Solder composition

A solder composition for bonding a semiconductor die to a plated or unplated metal package member. In one embodiment the solder composition comprises, in weight percent, 5-8 copper, 20-40 silver, and the balance tin. Such a composition is particularly efficacious for bonding to copper and copper alloy package members. A further embodiment particularly efficacious for bonding to nickel and nickel alloy members further comprises the addition of 0.5-3.0 weight percent selenium.

BACKGROUND OF THE INVENTION 
This invention relates to a solder composition and more particularly to 
solder compositions suitable for bonding a semiconductor die to plated and 
unplated metal package members. 
In the fabrication of semiconductor devices including transistors, diodes, 
integrated circuits, and the like it is conventional to fabricate a 
semiconductor die and then to attach that die to a metal package member or 
header. The metal package member ultimately serves for mounting support, 
protection, electrical connection, and dissipation of heat generated in 
the operation of the device. The attachment is accomplished by using an 
epoxy resin or a metal alloy solder. Epoxy resin is generally 
unsatisfactory in most applications because of its inherently high 
electrical resistance and low thermal conductivity. The metal and metal 
alloy solders used for die attachment are conventionally divided into hard 
solders and soft solders. The hard solders, typically gold alloys, form 
strong bonds which have excellent thermal and electrical properties; but 
the high strength of the hard solder can cause the semiconductor die to 
fracture or the die and metal member to deform elastically under thermal 
stress because the hard solder transfers the stress to the die without 
plastic deformation in the die bond. The conventional soft solders, 
typically lead or tin alloys, are plastic enough to accommodate the 
thermal expansion mismatch between the die and the package, but are 
susceptible to metal fatigue after repeated temperature cycles. Such metal 
fatigue can result in device reliability problems. Thicker layers of the 
soft solder can reduce the stress in the solder and lessen the fatigue 
problem but only at the expense of undesirably higher thermal resistance. 
Intermediate solders such as the silver-tin-antimony alloys disclosed in 
application Ser. No. 788,954 assigned to the assignee of the present 
application and filed Apr. 19, 1977 now U.S. Pat. No. 4,170,472, have 
characteristics intermediate between the hard and soft solders and are 
able to overcome many of the problems attendant with the prior art 
solders. But all of these solders, hard, soft, or intermediate, require 
special surfaces to which to bond. This usually entails, for example, the 
plating of the package member with a layer or layers of gold, silver, 
nickel, or the like. Similarly, the back of the die itself is covered, 
usually by evaporation, with a layer of a suitable metal such as gold or 
silver. These special metals on the package and on the die are required 
because the solders employed either will not wet or will not adequately 
wet, the conventional package materials under standard bonding conditions. 
This requirement of specially plated packages greatly adds to the expense 
of the package both because of additional processing steps required and 
because of the expense of the plating metal itself. 
Accordingly, a need existed to provide solder alloy compositions which 
would overcome the problems of prior art compositions to produce a high 
yielding, low cost composition and method for the attachment of 
semiconductor die to metal package members. 
It is therefore an object of this invention to provide an improved solder 
alloy composition for the attachment of semiconductor die to plated or 
unplated metal package members. 
It is a still further object of this invention to provide a solder 
composition for semiconductor die attachment having desirable electrical, 
thermal, and thermal fatigue resistant properties. 
SUMMARY OF THE INVENTION 
In one embodiment of the invention, a solder alloy composition is provided 
which comprises, in weight percent 5-8 copper, 20-40 silver, and the 
balance tin. This solder composition is particularly suited to the bonding 
of semiconductor die directly to copper based metal package members. In 
another embodiment of the invention the solder alloy composition further 
comprises about 0.5-3.0 weight percent selenium to render the composition 
particularly effective in bonding semiconductor die to nickel based metal 
package members. By using either composition the semiconductor die can be 
attached to the plated or unplated metal package member by applying the 
composition to either the package member or to the back of the die and by 
heating to a temperature of approximately 400.degree.-500.degree. C. to 
cause the solder composition to flow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The sole FIGURE shows a semiconductor die 10 which is to be attached to a 
metal package member or lead frame 12. Die 10 is to be attached using a 
solder alloy composition shown here as a solder preform 14. The die can 
be, for example, a silicon power transistor or other semiconductor device. 
Lead frame 12 is typically formed of copper, a copper alloy, or a nickel 
alloy. The lead frame shown is representative of a wide variety of lead 
frames of the TO-220 type. The die is attached to the lead frame, leads 
are bonded to the device electrodes, and the device and portion of the 
lead frame are encapsulated in plastic. The form of the package is not 
important to the invention and can be the plastic package as described, a 
metal can package such as the TO-3, a metalized glass frit or ceramic, or 
the like. The lead frame can be plated or unplated, but by using a solder 
composition in accordance with the invention, plating is not necessary. 
In accordance with the invention a solder alloy composition has been 
developed which provides an intermediate solder (that is, one having 
properties intermediate between the known hard and soft solders) which can 
be used for attaching die to either plated or unplated metal package 
members. The solder is based on copper, silver, tin ternary alloys with 
the possible addition of selenium to further enhance the wetting on nickel 
based package parts. 
The solder alloy composition is tin based to provide a low temperature 
alloy. By low temperature alloy is meant an alloy having a melting point 
between about 200.degree. C. and 500.degree. C., a temperature range 
compatible with semiconductor devices. Silver is added to the tin to 
provide an alloy which has improved wetting. Especially on copper, the 
wetting was found to increase slowly with silver content up to about 20 
weight percent and then to rise rapidly between 20 to 40 weight percent. 
The addition of silver also alters the kinetics of wetting. Pure tin tends 
to ball up upon melting and the addition of silver reduces this tendency. 
The length of the spreading period also increases with the silver content. 
When the silver content is smaller than 20 weight percent the solder 
spreads out with a series of surges before saturating. When the silver 
content is between 20 and 40 weight percent, continuous spreading is 
observed up to saturation. 
To the tin-silver binary alloy is added about 5-8 weight percent of copper 
to improve the mechanical strength of the alloy. The alloy properties are 
optimized when the ratio of silver to copper is in the range of about 
3.9-4.4 to 1 and especially when the ratio is about 4.3 to 1. 
To further improve the wetting characteristics of the solder alloy, 
especially on nickel based package parts, a small amount, about 0.5 to 3.0 
weight percent, of selenium is added to the alloy. The additional selenium 
has little effect when bonding a semiconductor die to a copper based 
package, but has an appreciable desirable effect when bonding to a nickel 
based substrate. 
One method for evaluating the wetting behavior of the various solder alloys 
is to observe the behavior of the solder when melted on a substrate under 
simulated bonding conditions. The various solders were formed into 
preforms and melted on the various substrate materials under controlled 
ambient conditions. To evaluate the wetting the normalized areal spread 
(NAS) was determined. The normalized areal spread is determined by first 
measuring the spread of the preform after melting and then normalizing 
this with the maximum cross sectional area of a sphere which possesses the 
same volume as the solder. The greater the wetting ability of the solder 
alloy, the greater will be the NAS. 
The following table compares the wetting behavior as measured by NAS for 
solder alloy compositions in accordance with the invention to various well 
known hard and soft solders. The various alloys have been identified by a 
letter designation. The alloys B, C, Bl, and Cl are illustrative of the 
invention, but the invention is not to be construed as limited to these 
examples. Alloys S, T, and U are hard solders and V and W are well known 
soft solders. The wetting behavior is shown on substrates of copper, alloy 
194 and alloy 155 which are copper based alloys, and on nickel, Kovar and 
alloy 42 which are nickel based alloys. 
______________________________________ 
N.A.S. 
Alloy Al- Al- Al- 
Desig- Composition 
loy loy loy 
nation (wt. %) 194 155 OFHC Ni Kovar 42 
______________________________________ 
S Au-12 Ge 8 8 0 0 0 
T Au-20 Sn 7 3 5 2 0 0 
U Au-3 Si 0 0 0 0 0 0 
V Pb-5 Sn 2 2 2 4 2 2 
W Pb-5 In- 0 0 0 4 0 0 
2.5 Ag 
B Sn-26 Ag- 18 20 20 3 2 3 
6.8Cu 
C Sn-35 Ag-8Cu 
30 25 22 4 2 4 
B1 Sn-25.5 13 13 12 5 2 6 
Ag-6.8Cu- 
0.5Se 
C1 Sn-34.5 12 13 12 7 5 7 
Ag-8Cu-0.5Se 
______________________________________ 
In almost every instance the alloys in accordance with the invention are 
far superior to either the hard or soft solder alloys. Alloys B and C 
without any selenium are best on copper and copper based substrates, while 
Bl and Cl each containing 0.5 weight percent selenium are superior on the 
nickel and nickel based substrates. 
Although the amount of selenium added to the alloy composition seems to be 
a small amount, it is important, especially on the nickel based 
substrates. The selenium has little effect on the wetting of copper based 
substrates if the amount of selenium is kept small. The addition of 3 
weight percent of selenium, for example, is found to reduce the NAS on 
copper based substrates by almost 50 percent. Including 0.5 weight percent 
of selenium in the alloy composition, however, retains 90 percent of the 
wetting ability. On nickel based substrates the effect of the selenium is 
much more evident. Adding about 0.5 weight percent of selenium to alloy C 
to form alloy Cl increases the NAS on a Kovar substrate by up to 300 
percent and on other nickel based substrates by up to 100 percent. The 
addition of more than 3 weight percent of selenium, however, causes a 
decrease in the wetting ability even on the nickel based substrates. From 
the standpoint of wetting ability, the best composition is found to 
contain between 0.5 and 1.5 weight percent of selenium. 
In accordance with the invention the improved solder composition can be 
used for the attachment of a semiconductor die to a metal or metalized 
package member. Oxides, oils, grease, and other contamination are almost 
always present on the metal substrate surface. The oils grease and other 
contamination can be removed by cleaning in a detergent solution and by 
subsequent degreasing in alcohol or other organic solvent. The substrates 
are then dried and stored in clean, non-oxidizing ambient until needed. 
Further cleaning with acids to remove the oxide film can improve the 
wetting characteristics but is not essential with the solder alloys in 
accordance with this invention. Also, fluxes which are designed to clean 
the surface are not needed for proper wetting. Because of its corrosive 
property and incompatability with semiconductor die, fluxing is not 
recommended and is usually strictly avoided in the die bonding process. 
The ambient atmosphere in which the die attachment takes place is very 
important to the spreading of the molten solder. NAS values for the free 
wetting of solder are reduced when the oxygen concentration in the ambient 
is more than about 100 parts per million (ppm). The die attachment can, 
however, be accomplished, especially when pressure assigned, when the 
oxygen concentration in the ambient is more than about 10,000 ppm if the 
bonding time is kept short enough. Preferably the oxygen concentration is 
kept as low as conveniently possible in a flowing and otherwise inert 
ambient. The bonding can be readily accomplished within an enclosure 
containing an atmosphere containing about 5 percent hydrogen in nitrogen. 
The oxygen content of this atmosphere is monitored and kept at the desired 
low level. High levels of oxygen in the atmosphere or long bonding times 
have the deleterious effect of causing an oxide film to form on the molten 
solder before the bonding process can be completed. 
For compatability with semiconductor devices, an acceptable temperature 
range for die bonding is between about 200.degree. and 500.degree. C. For 
optimum bonding which includes a strong die attach and a minimum 
time-temperature exposure of the semiconductor die, the die bond 
temperature is preferably maintained between 400.degree. and 500.degree. 
C. For alloy compositions in accordance with this invention a die bonding 
temperature of about 425.degree. C. is particularly suitable. 
The solder can be supplied to the bonding process in a number of different 
forms. If the solder is, for example, in a ribbon or wire form, a small 
portion of that ribbon or wire can be melted on the metal substrate member 
under conditions as described above to provide a presoldered area to which 
the die can be attached in a subsequent heating step. Alternatively, the 
solder alloy composition can be provided in a preform having a thickness 
of about 0.10 to 0.25 mm and having a circular or rectangular area of 
sufficient size to supply the needed solder for the particular size die 
being bonded. The preform can be used in the same manner as the ribbon or 
wire to form a presoldered area on the metal package member or the 
package, preform, and die can be stacked together and the die attached in 
a single heating step. In the latter heating step pressure may have to be 
applied to the die to ensure proper seating as the preform melts. In a 
preferred process the solder alloy composition is applied to the back of 
the semiconductor die. This can be done, for example, by a deposition 
process such as evaporation or sputtering which deposits the composition 
onto the back of the semiconductor wafer before that wafer is separated 
into individual die. In such a process a brittle alloy is desirable for 
back metallization. The brittleness is desired because in separating the 
wafer into individual die by a scribe and break procedure, solders which 
are not brittle can form hinges holding the die together and ultimately 
leading to die breakage, die scatter, metal removal, or other yield loss 
as the die are forceably separated. In this respect alloy C is the most 
brittle, and thus the most desirable. The hard and soft solders are all 
generally less brittle. If the solder alloy composition is applied to the 
back of the die in this manner, the die can be attached to a metal package 
member without providing additional solder as a preform or other form. 
This process, therefore, reduces assembly time and complexity. 
It is thus apparent that there has been provided, in accordance with the 
invention, an improved solder alloy composition which allows the bonding 
of a semiconductor die to either a plated or unplated metal or metallized 
package member. While the invention has been described in conjunction with 
the attachment of semiconductor die to package members formed of 
particular alloys, it is not intended that the invention nor its usage be 
so limited. The solder composition can also be used for the attachment of 
die to other copper and nickel based substrates, or to package members 
having a copper or nickel based coating. Other variations and 
modifications will, of course, be apparent to those skilled in the art in 
the light of the foregoing description. Accordingly, it is intended to 
embrace all such variations and modifications as fall within the scope of 
the appended claims.