Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-06-26
2003-10-14
Vo, Peter (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S846000, C029S848000, C338S258000, C338S308000
Reexamination Certificate
active
06631551
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to circuit boards and their fabrication. More particularly, this invention relates to a method for integrating stable resistive and capacitive materials with organic-based substrates, yielding resistors and capacitors that exhibit stability under temperature and humidity stress and over a wide range of resistance/capacitance values.
BACKGROUND OF THE INVENTION
Inorganic-based thick-film resistors are formed of materials that can provide a wide range of stable resistor values, and therefore have been widely used to replace discrete resistors on ceramic substrates of hybrid electronic circuits. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink that is typically composed of a glass frit composition, an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, and an organic vehicle. After printing, the ink is sintered or fired at a high temperature, typically about 850 to 900° C. The heating rate is controlled to first remove the organic vehicle, after which the glass frit composition is bonded together and to the ceramic substrate, forming a solid resistive mass that forms a thick-film resistor with a predictable and stable resistance when properly terminated. Other layers of material may then be printed over the resistor to yield a buried resistor in a multi-layer circuit board.
Thick-film inks of the type described above are fired at temperatures that organic substrates, such as those for printed wiring boards, cannot tolerate. Therefore, integrated resistors for organic substrates are typically formed from resistive materials such as NiP plated on copper which is subsequently laminated to the substrate, CrSi sputtered onto copper on polyimide, and polymer thick-film (PTF) inks that are printed on the substrate. PTF inks are generally composed of an electrically-conductive material (e.g., carbon) in an organic matrix material. After printing, PTF inks are heated to cure the matrix material to form an electrically-resistive film that adheres to the organic substrate. The temperature to which PTF materials must be heated is roughly about 200° C., which can be tolerated by organic substrates for a few minutes.
Processes required for organic substrates and their resulting integrated resistors have certain disadvantages as compared to resistors and processes for ceramic substrates. The NiP and CrSi materials are expensive and can be practically produced only with low sheet resistivities (up to about 1000 ohms/square). Consequently, large-value resistors must be very long, introducing electromagnetic interference (EMI) problems and occupying relatively large areas on or within the organic substrate. PTF resistors can be cost-effective but are relatively unstable because the organic matrix continues to cure during subsequent exposures to high temperatures (e.g., above 100° C.), causing drift in the resistance value. Extended cures at temperatures sufficient to eliminate this problem are not compatible with organic substrates.
In view of the above, it can be appreciated that integrated resistors that can be formed on organic substrates are often very large or less stable than integrated thick-film resistors formed on ceramic substrates. Similar differences can be seen between capacitors for ceramic and organic substrates. Stable integrated thick-film capacitors are formed on ceramic substrates using thick-film inks of high-dielectric constant (high &egr;
r
) materials (e.g., BaTiO
3
, perovskites) and an organic vehicle, the latter of which is burned off during firing. For organic substrates, high-&egr;
r
materials such as BaTiO
3
are mixed with organic binders such as epoxy to form a composite that can be deposited and cured to form a capacitor. High-&egr;
r
materials and one of the capacitor electrodes may also be sputtered roll-to-roll onto copper on polyimide for organic substrates, or a high-&egr;
r
film (such as polyimide with BaTiO
3
) may have conductors sputtered onto both sides. Other deposition methods include vacuum deposition and solution coating. Lower &egr;
r
organic (polymer) materials are deposited in thin layers and cured to form thin capacitors in organic substrates. In some applications, the low total capacitance of capacitors formed from lower-&egr;
r
organic materials can replace enough discrete capacitors to be cost-effective. However, lower-&egr;
r
organic materials generally cannot form small capacitors of appreciable capacitance values. Consequently, capacitors formed from these materials have found limited application, such as power filters where entire planes are formed of these materials to decouple power everywhere on a substrate.
The higher-&egr;
r
materials mixed in organic matrix materials noted above achieve higher capacitance values, and can be quite effective for filtering power. However, capacitors used as filters on signals would be desirable, and for these applications only one of the electrodes (ground) can be in common, such that there is no benefit to be gained from sharing capacitance (as with power filtering). Considerable development would be required to make the higher-&egr;
r
materials in organic matrices a suitable capacitor material for integrating discrete capacitors in organic substrates. Finally, sputtered high-&egr;
r
materials, whether applied directly as an oxide or applied as a metal and anodized, currently suffer from defects in addition to having high material and processing costs.
From the above, it can be appreciated that resistors and capacitors have been integrated into ceramic substrates using inorganic-based materials and are able to have a wide range of stable resistance and capacitance values. However, the inorganic-based materials that provide these benefits have not been used with organic substrates because the high firing temperatures required are incompatible with organic substrates. Organic-based materials, whose processing temperatures (e.g., cure) are compatible with organic substrates, do not yield resistors and capacitors that are as stable as inorganic-based resistors and capacitors, and their relatively poor resistive and capacitive properties do not allow for stable resistance and high capacitance values. Accordingly, what is needed are stable resistors and capacitors that can be formed to have a wide range of resistance/capacitance values, yet can be integrated on organic substrates.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a process for forming stable integrated resistors and capacitors on organic substrates. The resistors and capacitors are capable of a wide range of resistance and capacitance values, yet can be processed in a manner that does not detrimentally effect the organic substrate or entail complicated processing. Resistors and capacitors formed in accordance with this invention can be buried in a multi-layer circuit board.
The method of this invention generally entails the use of thick-film materials of the types used to form resistors and capacitors on ceramic substrates. The thick-film materials are applied to an electrically-conductive foil, such as a copper or stainless steel foil, and then heated to bond the thick-film material to the foil and form a solid mass. The solid mass will either be electrically resistive, capacitive or insulating, depending on the composition of the thick-film material. For example, the solid mass is electrically resistive if the thick-film material contains an inorganic composition, an electrically-conductive material, and an organic vehicle, but would be capacitive if the thick-film material contains a high-&egr;
r
constituent. To be useful as a capacitor, the dielectric constant of the solid mass should be very high, such as with the BaTiO
3
family of materials. For a capacitor, a second conductor layer is applied to the solid mass such that the solid high-&egr;
r
mass is sandwiched between with the conductive foil and the second co
Bowles Philip Harbaugh
Ellis Marion Edmond
Mobley Washington Morris
Parker Richard Dixon
Chmielewski Stefan V.
Delphi Technologies Inc.
Funke Jimmy L.
Nguyen Donghai
Vo Peter
LandOfFree
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