Radiation imagery chemistry: process – composition – or product th – Imaged product – Including resin or synthetic polymer
Reexamination Certificate
2002-05-09
2003-06-10
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaged product
Including resin or synthetic polymer
C430S280100, C430S311000, C428S901000, C522S101000, C522S102000, C525S524000, C525S482000
Reexamination Certificate
active
06576382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to photoimagable compositions, and more particularly to a solder mask having cationically polymerizable non-brominated epoxy resin system.
BACKGROUND OF THE INVENTION
There are many different instances where photoimagable solder masks are used in various industrial processes. In one particular process the solder mask is applied to printed circuit board, then photolithographic techniques are employed to reveal various underlying structures on the board while masking others so that solder may be applied to the exposed structures. During the solder applying process the solder will adhere to the exposed underlying components and be prevented from adhering where the remaining material operates as a solder mask.
It is necessary that the solder mask material be formulated such that it possesses rheological properties for effective coating. Further, the solder mask must permit efficient transmission of the light or other exposing radiation so as to photolyze the photoinitiator through whatever thickness of material is applied. Also, of course, the solder mask must possess appropriate physical and chemical properties to withstand the application of the solder without significant deterioration or degradation and maintain its coverage over the portions of the board wherein solder is to be masked.
There have been many prior art proposals for different photoimagable compositions including many that use epoxies.
Examples of these are found in the following U.S. Pat. Nos.: 4,279,985; 4,548,890; 4,351,708; 4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623; 4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197. All of these patents show various resins and photoinitiators for use in photoimagable compositions many of which are useful as solder masks. However none of them teach or suggest the specific composition of the present invention.
SUMMARY OF THE INVENTION
According to the present invention, an improved photoimagable cationically polymerizable epoxy based solder mask is provided that contains a non-brominated epoxy resin system and from about 0.1 to about 15 parts, by weight per 100 parts of resin system, of a cationic photoinitiator. The non-brominated epoxy resin system has solids that are comprised of from about 10% to about 80% by weight, of a polyol resin having epoxy functionality; from about 0% to about 90% by weight of a polyepoxy resin; and from about 25% to about 85% by weight of an difunctional epoxy resin. Since the photosensitive cationically polymerizable epoxy based solder mask does not contain bromine, it is particularly advantageous halogens in waste processing chemicals or in incinerated scrap circuit boards are regulated by environmental concerns. The photosensitive cationically polymerizable non-brominated epoxy based solder mask has a glass transition temperature greater than about 100° C., preferably greater than about 110° C. The solder mask dries to a tack-free film; thus, artwork used in the photoimaging process will not stick to the dried soldermask film.
The polyol resin which is a condensation product of epichlorohydrin and bisphenol A, has a weight average molecular weight of between about 40,000 and 130,000. The polyepoxy resin is an epoxidized multi-functional bisphenol A formaldehyde novolak resin having a weight average molecular weight of 4,000 to 10,000. The epoxidized diglycidyl ether of bisphenol A has two epoxide groups per molecule, a melting point of between about 80° C. and about 110° C. and a weight average molecular weight of between about 600 and 2,500. The invention also relates to a cationically polymerized solder mask.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a photoimagable solder mask which comprises a cationically polymerizable epoxy resin system and a photoinitiator. The solder mask is stable at 320° C. for 30 minutes and has a glass transition temperature greater than about 100° C., preferably greater than about 110° C., more preferably about 120° C. The photoimageable solder mask is applied to a substrate by conventional coating techniques such as roller coating, wound wire rod coating and curtain coating, for use as a solder mask. However, the photo solder mask can be used for other applications, such as an etch mask, permanent plating resist or protective coating. In this particular solder mask application, the photoimagable solder mask is typically coated by conventional coating techniques onto substrate to a thickness of about 0.5 to about 4.0 mils or more, dried, photoimaged and developed. The developed areas reveal the desired underlying metallized portions of the substrate wherein solder is to be applied, and the remaining solder mask material is cured and remains on the board as a solder mask during the application of solder by any conventional technique. Thus, the system of this invention must have satisfactory rheological properties for application techniques, it must be sensitive to exposure to radiation, which conventionally is in the 330 to 700 nm region, without any significant absorption to thereby allow penetration of the exposure radiation completely through the film; and it must also have the necessary physical and chemical properties to resist degradation during the soldering process.
For many applications the solder mask remains on the board after soldering. In formulating such a coating there are many competing factors that must be considered in order to provide a desired end product. No specific type of epoxy resin has been found which will satisfy all of the various requirements; however, a formulation of a combination or mixture of various epoxy resins according to this invention provides the required properties for a curtain coatable photoimagable solder mask composition.
The photosensitive epoxy solder mask comprises about 100 parts of non-brominated cationically polymerizable epoxy-resin system and about 0.1 to 15 parts by weight, of a cationic photoinitiator. The non-brominated epoxy resin system comprises between about 10% and about 80% by weight of a polyol resin of a high molecular weight and having epoxy functionality; between about 0% and about 90% by weight of a polyepoxy resin of a medium molecular weight; from about 25% to about 85% by weight of a difunctional, solid, low melting point epoxy resin. The epoxy resin system is characterized in that it does not contain brominated resins. The cationic photoinitiator is capable of initiating polymerization in the non-brominated epoxidized resin system upon exposure to actinic radiation. Preferably, the non-brominated resin system is further characterized by having an absorbance of light in the 330 to 700 nm region of less than 0.1 for a 2 mil thick film.
The Non-Brominated Epoxy Resin System
In general, the non-brominated epoxy resin system contains a phenoxy polyol resin which is a condensation product of epichlorohydrin and bisphenol A, herein also referred to as the “polyol resin”. The weight average molecular weight of the polyol resin is between about 40,000 and about 130,000 preferably from about 60,000 to about 90,000 and a weight per epoxide of from about 20,000 to about 65,000, preferably about 30,000 to about 45,000. Preferably the polyol resin is a difunctional epoxy resin. A suitable resin of this type was formerly sold by Union Carbide Corporation under the trademark PKHC and is currently sold by Phenoxy Resins Incorporated, also under the trademark PKHC. This resin has an epoxide value of about 0.03 equivalents per kg, a weight per epoxide of about 37,000, and a Tg, that is, glass transition temperature, of about 98° C.
The second resin in the system, although optional, is preferred. The second resin is an epoxidized multi-functional bisphenol A formaldehyde novolak resin with a medium range molecular weight and at least about 3 epoxy groups per molecule, also referred to herein as the “polyepoxy resin”: The weight average molecular weight of the polyepoxy resin is between about 4,000 and about 10,000 preferably from about 5000 to
Day Richard Allen
Glatzel Donald Herman
Russell David John
Hamilton Cynthia
Hogg William N.
International Business Machines - Corporation
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