High yield combined rigid and flexible printed circuits and method of manufacture

This is a printed circuit comprising multiple layers and rigid and flexible portions and including a sheet of flexible substrate material extending over the entirety of the rigid and flexible portions and paths of conductive material carried by at least one side of the sheet of flexible substrate material. This invention comprises, a sheet of flexible over-layer material extending over at least the entirety of all the flexible portions; a flexible adhesive material adhesively attaching the sheet of flexible over-layer material to the entirety of all the flexible portions; sheets of a rigid substrate material extending over the entirety of all the rigid portions; and, a rigid adhesive material adhesively attaching the sheets of a rigid substrate material to the entirety of all the rigid portions. The rigid adhesive attaching the sheets of rigid substrate to the rigid portions may extend out over the edge of the rigid portions onto the flexible portions to form a protective edge. The sheet of flexible over-layer material may also extend over the rigid portions and be attached with a rigid adhesive.

BACKGROUND OF THE INVENTION 
This invention relates to printed circuits and their method of manufacture 
and, more specificially, to a printed circuit comprising multiple layers 
and rigid and flexible portions comprising, a sheet of flexible substrate 
material extending over the entirety of the rigid and flexible portions; 
paths of conductive material carried by at least one side of the sheet of 
flexible substrate material; a sheet of flexible over-layer material 
extending over at least the entirety of all the flexible portions; a 
flexible adhesive material adhesively attaching the sheet of flexible 
over-layer material to the entirety of all the flexible portions; sheets 
of a rigid substrate material extending over the entirety of all the rigid 
portions; and, a rigid adhesive material adhesively attaching the sheets 
of a rigid substrate material to the entirety of all the rigid portions. 
The printed circuits to be described hereinafter are generally very thin in 
cross section being constructed of multiple layers of film materials 
(conductive, insulative, and adhesive). In the interest of ease of drawing 
and understanding only, it should be readily recognized and understood by 
those skilled in the art that the drawing figures which accompany the 
written descriptions are not to scale. 
Printed circuits combining rigid and flexible portions are known in the art 
and are popular in many applications such as automotive and aircraft. Such 
printed circuits allow madern printed circuit-mounted electronic 
components to be mounted and interconnected without the need for prior art 
wiring "harnesses" which were prone to damage, mis-wiring, and the like. 
As depicted in simplified form in FIG. 1 by way of example, such a 
combined printed circuit 10 may include several rigid portions 12 
interconnected by flexible portions 14. Typically, the rigid portions 12 
have components 16 and connectors 18 mounted thereon while the flexible 
portions 14 provide the interconnecting conductors 20 which replace the 
wires of the prior art wiring harnesses. 
As is well known to those skilled in the art, printed circuits can be made 
by various deposition and etching processes. For example, one may start 
with an insulative substrate and deposit conductive paths and portions on 
one or both sides. In the alternative, one may start with an insulative 
substrate having a conductive layer on one or both sides and form the 
conductive paths and portions by removing un-needed portions of the 
conductive layer(s) using, for example, a chemical etching process through 
a mask. Such techniques are applicable to both rigid and flexible 
substrates so as to form rigid and flexible printed circuits. Since the 
manufacturing techniques are applicable to both rigid and flexible printed 
circuits, combined rigid and flexible printed circuits can be formed in a 
single process using a common technique. 
While the first printed circuits were typically a single layer of substrate 
having conductive portions formed on one or both sides, many printed 
circuits employed now comprise several layers with the individual layers 
being adhesively bonded together into a unitary structure. To provide 
inter-layer electrical connections, aligned holes through the layers 
(known as vias) are internally plated with a conductive material. Under 
ideal conditions, the foregoing structure and general method of 
manufacture presents no problems. Under actual manufacturing conditions, 
however, various problems present themselves. The result is a diminishing 
of the yield of the manufacturing process; that is , the various problems 
to be described shortly result in defects in the resultant printed 
circuits which make them unreliable and, therefore, unusable. As those 
skilled in the art are well aware, process yield is a most important 
factor in electronics manufacturing. Pricing and competitiveness (as well 
as product quality and reputation) depend on high yields of reliable 
components and products produced therefrom. Thus, it is of vital 
importance that the manufacturing processes used to make such multi-layer, 
combined rigid and flexible printed circuits result in high yields and 
constantly reliable parts. 
Typical prior art approaches to the construction and manufacturing of 
combined rigid and flexible printed circuits can be seen with reference 
to, for example, U.S. Pat. No. 4,687,695 (Hamby) or 4,800,461 (Dixon et 
al.). A typical prior art approach and its associated problems is depicted 
in simplified form in FIGS. 2 and 3. As with the printed circuit 10 of 
FIG. 1, there is a multi-layer printed circuit 10' having rigid portions 
12 and a flexible portion 14. At the core or center of the circuit 10' 
there is a flexible substrate 22 having first conductors 24 formed on the 
outer surface thereof according to any of the many techniques known to 
those skilled in the art. The substrate 22 and first conductors 24 are 
protected on both sides by a flexible overcoating material 26 which 
adhesively bonds thereto. The rigid portions 12 are created by attaching a 
rigid substrate 28 over the overcoating material 26 employing a flexible 
adhesive material 30. The rigid substrate 28, of course, also includes 
second conductors 32 formed thereon as necessary. 
While the rigid substrates 28 with the second conductors 32 pre-formed 
thereon could be adhesively attached to overcoating material 26, for 
registration reasons, the creation of the plated vias, etc. it is more 
typical to attach the rigid substrate 28 to the overcoating material 26 
using aligning holes through the various layers (not shown) and to then 
form the second conductors 32 and the vias (shown ghosted as 34) in 
separate manufacturing steps. It is this approach which causes the 
manufacturing problems leading to reduced yield and reliability mentioned 
earlier herein. Because of the heat used in testing processes, there is 
considerable difference in the thermal expansion which occurs in the 
adhesive material 30 and the various other materials. As a result, voids 
are created in the adhesive material 30 and cracking may occur in the 
conductive plating material of the vias 34. This can cause patent defects 
which decrease the yield or, more serious, latent defects which can cause 
the final product to fail or malfunction suddenly and unexpectedly at a 
later time. 
Another problem in the prior art is adverse chemical reactions during the 
manufacturing process. Typically, windows 36 are formed in the rigid 
substrates 28 in areas which are to be flexible in the final printed 
circuit 10'. To prevent the chemicals employed in formimg the vias 34 
and/or second conductors 32 from damaging the underlying flexible portion 
14, a blocking piece 38 is located in each window 36. A blocking piece 38 
can be located in the window 36 as shown on the upper layer of FIG. 2; 
but, in such case, will be relatively ineffective in blocking the 
chemicals, which can simply flow around the edges and under the blocking 
piece 38. The blocking piece may also move out of registration. More 
typically, therefore, in such a prior art approach to manufacturing, the 
blocking piece 38 is held in place by adhesive 40. This aids, to aome 
degree, the chemical leakage problem--but doesn't always and completely 
solve it. On the other hand, it can introduce a new problem when removed 
after manufacture as depicted in FIG. 3. 
A solution to this latter problem is disclosed in copending application 
Ser. No. 07/209,826, filed 6.22.88, which is assigned to the common 
assignee of this application. It is also depicted in simplified form in 
FIG. 4 hereof. For ease of understanding, elements of the printed circuit 
10" of FIG. 4 which are common to the printed circuit 10' FIGS. 2 and 3 
are designated with common numbers. Thus, there is once again a flexible 
substrate 22 having first conductors 24 formed on the outer surface 
thereof at the core or center of the circuit 10". The substrate 22 and 
first conductors 24 are protected on both sides by a flexible overcoating 
material 26 adhesively bonded thereto. The rigid portions 12 comprise a 
rigid substrate 28 including second conductors 32 formed thereon as 
necessary. At that point, the construction of the printed circuit 10" 
changes to implement the novel aspects of that co-pending application. As 
depicted in the bottom layer of the printed circuit 10" of FIG. 4, a 
protective layer 42 is placed over the entirety of the overcoating 
material 26--but, only adhesively attached (as with adhesive 44) over 
those portions thereof which are to be part of the rigid portions 12. In 
other words, in the "window" areas 36 the protective layer 42 simply 
covers the overcoating material 26, but is not adhesively attached 
thereto. The rigid substrate 28 is then attached to the protective layer 
42 with adhesive material 30. The blocking pieces 38 or portions 38 are 
integral with but partially separated from the remainder of substrate 28 
so that they may be readily removed after manufacture. When the 
manufacturing process is complete and the chemicals have been washed from 
the completed printed circuit 10", the blocking pieces 38 and the portions 
of the protective layer 42 within the window 36 are removed as depicted in 
the top layer of the printed circuit 10" of FIG. 4. No damage occurs to 
the overcoating material 26 and the portions under it when the protective 
layer 42 within the window 36 is removed since it is not adhesively 
attached to the overcoating material 26. 
As those skilled in the art will readily recognize and appreciate, the 
novel approach of the above-described co-pending application solves the 
problem of the blocking pieces 38 and eliminates the possibility of 
chemical damage of the prior art over which it is an improvement; however, 
it does not address the above-described problem of voids being created in 
the adhesive layers and/or the problem of cracking of the plating within 
the vias from thermal expansion differences. 
Wherefore, it is the object of this invention to provide a construction for 
printed circuits combining rigid and flexible portions and a method of 
manufacture thereof which eliminates not only the possibility of chemical 
damage but also both the problem of voids being created in the adhesive 
layers and the problem of cracking of the plating within the vias from 
thermal expansion differences. 
Other objects and benefits of the invention will become apparent to those 
skilled in the art from the detailed description which follows hereinafter 
when taken in conjunction with the drawing figures which accompany it. 
SUMMARY 
The foregoing object has been achieved by the printed circuit comprising 
multiple layers and rigid and flexible portions of the present invention 
comprising, a sheet of flexible substrate material extending over the 
entirety of the rigid and flexible portions; paths of conductive material 
carried by at least one side of the sheet of flexible substrate material; 
a sheet of flexible over-layer material extending over at least the 
entirety of all the flexible portions; a flexible adhesive material 
adhesively attaching the sheet of flexible over-layer material to the 
entirety of all the flexible portions; sheets of a rigid substrate 
material extending over the entirety of all the rigid portions; and, a 
rigid adhesive material adhesively attaching the sheets of a rigid 
substrate material to the entirety of all the rigid portions. 
According to one variation, the paths of conductive material carried by at 
least one side of the sheet of flexible substrate material comprise a 
conductive foil adhesively attached to the sheet of flexible substrate 
material with a flexible adhesive in the flexible portions and with a 
rigid adhesive in the rigid portions. 
According to another variation, the rigid adhesive adhesively attaching the 
sheets of a rigid substrate material to the entirety of all the rigid 
portions extends out over the edge of the rigid portions onto the flexible 
portions over the sheet of flexible over-layer material whereby the sheet 
of flexible over-layer material is protected from edge portions of the 
rigid substrate material at points of flexing of the flexible portion 
adjacent thereto. 
According to yet another variation, the sheet of flexible over-layer 
material also extends over the entirety of all the rigid portions and a 
rigid adhesive material adhesively attaches the sheets of flexible 
over-layer material to the entirety of all the rigid portions.

DESCRIPTION OF THE PREFERRED EMBODIMENT: 
A printed circuit 10'" according to the present invention in one embodiment 
is shown in edge view in FIG. 5 and one end is shown enlarged and cutaway 
in FIG. 6. A second embodiment is similarly shown in FIGS. 7 and 8 while a 
third is shown in FIGS. 9 and 10. Again, for ease of comparison and 
understanding, elements of the printed circuits 10'" which are common to 
the printed circuit 10" of FIG. 4 and printed circuit 10' of FIGS. 2 and 3 
are designated with common numbers. As will be appreciated from the 
descriptions of the various embodiments of the present invention contained 
hereinafter, the common element and major point of novelty is the use of 
rigid adhesives in the rigid areas of the printed circuit and flexible 
adhesives in the flexible areas thereof. That is, unlike the prior art 
approaches which employ a uniform adhesive coating across the layers of 
the printed circuit during its steps of manufacture where needed, in the 
present invention the adhesive layers are comprised of different adhesives 
on the same layer. 
With reference first to the embodiment of FIGS. 5 and 6, as will be seen 
from the numbering there is once again a flexible substrate 22 having 
first conductors 24 formed on the outer surface thereof at the core or 
center. The substrate 22 and first conductors 24 are protected on both 
sides by flexible over-layers 46 bonded thereto with a flexible adhesive 
48 in the flexible portions 14 and a rigid adhesive 50 in the rigid 
portions 12. The rigid portions 12 again comprise a rigid substrate 28 
including second conductors 32 formed thereon as necessary. In the printed 
circuit 10'" of this invention, the flexible over-layers 46 take the place 
of the protective layers 42 of FIG. 4; but, perform the same protective 
function with respect to any possible chemical damage during later 
manufacturing steps. The rigid substrate 28 is attached to the over-layers 
46 with the rigid adhesive material 50 and the blocking portions 38 fill 
the windows 36 these being held in place by their connection to the 
remainder of the substrate 28 at the location of grooves 52 which 
facilitate later removal of the portiona 38. During manufacture the 
adhesive material layers 50 which attach the substrates 28 to the 
over-layers 46 extend under the blocking portions 38 throughout the 
windows 36. A release coating is provided between the layers 50 and the 
over-layers 46 to facilitate removal of the blocking portions 38 and the 
immediately underlying adhesive 50 after manufacture. Note in particular 
in the enlarged view of FIG. 6 that the rigid adhesive material 50 used to 
attach the substrate 28 to the over-layer 46 extends out slightly from the 
edge of the rigid substrate 28 adjacent the flexible portion 14. This 
extende portion 54 can be straight along its inner edge as depicted or, if 
desired, can be angled back towards the substrate 28. It can also be 
formed of a single layer of the adhesive material 50 or of multiple 
layers. Thus, during any subsequent chemical steps of the manufacturing 
process, there is complete protection by the substrate 28 including 
portions 38 and by over layers 46. 
To facilitate removal of the adhesive 50 in the windows, the layer 50 is 
die cut at the inner extension of the extended portions 54. The extended 
portions 54 act, in use, as stress relievers of the area of the circuit 
structures at junctions between rigid and flexible portions thereof. When 
the manufacturing process is complete and the chemicals have been washed 
from the completed printed circuit 10'", the blocking portions 38 and the 
adhesive 50 within the windows 36 are easily removed without damage to the 
underlying portions of the flexible portion 14. Moreover, since the rigid 
substrates 28 are attached to the over-layers 46 with the rigid adhesive 
material 50, there is no excessive thermal expansion of the adhesive 
material 50 during the subsequent testing steps and, therefore, no voids 
are formed and no cracking can take place in the plating material of the 
vias 34. 
The embodiment of FIGS. 7 and 8 is substantially the same as that of FIGS. 
5 and 6 with one exception. The over-layers 46 extend only over the 
windows 36 that comprise the flexible portions 14 of the final printed 
circuit 10'". The portion of the layer not including the over-layer 
therein under the substrate 28 comprises an additional quantity (or 
layer(s)) of the rigid adhesive 50. Thus, in this embodiment, with the 
exception of the flexible substrate 22 at its center, the entirety of the 
rigid portion 12 is comprised of rigid materials. 
Similarly, the embodiment of FIGS. 9 and 10 is substantially the same as 
that of FIGS. 5 and 6 also with one exception. The first conductors 24 are 
separate and attached to the flexible substrate 22 employing a layer of 
the rigid adhesive 50 in the rigid portions 12 and flexible adhesive 48 in 
the flexible portions 14 rather than being formed directly on the surface 
of the substrate 22. As those skilled in the art will readily recognize 
and appreciate, while not specifically depicted, a third embodiment could 
be made according to the present invention by combining the exceptions of 
FIGS. 7 and 8 with those of FIGS. 9 and 10 to have the over-layers 46 
extend only over the windows 36 that comprise the flexible portions 14 of 
the final printed circuit 10'" and having the first conductors 24 separate 
and attaching them to the flexible substrate 22 employing a layer of the 
rigid adhesive 50. 
In the preferred implementation of the present invention in its various 
embodiments as described above, the preferred material for the rigid 
substrate 28 is a polyimide or epoxy material. The preferred over-layer 46 
is a polyimide film such as those sold under the names Kapton, Upilex, and 
Apical. The preferred flexible adhesive 48 is an acrylic adhesive or 
similar flexible polymer. The preferred rigid adhesive 50 is an epoxy or 
polyimide prepreg. 
The presently contemplated best mode of the various embodiments described 
above consists of a central plane of Kapton or other polyimide film 
coextensive throughout the rigid and flexible regions. Copper foil is 
bonded to either or both surfaces thereof with a flexible adhesive 
material such as duPont Pyralux in the flexible areas and a rigid adhesive 
material such as Hitachi Chemical GIA-67N prepreg or Arlon 37N in rigid 
areas. This construction provides all-flexible materials in flexible areas 
and virtuallyb all-rigid material in rigid areas. The "virtual" limitation 
in the rigid areas is the polyimide film which, while actually flexible, 
is a low-expansion, high modulus material with a high T.sub.c and , 
therefore, acts like the rigid materials with respect to the thermal 
expansion problem described earlier herein. 
Thus, it can be seen that all the objectives for a multi-level printed 
circuit combining rigid and flexible portions as stated above are met by 
the present invention.