Bumpless method of attaching inner leads to semiconductor integrated circuits

A method of attaching inner leads to the metal electrodes of an integrated-circuit die, in which the tips of the inner leads are pressed onto the metal electrodes by a heated bonding tool. The heat of the bonding tool softens the tips of the inner leads, enabling the inner leads to be directly pressure-welded to the metal electrodes, without the need for intervening soft metal bumps.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of attaching inner leads to the 
metal electrodes of a semiconductor integrated-circuit die. 
Conventional methods of inner-lead attachment employed in tape-automated 
bonding, for example, make use of bumps of a soft metal such as gold, 
which are formed on either the metal electrodes or the inner leads 
themselves. While these soft metal bumps facilitate the bonding of the 
inner leads to the electrodes, the formation of the bumps is an expensive 
process requiring separate equipment and extra labor, and the technology 
of bump-formation takes time to learn. The bump-formation process thus 
adds significantly to the cost of the completed semiconductor device. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to attach inner leads 
to the metal electrodes of a semiconductor integrated-circuit die without 
requiring the formation of bumps. 
The invented method presses the tips of the inner leads onto the metal 
electrodes with a heated bonding tool. The heat of the bonding tool 
softens the tips of the inner leads, so that when pressed onto the metal 
electrodes, they are directly pressure-welded to the metal electrodes 
without the need for soft metal bumps. 
The invented method may comprise one or more of the additional steps of 
pre-softening the inner leads by annealing, roughening the surfaces of the 
inner leads that will be pressed against said metal electrodes, and 
forming mesas at the tips of the inner leads.

DETAILED DESCRIPTION OF THE INVENTION 
Embodiments of the invention will be described with reference to the 
attached illustrative drawings. Identical reference numerals will be used 
for equivalent or identical elements in different drawings. 
First embodiment 
FIG. 1 shows part of a plastic-film lead frame of the type employed in, for 
example, tape-automated bonding. The lead frame comprises a polyimide film 
2 with a central hole 4, and a plurality of inner leads 6 that are 
attached to the polyimide film 2 but extend over the central hole 4. The 
cross-sectional structure of the frame through line 2--2 is illustrated in 
FIG. 2. Each inner lead 6 comprises a copper-foil core 8 approximately 
thirty-five micrometers (35 .mu.m) thick, covered by gold plating 10 
approximately 5 .mu.m thick. The inner lead 6 is attached to the polyimide 
film 2 by an adhesive 12. 
The lead frame shown in FIGS. 1 and 2 is created by spreading adhesive 12 
on the polyimide film 2, using the adhesive 12 to bond a sheet of copper 
foil onto the entire surface of the polyimide film 2 (and so as to cover 
the central hole 4), patterning the copper foil to remove those portions 
that will not become inner leads 6, thus leaving the copper-foil cores 8 
of the inner leads attached to the polyimide film 2, then gold-plating the 
exposed surfaces of the copper-foil cores 8, including both upper and 
lower surfaces. The hardness of the resulting inner leads 6 normally has a 
value of 110 to 120 on the Vickers hardness scale (HV). 
Before the inner leads 6 are bonded to a semiconductor integrated-circuit 
die (referred to below as an IC chip), the lead frame is placed in an 
annealing kiln of the type shown in FIG. 3, and annealed at a temperature 
in the range from about 200.degree. C. to about 300.degree. C. for 
approximately thirty minutes, in a reducing atmosphere such as a nitrogen 
atmosphere. This annealing pre-softens the inner leads 6, reducing their 
hardness to approximately 80 to 100 HV. 
The annealing temperature can be varied according to the type of copper 
foil employed in the inner leads. FIG. 4 illustrates the relation between 
annealing temperature, shown on the horizontal axis, and lead hardness, 
shown on the vertical axis. The white dots are data points taken for one 
type of copper foil commonly employed in inner leads, for which an 
annealing temperature of about 300.degree. C. is appropriate. The black 
dots are data points for another type of copper foil commonly employed in 
inner leads, for which a lower annealing temperature is suitable. 
Referring to FIG. 5, the lead frame is next placed in a bonding apparatus 
so that the annealed inner leads 6 can be bonded to an IC chip 14, which 
is disposed within the central hole 4. The bonding is accomplished by a 
bonding tool 16. 
The bonding process is more clearly illustrated by the sectional view in 
FIG. 6. The IC chip 14 is placed on the center of a stage 18, which is 
heated to 100.degree. C. The bonding tool 16, which is heated to 
500.degree. C., is positioned over the center of the stage. A clamper 20 
is placed on the stage 18, and the polyimide film 2 is placed on the 
damper 20 and held in a position such that the tips of the inner leads 6 
are directly over the aluminum electrodes 22 of the IC chip 14. The 
bonding tool 16 is then lowered so that it presses the tips of the inner 
leads 6 against the aluminum electrodes 22, while simultaneously heating 
the tips of the inner leads 6 to 500.degree. C. Pressure is maintained 
with a force of one kilogram (1 kg) for eight tenths of a second (0.8 s). 
The combination of high temperature and pressure alloys the gold plating 
of the inner leads 6 with the aluminum of the electrodes 22, thereby 
pressure-welding the inner leads 6 directly to the aluminum electrodes 22. 
Contact with the 500.degree. C. bonding tool 16 softens the tips of the 
inner leads 6 to a hardness value of approximately 40 HV to 60 HV, as can 
be verified from FIG. 4. This hardness value is similar to the hardness of 
the gold bumps employed for inner-lead bonding in the prior art. The 
invented method is accordingly able to form bonds with a reliability 
comparable to that in the prior art, without the need for bumps. The 
result is a major saving in equipment and labor costs. 
A further advantage of the invented method is that the bonds have a 
uniformly low profile, as the leads are bonded directly to the aluminum 
electrodes, with no raised bumps. 
Second embodiment 
The second embodiment of the invented method is similar to the first, 
except that the step of annealing the inner leads is omitted, and the 
temperature of the bonding tool 16 is increased to 560.degree. C. The 
bonding pressure is again 1.0 kg, maintained for 0.8 s. With these 
temperature and time conditions, the tips of the inner leads 6 can be 
softened to 40 HV to 60 HV even though the initial hardness of the leads 
is from 110 HV to 120 HV. 
The second embodiment achieves effects similar to the first embodiment, and 
has the additional advantage of being shorter and simpler, since the 
annealing step is omitted. 
Third embodiment 
The third embodiment is similar to the first embodiment, but comprises the 
additional step of roughening the under-surfaces of the inner leads 6. 
The roughening is performed by etching the copper foil cores 8 of the inner 
leads 6 after the copper foil has been patterned, but before the leads 6 
are plated with gold. The gold plating 10 is deposited on all exposed 
surfaces of the leads, including the roughened surfaces. FIG. 7 is a 
sectional view illustrating the roughened surface 24 of an inner lead 6. 
The roughness scale (the spacing between adjacent valleys in the roughened 
surface) is substantially 1.0 .mu.m. Roughening does not alter the 
hardness of the inner leads 6, which remains about 110 HV to 120 HV before 
annealing. 
The annealing and bonding steps are carried out as in the first embodiment, 
so a description will be omitted. The advantage of the third embodiment is 
that even if the aluminum electrodes 22 of the IC chip 14 are covered by a 
thick natural oxide layer, when the roughened surface 24 is pressed 
against the electrode surface, the oxide layer is readily broken through, 
enabling the reliable formation of an aluminum-gold alloy weld as in the 
first embodiment. 
The third embodiment can be modified by omitting the annealing step, as in 
the second embodiment. The third embodiment can also be modified by 
roughening only parts of the under-surfaces 24 of the inner leads 6, 
instead of the entire under-surfaces, 24 provided the parts that make 
contact with the aluminum electrodes 22 are roughened. 
Fourth embodiment 
The fourth embodiment is similar to the first embodiment, but gives the 
tips of the inner leads a mesa shape, so that the inner leads can be 
bonded to the aluminum electrodes without exerting unwanted pressure on 
surrounding passivation films. 
FIG. 8 is a sectional view showing an inner lead 6 of the type employed in 
the fourth embodiment. The mesa 26 is formed after the inner leads 6 have 
been patterned by removing unwanted copper foil, and before the gold 
plating 10 is applied. The mesa 26 is formed by etching to reduce the 
thickness of the copper-foil core 8 in areas other than the mesa 26. The 
mesa may be square, round, hexagonal, diamond-shaped, or of any other 
shape suitable for bonding to an aluminum electrode 22. Mesa formation 
does not change the hardness of the inner leads 6, which remains 110 HV to 
120 HV as in the first embodiment. 
The annealing and bonding processes are carried out as described in the 
first embodiment. FIG. 9 illustrates the bonding of an inner lead 6 to an 
aluminum electrode 22 that is surrounded and partially covered by a 
passivation film 28. When the inner lead 6 is pressed down by the bonding 
tool, only the mesa 26 makes contact with the aluminum electrode 22. No 
part of the inner lead 6 is pressed against the passivation film 28. 
The joint strength of the bond formed between the mesa 26 and aluminum 
electrode 22 is substantially the same as the joint strength of the bond 
formed in the first embodiment. The advantage of the fourth embodiment is 
that the passivation film 28 cannot be cracked by the pressure of bonding. 
The fourth embodiment can be modified by omitting the annealing step, as in 
the second embodiment. 
The present invention is not limited to use in tape-automated bonding, but 
is applicable in any bonding process in which inner leads are attached to 
the metal electrodes of an IC chip by pressure welding, regardless of how 
the inner leads themselves are formed or mounted. 
The temperatures of 200.degree. C., 300.degree. C., 500.degree. C., and 
560.degree. C., and Vickers hardness values of 40 HV, 60 HV, 80 HV, 100 
HV, 110 HV, and 120 HV mentioned in the descriptions of the above 
embodiments have been given as general guides; the invention is not 
restricted to these exact temperatures or hardness values. When the 
annealing step is carried out, the annealing time is not limited to thirty 
minutes. The roughness scale of the roughened inner leads may be greater 
than 1.0 .mu.m, and the roughening method is not limited to etching. 
After inner-lead bonding, the IC chip may be individually packaged, or may 
become part of a multi-chip module. 
Those skilled in the art will recognize that further modifications are 
possible within the scope claimed below.