Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-09-03
2001-08-07
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S255000, C361S767000
Reexamination Certificate
active
06271481
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to multilayer circuit assemblies, and more particularly to pad shapes for improved etching of tri-metal-layered multilayer circuit assemblies.
2. Disclosure Information
U.S. Pat. No. 3,801,388 to Akiyama et al. (hereinafter “Akiyama”), U.S. Pat. No. 4,404,059 to Livshits et al. (hereinafter “Livshits”), and U.S. Pat. No. 5,738,797 to Belke, Jr. et al. (hereinafter “Belke”), all of which are incorporated herein by reference, disclose various methods for making electronic circuits which feature circuit crossovers or “air bridges” using various combinations of plating, masking, and etching steps. The methods disclosed in these references are useful for making multilayer circuits by selectively etching a tri-metal-layered precircuit such that the desired air bridge and circuit layout structures are created.
For example, the tri-metal precircuit may comprise a structure similar to that illustrated in
FIGS. 1-3
. Here, the middle layer
10
is a continuous 6-mil-thick aluminum sheet or foil having a top surface
14
and a bottom surface
12
, with a first (lower) conductor pattern
20
made of 2-mil-thick copper patterned onto the bottom surface
12
, and a second (upper) conductor pattern
40
made of 2-mil-thick copper patterned onto the top surface
14
. The lower conductor pattern
20
includes a plurality of base pads
26
, pedestal pads
22
, and circuit traces
24
, while the upper conductor pattern
40
includes a plurality of top pads
41
and bridging elements
42
. Each top pad
41
is arranged on the middle layer top surface
14
opposite a respective one of the base pads
26
. Each bridging element
42
has first and second enlarged ends
44
and a constricted portion
46
between and contiguous with the enlarged ends. The bridging elements
42
are arranged on the top surface
14
such that each enlarged end
44
is disposed opposite a respective one of the pedestal pads
22
and each constricted portion
46
is disposed opposite and transverse to a respective one of the circuit traces
24
.
This precircuit may then be affixed to a substrate
30
having an electrically insulative surface
32
by attaching the first conductor pattern
20
to this surface
32
. Then, the precircuit may be exposed for a predetermined amount of time to an etchant (e.g., sodium nitrate) which etches substantially only the aluminum, resulting in the final circuit structure illustrated in
FIGS. 4-6
. Here, those portions of the aluminum foil
10
which are sandwiched between an enlarged end
44
and a pedestal pad
22
, or between a top pad
41
and a base pad
26
, are more protected from attack by the etchant than are other portions of the foil
10
. After the predetermined amount of time has elapsed and the etching has ceased, most of the foil
10
has been etched away, except for a pedestal
48
of aluminum remaining sandwiched between (1) each matched pair of upper enlarged ends
44
and lower pedestal pads
22
, and between (2) each matched pair of upper top pads
41
and lower base pads
26
. This provides a plurality of air bridges
42
and a plurality of “towers”
43
, as shown in
FIGS. 4-6
.
The air bridges
42
created by this process serve as three-dimensional crossovers. For example, signal or current may flow from point A to point B along the lower conductor pattern, then rise to point C through an air bridge pedestal, then flow across the air bridge to point D, then down the other pedestal to point E, and then on across the lower conductor pattern to point F, thus allowing the circuit trace path ABCDEF to “cross over” the circuit trace path between points G and H.
Each tower
43
comprises a top pad
41
atop a pedestal
48
atop a base pad
26
, as shown in FIG.
6
. These towers
43
may be sized and arranged to serve in a variety of interconnect configurations. For example, a given set of towers
43
may serve as wirebond pads, solder joint pads (e.g., for reflowed chip components), direct chip attachment, and the like. A circuit trace
24
is typically attached to the base pad
26
of each tower element
43
.
The conductor patterns
20
/
40
may be formed on the aluminum sheet
10
by various methods disclosed in Belke, Livshits, and Akiyama. These references teach that the enlarged ends
44
and/or top pads
41
should be made a certain minimum size (e.g., 40 mils in diameter or smallest width, for the 2/6/2-mil example above) and the constricted portion
46
made a certain maximum size (e.g., no more than 5 mils wide), so that the foil
10
sandwiched between each pair of enlarged ends
44
and pedestal pads
22
and each pair of top pads
41
and base pads
26
is only partly etched through in the X direction leaving the desired pedestals
48
, while all other portions of the foil
10
(including those portions underneath the constricted portions
46
) are completely etched away.
Whenever there is ample circuit space available around a given air bridge or tower, one may design the enlarged ends
44
and/or top pads
41
well above the recommended minimum size, thus assuring the formation of sturdy pedestals
48
and a robust metallurgical connection between each pedestal and its upper enlarged end/top pad
44
/
41
. However, in fine-pitch applications, or where circuit layout is particularly crowded, it may not be possible to design the ends/pads
44
/
41
oversized; in fact, when the size of the ends/pads
44
/
41
is kept close to the recommended minimum size, or even more so when it is desired to make these features even smaller than recommended, there is often a danger of the ends/pads
44
/
41
becoming delaminated from their respective pedestals
48
during etching, as illustrated in FIG.
7
.
It would be desirable, therefore, to provide a way of keeping the size of the enlarged air bridge ends
44
and top pads
41
small, while minimizing the aforementioned risk of delamination.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the prior art approaches by providing specially designed multilayer precircuit and final circuit structures which enable the use of smaller air bridge and/or tower structures.
It is an object and advantage that the present invention provides precircuit and final circuit structures which have specially designed enlarged air bridge ends and/or top pads which minimize undercutting of these ends/pads, thereby decreasing their susceptibility to delamination.
Another advantage is that the present invention allows the use of smaller air bridge/tower structures with the same or better integrity and reliability as prior art approaches.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.
REFERENCES:
patent: 3801388 (1974-04-01), Akiyama et al.
patent: 4404059 (1983-09-01), Livshits et al.
patent: 4888665 (1989-12-01), Smith
patent: 4920639 (1990-05-01), Yee
patent: 5264664 (1993-11-01), McAllister et al.
patent: 5381307 (1995-01-01), Hertz et al.
patent: 5468917 (1995-11-01), Brodsky et al.
patent: 5738797 (1998-04-01), Belke, Jr. et al.
Baker Jay DeAvis
Goenka Lakhi Nandlal
Kajander John R.
Paladini Albert W.
Visteon Global Technologies Inc.
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