AU706730B1 - Photoimageable compositions for improved adhesion and processing times - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE

SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Nichigo Morton Co., Ltd.

Actual Inventor(s): Daniel E. Lundy Robert K. Barr Address for Service: PHILLIPS ORMONDE

FITZPATRICK

Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title:

PHOTOIMAGEABLE

PROCESSING TIMES

COMPOSITIONS

FOR IMPROVED ADHESION AND Our Ref 555966 POF Code: 1436/347739 The following statement is a full description of this invention, including the best method of performing it known to applicant(s):

PATENT

3553-35-00 PHOTOIMAGEABLE

COMPOSITIONS

FOR IMPROVED ADHESION AND PROCESSING TIMES The present invention is directed to negative-acting photoimageable compositions such as those used as photoresists used to form printed circuit boards. The photoimageable compositions utilize a binder polymer of relatively low molecular weight and contain, as at least a portion of a photopolymerizable component, a photopolymerizable urethane oligomer. This combination results in improved adhesion and processing times, yet flexibility is maintained.

Background of the Invention This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.

A variety of such photoimageable compositions are described. Essential components of compositions of the type to which the present invention is directed are A) a binder polymer; B) photopolymerizable a,p-ethylenically unsaturated compound(s), and C) a photoinitiator chemical system. The binder polymer A) has sufficient acid functionality, generally carboxylic acid functionality, that it is soluble in alkaline aqueous solution and thereby renders the photoimageable composition developable in alkaline aqueous solutions.

It is generally desirable to shorten processing times of photoresists, whereby greater production volumes are achieved.

Likewise, photoimageable compositions must adhere to a substrate; current formulations are expected to adhere to smoother surfaces and resolve smaller circuit geometries. Thus, improved adhesion is a goal of the present invention.

At the same time, flexibility is a desirable property of photoresists. Printed circuit -2boards generally have through-holes by which circuitry on one side of the board is connected to circuitry on the other side of the board. Photoresists are required to "tent" these through-holes during processing. Through-holes in printed circuit boards are increasingly becoming larger; accordingly, tenting strength is becoming increasingly important.

Hence, the present invention is directed to photoimageable compositions useful as photoresists having improved adhesion and processing time while retaining good flexibility.

Summary of the invention The negative-acting photoimageable composition comprises

A)

between about 29 and about 69 wt% based on total weight of A) plus B) plus C) of an organic polymeric binder(s) having sufficient acid functionality to render the photoimageable composition developable in alkaline aqueous solution, the binder polymer or portion of the binder polymer at between about 3 and about 69 wt%, preferably at least about 10 wt%, of the total of A) plus B) plus C) being a polymer having a weight average molecular weight between about 5000 and about 40,000, preferably between about 20,000 and about 40,000, balance of the binder polymer, preferably up to about 5 to 66 wt%, of the total of A) plus B) plus being a polymer having a weight average molecular weight between about 41,000 and about 200,000,

B)

between about 30 and 60wt% (calculated relative to total weight of A) plus B) plus of an addition-polymerizable component, component B) comprising a nongaseous ethylenically unsaturated compound or compounds capable of forming a polymer by free-radical initiated chain-propagating addition polymerization, and between about 0.5 and about 15 wt% of C) (calculated relative to total weight of A) plus B) plus of an organic, radiation-sensitive free-radical generating system activatable by actinic radiation to initiate chainpropagating addition polymerization of the addition-polymerizable material B).

Component B) comprises an isocyanate trimer having tri-a,p-ethylenically unsaturated functionality with the formula: IC C:%WINWORDJLONASHARON"SPECSP840

.DOC

_~1131~ -2a-

R

N

Q=C C=o N N R C R

II

where R is -(CH2)p-NH-COO-(CHY-CHY-O)m-CO-CX=CH2, where X is H or CH 3 Y is H, CH 3 or C 2

H

5 p is an integer from 1 to 36 and m is an integer from 1 to 14, said trimer B' comprising between about 2 and about 30 wt% relative to total weight of A) plus B) plus balance of B, up to 57 wt%, preferably at least about 5 wt%, relative to total weight of A) plus B) plus C) of B" comprising other a,p-ethylenically unsaturated monomers. In addition, compositions in accordance with the invention preferably contain between about 1 and about 8 wt%, preferably between about 2 and about 6 wt% (calculated relative to total weight of A) plus B) plus C) of D) a dibenzoate plasticizer.

IC C:\WIORDULONANSHARON\ JSPECrPSo1 .DOC ~111131 -3 Detailed Description of Certain Preferred Embodiments Herein, unless otherwise noted, all percentages are weight percentages. Component A) (the binder polymer), Component B) (the photoimageable compounds(s)) and Component C) (the photoinitiator chemical system) are herein considered to equal 100 wt%, and other components, such as plasticizer, are calculated as parts relative to 100 parts of A) plus B) plus Molecular weights of polymers and oligomers, unless otherwise stated, are weight average molecular weights.

The invention is directed to photoimageable compositions which are developable in alkaline aqueous solution and which therefore have substantial acid functionality. Such photoimageable compositions typically have a binder polymer A) having acid functionality, 1o typically an acid number of at least about 80, preferably at least about 100 and more preferably about 150 or more, up to about 250. The acid functionality is typically carboxylic acid functionality, but may also include, for example, sulfonic acid functionality or phosphoric acid functionality.

The binder polymer A) is typically derived from a mixture of acid functional monomers and non-acid functional monomers. Some specific examples of suitable acid functional monomers are acrylic acid, methacrylic acid, maleic acid, fumaric acid, citraconic acid, 2 -acrylamido-2-methylpropanesulfonic acid, 2-hydroxyethyl acrylolyl phosphate, 2-hydroxypropyl acrylol phosphate, 2-hydroxy-alpha-acryloyl phosphate, etc.

One or more of such acid functional monomers may be used to form the binder polymer.

The acid functional monomers may be copolymerized with non-acid functional monomers, such as esters of acrylic acids, for example, methyl acrylate, methyl

PATENT

3553-35-00 methacrylate, hydroxy ethyl acrylate, butyl methacrylate, octyl acrylate, 2-ethoxy ethyl methacrylate, t-butyl acrylate, n-butyl acrylate, 2-ethyl hexyl acrylate, n-hexyl acrylate, diacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanediol diacrylate, 2,2-dimethylol propane diacrylate, glycerol diacrylate, tripropylene glycol diacrylate, glycerol triacrylate, 2 2 -di(p-hydroxyphenyl)-propane dimethacrylate, triethylene glycol diacrylate, polyoxyethyl-2-2-di(p-hydroxyphenyl)-propane dimethacrylate, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate, ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 2 ,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, 1-phenyl ethylene-i ,2-dimethacrylate, pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanediol dimethacrylate, and 1,4-benzenediol dimethacrylate; styrene and substituted styrene, such as 2-methyl styrene and vinyl toluene and vinyl esters, such as vinyl acrylate and vinyl methacrylate to provide the desired acid number.

Examples of such polymers and photoimageable compositions using such polymers are found, in the following U.S. Patent Nos: 3,953,309, 4,003,877, 4,610,951, and 4,695,527 the teaching of each of which are incorporated herein by reference.

Binder polymer of the photoimageable compositions of the present invention has a weight average molecular weight between about 5000 and about 40,000 preferably at least about 20,000. These molecular weights are relatively low for this type of binder polymer; however, the improved adhesion and processing time of the photoimageable composition of the present invention is largely attributable to the utilization of low molecular weight binder polymer. Binder polymer may be used alone, or in admixture with a higher molecular weight binder polymer Higher molecular weight binder polymer if used, is used to enhance tenting strength.

The photoimageable, acrylate functional, isocyanate trimer which comprises a

PATENT

3553-35-00 portion of, and in some cases all of, the photopolymerizable component B) has the general formula:

R

N

O=C C=O I I N N R C R

II

0 where R is -(CH2)-NH-COO-(CHY-CHY-O)m-CO-CX=CH2, where X is H or CH 3 Y is H, CH 3 or C 2

H

5 p is an integer from 1 to 36 and m is an integer from 1 to 14. Such trimers are described in European Patent Application EP 0 738 927 A2.

While the use of low molecular weight binder polymer enhances adhesion and processing time, the short chain polymers tend to reduce overall flexibility of the photoimageable composition. Poor flexibility can cause tented holes to fail and lines to fracture during the many mechanical handling steps, defects which necessitate scrapping printed circuit boards having such defects. The flexibility problem is addressed herein by the use, in conjunction with low molecular weight binder polymers, of the acrylatefunctional isocyanate trimer (also referred to herein as the "urethane oligomer"). To this end, the molecular weight of the isocyanate trimer should be at least about 1000. The relatively high molecular weight of the urethane oligomer compensates for the lower molecular weight of the binder polymer, producing a photoimageable composition with flexibility comparable to those formulations using higher molecular weight polymers.

The balance of photopolymerizable component used at 0 to about 30 wt% of the photoimageable composition (calculated relative to total weight of A) plus B) plus C) is typically a monomer, dimer or short chain oligomer having ethylenic unsaturation,

PATENT

3553-35-00 particularly a,p-ethylenic unsaturation, including monofunctional compounds and compounds having a,P-ethylenic unsaturation functionality 2 or greater. Typically, a mixture of mono-functional and multi-functional monomers will be used. Suitable photopolymerizable compounds include, but are not limited to, the monomers recited above as suitable for forming binder polymers, particularly the non-acid functional compounds.

To initiate polymerization of the monomers upon exposure to actinic radiation, the photoimageable composition contains photoinitiator chemical system. Suitable photoinitiators include, for example, 9-phenyl acridine, benzoin ethers, benzil ketals, acetophenones, benzophenones and related compounds with amines. Also, suitable 9phenyl acridine homologues, such as those described in U.S. Patent No. 5,217,845, the teachings of which are incorporated herein by reference, are useful photoinitiators.

As a preferred aspect of the invention, flexibility, is further improved through the use of dibenzoate plasticizers. This combination further contributes to fine line adhesion and good stripping characteristics. Dibenzoate plasticizers in accordance with the invention have the general formula:

C

6 6

H

5 where R -CHX-CHX-O- where one or both Xs are H or one X may be CH 3 and the other H; n 1 to 10, and R' is -CH 2

-CH(CH

3 -CH 2

-CH

2 -OOC-, or -OC-.

Specific examples of suitable dibenzoates include, but are not limited to dipropyleneglycol dibenzoate, diethylene glycol dibenzoate, polypropyleneglycol dibenzoate, and polyethylene glycol dibenzoate. The dibenzoate plasticizer if used, is used at levels of between about 1 and about 8 wt% relative to total weight of A) plus B) plus typically at between about 2 and about 6 wt%.

Compared to other plasticizers tested, the dibenzoates produced a significant improvement in tent strength. Coupled with improved flexibility, the combination of dibenzoate plasticizer and the isocyanate trimer produces a fine line (less than 75 microns) resist sidewall that adheres better to the copper surface. It is believed that the addition of

PATENT

3553-35-00 the dibenzoate contributes to a lower glass transition temperature (Tg) of the formulation.

The lower Tg allows for better flow at lamination and better conformation to the copper surface. This property is particularly important on copper surfaces with nicks or scratches.

Because the composition with the dibenzoate conforms better to the copper, more photoresist surface area is in contact with the copper surface. This results in more potential for chemical bonding to the copper surface and therefore, improved adhesion properties. Because the dibenzoate cannot be incorporated into the backbone of the exposed acrylic monomer system, the inclusion of the dibenzoate compounds results in less overall shrinkage of the composition. This reduced shrinkage probably produces less stress on the copper/photoresist interface, contributing to the improved adhesion. Less shrinkage is particularly observing if 9-phenyl acridine is used as the photoinitiator, as this initiator produces a high degree of cross-link density.

Processing of the photoimageable composition is in a conventional manner. In a typical procedure, a photoimageable composition layer, either formed from a liquid composition or transferred as a layer from a dry film, is applied to a copper surface of a copper-clad board. The photoimageable composition layer is exposed to actinic radiation through appropriate artwork. Exposure to actinic radiation polymerizes the monomer in the light-exposed areas, resulting in a cross-linked structure that is resistant to developer.

Next, the composition is developed in dilute alkaline aqueous solution, such as a 1 sodium carbonate solution. The alkali solution causes salt formation with the carboxylic groups of the binder polymers, rendering them soluble and removable. After development, an etchant may be used to remove copper from those areas where the resist was removed, thereby forming a printed circuit. The remaining resist is then removed using an appropriate stripper.

Throughout the description and claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

The invention will now be described in greater detail by way of specific examples.

Examples A-D (A-C comparative: D in accordance with the invention Photoimageable compositions A-D are prepared having the following components (Table 1):

PATENT

3553-35-00 1 Low Molecular Weight CoPolymer** MAA, 65% Styrene) Urethane Oligomer McesEthoyl Krietyol pne able 1 Listing of Photoresist Ingredients (all amounts listed in grams Of 100% solid material) Formula Formula Formula "C"9 40 40 Formula

"D"

1 2 0.20 0.05 0.075 0.10 0.03 0.20 0.05 0.075 0.10 0.03 0.20 0.05 0.075 0.0 0.20 0.05 0.075 0.10 0.03 0.18 0.18!: 0.18 0.18 Dipropylene glycol 3.50 3.503.0.5 dibenzoateI1350.0 *metha 85,000 20,000 M A =melhac fic =cid ,A=.-luyl -acrae MMA-= methyl 8

PATENT

3553-35-00 All compositions were prepared in 7:1 2 -Butanone:2-Propanol at approximately solids. The solutions were coated onto biaxially oriented 80 gauge polyester film and dried to approximately 1% or less retained solvent. The coated mixtures were then laminated onto mechanically scrubbed 1 oz./FR-4/1 oz. clad copper composite using a hot roll laminator at 110°C at 2 meters/minute and 3 bar pressure.

The laminated material was then imaged on a UV printer through an appropriate phototool with an adjusted exposure to obtain a copper step of 7 as measured with a Stouffer® 21 step wedge (approximately 20 mJ/cm 2 The exposed panels were then developed in a 1% sodium carbonate monohydrate solution at 29C using a conveyorized spray developer at about 26 psi with residence time adjusted so that the break point occurred at 40% to 50% of the chamber length (unless otherwise noted in the specific example), followed by several spray rinses using tap water and the deionized water.

Etching was accomplished using a 2N cupric chloride/hydrochloric acid solution at 48 0 C in a conveyorized etcher equipped with multiple spray nozzles. The etched boards were then stripped of the imaged, developed and etched photo resist in a 3 sodium hydroxide solution at 54C in a conveyorized stripping unit equipped with mutiple spray nozzles followed by a spray rinse of tap water.

Process responses for the examples are cited at various points throughout the above procedure. Results are shown in Table 2:

PATENT

3553-35-00 Table 2 Performance Results indicates desired performance) Testing Parameter Fine Line Adhesion (minimum line adhered Wit 400,u space) Tent Flexibility (grams force to break) Developing Time Na 2 CO, 30 0

C)

Stripping Time NaOH, 50 -C) Cross Hatch flexibility Resist 'Intact) Formula J Formula "B" Formula "C" Formula "D" 50 AM 50 jum F 350 grams *570 grams 140 grams *500 grams 52 seconds 54 seconds *38 seconds 1 *38 seconds 80 seconds *52 secods *54 seconds 70% *80 50% *75 Note: formula appears to indicate a synergy between the low molecular weight polymer and the urethane oligomer

Claims (4)

1. A photoimageable composition comprising: A) between about 29 and about 69 wt%, based on total weight of A) plus B) plus of an organic binder polymer(s) having sufficient acid functionality to render said photoimageable composition developable in alkaline aqueous solution, A) comprising between about 3 and about 69 wt% relative to total of A) plus B) plus C) a binder polymer having a weight average molecular weight between about 20,000 and about 40,000, and to 66 wt% relative to total of A) plus B) plus C) a binder polymer having a weight average molecular weight between about 41,000 and about 200,000, B) between about 30 and about 60 wt%, based on total weight of A) plus B) plus of an addition polymerizable component comprising a,p-ethylenically unsaturated compounds, said component B) comprising an isocyanate trimer having [tri-a,13-ethylenically unsaturated functionality] the formula: R N O=C C=O N N R C II where R is -(CH 2 )p-NH-COO-(CHY-CHY-O)m-CO-CX=CH 2 where X is H or CH 3 Y is H, CH 3 or C 2 H 5 p is an integer from 1 to 36 and ICC:\WINWORDMLONA\SHARON\SJJSPECSP88401 .DOC i-o n~ ~1111 -12- m is an integer from 1 to 14, said trimer being present at between about 2 to about 30 wt% based on total weight of A) plus B) plus and 0 to about 57 wt% based on total weight of A) plus B) plus C) of other a,-ethylenically unsaturated compounds, and C) between about 0.5 to about 15 wt%, based on total weight of A) plus B) plus C) of an organic, radiation-sensitive free-radical generating system.

2. The photoimageable composition according to claim 1 wherein said free-radical generating system C) comprises 9-phenyl acridine.

3. The photoimageable composition according to claim 1 further comprising D) between 1 and 8 wt%, calculated relative to total weight of A) plus B) plus C) of a dibenzoate plasticiser.

4. A photoimageable composition according to claim 1 substantially as hereinbefore described with reference to any of the examples. DATED: 15 April, 1999 PHILLIPS ORMONDE FITZPATRICK Attorneys for: NICHIGO MORTON CO. LTD. IC C:%WIMORDMLONAHARON\SUSPECI.SPO 1 .DO