Copper-based alloy having a high electrical conductivity and a high softening temperature for application in electronics

A method for forming supports for use in electronic components. A plate of copper-based alloy including from 0.1 to 1.0% by weight nickel, and from 0.005 to 0.1% by weight of phosphorus is melted and cast. The alloy includes fine precipitates of nickel phosphides throughout the copper matrix. The plate is subjected to a series of deformation operations including, rolling and intermediate annealing at a temperature in the range of 400.degree. to 600.degree. C., with the annealing temperature being maintained for two to four hours, thereby maximizing the production of fine precipitates of nickel phosphides within the alloy. After alloy formation, the plate is coated with a layer of nickel, cut into a desired shape, and secured to an electronic component.

DESCRIPTION OF THE INVENTION 
BACKGROUND AND SUMMARY 
The present invention relates to copper-based alloys for use in electronics 
for the manufacture of supports for components. 
Copper, which, as is well known, is an excellent conductor of electricity, 
is used in many applications, especially in electronics; it is used as a 
support for electronic circuit components (lead frame) for the most varied 
components and especially electronic chips. In the production of circuits, 
the components are generally brazed, adhesively bonded and/or crimped and 
then hot-coated with plastics material on the copper support which must 
thus be temperature-resistant and preserve its mechanical characteristics. 
Owing to this temperature resistance (restoration strength), copper-based 
alloys have been used; that enables the restoration strength to be 
increased while preserving a good conductivity. 
The temperature strength, or what is referred to as restoration strength, 
corresponds to a mechanism leading to the softening of the copper alloy by 
activation of dislocation annihilation by re-heating to a high 
temperature. Restoration resistance is characterised by the maximum 
duration (for example longer than 10 minutes) of maintenance at an 
elevated temperature (for example 450.degree. C.) after which the hardness 
of the metal remains higher than a predetermined value. 
The measured conductivity of the alloy, given as a percentage, corresponds 
to the 100% conductivity of pure copper. This percentage conductivity is 
called IACS conductivity. 
By way of example, the alloy Cu Sn 0.15, which is an alloy of copper and 
tin, is used. 
The copper supports used in electronics must not only offer good mechanical 
strength and good temperature strength but they must also exhibit 
excellent solderability and/or brazing suitability; to that end, the 
copper alloy is coated with a layer of nickel. This layer of nickel is 
applied to the alloy before cutting the products, such as supports. This 
results in a substantial amount of nickel-coated copper alloy waste, which 
is expensive to recover because it is necessary to use electrolysis to 
separate the copper from the nickel and to recover it.

DETAILED DESCRIPTION OF THE INVENTION 
The aim of the present invention is to improve copper-based alloys for use 
in electronics in order to obtain alloys having high temperature strength 
and high conductivity and facilitating the recovery of manufacturing 
waste. 
To that end, the invention relates to the use, for the manufacture of 
supports for electronic circuit components that are to be brazed, 
adhesively bonded and/or crimped at high temperature onto the support, of 
a copper-based alloy containing, as a percentage by mass, from 0.1 to 1% 
of nickel and from 0.005 to 0.1% of phosphorus, the remainder being copper 
or principally copper. 
According to the invention, such an alloy may, in addition, contain up to 
0.1% of iron and/or up to 0.5% of zinc. 
This copper-based alloy has a good conductivity generally higher than 80% 
IACS in the composition ranges proposed, and also an excellent temperature 
strength, that is to say, restoration resistance, associated especially 
with the addition elements: nickel and phosphorus. 
The copper-based alloy according to the invention is also very valuable 
from the economic point of view because it facilitates recycling of the 
waste occurring during the manufacture of supports or elements for 
electronics because, in this case, the alloy according to the invention is 
coated with a layer of nickel. The mechanical characteristics of this 
alloy are especially valuable. 
The alloys according to the present invention offer numerous advantages. 
For example, their electrical conductivity is very good. It is easy to 
obtain an electrical conductivity higher than 70% IACS. It is even 
possible, as demonstrated by the Examples hereinafter, to ensure a 
conductivity higher than 80% IACS by varying the addition of phosphorus as 
a function of that of nickel and iron, and by limiting the content of 
residual elements (zinc, . . . ): special production programmes are 
therefore to be observed in order to optimise the annealing cycles and the 
formation fine precipitates of Nickle Phosphides. 
The content of residual nickel and phosphorus in solution, after the most 
extensive precipitation possible, ensures very good resistance to 
restoration: softening is very slight, as demonstrated by the Examples 
hereinafter, when the alloy is maintained in a furnace even beyond 
450.degree. C. and would be negligible in the case of soldering, brazing 
or plastic encapsulation at temperatures of between 370 and 425.degree. C. 
The precipitates formed (of Ni.sub.5 P.sub.2 according to the most 
up-to-date thermodynamic calculations or more certainly of Ni.sub.2 P 
according to analyses effected by loss of energy in transmission 
microscopy) permit significant hardening of the alloys according to the 
present invention. At the same time, they increase resistance to stress 
relief. 
The alloys according to the present invention are inexpensive. They use 
only conventional addition elements. They also enable the nickel-coated 
copper waste to be recycled economically. Small amounts of impurities 
(zinc, silicon, . . . ) can be tolerated: according to known laws, they 
degrade the conductivity of the product. The marginal addition of other 
alloy elements, such as iron (up to 1000 ppm but preferably less than 100 
ppm) can permit acceleration of annealing and an improvement in mechanical 
characteristics while hardly affecting conductivity. 
The alloys according to the present invention are therefore especially 
suitable for electronic applications (grids, power components, . . . ) and 
would advantageously replace alloys such as Cu Sn 0.15. 
The alloy according to the invention can be manufactured by casting 
processes normally used for copper-based alloys. The particular process 
selected for casting the alloy has no particular critical influence on the 
product obtained. 
However, prior homogenisation of the alloy by dissolving all the alloy 
elements at high temperature (800.degree. C. or more) is very desirable, 
especially where, for example, iron is added. 
In order to obtain boards, it is possible, for example, to cast the alloy 
in strips, to mill it, then, after slight work-hardening, to subject it to 
homogenising annealing (from 800 to 850.degree. C. for approximately 1 
hour) followed by quench-hardening. It is also possible, and preferable, 
to cast the alloy in plates of conventional dimensions, and then first to 
hot-roll them (at from 650 to 1000.degree. C. depending on the alloy 
elements) to a thickness of a few millimetres and then to cold-roll them. 
The alloy can then be cold-rolled to the desired thickness with 
intermediate annealing operations. 
The greatest possible reduction, and at least 50%, is preferable between 
two consecutive annealing operations: the duration of each annealing 
operation is thus substantially reduced with an improved final 
conductivity. The optimum annealing temperatures are between 400 and 
600.degree. C., with maintenance at the annealing temperature for at least 
two hours and, if possible, four hours. Longer durations generally ensure 
greater conductivity, except in the unfavourable case of competitive 
precipitations of addition elements with the phosphorus, for example. 
The present invention will be explained hereinafter by means of two 
embodiments of copper-based alloys. 
The results of hardness and conductivity measurements are given in appended 
FIGS. 1 and 2. FIG. 1 is a diagram of temperature strength at 425.degree. 
C.; the time has been shown on the abscissa and the HV hardness has been 
shown on the ordinate. The diagram gives the graphs of Cu Sn, Cu Ni 0.4, 
Cu Ni 0.2 and the alloy FPG, that is to say, an alloy of copper containing 
from 950 to 1000 ppm Fe and from 330 to 370 ppm P. 
The test consisted in increasing the temperature to 425.degree. C. and 
remaining at that temperature for a period extending beyond the scale of 
the diagram. 
FIG. 2 shows the conductivity graphs for various IACS percentages, the 
abscissa representing the mass in ppm of Ni and the ordinate representing 
the mass in ppm of P in the copper-based alloy. 
EXAMPLE 1 
The alloys of this Example are prepared in the manner indicated 
hereinafter. Cuttings of copper-phosphorus alloys (Cu-b1, Cu-b2) coated 
with nickel are melted in a channel induction furnace: at the end of the 
melting process, on the basis of a spectrometer analysis, adjustment of 
the content of phosphorus enables the desired composition to be obtained. 
The molten mass is then maintained for a few minutes at the same 
temperature (approximately 1200.degree. C.) under a reducing cover of 
charcoal. Casting is effected in a water-cooled ingot mould measuring 
200.times.400 mm, for example. The composition of the alloys prepared for 
this Example is given in the following Table. 
______________________________________ 
Ni P Fe Zn 
______________________________________ 
Cu Ni 0.2 2060 305 -- 3200 
Cu Ni 0.4 4410 300 -- 800 
______________________________________ 
(All the contents are given in mass ppm.) 
The plates thus cast are reheated to a temperature higher than 840.degree. 
C. and then hot-rolled to from 200 to 13 mm. They can then, at a 
temperature higher than 600.degree. C., be either quench-hardened or not, 
as desired. The blank is then milled, cold-rolled to a thickness of 1.5 mm 
and then annealed under a hood with maintenance at 480.degree. C. for 4 
hours. The hardness in the annealed state is between 54 and 57 HV. The 
conductivities of the alloys Cu Ni 0.4 and Cu Ni 0.2 measured in this 
state are, respectively, 78.1% IACS and 79.4% IACS. 
The high content of residual zinc has a substantial effect on conductivity. 
On the basis of the known effect of zinc in solution on conductivity, it 
is possible to estimate that Cu Ni 0.2 and Cu Ni 0.4 alloys containing no 
addition element other than nickel and phosphorus at the contents 
indicated, would have conductivities of 83% IACS and 79% IACS, 
respectively. 
In that metallurgical state, after a fresh reduction by rolling of 20%, the 
conductivity hardly varies and the hardness reaches from 107 to 110 HV. It 
is equivalent to that obtained under the same conditions with a Cu Sn 0.15 
alloy. 
At this level of work-hardening, strip samples are annealed at different 
temperatures from 360 to 480.degree. C. for 10 minutes. The fall in 
hardness with temperature in the case of the Cu Ni 0.4 alloy is compared 
with that measured for a Cu Sn 0.15 alloy. The softening temperature of 
the Cu Ni 0.4 alloy is higher than 460.degree. C. when that of the Cu Sn 
0.15 alloy is of the order of 440.degree. C. 
EXAMPLE 2 
New alloys, according to this Example, were prepared in the manner 
indicated hereinafter. High-purity copper is melted in a channel induction 
furnace: the introduction of alloy elements is effected in the form of 
pure nickel, copper phosphide 85-15 and metal silicon until the desired 
composition is obtained. The molten mass is then maintained at the same 
temperature (approximately 1200.degree. C.) under a charcoal cover. The 
composition is gradually modified in order to obtain a wide range of 
different alloys. Billets are taken from the bath and cast for each new 
composition (diameter: 25 mm, height: 40 mm). The composition of each 
alloy prepared for this Example is within the ranges indicated in the 
following Table. 
______________________________________ 
Ni P Fe Si 
______________________________________ 
minimum 2870 &lt;10 &lt;10 0 
maximum 4300 910 80 100 
______________________________________ 
(All the contents are given in mass ppm.) 
Each of the billets is homogenised by being maintained at 850.degree. C. 
for 1 hour and then being quench-hardened in water. In that state, they 
are deformed by more than 70% (reduction in height) by compression in a 
hydraulic press. They are then annealed in such a manner that, for each 
alloy, the maximum conductivity is obtained. Correlations were then 
established between these measured conductivity values and the composition 
of the alloys. The correlations also take into account the previous 
characterisations, indicated in Example 1. 
Lines of the same conductivity can then be plotted in the plane of the 
nickel and phosphorus contents, without any other addition element, in the 
case of pure copper-nickel-phosphorus alloys. The results are indicated in 
FIG. 2.