Multi-layer flexible film module

A high density electronic circuit module which includes a thin flexible film chip carrier having circuitry on both sides thereof. The double-sided thin, flexible circuitry is fabricated by depositing a first layer of chrome-copper-chrome circuitry on an aluminum substrate. This first layer of chrome-copper-chrome circuitry is covered with a layer of polyimide. Vias are etched into the polyimide. Next, a second layer of chrome-copper-chrome circuitry is deposited on top of the polyimide. The first and second layers of circuitry are connected through the etched vias. Finally, hydrochloric acid is utilized to etch away the aluminum substrate carrier. It is noted that hydrochloric acid etches aluminum whereas it does not etch chrome-copper-chrome circuitry.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electronic packaging and more particularly to 
high density, low cost packaging of electronic circuitry. 
2. Description of the Prior Art 
The prior art shows electronic packages where integrated circuit chips are 
mounted on thin, flexible sheets of insulating material. For example, see 
U.S. Pat. Nos. 3,780,352 and 3,781,596. 
The prior art also teaches various techniques for avoiding problems caused 
by thermal mismatch between integrated circuit chips and the support on 
which the chips are mounted. One known technique for avoiding problems due 
to thermal mismatch is to mount the chip on a flexible film carrier. For 
example, U.S. Pat. No. 4,231,154 shows an integrated circuit chip mounted 
on a very thin layer of circuitized polyimide. 
Double sided circuitry is also well known. Such circuitry generally 
includes a carrier with circuitry on both sides of the carrier. The 
circuitry on the two sides of the carrier is generally interconnected by 
vias or thru holes. The carrier in such circuitry is generally a material 
such as epoxy glass which can be etched or drilled to provide passages for 
the via interconnections. 
None of the prior art teaches how to fabricate circuitry on both sides of a 
thin polyimide carrier where the circuitry on the two sides of the carrier 
is interconnected by vias which are etched before the polyimide is cured. 
It is noted that in order to etch vias into polyimide one must have uncured 
polyimide. The polyimide that is generally available in sheet form is 
cured polyimide. The reason that only cured polyimide is generally 
available in sheet form is that the process normally used for making sheet 
polyimide cures the polyimide during the fabrication process. If one 
fabricates circuitry on both sides of a "cured" polyimide sheet, one 
cannot etch thru holes or vias into the polyimide sheet using conventional 
photoresist and etch techniques. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of making high 
density electronic packages. 
It is further an object of the present invention to provide an electronic 
package that includes an electronic chip mounted on a thin flexible film 
which has circuitry on both sides of the thin, flexible film. 
It is further an object of the present invention to provide an electronic 
package that includes an electronic chip mounted on a thin flexible film 
which has circuitry on both sides of the film wherein the circuitry is 
interconnected by means of vias that have been etched in the film. 
Another object of this invention is to provide a method of manufacturing 
the assembly described above utilizing a temporary support member. 
In accordance with the present invention, an electronic package is provided 
which includes an electronic chip mounted on a thin layer of polyimide. 
The thin layer of polyimide includes circuitry on both of its surfaces. 
The two layers of circuitry are interconnected by means of vias that have 
been etched into the polyimide. 
The polyimide with circuitry on both of its surfaces is fabricated by 
depositing a first layer of conventional chrome-copper-chrome circuitry on 
an aluminum carrier. This first layer of circuitry is covered with 
polyimide. Vias are etched into the polyimide and then the polyimide is 
fully cured. A second layer of conventional circuitry is deposited on the 
top surface of the polyimide. The aluminum carrier is then etched away 
using Hydrochloric acid. 
The result is a thin film having circuitry on both of its surfaces. The two 
layers of circuitry are interconnected by means of etched vias. The 
aluminum can be etched away without damaging the circuitry since 
hydrochloric acid attacks aluminum but it does not attack chrome. 
The foregoing and other objects, features, and advantages of the invention 
will be apparent from the following more particular description of a 
preferred embodiment of the invention, as illustrated in the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the base layer of aluminum 10 used as a substrate in the 
present invention. This substrate can be a conventional, highly polished 
aluminum sheet 1/16 inch thick. 
In the first step of the invention, conventional chrome-copper-chrome 
circuitry 20 is deposited on top of the aluminum carrier as shown in FIG. 
2. Circuitry 20 can be evaporated, sputtered or otherwise deposited in a 
conventional manner. Circuitry 20 can, for example, consist of 
conventional sputtered chrome-copper-chrome circuitry which is fabricated 
by sputtering 800 angstroms of chrome, followed by 40,000 anstroms of 
copper, and finally 800 angstroms of chrome. It should be noted that in 
the drawings the thickness of layer 20 (and likewise of layers 30 and 40) 
is exaggerated for ease of illustration. 
The chrome-copper-chrome layer 20 is circuitized using standard photoresist 
and etching techniques. The techniques used to deposit the 
chrome-copper-chrome and to form a circuit pattern using photoresist can 
be the same techniques as those shown in the prior art. For example see 
U.S. Pat. No. 4,231,154. 
Next, a layer of liquid polyimide 30 is applied on top of the layer of 
chrome-copper-chrome circuitry 20 as shown in FIG. 3. The polyimide 30 
can, for example, be commercially available material such as Dupont 5057. 
The polyimide 30 can be applied by various conventional techniques such as 
spinning or spraying. Layer 30 can be very thin. For example it may be in 
the range between 0.0002 and 0.0003 inches thick. 
After the polyimide layer 30 is applied it can be stabilized by curing it 
to the B stage by the application of heat for a relatively short time. At 
this stage in the process, care must be taken not to fully cure the 
polyimide, since in the next step holes will be etched into the polyimide 
and it is not practical to etch holes in fully cured polyimide. 
Next, holes 30A and 30B are etched into the uncured polyimide utilizing 
conventional photoresist and etch techniques. Holes 30A and 30B will be 
used as vias to make connections between the circuitry on the two sides of 
the polyimide film. 
After holes 30A and 30B have been etched into the polyimide film, the 
polyimide is cured by a conventional baking step. 
In the next step of the process, a second layer of chrome-copper-chrome 
circuitry 40 is deposited on the top of the layer of polyimide 30. The 
resultant structure is shown in FIG. 4. The top layer of 
chrome-copper-chrome circuitry 40 is deposited and circuitized in a 
conventional manner. 
When top layer of circuitry 40 is deposited, metal is also deposited in 
vias 30A and 30B making connections between circuitry 20 and circuitry 30. 
Added reliability of the via interconnections can be achieved by first 
depositing a layer of interface material such as gold in vias 30A and 30B 
as shown in co-pending application Ser. No. 165,320 filed July 2, 1980 and 
entitled "Multiplanar Integrated Circuit Substrate with Multilayer Metal 
Conductors". 
After the top layer of circuitry 40 has been formed the aluminum substrate 
10 is selectively etched to form cavity 10A. Substrate 10A is selectively 
etched by covering all of the surface 10B of substrate 10 (except area 
10A) with a protective film (not shown) that is impervious to hydrochloric 
acid. Such films are commercially available. Next the substrate is exposed 
to hydrochloric acid to etch cavity 10A. It is noted that hydrochloric 
acid will etch the aluminum substrate 10 but it will not attach the 
chrome-copper-chrome circuitry 20. 
In its final form, as shown in FIG. 5, the package fabricated in accordance 
with the present invention includes an integrated circuit chip 55 which is 
connected to circuitry 20 and 30 by means of solder ball joints 50 
(generally referred to as C4 joints). In FIG. 5 the circuit pattern which 
has been etched into layer 20 is designated 20C. The circuitry that has 
been etched into layer 40 is designated 40A and 40B. In FIG. 5 only one of 
the via connections 30B between circuit layer 20 and circuit layer 30 is 
shown. 
The technique of the present invention allows one to form circuitry on both 
sides of a very thin flexible polyimide film 30 which includes etched vias 
such as 30A and 30B. Prior techniques for forming two sided interconnected 
circuitry required a much thicker and rigid substrate. Since substrate 30 
is thin and flexible it can absorb any thermal mismatch between integrated 
circuit 55 and the aluminum carrier 10. 
A more detailed explanation of the various steps in the particular 
embodiment of the process shown herein is as follows: 
1. Form a layer of circuitry 20 on a highly polished aluminum carrier 10. 
Circuit layer 20 can for example be formed by evaporating, sputtering, or 
otherwise depositing 800 angstroms of chromium followed by 40,000 
angstroms of copper, followed by 800 angstroms of chromium. Sputtering can 
be done using conventional techniques. 
2. Circuitize the chrome-copper-chrome metallurgy 20 using standard 
photoresist and etching techniques. (for example see steps 18 to 22 
below). 
3. Apply a 0.0002 to 0.0003 inch thick layer 30 of liquid polyimide (e.g., 
Dupont 5057) onto the chromium surface by conventional spinning, or 
spraying techniques. 
4. Cure polyimide layer 30 to the B stage only by heating to 120 degrees F. 
for 20 minutes. 
5. Apply a layer of photoresist to top of the polyimide layer 30. 
Conventional photoresist such as KTFR can be used. 
6. Expose the desired via pattern (for example vias 30A and 30B). 
7. Develop the via pattern. 
8. Etch the vias with KOH (potassium hydroxide). 
9. Strip the photoresist with J100 (Kodak). 
10. Cure the polyimide 30 with a high temperature, e.g., 350.degree. C. for 
5 minutes in a nitrogen or other inert atmosphere. 
11. Deposit a second layer of chrome-copper-chrome circuitry 40 by 
evaporating, sputtering, or otherwise depositing 800 angstroms of 
chromium, 40,000 angstroms of copper and, finally 800 angstrom top layer 
of chromium onto the cured polyimide. 
12. Apply photoresist (KTFR) onto the chromium surface. 
13. Expose the the C4 land patterns 40B. 
14. Develop out C-4 areas 40B. 
15. Etch top chromium from land patterns 40B with KMnO /KOH (potassium 
permanganate/potassium hydroxide). 
16. Gold plate 20-50 microinches of gold onto the tabs 40B (for example Sel 
Rex Pure A Gold 125 could be used). The tabs for C-4 joints could also 
just be tinnned. 
17. Strip photoresist. 
18. Reapply photoresist KTFR (Kodak Thin Film Resist). 
19. Expose circuit pattern desired in layer 40. 
20. Develop photoresist to the desired circuit pattern in layer 40. 
21. Etch circuitry into layer 40 with standard etchants. For example use 
KMnO /KOH, FeCl (ferric chloride), and KMnO /KOH. 
22. Strip the photoresist. 
23. Protect the front side circuit layer 40 and area of substrate 10 other 
than cavity 10A with a temporary organic layer (e.g., Apiezon wax, 
commercially available from the James Biddle, Co.) or with a mechanical 
cover. 
24. Etch the aluminum carrier in area 10A with HCl. 
25. Place device chip 55 onto the C-4 pads 40B. 
26. Reflow device chip 55 to the C-4 pads in a conveyor furnace (standard 
C-4 joining). 
While the invention has been particularly shown and described with 
reference to preferred embodiment thereof, it will be understood by those 
skilled in the art that various changes in form and detail may be made 
therein without departing from the spirit and scope of the invention.