Metallization of Polymers - American Chemical Society

Chapter 35

Polyetherimide Surfaces Chemically Treated To Improve Adhesion to Electroless Copper 1

Donald F. Foust and William V. Dumas Downloaded by PENNSYLVANIA STATE UNIV on March 14, 2015 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch035

Corporate Research and Development, General Electric Company, P.O. Box 8, Schenectady, NY 12301

A method for reliably obtaining 150-250 g/mm peel strength between glass-filled U L T E M polyetherimide surfaces and electroless copper is described. Unlike existing commercial technologies for metallizing plastics which rely on the swelling of the polymer followed by the etch ing of mechanical anchors with strong oxidizers such as chromic acid, this new method utilizes a chemical bond between the copper and plas tic surface. The bulk properties of the plastic substrate are unaffected by the new system. Through a series of chemical treatments, the outer layer of U L T E M resin (less than 10 microns) is removed, the new sur face is cleaned, a chemical with the ability to improve adhesion is applied, and the sample is metallized with electroless copper. SEM/XPS analyses reveal the changes that occur during each step of the sequence. A broad class of adhesion promoting materials is also described. The production of printed circuit boards has become a well established indus try, primarily relying on sheets of copper clad glass-epoxy laminate as the base. Usage of injection moldable engineering plastics as dielectric materials offers numerous advantages [1-2]. Plastics such as General Electric's U L T E M polyetherimide have a reduced dielectric constant, increased glass transition temperature, lower Ζ axis expansion and a reduced coefficient of thermal expansion in comparison to glass-epoxy composites. In addition to improved properties of the substrate, the plastic can be molded into various three dimen sional shapes. This design flexibility enables the plastic base to be more than just the dielectric material holding electrical circuitry; now structural ribs, com ponent supports and mounting features can be incorporated into the circuit device, reducing assembly costs as well as parts inventory. Creation of through holes and via can be accomplished during molding thus eliminating the need for drilling and deburring. The holes can be accurately reproduced with each 1

Current address: General Electric Armament & Electrical Systems Department, Burlington, VT 05401 0097^156/90ΛΜ40-0485$06.00/0 © 1990 American Chemical Society In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Downloaded by PENNSYLVANIA STATE UNIV on March 14, 2015 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch035

486

METALLIZATION OF POLYMERS

molding. Tapered holes are also a design option with molded circuit boards, enabling easier assembly of through hole components. The manufacture of a three-dimensional circuit device from a molded plastic such as the demonstration part shown in Figure 1 differs from the tradi tional printed circuit board. Different imaging techniques are required due to the three-dimensional features of the devices. In addition, the metal compris ing the traces on the surface of the substrate are now deposited rather than formed from the laminated copper foil. A number of novel techniques of producing conductors on plastic bases have been proposed. The use of photo-sensitive catalysts [3], recessed circuitry [4], laser decomposition of the plastic to produce electrically conductive carbon [5], laser induced deposition of metals [6], and the screen printing of conduc tors [7] have been examined. Ahesion of the deposited metal to the substrate is critical to the integrity of the device. Several theories have been developed to explain the bonding of the metal to the plastic including physical and chemical interactions [8]. Early adhesion work focused on techniques developed for the plating of acrylonitrile - butadiene -styrene substrates [3] using a roughened plastic sur face for physical bonding. Chromic acid solutions were used to create micro pores on the surface of the plastic. The use of organic solvents to swell the plastic surface prior to etching were found to enhance the etching process [9,10]. With the development of more chemically resistant plastics, more rigorous swelling and etching treatments were required in order to produce a microporous surface suitable for metallization. Variability in molding condi tions across the part brought another factor to the forefront, variable moldedin stresses. Plastic under differing stress reacts with varying degrees to the swelling solvent. This affects the amount of etching which leads to variable adhesion of the metal to the plastic. Figure 2 is a cross section of a copper-plated sample of a glass- filled U L T E M resin pretreated with swellant and chromic acid. Cracks from the swell and etch processing extend deep into the plastic ( > 3 0 0 μ ) . Such a treat ment affects the bulk properties of the plastic. As shown in Table I, reduction in mechanical and electrical properties occur to the plastic treated with swell and etch chemistry. Etching, extensive enough to cause sufficient adhesion, results in an irregular plastic surface. This may translate into a rough metal lized surface (Figure 3). Subsequent assembly processes such as wire bonding and the surface mounting of components may be adversely affected. Still another method of etching plastics involves the use of reactive gases [11-12]; however, the production of large numbers of complex 3-D shaped plastic cir cuit devices makes this approach unattractive. A number of inventions dealing with the chemical treatments of polyimides to improve adhesion to deposited copper have been described [13-16]. In these cases, a chemical modification of the plastic is believed to occur. The use of organic swellants and strong etching oxidants such as chromic acid are not required. A chemical process for obtaining high peel strengths between polyetherimide surfaces and copper without the use of swell and etch tech niques is described in this work.

In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


FOUST AND DUMAS

Polyetherimide Surfaces

Figure 1. Example of 3-D, metallized U L T E M 2312

Figure 2. Example of swell and etch treated U L T E M 2312



488


Table I. Comparison of Polyetherimide Properties Following Adhesion Treatment

Traditional Swell & Etch Flexural Strength Weight Loss Electrical Electrical 48/50 (humidity)

30% reduction 10-15% reduction No change 25% reduction

This Work No change No change No change No change

Figure 3. Example of swell and etch treated U L T E M 2312


35. FOUST AND DUMAS

489


EXPERIMENTAL


Since the molded circuit board industry requires high volumes and high throughputs of product, deposition of the initial metal onto the plastic was performed using electroless techniques. Further thicknesses of metal were obtained by electrolytic methods. The plastic substrate in this study was based on General Electric's U L T E M polyetherimide resin. In order to withstand the severe conditions required for soldering components onto the circuit board and for an improved coefficient of thermal expansion and increased rigidity, a glass-filled grade of polyetherimide was also examined (Table II). Table II. U L T E M Polyetherimide Properties

Sample

Filler

Deflection Temperature at 1.82 MPa(°C)

Flexural Modulus (MPa)

Coefficient of Thermal Expansion (m/m/°C)

1000

None

200

3,300

5.6xl0"

2312

30% Glass

210

9,000

2.0xl0-

5

5

Samples of unfilled resins ( U L T E M 1000) were utilized to examine the changes occurring to the plastic during the chemical treatments. Adhesion data was generated using the glass-filled resin ( U L T E M 2312). Peel strength values were obtained by peeling 32 mm (0.125 inch) copper strips from the substrate. A n end of each strip was clipped to an Ametek digital force measuring gauge which was connected to a computer processor. Force values required to lift the metal strips from the substrate were converted by the computer into grams per millimeter peel values. Multiple peel values for each strip were obtained and then averaged. R E S U L T S A N D DISCUSSION As an initial step in the plating sequence, (Figure 4) molded plastic samples (Figure 5) were cleaned (Figure 6). Greases and fingerprints from handling as well as silicones from molding release agents were removed prior to plating. X-Ray Photoelectron Spectroscopy (XPS) data presented in Table III show that a number of cleaners are capable of removing surface impurities from the plastic.



490


Figure 4. Metallization sequence for U L T E M resins

Table III. XPS Surface Composition, Atom & Substrate Cleaning Process

Treatment None

C 71.9

Ν 2.1

Ο 17.2

Si 8.8

A L L detergent

81.5

3.1

13.4

2.0

SHIPLEY Acid Cleaner 1118

81.1

3.1

11.1

4.5

FreonTF

78.6

2.3

12.3

5.2

Freon TMS

84.9

3.2

10.3

1.3

Fréon T A

83.2

3.9

11.5

0.4

U L T E M theoretical

82.3

4.4

13.3

__

Freon-type cleaners are preferred because of their volatility, enabling ease of drying of the sample. Drag-in problems to subsequent baths are thus eliminated. Degreasing is insufficient to ensure consistency; the surface of the plastic must be removed. A n inconsistent surface may be physical or chemical in nature. Physical differences may arise from handling or from the molding process. Chemical differences can arise from exposure to ultraviolet irradiation [17] or thermolysis during molding. Certain reagents will attack the surface of U L T E M polyetherimide such as dimethylformamide or sulfuric acid. Concentrated sulfuric acid was chosen



FOUST AND DUMAS


Figure 5. S E M and XPS analyses of molded U L T E M 1000 polyetherimide

Figure 6. S E M and XPS analyses of degreased U L T E M 1000 polyetherimide



492


as a surface removal agent for environmental and flammability reasons. Following immersion on sulfuric acid and a water rinse, a non-adherent, white residue is present on the surface of the plastic. XPS data shows this residue to be highly oxidized and sulfur-containing (Figure 7). Figure 8 shows the amount of material removed during treatment at room temperature in 96% sulfuric acid. In general, removal of less than 5 microns of plastic is required for consistent adhesion. The white residue is next modified by treatment with a strong base in alcohol. It is known that bases can hydrolyze the imide ring of U L T E M polyetherimide (18). Figure 9 shows that both chemical and physical changes to the residue have occurred following immersion in methanolic potassium hydroxide. X P S results are consistent with imide ring hydrolysis and formation of the potassium salt of a carboxylic acid. Following treatment with the alcoholic base and an alcohol rinse, the modified white residues can be removed by immersion in a mixture of an alcohol and an aggressive solvent. S E M and XPS analysis of a polyetherimide sample following treatment in a 50/50 mixture of dimethylformamide/methanol is shown in Figure 10. Little physical change to the plastic surface has occurred. The plastic surface properties, however, have now been altered. The sample is now water wettable.

Figure 7. S E M and XPS analyses after sulfuric acid treatment of U L T E M 1000 polyetherimide




35. FOUST AND DUMAS

Figure 9. S E M and XPS analyses after KOH/methanol treatment of U L T E M 1000 polyetherimide


493


494


Figure 10. S E M and XPS analyses after DMF/methanol treatment of U L T E M 1000 polyetherimide

Catalyzation with a tin/palladium colloid, acceleration by removal of the tin coating and electroless copper deposition with commercially available products results in an adherent copper layer to the plastic. The plated sample is then heat treated, electroplated with copper to a thickness of 37.5μ and then heat treated again. It was found that an enhancement in peel strength could be affected by immersion of the sample in a solution of hydroxylamine following residue removal in dimethylformamide/methanol and prior to catalyzation. Peel strength values on U L T E M 2312 increased from 125 g/mm to 150 g/mm with this change. Through the use of model compounds, it was found that the por tion of hydroxylamine responsible for the adhesion increase was the -N-O- seg ment. Compounds containing - N - H (ethylamine), - N - C H (trimethylamine), - O - H (methanol) or - N = 0 (pyridine -N-oxide, trimethylamine -N-oxide) bonds show no improvements in adhesion (125 g/mm or less), while reagents containing C H 3 - N - O - (N, N - dimethylhydroxylamine), - N - O - C H 3 - (methoxylamine), CH3-N-O-CH3 (O, N - dimethylhydroxylamine) or = N - 0 - H (acetone oxime) show higher peel strength. As a result, a number of other adhesion promoters could be added to the system, with peel strength values as high as 206 g/mm being obtained (Table IV). 3


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495


Table IV. Adhesion Promoting Agents for Polyetherimide (ULTEM 2312)

Adhesion-Promoting Compound

Peel Strength (g/mm)

None Hydroxylamine#HCl N,N-dimethylhydroxylamine«HQ Methoxylamine*HCl N-methylhydroxylamine»HCl O, N-dimethylhydroxylamine*HCl Hydroxylamine-O-sulfonic acid Formaldoxime trimer Acetaldoxime Acetone oxime Çyclohexanone oxime 2-pyridinealdoxime N-hydroxysuœinimide N-hydroxyphthalimide l-hydroxybenzotriazole*H 0 Cycloserine Hydroxyurea Acetohydroxamic acid

125 152 155 157 172 206 147 175 138 204 159 197 152 193 191 186 129 154

2

Adhesion promotion could be combined with cleaning of the modified white residue without deleterious effects. Therefore a dimethylformamide/methanol solution of hydroxylamine hydrochloride could be utilized. Peel strength was also affected by the heat treatment steps between metal deposition. The effect on adhesion of the bake time after electrolytic copper deposition is shown in Figure 11. Peel strength as high as 250 g/mm were observed on copper plated U L T E M 2312 with extended heatings at 110°C. Without a heat treatment, peel strength values were 50 g/mm. Figure 12 represents a cross section of a plated sample of U L T E M 1000 having a peel strength of 118 g/mm. Little physical surface change of the plas tic has occurred as a result of the pretreatment steps. Figure 13 is a cross sec tion of copper plated U L T E M 2312. While a mechanical component to adhe sion is present, it is much less than that found in traditional swell and etch treatment (Figure 2). Physical alteration of the plastic is confined to the outer most 25μ. The bulk properties of the plastic (flexural strength, electrical resis tivity) are unaffected by this new process (Table I). Failure during peel is occurring cohesively in the plastic. S E M analysis shows fractured plastic on both the polyetherimide and metal side of the peel for both U L T E M 1000 (Figure 14) and U L T E M 2312 (Figure 15). Glass fibers can also be found on the metal side of the peel for U L T E M 2312.



496


300 π

U L T E M ® 2312 T = 110°C

1

"Ί

10

100

— ι —

1000

TIME (hours)

Figure 11. Effect of heat treatment on peel strength

Figure 12. Cross section of plated U L T E M 1000



35. FOUST AND DUMAS


Figure 13. Cross section of plated U L T E M 2312


497


498


Figure 14. Peeled U L T E M 1000

Figure 15. Peeled U L T E M 2312


M

m

35. FOUST AND DUMAS


499

SUMMARY


A method for obtaining peel strength ranging from 150 to 250 g/mm between copper and glass-filled U L T E M polyetherimide has been developed without the use of swell and etch techniques. Peel failure occurs cohesively within the plastic substrate. The bond to the glass-filled polyetherimide has both a mechanical and a chemical component. Adherence of copper to unfilled polyetherimide is basically through a chemical bond. Organic compounds con taining the - N - O - moiety are found to promote the chemical bond. Such a sys tem is of value to the manufacture of molded circuit devices. ACKNOWLEDGMENTS The authors wish to thank B. R. Karas and E . J. Lamby for their inputs to the metallization of plastics as well as the metallography, XPS ( M . Burrell, J. Chera) and S E M (L. King) analytical services. L I T E R A T U R E CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Travis, J.; Ganjei, J. Proc. 41st Reinforced Plastics/Composites Institute, 1986, Session 8-A, ρ 1. Mihelcic, J. M . Printed Circuit Design, February 1988, p. 41. Frisch, D. C., Weber, W. U.S. Patent 4 594 311, 1986. Elarde, V . D . U.S. Patent 4 689 103, 1987. Lyons, A. M . ; Mendenhall, F. T. Jr.; Robbins, M . ; Quick, N. R.; Wilkins, C. W. Jr.; U . S. Patent 4 691 091, 1987 Cole, H . S.; L i u , Y.S.; Rose, J. W.; Guida, R. Applied Phys. Lett. 1988, 53, 2111. Wolf, G . D.; Giesecke, H . ; Sirinyan, K. Gaivanotecknik 1989, 79, 2154. Townsend, W. P. In Handbook ofAdhesives, 2nd ed.; Skiest, I., Ed.; Vos Nostrand Reinhold: New York, 1977, ρ 846. Saubestre, Ε. B. In Modern Electroplating, 3rd ed.; Lowenheim, F. A., Ed.; Wiley-Interscience: New York, 1974; Chapter 28. Yoshino, K.; Ebina, N. Proc. Sym. Electroless Metal Deposition, Electro chem. Soc. 88-12, 1988, ρ 268. McCaskie, J. E. U . S. Patent 4 520 046, 1985. Paik, K. W.; Ruoff, A. L.J.Adhesion Sci. Technology 1988, 2, 245. Mahlkow, H . ; Strache, W. German Patent D E 3 612 822, 1987. Mahlkow, H . ; Romer, M.; Rosskamp, G.; Seidenspinner, H.-M.; Stein, L.; Strache, W. German Patent D E 3 708 214, 1988. Grapentin, J.; Mahlkon, H.; Skupsch, J. U . S. Patent 4 517 254, 1985. Dumas, W. V.; Foust, D. F. U . S. Patent 4 775 449, 1988. Mance, A. M.; Waldo, R. A. J. Electrochem. Soc. 1988, 135, 2729. Burrell, M.C.; Karas, B. R.; Foust, D. F.; Dumas, W. V.; Lamby, E . J.; Grubb, W. T.; Chera, J. Electrochem. Soc. Mtg., October 1988, in press.

RECEIVED May 16,

1990


Metallization of Polymers - American Chemical Society

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