Adhesion primers for encapsulating epoxies

A copper plastic laminate is provided having a high bond strength. The copper is selected from the group consisting of copper and copper alloys and has on its surface a uniform glassy-like, substantially pore-free coating of copper phosphate. A plastic encapsulating material containing a mold release agent is adhesively bonded to the copper.

While the invention is subject to a wide range of applications, it is 
especially suited for encapsulating electrical leadframes with integrated 
circuits attached thereto and will be particularly described in that 
connection. 
In the electronics industry, one of the basic types of semiconductor 
packages used for integrated circuits is the plastic molded package. The 
package may include a leadframe, having an electronic device attached 
thereto, molded into an encapsulating plastic. This type of plastic 
package often has reliability problems created by failures in the plastic 
to metal bond between the encapsulating plastic and the leadframe. The 
bond failure provides an avenue through which moisture and other 
atmospheric contaminants can reach the electric device and cause corrosion 
problems. Such failures are more fully explained and documented in an 
article entitled "Factors Governing Aluminum Interconnection Corrosion in 
Plastic Encapsulated Microelectronic Devices" by Neighbour and White 
published in Microelectronics and Reliability, by Pergamon Press in Great 
Britain, Volume 16, 1977, pages 161-164. 
It is known in the prior art to produce a laminate of copper or copper 
alloys and plastic film. A laminating adhesive is applied between the 
metal and plastic film to bond them together. A high bond strength results 
between the copper or copper alloys and the plastic film due to a 
phosphate coating provided on the metal. Examples of this are disclosed in 
U.S. Pat. Nos. 3,677,828, 3,716,427, 3,728,177, 3,728,178, 3,764,399, 
3,833,433, 3,837,929, 3,853,691, 3,940,303, 3,941,627, 3,941,628, and 
3,944,449. However, the plastic film laminate disclosed in these patents 
do not contain mold release agents as set out in the present invention. 
Since the adhesive is commonly required only between the film and the 
metal, there is no reason to use a mold release agent. Further, if a mold 
release agent were used, it would only decrease the bond strength. The 
absence of mold release agents is significantly different from the present 
invention where the mold release agents are definitely required to prevent 
an encapsulating epoxy from sticking to a mold surface. Unfortunately, the 
mold release agent also hinders the encapsulant from sticking to an 
encapsulated metal strip such as a leadframe. The resulting bond between 
the encapsulant and the leadframe is often deficient and creates an avenue 
for contaminating an encapsulated electronic element attached to the 
leadframe. 
It is a problem underlying the present invention to provide an encapsulated 
package for an electrical or electronic component which is highly 
resistant to atmospheric contamination. 
It is an advantage of the present invention to provide a casing for an 
electrical or electronic component which obviates one or more of the 
limitations and disadvantages of the described prior arrangements. 
It is a further advantage of the present invention to provide a hermetic 
casing for an electrical or electronic component which is substantially 
resistant to the diffusion of contaminants. 
It is a yet further advantage of the present invention to provide a casing 
for an electrical or electronic component which is relatively inexpensive 
to manufacture. 
Accordingly, there has been provided a casing adapted for containing an 
electrical or electronic component and a method of producing the casing. 
The casing includes a copper or copper alloy leadframe. An adhesion primer 
comprising a uniform, glassy and substantially pore-free phosphate coating 
is applied to the leadframe. Also, an encapsulating material containing a 
mold release agent is bonded to the copper leadframe to form a hermetic 
casing. In addition, the encapsulating epoxy may be adhered to copper or 
copper alloy material having an adhesion primer thereon whenever a strong 
bond is required. 
The invention and further developments of the invention are now elucidated 
by means of the preferred embodiments shown in the drawings.

The present invention is primarily concerned with forming a strong bond 
between copper or copper alloy and an encapsulating epoxy or plastic. This 
is particularly important in semiconductor leadframe applications where 
the leadframe and semiconductor attached thereto are encapsulated by an 
encapsulating epoxy. Semiconductor devices are particularly sensitive to 
degradation and failure from exposure to moisture and/or penetration of 
atmospheric pollutants into the package through a path between the 
encapsulating molding compound and the leadframe. Although this is the 
primary application described herein, it is within the scope of the 
present invention to apply the treatment described herein wherever copper 
or copper alloys are molded into the general class of plastics or epoxies 
which contain mold release agents. The mold release agent also known as an 
abherent allows the hardened epoxy to release from the encapsulating mold 
and minimize redressing the mold after each use. Just as the mold release 
agent prevents the plastic or elastomer from sticking to the mold wall, it 
also reduces the strength of the bond between the metal or alloy leadframe 
and the plastic or elastomer. 
The metal or alloy which is used in this invention is preferably copper or 
copper alloy. Also, this metal or alloy may be in any form such as sheets, 
strip, or foil. 
The metal or alloy is coated with a uniform, glassy-like, substantially 
pore-free phosphate coating. This coating may be in the range of thickness 
from about 20 to about 100 Angstrom units. It may be applied using various 
techniques as set forth in the patents enumerated above. For example, the 
coating may be obtained by applying a phosphoric acid solution containing 
from about 3.5 grams per liter up to the solubility limit of sodium 
dichromate (Na.sub.2 Cr.sub.2 O.sub.7 2H.sub.2 O) or potassium dichromate 
(K.sub.2 Cr.sub.2 O.sub.7) or mixtures thereof to the copper alloy 
material. Normally, the application of the aforementioned solution is by 
immersion of a sheet or strip of material into a bath of the 
abovementioned acid solution. 
Another treatment which is throught to be advantageous in the present 
invention is disclosed in U.S. Pat. No. 4,264,379. 
The treatment may be effected with an aqueous solution containing a low to 
moderate concentration of the phosphonic acid component or components, 
preferably ranging from about 0.1 to about 30 volume percent for liquid 
acids or corresponding weight percent limits for solid phosphonic acids, 
preferably in the range of about 0.1 to 40 percent by weight. 
The treating solution also preferably includes a low to moderate 
concentration, such as about 0.1 to about 15.0 percent by weight, 
preferably 0.2 to 5.0 percent by weight of oxidizing agent, such as sodium 
or other alkali chromate or dichromate, or nitric acid (100 percent) at a 
concentration of about 0.05 to about 10.0 volume percent, preferably about 
0.05 to about 2.0 percent by volume HNO.sub.3. Other known oxidizing 
agents of similar activity may be used at a comparable dilute or moderate 
concentration effective for the purpose, but generally with avoidance of 
such vigorous oxidizing conditions as might cause substantial 
decomposition of the phosphonic acid. 
Following the aforementioned immersion step, the copper alloy strip is 
rinsed and dried. The rinsing is normally carried out in running water 
although a spray rinse may also be readily employed. Drying may be 
accomplished by an air blast, rinsing in an alcohol solution such as 
methanol and allowing to dry, or merely by exposure to the atmosphere. 
Following rinsing and drying, the treated surface of the copper sheet or 
strip is prepared for the attachment of semiconductors and the attachment 
of lead wires from the semiconductors to the coated leadframe or strip. In 
manufacturing the leadframe with a semiconductor device attached thereto, 
it may be desirable to silverplate a spot on the leadframe to enhance the 
connection of the wires from the semiconductor device to the leadframe. 
Further, the ends of the leadframe may be electrosolder plated to enhance 
the connection between the leadframe and another element to which it may 
be attached by a soldering operation. In any case, the glassy-like 
phosphate coating of the leadframe, in accordance with the present 
invention, is preferably accomplished after the addition of the 
silverplated spot and/or electrosolder plate because the coating does not 
substantially adhere to either the silver or the solder. Conversely, the 
silverplating and/or electrosolder plate may be applied after the 
phosphate coating has been applied because the coating does not prevent 
the adherence of the plating. 
The semiconductor device is attached to the plated leadframe in any 
conventional manner. The leadframe may then be placed within a mold and 
encapsulated in any conventional manner with an encapsulating epoxy, 
preferably containing both mold release agents and inert fillers which are 
suitable for the invention described herein. The encapsulating epoxies are 
generally in the class of thermosetting plastic or polymers. They 
preferably contain mold release agents, such as low melting temperature 
organics, which allow release of the hardened epoxy from the mold and 
minimize mold redressing between uses. These mold release agents or 
abherents may include materials such as silicones, stearates and fatty 
acids. Examples of other abherents are described in the Encyclopedia of 
Chemical Technology, 3rd Edition, by Kirk-Othmer, published by John Wiley 
& Sons, Volume 1, pages 1-9. The encapsulating epoxies may also contain 
inert fillers to provide dimensional and temperature stability. These 
fillers may be discrete particles such as glass fibers or silica. 
Epoxy encapsulants, containing mold release agents, which are suitable for 
the present invention are Morton Polyset 410B manufactured by Morton 
Chemical Company and Plaskon 3100 and 3200 manufactured by Plaskon 
Electronic Materials, Inc. 
FIGS. 1-3 illustrate the experimental results exhibited during lap shear 
strength testing of samples. The lap shear strength test measures the bond 
strength in terms of shear strength. It is performed by overlapping two 
strips of the desired material and bonding them together with an epoxy. 
Then, the strips are pulled apart from each other exerting a shear stress 
at the bond. 
Referring to FIG. 1, there is shown samples of uncoated CDA copper strip 
subjected to the lap test described above. The encapsulating epoxy used in 
this test was Morton Polyset 410B. FIG. 2 illustrates samples of CDA 
copper strips coated with glassy-like phosphate and bonded together with 
Morton Polyset 410B. Comparison of FIGS. 1 and 2 indicates that the median 
bond strength is nearly double for the coated copper alloy samples as 
compared to the clean samples of the same alloy. 
FIG. 3 illustrates the experimental results of a durability test. The 
durability test measures the effect of the moisture on the bond strength. 
The data illustrated resulted from bonding coated and uncoated copper 
alloy CDA 194 strips with an encapsulating epoxy, Morton Polyset 410B. The 
eposy was cured for 24 hours at 165.degree. C. Then, the bonded strips 
were placed in boiling water for different amounts of time. Afterwards, 
they were pulled apart using the aforementioned lap strength test. The 
slope through the median of the bond strength ranges of the coated and 
uncoated or clean copper 194 indicates that the coated alloy had a larger 
bond strength. This is an example of a specific cure cycle. The same type 
of effects can be expected with any equivalent cure cycle. Even in an 
uncured condition, specific bond improvement of about 285 PSI for coated 
copper alloy vs. 200 PSI for uncoated copper alloy was found. 
The durability tests indicate the increased bond strength of coated strip 
in accordance with the present invention, as compared to uncoated strip 
when both were subjected to similar conditions of heat and moisture. The 
test is a strong indicator of the hermeticity of a package comprising a 
semiconductor device attached to a leadframe and encapsulated with epoxy 
containing mold release agents as well as inert fillers. The hermeticity 
has been substantiated by vacuum testing plastic metal adhesion bonds with 
and without the adhesion primers. In the absence of an adhesion primer, no 
vacuum is obtained. However, with coated samples, vacuum measuring in the 
range of 10.sup.-6 and 10.sup.-7 Torr was achieved. 
The coating of the present invention is thought to provide an additional 
advantage in preventing bond failure in the shear mode due to differences 
in thermal expansion between the metal and the epoxy. The expansion 
coefficient of epoxy may be different from the expansion coefficient of 
the copper or copper alloy. For example, the coefficient of thermal 
expansion of copper is 10.times.10.sup.-6 in/in/.degree.C. and the 
coefficient of linear expansion of a typical encapsulating epoxy is 
27.times.10.sup.-6 in/in/.degree.C. The difference in coefficient of 
expansions creates a high shear stress condition when the composite is 
placed in a cyclic temperature environment. However, the adhesion primers 
of the present invention are thought to be slightly flexible and have been 
shown to adjust to some expansion. The improved bond strength achieved 
with the present invention should prevent bond failure over a wider shear 
displacement range, such as exists in the range of temperatures between 
-50.degree. C. and 165.degree. C. These temperatures present the extremes 
in which these devices are normally used. This may be particularly 
important in adverse environmental conditions where encapsulated 
semiconductor devices are used. 
The patents and publications set forth in the specification are intended to 
be incorporated by reference herein. 
It is apparent that there has been provided in accordance with this 
invention a copper plastic composite and a method of forming the composite 
which fully satisfies the objects, means, and advantages set forth 
hereinabove. While the invention has been described in combination with 
the specific embodiments thereof, it is evident that many alternatives, 
modifications, and variations will be apparent to those skilled in the art 
in light of the foregoing description. Accordingly, it is intended to 
embrace all such alternatives, modifications, and variations as fall 
within the spirit and broad scope of the appended claims.