Circuitized substrate with material containment means and method of making same

A circuitized substrate having conductive circuitry thereon and a barrier located adjacent at least portions of the circuitry to serve as an effective constraint for liquid material (e.g., encapsulant) applied to cover and protect the circuitry. The barrier can be formed concurrently with circuitry formation and formed of materials (e.g., copper, nickel, gold) similar to those used for the circuitry. The barrier is of two-part construction and of a particular shape wherein one part affords a greater surface tension than the other such that the material may actually lie on one part while being prevented from engagement with the other. Providing such dual (or "progressive") surface tensions successfully constrains the liquid material at least until solidification thereof occurs.

TECHNICAL FIELD 
The invention relates to circuitized substrates and to methods for the 
manufacture of such circuitized substrates. Particularly, the invention 
relates to such circuitized substrates as may be used in electronic 
packaging for use in information handling systems (computers). 
BACKGROUND OF THE INVENTION 
Many electronic components used on circuitized substrates need to be 
encapsulated with a polymer material or the like after being attached to 
the substrate, which in turn may comprise a ceramic carrier, printed 
circuit board/card, or other type of electronic packaging medium. The 
encapsulation material typically serves to protect the component (and 
adjacent circuitry) from moisture, corrosion, and physical damage. 
It is also a common requirement of many packages that the encapsulation 
material be constrained to a specified region of the package, typically on 
the upper surface of the substrate. Such a requirement may be desired to 
save materials, time, and, therefore, costs of the final assembly. 
Circuitized substrates such as those described above for use in electronic 
packaging are known, with examples shown in U.S. Pat. Nos. 5,173,755 (Long 
et al) and 5,336,931 (Juskey et al), while various methods used to 
manufacture such circuitized substrates (e.g., printed circuit boards) are 
described in U.S. Pat. Nos. 5,001,604 (Lusby), 5,284,548 (Carey et al) and 
5,382,759 (Kei Lau et al). 
In U.S. Pat. No. 5,173,766, encapsulant is constrained by a casting frame 
of approximately the same height as the required encapsulation material. 
The casting frame is produced independent of the substrate, and is added 
in an assembly process some time after the circuits have been formed on 
the substrate. An inner encapsulant dam, made of an insulating material, 
acts as a first encapsulant dam, temporarily constraining a first 
encapsulant material. This material must of necessity be an insulator 
because if conductive it could short together circuits in the structure. 
The insulating material in this structure is produced in a separate 
process step or steps, and is subsequently laminated to a conductive film. 
In U.S. Pat. No. 5,336,931, a polymer-based dam material is dispensed on 
the substrate in a separate step from that which forms the article's 
circuitry. The encapsulant material is dispensed such that it does not 
flow over the dam material, and is shown to be constrained within the 
inner surface of the dam (not flowing over the dam's top edge and being 
constrained on the outer edge). 
In accordance with the teachings herein, the present invention defines a 
circuitized substrate in which an effective material (e.g., encapsulant) 
barrier is provided which is relatively easy to form on the substrate's 
surface substantially concurrently with formation of the substrate's 
circuitry. The barrier is made of similar materials as the circuitry and, 
by its unique configuration, is able to provide effective liquid material 
constraint while not being of a relatively tall, possibly obstructive 
configuration in comparison to the thickness of the corresponding 
substrate circuitry. This barrier can be provided without extensive 
modification to the apparatus and processes currently used to produce such 
substrates, thus assuring relatively low costs for such production. 
It is believed that a circuitized substrate and process for making same 
which afford the above advantages as well as others discernible from the 
teachings herein would constitute a significant advancement in the art. 
DISCLOSURE OF THE INVENTION 
It is an object of the invention to enhance the circuitized substrate art. 
It is another object of the invention to provide a circuitized substrate 
comprising circuitry and a barrier member for containment of encapsulation 
material wherein the barrier member can be formed concurrently with 
formation of the substrate's circuitry, thereby reducing time and other 
costs for making such a product. 
It is still another object of the invention to provide a method for forming 
a substrate comprising circuitry and a barrier member for containment of 
encapsulation materials. 
It is yet another object of the invention to provide such a method for 
making such a product which is readily adaptable to mass production 
operations, thereby further reducing costs associated with production. 
In accordance with one aspect of the invention, there is provided a 
circuitized substrate including a dielectric layer including a first 
surface, an electrical conductor positioned on the first surface of the 
dielectric layer, and a barrier member for preventing the flow of liquid 
material thereover. The barrier member includes a first portion of a first 
configuration positioned on the first surface of the dielectric layer 
adjacent the electrical conductor and a second portion of a second 
configuration positioned on the first portion. The first portion of the 
barrier member provides a greater surface tension with respect to the 
liquid than the second portion of the barrier member. 
In accordance with another aspect of the invention, there is provided a 
method of forming a circuitized substrate which comprises the steps of 
providing a dielectric layer having a first surface, positioning an 
electrical conductor on the first surface of the dielectric layer, and 
forming a barrier member on the first surface of the dielectric layer for 
preventing the flow of liquid material thereover by initially forming a 
first portion on the first surface of the dielectric layer adjacent the 
electrical conductor and thereafter forming a second portion on the first 
portion of the barrier member. The first portion of the barrier member is 
formed of a first configuration and the second portion of the barrier 
member is formed of a second configuration, the first portion providing a 
greater surface tension with respect to the liquid material than the 
second portion.

BEST MODE FOR CARRYING OUT THE INVENTION 
For a better understanding of the invention, together with other and 
further objects, advantages and capabilities thereof, reference is made to 
the following disclosure and appended claims in connection with the 
above-described drawings. It is understood that like numerals will be used 
from FIG. 1 to FIG. 8 to identify similar elements. It is also understood 
that the various elements of the invention as illustrated herein are not 
necessarily to scale and in fact may be exaggerated in size for enhancing 
illustration and description thereof. 
In FIG. 1, there is shown a first step in the manufacture of a circuitized 
substrate 10 in accordance with one embodiment of the invention. In FIG. 
1, a seed layer 26 (e.g., chromium) is deposited on the first surface 14 
of the dielectric material 12 (e.g., fiberglass-reinforced polymer resin, 
also known as "FR4" material) which forms the base layer for the 
invention. Material 12 is preferably polyimide, a known polymer. The seed 
layer may be deposited using sputter vacuum deposition techniques known in 
the integrated circuit industry, or alternatively may be laminated, 
chemically activated, or otherwise deposited, also using well-known 
procedures. Further description of these processes is thus not believed 
necessary. In a preferred embodiment, layer 26 is formed of two individual 
metals or metal alloys (27, 28), the first metal or metal alloy preferably 
chromium (as stated) and the second metal or alloy 28 preferably copper. 
In a preferred embodiment, chromium was deposited to a thickness of only 
about 0.8 microinches and a layer of copper having a thickness of about 24 
microinches added thereto. 
In FIG. 2, there is shown a subsequent step in the manufacture of 
circuitized substrate 10. In FIG. 2, a layer of photoresist material 29 is 
deposited on seed layer 26, irradiated in accordance with the 
recommendations of the photoresist manufacturer, and chemically processed 
to remove desired portions of said photoresist. A preferred photoresist 
material for use in the invention is Hercules CFI dry film resist 
available from Hercules, Inc., Wilmington, Del. This is a negative 
photoresist. In one embodiment, layer 29 had a thickness of about 0.001 
inches, and was applied using conventional lamination technology. 
In FIG. 3, a barrier member 18 and electrical conductor 16 are formed 
simultaneously, preferably using a known electroplating process. More 
specifically, the substrate is immersed in a solution containing an ionic 
source of deposited copper, such as copper sulfate. Direct current is 
passed through the solution, with the substrate being the negative 
terminal of the circuit in the solution, thus causing the reduction of 
ionic copper to metallic copper on the substrate in areas defined by the 
layer of photoresist material. Two barrier members 18 are shown, it being 
understood that only one can be used in accordance with the teachings 
herein, e.g., to only control material on one side of conductor 16. 
Conductor 16 may function as a circuit line and/or conductive pad in the 
final circuitry of substrate 10. Using the above electroplating process, 
conductor 16, and, of course, barrier member 18, are of bi-layered 
construction, including a first portion 19 of a first layer of copper 
having a thickness of about 0.0005 to 0.0015 inches and a second portion 
of a second layer 20 of a different metal or metal alloy, preferably gold, 
from layer 19. Layer 20 is preferably about 20 to about 90 microinches 
thick. As described below, layer 20 is preferably comprised of two parts. 
In FIG. 4, a subsequent process step is shown illustrating the formation of 
a hole 34 in dielectric layer 12. In FIG. 4, a second layer 35 of 
photoresist 36 is deposited on the dielectric layer 12 and completely 
covers both the barrier members 18 and adjacent electrical conductor 16. 
This second photoresist is then exposed in accordance with the 
recommendations of the photoresist manufacturer, and chemically processed 
to remove portions thereof. The underlying dielectric material is then 
removed, preferably using chemical etching. This second photoresist 
material is preferably different than photoresist material 29 and, 
specifically, a photoresist having adequate chemical resistance to 
etchants used in subsequent processing of the invention. An example is 
duPont 330R, available from the E. I. duPont de Nemours Company, 
Wilmington, Del. Understandably, layer 35 extended entirely across 
dielectric material 12 on both the upper and lower surfaces. A region 35' 
is removed, exposing material 12 thereunder. This part of material 12 is 
then removed (etched) to the tapered configuration illustrated. 
In FIG. 5, a second part 41 of the electrical conductor 16 is formed on the 
exposed undersurface 43 of conductor 16 that bridges hole 34. This is 
preferably accomplished using electroplating. Second part 41 of conductor 
16 projects through hole 34, and preferably includes a thin layer of gold 
45 thereon, preferably formed by electroplating. As seen, part of 
photoresist material 36 has been removed, this occurred following 
formation of second part 41. In a preferred embodiment, second part 41 had 
a thickness of about 0.001 inch and gold layer 45 a thickness of about 20 
to 90 microinches. Part 41 spanned the entire width of hole 34 beneath the 
already formed first part of conductor 16, for a total distance in one 
embodiment of about 0.015 inch. One example of a use for second pad 41 is 
to provide an electrical and/or thermal path to an external component such 
as a circuit board or heat sink. 
In FIG. 6, portions of seed layer 26 that were unprotected by barrier 
member 18 and electrical conductor 16 are removed, preferably using a 
chemical etching process. This was accomplished by developing and removing 
photoresist above layer 26 that such removal was desired for (now shown in 
FIG. 6). The mentioned chemical etching process also resulted in a slight 
tapering of the side walls of both conductor 16 and barrier members 18. 
Such tapering represents a significant aspect of the invention, as 
described hereinbelow. 
In FIG. 7, a liquid material 50 is added. Preferably, liquid material 50 is 
encapsulant material, a preferred example being Hysol 4450, available from 
Dexter Corporation of Olean, N.Y. This material is dispensed using 
standard needle dispensing equipment. Most significantly, the liquid 
encapsulant extends readily outwardly to both barrier members 18 and is 
prevented from additional flow beyond said members. Of further 
significance, the liquid was able to form the substantially domed 
configuration in FIG. 6, having in one embodiment, a thickness of 0.020 to 
0.050 inch above the upper surface of conductor 16. Material 50 was then 
allowed to solidify. Such a thickness assures adequate coverage (and 
protection) of the underlying conductor 16, particularly if said conductor 
includes added structures, e.g., wires (e.g., gold) wirebonded thereto. 
In FIG. 8, a greatly enlarged view of barrier member 18 is provided for 
illustration purposes. The etching process described in FIG. 6 forms a 
first angle 60 (e.g., 45 to 90 degrees) between surface 14 of dielectric 
layer 12 and the first portion 19 of member 18. Also formed is a second 
angle 64 (e.g., 45 to 135 degrees) between this first portion and second 
portion 20. Second portion 20 is understood to comprise at least one thin 
layer 66 of a metal or metal alloy different from the metal or metal alloy 
used for first portion 19. A preferred example of such a second metal or 
alloy is gold. Significantly, this second metal or alloy provides a lesser 
surface tension with respect to liquid material 50 when this material 
attempts to flow thereacross compared to the surface tension exhibited by 
portion 19. It is believed that the use of different metals and/or metal 
alloys for both portions 19 and 20 of member 18, combined with selection 
of different angular orientations of such portions, accounts for the 
increased surface tension by portion 19 to thereby prevent material 50 
flow beyond the location of intersection substantially between portions 19 
and 20. Preferably, portion 20 is extremely thin in comparison to the 
corresponding thickness of underlying portion 19, as shown in FIG. 8. As 
also seen in FIG. 8, portion 20 conforms to the upper surface of portion 
19. 
As stated above, second portion 20 of barrier member 18 is preferably 
comprised of two parts (or separate, thin layers), shown in FIG. 8 by 
numerals 66 and 67. Layer 67, located immediately on underlying copper 
portion 19, is preferably of nickel having a thickness of about 20 to 
about 90 microinches. Thin layer 66 is preferably of gold, also with a 
thickness of about 20 to about 90 microinches, or about the same as layer 
67. 
Although the surface of the barrier member's side walls are shown as being 
straight (planar), it is readily possible to provide this surface with a 
slight curvature (shown in phantom in FIG. 8) such that it will appear 
slightly convex. Such a curved configuration is possible using a chemical 
etching process and adjusting the various parameters thereof during 
material etch. 
While there have been shown and described what are at present considered 
the preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications can be made 
therein without departing from the scope of the invention as defined by 
the appended claims.