Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-10-20
2001-07-24
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S284100, C525S451000, C560S222000, C560S221000, C522S095000
Reexamination Certificate
active
06265132
ABSTRACT:
The present invention is directed to photoimageable compositions useful as photoresists in the formation of electronic circuitry. Particularly advantageous aspects of the invention relate to photoresists used as solder masks, particularly solder masks which are subject to harsh processing conditions such as are encountered in nickel/gold electroless plating.
BACKGROUND OF THE INVENTION
Novel oligomers in accordance with the present invention provide flexibility to photo-imaged photoresists. This flexibility is desirable in all photoresist compositions, including primary imaging photoresists.
However, currently, tie most important benefits of the novel oligomers are in solder mask-forming photoimageable compositions. Such photoresists form a hard, permanent layer on a printed circuit board. A solder mask must be hard, durable, and resistant to chemicals, such as organic solvents and strong acids and bases. In this regard a solder mask should meet the standards of IPC/ANSI SM-840B Table 12 tests, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting Circuits). Herein, a solder mask is defined as a layer which meets those tests. Examples of solder mask-forming compositions that are developable in alkaline aqueous solutions are U.S. Pat. Nos. 5,229,252 and 5,364,736, the teachings of each of which are incorporated herein by reference.
Previously, solder was applied directly to exposed pads of the copper electronic circuitry traces for electrical connection to surface mounts and the like. Problems with soldering directly to the circuitry traces include expense of the solder, toxicity of the solder, copper migration into the solder, and tarnishing of the solder. An improved process involves nickel/gold plating of the pads in preparation for electrical connection. The exposed pads are initially plated with nickel by an electroless process to a thickness of between about 100 and about 300 micro inches. Then a thin layer, e.g., 6 micro inches, of gold is plated over the nickel by an electroless process. Nickel/gold plating is advantageous in that the layers are planer and the nickel gold protects the copper from migrating. Also, the gold does not tarnish.
However, the electroless plating processes are harsh, requiring solder masks to stand up to the plating conditions. Particularly, nickel electroless plating is typically conducted at about 90° C. for 15 minutes. Often, lifting of the solder mask in the regions around the pads is observed.
Lower molecular weight binder polymers of solder mask-forming photoimageable compositions tend to shrink less during processing and result in less lifting of the solder mask. However, lower molecular weight binder polymers tend to be tacky, a trait undesirable because contact imaging, i.e., direct contact between the artwork and the photoimageable composition, is required for maximum resolution.
Higher molecular weight binder polymers reduce tack but shrink more, resulting in a greater amount of solder mask lifting during nickel/gold plating.
The present invention is directed to flexible photopolymerizeable oligomers which reduce the effect of property trade-offs, such as tack versus lifting in a solder mask composition exposed to nickel/gold plating.
SUMMARY OF THE INVENTION
The invention provides an oligomer having tetra-acrylate functionality for efficient photopolymerization as well as flexibility. The acrylate-functional oligomer may be advantageously used in primary imaging resists. Most significant advantages of the oligomer, however, are currently seen in use in solder mask-forming photoimageable compositions, particularly in compositions which form solder masks that are subsequently exposed to nickel/gold plating or similar harsh processing conditions.
The oligomer of the present invention has the general formula (I):
(CH
2
═C(H or CH
3
)—CO—X—R—CH(OZ)—CH
2
—O—&phgr;—)
2
—C(CH
3
)
2
where
R is an alkylene group of from 1 to 40 carbons;
X is nothing or a diol selected from the group consisting of alkylene diols, polyester diols and polyether diols of molecular weight between 50 and 2,800; and
Z has the formula:
—CO—NH—Y—NH—CO—O(—X′—O—CO—NH—Y—NH—CO—O)
n
—Q—O—CO—C(H or CH
3
)═CH
2
;
where
the Ys are alkylene or aromatic hydrocarbon groups;
n=1-4, preferably 1;
X′ is a diol selected from the group consisting of alkylene diols, polyester diols, and polyether diols; and
Q in an alkylene group of 2 to 6 carbon atoms.
Primary imaging photoresist in accordance with the invention include, based on weight of (A) plus (B);
(A) between about 30 and about 80 weight percent binder polymer having sufficient carboxylic acid functionality such that the photoresist is developable in alkaline aqueous solution;
(B) between about 20 and about 70 weight percent of a photopolymerizeable component comprising &agr;,&bgr;-ethylenically unsaturated oligomers and optional additional &agr;,&bgr;-ethylenically unsaturated monomers, at least about 2 weight percent, preferably at least about 35 weight percent, based on total weight of (A) plus (B) comprising oligomer of formula (I); and
(C) between about 0.1 and about 10 weight percent of a photoinitiator chemical system.
Photoimageable compositions which are to intended to provide permanent layers, such as solder masks, further comprise, in a part separate from (A), (D) between about 5 and about 40 weight percent based on total weight of (A) plus (B) an epoxy resin. Compositions containing (D) preferably also contain (E) between about 1 and about 10 weight percent based on total weight of (A) plus (B) of an epoxy cross-linking agent and/or an epoxy cure catalyst.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
The novel oligomers of the present invention are conveniently prepared from commercially available bisphenol A epoxy acrylates having the general formula (II): (CH
2
═C(H or CH
3
)—CO—X—R—CH(OH)—CH
2
—O—&phgr;—)
2
—C(CH
3
)
2
where X and R are as defined above. Such bisphenol A epoxy acrylates have molecular weights ranging from about 500 to about 1500. Commercial examples of such compounds include Novacure® 3701, Ebecryl® 600 and ECN® 117. While these commercial compounds have diacrylate functionality, they do not enjoy great use in present day photoimageable compositions, primarily because they have relatively high molecular weights per acrylate functionality and, if used in photoimageable compositions, provide insufficient photospeed. In accordance with the invention, these bisphenol A epoxy acrylates are modified to provide additional acrylate functionality in oligomer molecules that allow the acrylate functionalities to enter into the photopolymerization reaction without stearic hindrance so as to provide adequate photospeed. At the same time, the oligomers of the present invention are flexible, providing flexibility to photoimaged compositions. This flexibility, in particular, helps to reduce solder mask lifting under harsh processing conditions, such as in nickel/gold plating. Accordingly, higher molecular weight binder polymers can be utilized, thereby reducing tack. Thus, preferred solder mask compositions in accordance with the invention can be contact-imaged, have adequate photospeed, and withstand nickel/gold plating conditions.
It is to be noted in formula (II) that in addition to the two acrylate functionalities of the bisphenol A epoxy acrylates, the compounds have two hydroxy functionalities through which group Z of formula (I) is attached, adding two additional acrylate functionalities.
In the currently preferred synthetic route, the bisphenol A epoxy acrylate is reacted with a diisocyanate, two moles of diisocyanate being reacted so as to link at one end to each of the hydroxy functionalities and provide two isocyanate moieties available for further reaction. The diisocyanate may be any commonly available diisocyanate, such as hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), methylene diisocyanate (MDI), methylenebiscyclohexylisocyanate, trimethylhexamethyl diisocyanate, hexane diisocyanate, hexamethylamine diisocy
Barr Robert K.
Eramo, Jr. Lincoln
Male Mitchell G.
Hamilton Cynthia
Rohm and Haas Company
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