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
2001-07-31
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, C522S095000, C522S092000, C522S096000, C522S097000, C560S221000, C560S222000, C525S451000
Reexamination Certificate
active
06268111
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, most particularly solder masks which are subject to harsh processing conditions such as are encountered in nickel/gold electroless plating.
BACKGROUND OF THE INVENTION
Novel binder oligomers in accordance with the present invention provide flexibility to photo-imaged photoresists and substantially improve performance of solder mask-forming photoresists when exposed to conditions of nickel/gold plating. While most important benefits of the binder oligomer are seen in solder mask-forming photoresists, flexibility is desirable in all photoresist compositions, including primary imaging photoresists.
A solder mask-forming photoresists forms 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 withstand 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.
Many current solder mask-forming photoresists are based on a styrene-maleic anhydride binder polymer. Because of the inflexibility of styrene/maleic anhydride polymers, solder mask-forming photoresists based on such polymers tend to hold up poorly under conditions of nickel/gold plating.
The present invention is directed to flexible photopolymerizeable binder 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 a binder oligomer having tetra-acrylate functionality for efficient photopolymerization as well as flexibility. The binder oligomer also has sufficient acid functionality that a photoimageable composition in which it is incorporated is developable in alkaline aqueous solution. The acrylate- and acid-functional binder oligomer may be advantageously used in primary imaging resists. Most significant advantages of the binder 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 binder 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 2,000; and
Z has the formula:
—CO—NH—Y—NH—CO—O(—VW
1-5
—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;
V is an alkylene, aromatic, alkylaryl, or arylalkyl hydrocarbon group, and the Ws are the same or different and are each an acidic group selected from the group consisting of COOH, SO
2
H, and PO
4
H(H or C
1-18
alkyl);
n=1-4, preferably 1;
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 50 and 100 weight percent of binder molecules having sufficient carboxylic acid functionality such that the photoresist is developable in alkaline aqueous solution, between about 5 and 100 weight percent of the composition (based on total weight of (A) plus (B)) being (A1) the oligomer of formula (I), and between 0 and about 50 weight percent of the composition comprising (A2) additional acid functional binder polymer, preferably at least about 10 weight percent comprising said additional acid functional binder polymer (A2);
(B) 0 to 50 weight percent, preferably at least 2 wt %, of a photopolymerizeable, non-acid functional component comprising &agr;,&bgr;-ethylenically unsaturated monomers and/or &agr;,&bgr;-ethylenically unsaturated oligomers exclusive of the oligomer of formula (I),
(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-lining agent and/or an epoxy cure catalyst.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENT
Herein, percentages are by weight, unless otherwise noted. Molecular weights are weight average molecular weights. The term “oligomer” is used herein to refer to polymerized molecules having weight average molecular weights of 500 to 5000; the term “polymer” is used herein to polymerized molecules having weight average molecular weights above 5000.
The novel binder oligomers (A1) of the present invention are conveniently prepared from commercially available bisphenol A epoxy acrylates having the general formula (I): (CH
2
═CH(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 functionalites to enter into the photopolymerization reaction without stearic hindrance so as to provide adequate photo
Barr Robert K.
Eramo, Jr. Lincoln
Lundy Daniel E.
Male Mitchell G.
Hamilton Cynthia
Rohm and Haas Company
LandOfFree
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