bottom of the crucible, and then there are added two hg of 
Mg and 7 gr of Ca. Having taken samples for analysis casting into ingot 
moulds proceeds and subsequently hot rolling (indicated for brevity as HR) 
of the ingots down to a thickness of 11 mm operating at a temperature 
lying between 860.degree. and 890.degree. C.; after milling or "scalping" 
of the thus-obtained ingots to remove the oxidised layer these are 
subjected to different working cycles comprising a cold rolling (indicated 
for brevity as CR) effected in such a way as to cause a reduction in 
section lying between 50% and 80% and a possible artificial age-hardening 
heat treatment consisting in holding it for a determined time at a 
temperature lying between 365.degree. and 425.degree. C. The thus-obtained 
ingots were finally subjected to hardness tests (Vickers method 100 
gr./30") and standard conductivity tests according to the IACS 
(International Annealed Copper Standard) rules expressing the conductivity 
as a percentage of that of the IACS test strip at 20.degree. C., which, as 
is known, presents a resistivity of 1.7241 microhms-cm. The results 
obtained are plotted in table I and demonstrate the capacity of the alloy, 
with the same chemical composition, to assume different physical and 
mechanical characteristics according to the type of treatment. The 
capacity of the alloy to resist softening when hot has also been 
investigated; the results obtained (Vickers micro hardness after 1 hour at 
the various temperatures) are plotted in FIG. 1. 
TABLE 1 
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Electrical 
Conductivity 
Working % IACS Hardness HV 
______________________________________ 
HR 60 70-90 
HR + CR 67% 56 130-150 
HR + CR 67% + 365.degree. C. .times. 1 h 
68 155 
HR + CR 67% + 380.degree. C. .times. 1 h 
71 155 
HR + CR 67% + 400.degree. C. .times. 1 h 
78 96.5 
HR + CR 67% + 415.degree. C. .times. 1 h 
81 88 
HR + CR 67% + 425.degree. C. .times. 1 h 
81 87.6 
HR + CR 67% + 435.degree. C. .times. 1 h 
81 86.7 
HR + CR 67% + 450.degree. C. .times. 1 h 
81 84.6 
HR + CR 85% 52 160-170 
HR + CR 85% + 415.degree. C. + 2 h 
80 92 
HR + CR 85% + 425.degree. C. + 2 h 
82 90 
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EXAMPLE II 
Operating as in Example I, but in an industrial induction furnace having a 
capacity of 4 tonnes and associated with a semi-continuous casting 
position and proportionately adapting the quantities of copper and 
alloying elements to the different capacity of the furnace, ingots are 
obtained which are hot rolled at a temperature of 870.degree. C. down to a 
thickness of 11 mm throughout; then the rolled ingots thus obtained are 
further cold rolled with a reduction in section of 50%, obtaining a rolled 
ingot of 5.5 mm in thickness; this, after having taken samples, is 
separated into two parts respectively indicated A and B and subsequently 
treated in an electric furnace with a heat cycle involving two hours of 
heating, two hours of remaining at the temperature and five hours of 
cooling; the part A is treated at 425.degree. C. whilst part B is treated 
at 370.degree. C. Each part, after the heat treatment, is further 
subdivided into sub groups indicated with the numerals 1, 2 and 3; the sub 
groups 1 are cold rolled with a reduction in section of 20% in such a way 
as to produce a mild work-hardening; the sub groups 2 are rolled to a 45% 
reduction in section in such a way as to obtain a greater work-hardening 
(semi-hard state), whilst sub groups 3 are rolled to a 98% reduction in 
such a way as to make the rolled ingot strongly work-hardened (hard 
state). Samples of parts A and B are taken before the further rolling and 
from each sub group 1, 2 and 3 after rolling, and subjected to the normal 
tests of mechanical strength and conductivity. The results obtained are 
plotted in Tables II and III. 
TABLE II 
______________________________________ 
Characteristics of the alloy after age-hardening 
Type A Type B 
______________________________________ 
Electrical Conductivity (*) 
80% IACS 70% IACS 
Thermal Conductivity (Kcal/hm.degree. C.) 
274.7 240.3 
Density (kg/dm.sup.3) 
8.796 8.796 
______________________________________ 
(*)Expressed as a percentage of the conductivity of the International 
Annealed Copper Standard test strip at 20.degree. C. 
TABLE III 
______________________________________ 
Characteristics of the alloy in different 
physical states 
Resist- 
Test ance to Yield Number Electrical 
Strip 
tension strength of alter- 
Conductivity 
Type N/mmq N/mmq A% HV nate folds 
% IACS 
______________________________________ 
A 1 350 260 21 100 20 80 
A 2 460 420 8 140 15 78 
A 3 550 510 2 160 10 76 
B 1 472 428 15 150 26 70 
B 2 550 480 4 170 15 68 
B 3 710 650 13 190 6 63 
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EXAMPLE III 
Operating as in Example II there are produced three tons of an alloy having 
the following percentage composition by weight: 
EQU 0.25% Mg 0.20% P 0.01% Ca 0.10% Sn Remainder Cu 
The alloy produced is subdivided into two parts indicated "Type A" and 
"Type B" and subjected to different rolling and age-hardening cycles 
operating as in Example II; the resulting rolled ingots were then tested 
as in Example II and the results obtained plotted in graphical form and 
compared with the performances, again expressed in graphical form, of some 
of the principle copper alloys for electronic use at present on the 
market; the graphic result is plotted in FIG. 2; from this it can be 
appreciated that the alloy of the invention with absolutely the same 
chemical composition, can assume different physical characteristics 
according to the type of working to which it is subjected ("Type A" and 
"Type B" parts) as to find itself occupying positions only covered by 
known alloys having a completely different chemical composition (and not a 
different treatment). In particular, the alloy of the invention worked 
according to the cycle indicated in Example II for "Type A" and indicated 
with the reference LMI 108 A is close in performance to that of the alloy 
Wieland K72 (0.3 Cr- 0.15 Ti-0.02 Si-Cu), whilst the same alloy, worked 
according to the cycle indicated in Example II for "Type B" and indicated 
with the reference LMI 108 B has a performance close to that of the alloy 
Olin C197 (0.6 Fe-0.05 Mg-0.20 P-possible 0.23 Sn-Cu). 
EXAMPLE IV 
Operating exactly as in Example I there are prepared alloys of different 
chemical composition to test the influence of the content of the various 
alloying elements; the samples produced and subjected first to a hot 
extrusion at 870.degree. C. in such a way as to bring it down to a 
diameter of 24.5 mm and then cold drawing to bring it down to a diameter 
of 14.5 mm are then age-hardened at different temperatures and then tested 
with a standard conductivity test and with a Vickers hardness test; the 
results obtained are indicated in Table IV. 
TABLE IV 
______________________________________ 
Influence of the alloying elements 
Alloying Elements (% by weight) 
(Remaining Cu 99.9 ETP) 
Heat Conduct- 
Mg P Ca Sn Ag Treatment 
ivity HV 
______________________________________ 
0.22 0.20 0.0056 0.15 0.03 365.degree. C. .times. 1 h 
67 155 
0.22 0.20 0.0056 0.15 -- 365.degree. C. .times. 1 h 
66 155 
0.22 0.20 0.0070 0.08 -- 365.degree. C. .times. 1 h 
69 155 
-- 0.20 0.02 -- -- 365.degree. C. .times. 1 h 
88 50 
0.20 0.20 0.02 -- -- 365.degree. C. .times. 1 h 
68 154 
0.20 0.20 0.02 -- -- 380.degree. C. .times. 1 h 
71 154 
0.20 0.20 0.02 -- -- 415.degree. C. .times. 1 h 
81 87.5 
0.20 0.20 0.02 0.10 -- 415.degree. C. .times. 2 h 
82 88 
0.29 0.22 0.0258 0.10 -- 415.degree. C. .times. 2 h 
81 88 
0.22 0.25 0.025 0.10 -- 380.degree. C. .times. 1 h 
74 155 
0.22 0.25 0.025 0.10 -- 415.degree. C. .times. 1 h 
75 152 
0.22 0.18 0.05 0.10 -- 380.degree. C. .times. 1 h 
71 151 
0.22 0.18 0.05 0.10 -- 415.degree. C. .times. 1 h 
71 149 
1 0.90 0.04 0.15 -- 380.degree. C. .times. 1 h 
72 155 
1. 0.90 0.04 0.15 -- 415.degree. C. .times. 1 h 
81 90 
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