Thermocompression bonding of copper leads to a metallized ceramic substrate

Copper leads having an average grain size from 0.035 to 0.055 mm are thermocompression bonded to a metallized portion of a ceramic substrate. Surprisingly, such large grain size substantially decreases the "ceramic pullout" of the bonded leads.

TECHNICAL FIELD 
The instant invention relates to a method of thermocompression bonding of 
copper leads to metallized portions of a ceramic substrate. 
BACKGROUND OF THE INVENTION 
It is well known to thermocompression bond copper leads to metallized 
portions of ceramic substrates. Such a technique is described in copending 
U.S. Patent application to B. H. Cranston, Ser. No. 546,714, filed on Oct. 
28, 1983 and titled "A Lead Frame and Dual-in-Line Package Fabricated 
Using Said Frame", now U.S. Pat. No. 4,563,811, which is assigned to the 
instant assignee and is hereby incorporated by reference herein. That 
application discloses, inter alia, the thermocompression bonding of soft 
copper leads of a lead frame to metallized bonding pads on a ceramic 
substrate. 
One known disadvantage of such thermocompressive bonding is "ceramic 
pullout" which may result either during bonding of the copper lead to the 
metallized bonding pad and/or the testing of the bond. Ceramic pullout 
occurs when the lead is being deformed during bonding or when a pulling 
force is applied to the bonded lead until a failure occurs in which the 
lead separates from the substrate with a portion of the ceramic substrate 
attached to the lead. Such ceramic pullout results in the loss of the 
device. 
Various techniques have been tried to lessen the ceramic pullout problem. 
In particular, the parameters of time, temperature, pressure and materials 
associated with the thermocompression bonding process have been adjusted. 
However, such adjustments have not resulted in a solution to the problem. 
Thus, there is a need for a technique to decrease the ceramic pullout 
failures associated with thermocompression bonding of copper leads to 
metallized ceramic substrates. 
SUMMARY OF THE INVENTION 
The foregoing problems have been overcome in an electronic device comprised 
of: a ceramic substrate having metallized patterns thereon; a 
semiconductor chip bonded to first ends of the metallized patterns; and at 
least one copper lead, having an average grain size from 0.035 to 0.055 
mm, thermocompressively bonded to the second ends of the metallized 
patterns. 
Surprisingly, it has been discovered that such large grain size in the 
copper lead results in a substantial decrease in ceramic pullout.

DETAILED DESCRIPTION 
FIG. 1 is an exploded view of the components that comprise a dual-in-line 
package (DIP). A semiconductor beam leaded chip 5 is bonded to first ends 
6--6 of metallic leads 7--7 on a ceramic substrate 8--8. The leads 7--7 
are of a tri-metal material with titanium and palladium layers plated with 
gold. Copper leads 9--9 are thermocompression bonded to the second ends 
10--10 of the leads 7--7. The copper leads 9--9 have a nickel-gold coating 
on the ends to be bonded. 
A typical technique used to fabricate a DIP 11 is shown in FIG. 2. A lead 
frame 12 is comprised of a pair of lateral support members 14--14 
connected together by a plurality of tie bars 16--16 with a repetitive 
lead pattern 17 therebetween. Additionally, a plurality of support members 
18--18 are connected between lateral support members 14--14. Each lead 
pattern 17 is comprised of a plurality of internal leads 9--9 which extend 
from the support members 18--18 to a centrally located rectangular array 
of ends 24--24. A plurality of external leads 26--26 is parallel to the 
lateral support members 14--14 and each lead is connected between the tie 
bars 16--16 and the support members 18--18. 
Typically, the lead frame 12 is fabricated by indexing a continuous soft 
metal (e.g., CDA 102 copper or the like) strip 27 into a punch press (not 
shown) which repetitively punches out the lead pattern 17. The ends 24--24 
of the leads 9--9 and leads 26--26 are then coated with nickel which is 
then electroplated with gold. The metallized ceramic substrate 8 having a 
semiconductor chip 5 (see FIG. 1) bonded thereto is placed in contact with 
the lead frame 12 and the ends 24--24 of the internal copper leads 9--9 
thermocompression bonded to the respective leads 7--7. 
The ceramic substrates 8--8 may then be encapsulated with RTV silicone 
rubber (not shown) in a well known manner and the lead frame 12 placed in 
a molding machine wherein a body 38 of plastic material (e.g., epoxy, 
silicon, etc.) is formed about a portion of each pattern 17 as shown in 
FIG. 2. The lead frame 11 is then cut into individual sections and the 
leads trimmed and formed into a DIP 11. 
Copper lead frame material specifications have historically been directed 
to mechanical properties such as: yield strength, tensile strength, 
microhardness and percent elongation. The internal physical properties 
such as grain formations were not a consideration. This has been justified 
by the rationale that the deformation which occurs during 
thermocompression bonding is a function of the compression yield strength. 
However, we have discovered that ceramic pullout is related to the grain 
size of the copper prior to bonding. In particular, we found that large 
grain size (e.g., from 0.035 to 0.055 mm) results in less ceramic pullout. 
Although the mechanism is not fully understood, it is believed that there 
are lower localized stresses imposed on the ceramic by larger grains which 
is related to the mechanics of plastic deformation. It is known that in 
polycrystalline metals grain boundaries interfere with slip because the 
planes on which dislocations move are not continuous. Therefore, a larger 
grain material tends to be more ductile at ambient temperatures where 
grain boundaries are normally stronger than the internal grain structure. 
However, at the temperature required for thermocompression bonding (e.g., 
300.degree. C.), the principle deformation processes also include 
sub-grain formation and grain boundary sliding. This type of deformation 
is characterized by extreme inhomogeneity which would result in imposed 
stress concentrations upon the ceramic surface which leads to fracture of 
the ceramic in the bond region - ceramic pullout. 
Initially, grain size is a function of solidification of the metal from a 
melt. This can be controlled to a large extent by varying cooling rates. 
Due to the kinetics of grain formation and growth, quickly cooled metals 
will have smaller grains. Grain size also varies with the number of 
nucleation sites available during solidification. 
After solidification, grain size can be altered by annealing or cold-work 
processes. For example, annealing a small grain metal to promote 
recrystallization can result in either smaller or larger grains depending 
on internal stresses within the grains and nucleation sites. However, 
carrying the annealing process further (higher temperature-longer times) 
will result in grain growth where larger grains are formed as small grains 
are consumed. Once solidified, smaller grains can be formed through 
extreme cold-work of the metal. Usually, excessive deformation will cause 
the larger grain to fragment and form sites for smaller highly stressed 
grains. These grains with proper heat treatment can be stress relieved 
without promoting grain growth. 
Considering copper lead frame materials, the grain size can be controlled 
in two ways: (1) the metal can be heat treated to a very soft, large 
grained material and then cold-worked to obtain smaller recrystallized 
grains, or (2) the metal can be cold-worked first and then annealed to 
promote recrystallization or even grain growth. It is difficult to 
theoretically predict grain size and the required heat treatment, but once 
the proper treatment or cold-work process has been determined 
experimentally, it is repeatable. 
Standard copper lead frame materials are provided with an ASTM 9-10 grain 
size (average grain diameter of 0.015 mm). The large grained copper which 
has been found to produce a more homogeneous stress on the ceramic are 
typically ASTM 6-7 (average grain diameter of 0.045 mm). 
It is known that thermocompression bondability of copper to a ceramic 
substrate is improved by using a softer material. However, there is no 
direct correlation with the grain size of copper and microhardness as can 
be seen in Table I. For example, the grain size of copper material A 
having a grain size of 0.015 mm has a microhardness of 104 while material 
B having a large grain structure of 0.065 had a microhardness of 112. 
TABLE I 
______________________________________ 
Grain Size and Microhardness In Cross-Section 
Copper GS Microhardness (DPH.sub.100) 
Material 
(mm) -.chi. .eta. 
.sigma. 
max min range 
______________________________________ 
A .015 104 25 3.0 110 98 12 
B* 
all gr .045 109 25 6.2 126 101 25 
sm gr .025 106 12 4.2 116 101 15 
lg gr .065 112 13 6.5 126 104 22 
C .040 111 25 5.2 121 102 19 
______________________________________ 
GS Grain size which is determined by comparing the microstructure with 
ASTM nonferrous grain size standard and is reported as normal diameter in 
mm. 
-.chi. average microhardness taken in a crosssection of the material. 
.eta. number of reading in the crosssection. 
.sigma. standard deviation. 
*material B is a duplex grain structure. The data is the average for all 
grains combined and separately for the larger and smaller grains. 
The standard material used in the past has been a CDA 102 copper material 
shown as material A in Table I. 
The results of a study done to determine the effects of grain size of 
copper on ceramic pullout are shown in Table II. 
TABLE II 
__________________________________________________________________________ 
Thermocompression Bonding Study 
FMA 
Copper Temp Def 
Leads 
LF PLF INT 
CP 
Material 
GS (.degree.C.) 
Psig 
(%) 
Tested 
(%) 
(%) (%) 
(%) 
__________________________________________________________________________ 
A 9-10 
385 240 
64 1600 
55.9 
16.9 
0 27.2 
385 255 
65 1600 
60.0 
19.6 
0 20.4 
385 275 
66 1600 
64.7 
21.1 
0 14.2 
385 300 
69 1584 
65.0 
21.3 
0.1 
13.6 
B 6-7 385 240 
67 1568 
74.8 
16.8 
0.9 
7.5 
385 255 
69 1568 
84.0 
12.6 
0.1 
3.3 
385 275 
70 1600 
90.6 
7.9 0 1.5 
385 300 
71 1584 
90.6 
8.5 0 0.9 
C 6-7 385 240 
67 1584 
87.2 
10.4 
0.1 
2.3 
385 255 
68 1584 
90.3 
8.7 0 1.0 
385 275 
69 1552 
94.0 
5.9 0 0.1 
385 300 
72 1584 
95.5 
4.2 0 0.3 
__________________________________________________________________________ 
Psig The thermocompression bonding pressure is applied in two stages, th 
first stage is 70 psig for 9 seconds and second stage indicated in Table 
II for 30 seconds. Normal production setting is 240. 
Def The average deformation is determined by checking one inside lead on 
each side of 4 substrates. 
FMA Failure Mode Analysis determined by destructive push testing all 16 
leads simultaneously on each ceramic. 
a. LF is 100% lead failure where the entire bonded area remains intact. 
b. PLF is a partial lead failure, 100% &gt; LF &gt; 0%. 
c. INT is an interface or gold to gold failure, poor bond between lead an 
substrate. 
d. CP is a ceramic pullout with no lead left bonded. 
The information in Table II clearly indicates that the larger grain 
material - from 0.035 to 0.055 mm nominal diameter - had less ceramic 
pullout that the small grain at all identical parameter settings.