Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-09-24
2001-03-20
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S266000, C174S265000, C029S852000
Reexamination Certificate
active
06204456
ABSTRACT:
BACKGROUND OF THE INVENTION
In printed circuit board fabrication, SLC circuit layers and circuit layers within the conventional printed circuit board are interconnected by through holes which have been drilled and plated. Due to drilling and plating limitations, these plated through holes are quite large with respect to the SLC wiring, and remove valuable wire routing real estate from the SLC layers.
One method of filling holes involves forcing a fill composition, typically a copper filled epoxy composition, through a mask into the holes of the printed circuit board. However, the fill material can bleed between the mask and/or leave nubs at the surface of the filled through hole. This fill process further requires a polishing step after to remove the nubs and the bleed from the surface. The polishing process itself is not perfect. Often times, a small nub will be left after polishing or the fill material may tear out of the hole, leaving a depression at the center of the hole. Both residual nubs and depressions can cause problems during subsequent circuitization steps. The technique of forcing a fill composition through a mask also requires expensive paste, and requires the step of drilling the mask.
Other techniques for filling of holes involve screen printing a liquid epoxy material into the holes, or covering the holes with a dry film. Both of these techniques tend to trap a small air pocket at the center of the hole which can lead to cracking of the overlying circuit lines during thermal cycling.
The filling of vias also encounters similar problems. The screening or tenting processes can cause cracks in the encapsulant materials during thermal cycling. These cracks in the encapsulant will allow moisture penetration to the device, and can also propagate through the encapsulant and cause chip cracking.
It would be desirable to have an inexpensive and efficient technique for reliably and completely filling holes that does not trap air in the through hole, cause bleeding, or leave nubs or depressions at the surface of the filled through holes.
SUMMARY OF THE INVENTION
The present invention provides a novel method of filling apertures, for example, through holes, in substrates. The method utilizes a dielectric film, preferably a photoimageable dielectric film, which is employed to fill the apertures and to form a layer of dielectric film disposed above the substrate at the same time. As a result, the aperture fill is the same material as, and indeed continuous with, the dielectric film which is disposed on the substrate. The method employs the following steps: providing a substrate having apertures; providing a dielectric film disposed atop the substrate covering the apertures, reflowing the dielectric film to flow into the apertures and to form a dielectric film adherent to the substrate, to provide a continuous dielectric extending from the dielectric film into the apertures. In certain embodiments, after filling the apertures in the substrate, the dielectric film is patterned. For example, apertures, such as vias, are defined preferably by photoimaging or laser ablation, in the dielectric film. Preferably, the vias are then metallized, and circuitry formed atop the dielectric film.
The dielectric film preferably has solids which comprise 0 to about 50% of an inorganic particulate and about 50% to about 100% parts of an epoxy resin system; the epoxy resin system having: from about 10 to about 80% of phenoxy polyol resin which is the condensation product of epichlorohydrin and bisphenol A, having a molecular weight of from about 40,000 to 130,000, from about 20 to 90% of an epoxidized multifunctional bisphenol A formaldehyde novolac resin having a molecular weight of from about 4,000 to about 10,000, from about 35% to about 50% of a diglycidyl ether of bisphenol A having a molecular weight of from about 600 to about 2,500; and from about 0.1 to 15 parts by weight of the total resin weight, a cationic photoinitiator.
The invention also relates to circuitized structures produced according to the methods.
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Lauffer John M.
Markovich Voya R.
Palomaki Cheryl L.
Wilson William E.
Cuneo Kamand
Gaffin Jeffrey
Hogg William N.
International Business Machines - Corporation
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