US 3892646 A
Selected electrical connector contacts on a flexible support, which are to be mated to corresponding contacts on a rigid circuit board, are electrophoretically coated with a blend of a polyethylene ionomer emulsion and an epoxy ester polymer emulsion by selectively isolating from the electrophoretic deposition current those contacts which are not to be coated.
Description (OCR text may contain errors)
United States Patent 1191 Lazzarini et al.
l July 1,1975
 Assignee: International Business Machines Corporation, Armonk, NY.
 Filed: Aug. 17, 1970  App]. No.: 64,424
11/1969 Scheiber et al..... 1.. 204/181 2/1972 Cornish et al. 204/181 OTHER PUBLICATIONS Drake, IBM Technical Disclosure Bulletin, Vol. 11, No. 8, (Jam, 1969), pp. 949-950.
Primary Examiner-Howard S. Williams Attorney, Agent, or Firm-Joseph G. Walsh ABSTRACT Selected electrical connector contacts on a flexible support, which are to be mated to corresponding contacts on a rigid circuit board, are electrophoretically coated with a blend of a polyethylene ionomer emulsion and an epoxy ester polymer emulsion by selectively isolating from the electrophoretic deposition current those contacts which are not to be coated.
An electrophoretic coating bath useful therefor comprises an aqueous blend of the above polymers and yields a very thin electrophoretic coating which is a blend of the above polymers and which exhibits both high insulation values and excellent mechanical resistance to cracking and compression when mated to rigid surfaces at pressure loads of over 50 psi.
An electrical connector assembly comprises a flexible backing with a number of lengthy, narrowly spaced electrical contacts thereon. Alternate contacts are coated with a thin electrophoretically deposited coating, as described above, less then 0.0007! inch thick. The coating comprises 60-40 parts polyethylene ionomer and 40-60 parts epoxy ester polymer. When the electrical connector is mated, under pressure, to a corresponding electrical connector assembly, shorting is prevented. The mated connector assembly is also claimed.
7 Claims, 6 Drawing Figures "w vmu I975 3, 9
SHEH 1 FIG. 1
FIG. 20 FIG. 2b
FORCE 17 9 FIG. 3
14 -1 rfi r"! Fax-43 INVENTORS DONALD J LAZZARINI LEWIS K SCHULTZ 15 BY SMakVQU M} fiTi-" HULT TOT: 892,646 SHEET 2 800 A: FORCE REQUIRED FOR CONTACT AT FORCE 0.3 NNLs THICK,
(lbs) 700 a: FORCE REQUIRED FOR CONTACT AT 0.6 MILS THICK. C FORCE REOOTREO FOR BREAKDOWN 600 AT 0.3 ANLs THICK.
O FORCE REQUIRED FOR BREAKDOWN AT 0.6 MILS THICK. 500
FIG. 4 400 O so EPOXY FIG. 5
1 PROCESS FOR SELECT IVELY FORMING ELECTROPHORETIC COATINGS ON ELECTRICAL CONTACTS BACKGROUND OF THE INVENTION l. Field of the Invention The present invention relates to electrophoretic coating processes for electrical components, electrophoretic coating baths utilizable therein and electrophoretically coated electrical connectors.
2. Description of the Prior Art The electrophoretic deposition of various paint compositions is well known, as typified by US. Pat. No. 3,304,250. However, such coatings rely upon dyes or pigments typical in paint formulations and do not pro vide the necessary insulative or mechanical resistance values required for many electrical applications.
The electrophoretic deposition of aqueous polymer latices, particularly polytetrafluoroethylene, is known, as typified by US. Pat. No. 2,820,752. However, as described therein, the electrophoretic coating does not provide the necessary qualities which insulative coatings for microelectronic components must exhibit.
Many other means of forming coatings are available to the prior art, such as, encapsulating by a clipping process (U.S. Pat. No. 3,085,075) or using a fluidized bed (US. Pat. No. 3,058,95l). Neither of these prior art processes permits the control which is required in the coating of many modern electronic components.
SUMMARY OF THE INVENTION The present invention has three primary features.
First, there is provided a process for selectively, electrophretically coating preselected electrical connections on a flexible substrate. The process provides very thin (less than 0.00071 inch) coatings which can be accurately deposited on the selected electrical connections, a requirement imposed by the narrow spacing (5 mils) between connections.
Secondly, there are provided novel electrophoretic bath compositions which permit electrophoretic coatings to be formed which illustrate both high electrical insulation and excellent mechanical resistance to cracking and compression under elevated pressure loads during mating to complementary contacts on a rigid circuit board.
Thirdly, there is provided an electrical connector assembly which comprises a flexible backing with a number of lengthy, narrowly spaced electrical contacts thereon. Alternate contacts are coated with a thin electrophoretically deposited coating less than 0.00071 inch thick. The coating comprises 60-40 parts polyethylene ionomer and 40-60 parts epoxy ester polymer. When the electrical connector is mated, under pressure, to a corresponding electrical connector assembly, shorting is prevented. The coating has a high insulation value and excellent mechanical resistance at elevated applied pressures. A mated assembly as described is also provided by this invention.
The preferred electrophoretic coating composition of the present invention comprises: a polymeric blend of a polyethylene ionomer emulsion and an epoxy ester polymer emulsion. A blend of these two polymers can be electrophoretically deposited from an aqueous suspension to very low thicknesses, i.e., lower than 0.0007l inch. The resultant electrophoretic coating is a polymer blend with excellent insulative properties and excellent mechanical resistance to cracking and compression under elevated loads. Such a coating finds particular application in protecting selected flexible electrical connectors and miniature circuitry that requires mating to a rigid complementary surface under pressures in excess of 50 psi.
[n the preferred process of the present invention, closely spaced alternate electrical connectors on a flexible backing are isolated from the electrophoretic deposition current whereby an electrophoretic coating deposited from a bath as described above is selectively coated onto alternate electrical connectors carried on the flexible backing. [n this way minor mis-registrations between the two elements are prevented when the electrical connectors or contacts are mated to a complementary rigid circuit board, thereby stopping electrical shorting.
Extremely low coating thicknesses are necessary to meet two device requirements. First, to prevent electrical openings due to a lack of intimate contact between the electrical connections or contacts and the rigid circuit board to which the contacts are mated under pressure. Secondly, due to the narrow spacing between alternate electrical contacts, on the order of 5 mils, the coatings must be narrow to avoid undue compressing during mating. Under severe compression, the coating would not serve as an adequate insulator, and would cause shorting.
The present invention provides a process, an electrophoretic bath useful therein and a novel product, all of which meet the above requirements, and which are not subject to the disadvantages of the prior art, i.e., the present invention provides a coating with superior insulation and applied pressure resistance which can be formed at the tolerances required for usage in miniaturized electronic components.
It is thus one object of the present invention to pro vide a novel electrophoretic coating composition which will yield a coating illustrating both excellent electrical insulation properties and excellent mechanical resistance at high applied pressures.
[t is a further object of the present invention to provide a process for electrophoretically coating selected electrical connections or connector contacts on a flexible backing.
It is still another object of the present invention to provide a novel assembly of electrical connections or connector contacts on a flexible backing finding particular application in combination with a rigid circuit board to which the contacts on the flexible backing must be mated.
These and other objects of the present invention will become apparent upon a review of the drawings and the detailed descripton of the preferred embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates the electrical contacts of the present invention on a flexible support during electrophoretic deposition.
FIGS. 2a and 2b represent, respectively, top views of a correctly registered and a misregistered flexible connector/rigid circuit board assembly.
FIG. 3 is an overall side cross-sectional view of a complete flexible connector positioned for mating registration with a rigid printed board.
FIG. 4 is a plot of the mating force, in pounds, required for various contact coating thicknesses plotted against percent of epoxy ester polymer in the coating.
FIG. is a plot of voltage versus thickness in mils, for one coating composition in accordance with the pres ent invention.
DETAILED DESCRlPTlON OF THE PREFERRED EMBODIMENTS The following description is offered with respect to a preferred form of the present invention. It will be un derstood from the concepts and practical applications described therein that the present invention will find wider application.
Unless otherwise indicated, all percentages are in percent by weight and all parts are by weight.
ln its most preferred form, the present invention provides, from a process aspect, a process for applying a polymeric insulation to selected electrical connectors or circuit elements carried on a backing, such as a flexible polyimide backing. Broadly, the process would find application in forming an insulative coating which is resistant to applied pressure. For instance, the coating can be applied to any flexible connector used in miniature circuitry that requires mating to rigid surfaces under an applied pressure load.
Such flexible electrical connectors are used in various applications. In one specific application, selected connector contacts or electrical connections (hereinafter the terms will be used interchangeably) are mated under pressure to corresponding contacts on a rigid circuit board. When such an assembly is formed, insula tion on alternate connector contacts is required, in order to prevent electrical shorting in the case of minor mis-registrations between the flexible electrical connection and the rigid circuit board during high pressure mating. Extremely low coating thicknesses, less than 0.0007l inch, are required to prevent electrical cpenings due to a lack of intimate contact between the ends, or tabs, of the connector contacts and the corresponding contacts on the printed circuit board. Accordingly, the coating utilized must be one which is capable of being coated to an extremely low coating thickness, and it must be one which has high electrical insulative properties to prevent shorting. Further, since pressure loads in excess of 50 psi are often used during mating of the flexible electrical connection to a rigid circuit board, the electrophoretically, deposited coating must exhibit excellent mechanical resistance under the applied pressures, i.e., it must not crack or be severely compressed during mating.
ln greater detail, the present invention in a preferred embodiment provides a process for selectively coating selected electrical connections on a flexible backing as described, and reference should now be made to FIG. 1 of the drawings which schematically illustrates a flexible electrical connection immediately after coating in the electrophoretic bath of this invention. In this drawing, a flexible polyimide backing is represented by numeral l. The connector contacts which are coated with the electrophoretic coating of the present invention are represented by numeral 2, and the electrical contacts which are not coated with the electrophoretic coating are represented by numeral 3. The spacing between adjacent contacts 2 and 3 is very small, about 5 mils, and accordingly conventional coating procedures cannot be used, because conventional coating procedures,
such as spraying, silk-screening, photoresist techniques, and the like, cannot meet the fine deposition tolerances required.
With reference to FIG. 1, the coated electrical connectors 2 are shown in electrical contact with electrophoretic power line 5 via tabs 4, the tabs 4 being covered by mask 6. However, non-coated electrical connectors 3 are not joined to the electrophoretic power line 5. At the end opposite the junction with the electrophoretic power line 5, both connectors 2 and 3 are shown masked with mask 7.
By the application of an electrophoretic coating current via line 5 to the assembly shown when it was immersed in the bath of this invention, lines 2 were electrophoretically coated since an electrophoretic deposition current passed therethrough, while lines 3 remained uncoated.
With reference now to FIG. 2a, this figure shows a representation of correctly registered electrical connector carried on a flexible backing (not shown) with pads carried on a rigid printed board (not shown). The view is from above, and it can be seen that the connectors 8a carried on the flexible backing completely overlie and are approximately centered on the underlying pads carried on the rigid board. Approximate dimensions are shown.
In FIG. 2b, the same numerals with the notation (b) are used to denote a misregistered assembly which would lead to a short circuit if insulation breaks down at any time. From the representative dimensions shown, it will be apparent that if a connector 8b is misregistered as little as 0.0025 inches, it will short two of the underlying pads 9b.
With reference now to FIG. 3, there is shown a flexible assembly comprising a flexible backing 10 formed of polyimide carrying thereon a solder pad 11 which has deposited on portions thereof gold pressure contacts 12. Positioned immediately beneath the gold pressure contacts 12 are corresponding gold pressure contacts 13 carried on a solder pad 14 which is attached to the rigid circuit output board 15. The coating of the present invention is shown by numeral 16 as overlying certain selected gold pressure contacts 12. Upon application of pressure by means of pressure plate 17, the flexible connector will be pressure mated to the rigid circuit board.
FlG. 4 is a plot of pressure in pounds versus the percent epoxy ester polymer in the coating of this invention, the balance being polyethylene ionomer, required for contact and for breakdown at various coating thicknesses.
FIG. 5 is a plot of applied voltage during electrophoretic deposition versus thickness of the coating, in tenths of a mil, for the system used in the example at a 60 second voltage duration.
Having thus set the basic environment of the present invention, it is appropriate to turn to the polymer blend used in the electrophoretic coating of the present invention. As heretofore indicated, high insulation and high mechanical resistance to cracking and compression under pressure loads are required in the electrophoretic coating of this invention. The high insulation is necessary because the coatings are extremely thin, and good mechanical resistance is required since mating will be at pressures of over 50 psi.
The inventors have found that a system which meets all of the above requirements, comprises an aqueous blend of polyethylene and an epoxy ester polymer. Preferably, the polyethylene is a polyethylene ionomer, and for ease of formation, the electrophoretic bath used to form such a blend can be formed by blending two aqueous emulsions of the above materials.
Polyethylene ionomer resins are ion-linked acidmoditied ethylene interpolymers. Coatings made from polyethylene ionomer dispersions offer superior grease resistance, improved adhesion, improved clarity and higher gloss than coatings made from polyethylene. Polyethylene ionomer coatings have a moisture vapor barrier value approximately equivalent to that of low density polyethylene, and a heat sealing temperature range approximately equal to low density polyethylene.
Upon evaporation, ionomer dispersions coalesce into a continuous film at temperatures generally from about 85 to about l70F, depending upon the exact composition. Full strength of the ionomer film is realized when the film is heated to about 250F, or somewhat higher. Obviously, temperatures which decompose the ionomer must be avoided. Typically, an ionomer will have the structure:
where A is a lower alkyl group, preferably CH and B is a cation, preferably an alkali metal cation. The numeral n can vary with the lower alkyl chain used, but preferably will yield a polyethylene ionomer. The alkyl ratio to the carboxyl portion of the polyethylene ionomer is from 4/1 to 6/1 distributed in a semi-block manner, and X represents the molecular weight required to provide an ionomer which, upon blending with the epoxy ester polymer, meets the heretofore cited requirements. The preferred molecular weight is from about 200,000 to about 500,000.
The carboxyl groups and ion-linking capability of the polyethylene ionomer are believed to be the primary factors which provide the improved qualities cited above. By ion-linking is meant that the linkages in this polymer are ionic bonds as well as covalent bonds, i.e., there are positively and negatively charged groups present which are not associated with each other. This polar character makes the polyethylene ionomers unique in this respect. The ionized" carboxyl groups improve the adhesivity of a blend containing the polyethylene ionomer to various substrates.
Polyethylene ionomer dispersions useful in the present invention are dispersions in water and can be used directly as commercially available at a solids content of from 33 to 45% by weight ionomer. The thermoplastic polyethylene ionomer can, if desired, be chemically crosslinked due to the presence of carboxyl reactivity, and the carboxyl reactivity also provides sites for further reactions that could modify the basic property of the polyethylene ionomer for various purposes.
Typically, the high molecular weight ionomer will have a molecular weight within the range of about 200,000 to about 500,000, a melting point in the range of to 102C and a density within the range of 0.92 to 0.97. An example of such material is Surlyn DD- l230, commercially available and manufactured by the E. l. duPont deNemours Chemical Co. This product has a solids content of 33-45% by weight, and of the particles are 0.2 micron.
Surlyn DD-l230 is a polyethylene ionomer within the heretofore described formula where A is CH 8* is sodium, n sets the polyethylene chain, and which has a molecular weight of 200,000 and 500,000.
Most preferably there is used in combination with the polyethylene ionomer an epoxy ester polymer.
The epoxy ester resins or polymers are well known to the art. Typically, they are based upon the reactivity of the epoxy group,
and generally contain the repeating group O C l-L'CMe 'C H O-CH -CH CHOHCH where n is l to 9. The epoxy ester resins of this invention are well known, and can be formed, for example, by the following two-step process:
STEP A The liquid epoxy resin bisphenol A, a fatty acid, lithium naphthenate, and xylene are charged into a stirred autoclave fitted with a condenser, water trap and inert gas inlet. The reaction is blanketed with an inert gas and heated rapidly to 390F where an exothermic reaction occurs. A temperature rise of 90l00F will be noted in 5 to 10 minutes. The reaction is cooled to 470F and held at this temperature for 30 minutes. Additional fatty acid and triphenyl phosphite are then added to the reaction and heated to 500F. It is neces sary to distill off part of the xylene to reach 500F. The temperature is held at 500F with a vigorous xylene reflux for one hour; the water is removed in the water trap. The reaction is cooled to 450F and held at this temperature until an acid number of 12 to 14 is obtained. The epoxy ester is then cooled to 350F and discharged, or cooled to 2 l0F and emulsified in the same reactor.
In the above process, the following proportions are used:
Component Weight (grams) E xy resin l36.38 Btsphenol A 79.62 Refined Tall Oil Acids 1% rosin] 3.46 Lithium Naphthenate |.4% in water) 0.32 Xylene 8.77 Refined Tall Oil Acids 1% rosin) [94.40 Triphenyl Phosphite 0.82
Component Weight grams) Epoxy Ester from Step A 4l2.0
(97-98% NV.) Morpholine l2,5 Water 4245 TOTAL 8490 The product of Step B is an epoxy ester emulsion (50% N.V.) used in the present invention. The above data is a commercial process, and is only illustrative of the well known procedures to form epoxy ester resin emulsions,
Stabilizers such as ethanol amine, ethylene glycol, etc, can be added for long pot life, if desired. Upon di' lution to the desired concentration with DI. water, if necessary, the resin is ready for blending in this invention.
Epoxy ester polymeric resins as described above are commercially available from the Ciba Products Co., Dewey & Almy Division, as Araldite PR-SOS, from Mobile Chemical Company as epoxy ester polymer IR- 1579 (modified alkyd type) and from the Union Carbide Chemical Corporation as epoxy ester polymer 3060.
No criticality is attached to the exact epoxy ester polymeric resin used in this invention so long as it provides the requisite insulation and mechanical resistance to cracking and compression during mating at over 50 St. p Araldite PR-805, used in the example, is an epoxy ester polymer which can be formed using the procedural steps described above.
Most preferably, though non-limiting, these epoxy ester resin polymers have a molecular weight less than about 10,000.
It has been found that when the polyethylene ionomer in emulsion form and the epoxy ester in emulsion form, as described above, are used in an electrophoretic bath in proportions or relative amounts of from 60 to 40 parts polyethylene and from 40 to 60 parts epoxy ester polymer, based on the polyethylene and epoxy ester polymer, this provides a flexible electrophoretic coating composition which illustrates excellent insulating properties, has a mechanical strength able to with stand loads of over 50 psi, has excellent flex life, abrasion resistance, excellent adhesion to metallic connectors and, most importantly, is able to be deposited at thicknesses below 0.00071 inch. Optimum results are obtained with a 5050 mixture.
It has been found important to observe the above ranges in formulating the polyethylene/epoxy ester polymeric blend because of the following factors:
Firstly, the coating of the present invention must be a good mechanical and electrical insulator, and must always retain these properties after exposure to cleaning solvents such as are utilized in soldering of electrical devices and the like. For instance, solvent cleaning in the present example is used to remove flux residue after soldering. The coating must resist such solvents such as Freon/isopropanol mixtures. Typically, ultrasonic agitation is used for about ten minutes after immersing the coated device in such a solvent, and it will be seen that such agitation necessitates good mechanical insulation.
Secondly, the film must be capable of being compressed so that the contacts are not held apart in the event of misregistration.
Balancing the above factors, an increase in the polyethylene ionomer concentration results in a more compressive film which exhibits somewhat lowered mechanical strength and less resistance to solvents such as Fl. Circuit Cleaner (Freon/isopropanol). On the other hand, an increase in the epoxy ester polymer concentration reduces the compressiveness of the coating, but increases the mechanical strength and the resistance to solvents. Within the heretofore recited range, both the mechanical and electrical insulation properties, are acceptable.
It was found that if amounts outside the 40 to parts epoxy ester polymer (or 60 to 40 parts polyethyl ene) were used, a blend resulted which would not provide the required properties of mechanical strength, solvent resistance, etc.
It will be understood, of course, that in addition to the two specific materials described above, which are most preferred, other materials are operable in the present invention. For instance, instead of the ionomer described, a dispersion of a nylon in water could be utilized, such as Genton l 10, which is a water based dispersion of Zytel nylon (approx. 10% solids by weight), Although Zytel nylon appears to be a proprietary material, from the film properties thereof, it is believed that this is a nylon type 6 or type 6/6 material. This material does not provide results as good as are achieved with a polyethylene ionomer.
An epoxy ester polymer as described is essential to the present invention, and, at present, no substitutes therefor are known.
The above polymeric components are preferably used in an aqueous suspension at a total solids percentage in the suspension of 10 to by weight based on the total suspension, with a preferred range being 40 to 55% by weight based on the suspension. Variation is possible from the above ranges and coating will proceed, but the above ranges permit greater ease of operation. Typical state of the art electrophoretic additives to prevent foaming and the like may be used, if desired.
Generally, the suspension of the above polymers will comprise small particles having an average size of about 0.15 microns, with a general range of from 0.10 to 0.18 microns being preferred. No criticality is attached to the particle size, so long as the final electrophoretic film has the properties heretofore enumerated.
The electrophoretic bath of the present invention may be formed simply by physically blending suspensions of the desired polymers with the addition of the required amounts of water thereto, or by any equivalent method.
Any standard state of the art electrophoretic deposition system may be utilized in the process of this invention. For instance, such a system will generally comprise a container to hold the bath which is provided with appropriate electrical connection means. Electrical connection to made to the lines on the connector assembly to be coated by the use of clips, pressure contacts, etc. The connector assembly and contacts are immersed in the bath and the required electrophoretic coating voltage is applied thereto.
The deposition mechanism for electrophoretic deposition is well known in the art, essentially a hydroxyl ion being discharged and varying the pH in the vicinity of the anode, thereby reducing the negative charge on the resin or polymer particles carried in the aqueous suspension so that the particles coagulate to form the electrophoretic coating.
Film formation proceeds in accordance with the theory that as anion concentration increases and water in the area of film formation decreases, the phase-volume relationship exceeds the limit for oil-in-water stability, thereby leading to film formation.
Deposition on soluble anodes, in contradistinction from the above description relating to insoluble anodes, involves the reaction of charged polymer with metallic ions being oxidized at the anode and traveling toward the cathode. The result may be termed ionic coagulation, and is not in stoichiometric proportions.
For the electrophoretic coating baths of the present invention, deposition will usually occur at a voltage of from 6.5 to 8.0 volts for a period of from 55 to 65 seconds and at room temperature. Preferred conditions of deposition are from 7.25 to 7.75 volts for a period of from 55 to 65 seconds.
The temperature of electrophoretic deposition is not critical, and any state of the art temperature can be used below the boiling point of the bath. Generally, room temperature operation is preferred as this avoids the necessity for any complicating heat or cooling means. The pressure of coating is not important, and again to avoid unnecessary equipment operation is at atmospheric pressure.
After coating is completed to a preferred thickness of from 0.5 to 0.6 mils (l* inches), the coated article is removed from the electrophoretic bath, washed to remove excess material adhering thereto, and baked to harden or fuse the polymeric coating blend, typically for from I to 2 hours at 102 to 140C. These ranges are non-limiting, and the only important parameters to consider with respect to the baking time/temperature are that the upper temperature limit must not be so high as to destroy adhesives used in forming laminated materials which are coated, and that the lower limit be sufficient to fuse the coating.
Having thus described the general principles of the present invention, the following specific example is offered to describe the preferred processing scheme and electrophoretic bath used in the present invention.
EXAMPLE I An electrophoretic coating composition was prepared by mixing 50 grams of Suryln DD-l230 (40% solids by weight) and 50 grams of Ciba Araldite PR- 805 (40% solids by weight). A suspension was formed by agitation at room temperature (approximately 70F). The average particle polymer size of the electrophoretic suspension was about 0. 15 microns. The blend was homogeneous.
A signal overlay edge connector as shown in FIG. 1 was selected as the flexible electrical connector to be selectively coated. The specific assembly shown, with reference to FIG. 1, comprised a polyimide support (Kapton), which was highly flexible, having deposited thereon contacts 2 and 3 which were standard state of the art nickel-gold metal stripes plated over copper. The copper thickness was 0.7 mils and the nickelgold thickness was microinches. The stripe was 5-l0 mils wide, and spaced from adjacent contacts 3 about 5 mils. [n the bath, the nickel-gold contacts 2 served as a non-corrodible anode, rather than a corrodible anode. Of course, any comparable metal connectors can be coated which find application in the field of microelectronics.
Prior to immersion in the electrophoretic coating bath, the connectors 2 were cleaned or degreased by immersion in reagent chloroethane (inhibited l,l ,ltrichloroethane). The electrical connections 2 were then connected to an appropriate electrophoretic power supply by leads attached to tabs 4. Pressure sensitive insulating tape was then applied to the portion of the electrical connectors 2 that must be protected from the electrophoretic coating, these portions being the contact tabs 4 which are represented in FIG. 1 as being immediately under the masks 6 and 7. The adhesive on the tape should be one which does not come off on the tabs, thus increasing contact tab resistance. Further, it is necessary that the tape not be attacked by the electrophoretic solution. A tape that meets these requirements, though any material which meets the aboveparameters can be used, is Mystik Mylar White Tape 7300.
After the above operations were performed, the assembly was immersed in the electrophoretic suspension, and a constant voltage of 7.5 volts was applied for 60 seconds.
Thereafter, the flexible electrical connector was removed, thoroughly cleaned in de-ionized water to remove excess coating, and placed in an oven at l20C for two hours to fuse the resin mixture.
After fusion, a thin uniform coating was obtained only on electrical connections 2. The coating had a thickness of .0006 inch which resisted cracking (insulation breakdown) at applied pressures up to 200 psi. Of course, the insulation characteristics are excellent under an applied load in excess of 50 psi, and the assembly was pressure-mated to a rigid printed circuit board resulting in compliance with all of the parameters heretofore indicated.
it will be appreciated that the above electrophoretic coating process conditions can be varied from the 7.5 volt-60 second condition recited. If the voltage is less than about 6.5 volts, not only will a larger time be required to achieve uniform thickness, but the total obtainable coating thickness may be too low, i.e., the resulting coating will not be an acceptable insulator for all applications.
If the voltage is greater than about 8.0 volts, then a much thicker, non-uniformly distributed film results with some polymer degradation. Further, the deposited film is much more likely to crack under pressure, and is too thick to insure electrical contact of uncoated tabs under pressure.
The minimum permissible coating voltage is 0.5 and the maximum acceptable coating voltage is about 8.0, with a slight variation being acceptable in the maxi mum value depending upon device requirements. However, for a product which meets all the requirements heretofore posed with maximum insulation and mechanical resistance, one must observe the 6.5 to 8.0 volt range, that is, while a coating produced outside this range is superior to the prior art, it will not meet all of the stringent requirements heretofore posed.
As heretofore indicated, one of the primary features of the present invention comprises the ability to deposit coatings less than 0.0007] inch thick. As a rule, to meet the stringent requirements heretofore set forth, the minimum coating thickness is about 000048 inch. It will be apparent that where lesser user requirements are posed, i.e., lowered electrical and mechanical prop erties, these limits can be exceeded. However, to achieve the maximum unique balance of electrical insulative and mechanical resistance properties heretofore enumerated, one must follow the guidelines set forth. A product outside the thickness limits recited will still be far superior to the prior art, however.
Generally, voltage and time of application are inversely correlated.
Further, it will be apparent that tape need not be used to isolate the tab end connections, and that any comparable material or means could be used. It is only important that the tabs be protected from contact with the electrophoretic polymers so that they are not coated and thereby require an extra process step for polymer removal.
From the above discussion it will be clear that the present invention will find application with a great variety of flexible substrates which carry metal conductor stripes thereon. For instance, in addition to the polyimide substrate of the example, other representative and non-limiting flexible substrate materials in common use include epoxy-glass combinations, Mylar (polyester) materials and polyamide-imide materials. Typically, these are copper-clad.
Further it will be apparent that in addition to the metal connector stripes heretofore set forth, other state of the art metals used for electrical connection can be coated in accordance with the present invention, for instance: nickel (plated or electroless), nickel-gold, immersion tin, copper and tin-lead eutectics.
Although the heretofore offered discussion has been primarily in terms of flexible backings, it will be apparent that the present invention, though finding particular utility where flexible backings are used, will also find application in coating selected conductor stripes on a rigid backing. Of course, the present invention finds peculiar application where, other than with use in combination with a flexible backing, the coating requirements heretofore delineated are required, i.e., good insulation at a thin thickness, and good mechani cal resistance to compression at elevated pressure loads.
While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. In a process for forming a thin, insulative coating having good mechanical resistance to cracking and compression at elevated pressure, said process comprising immersing a plurality of metal electrical contacts carried on a backing in an electrophoretic coating composition, some of said metal electrical contacts being in electrical isolation from the source of electrical current, and some of said metal electrical contacts being connected to said source of electrical current, said metal electrical contacts comprising a plurality of longitudinally arranged, narrowly spaced members, and electrophoretically depositing said coating composition as a coating on said metal electrical contacts whereby a homogeneous, thin polymeric coating is formed only on said metal electrical contacts which are connected to said electrical current, the improvement according to which the electrophoretic coating composition comprises an aqueous polymeric blend of from 60 to 40 parts polyethyiene ionomer and from 40 to 60 parts epoxy ester polyer, all parts being by weight.
2. The process of claim 1 wherein said polyethylene ionomer contains the repeating structure where A is a lower alkyl group, X is the number of ionomer units required to provide a molecular weight of from about 200,000 to about 500,000, B is an alkali metal cation, and n is a positive integer wherein the ethylene ratio to the carboxyl portion of the ionomer is in the range 4/1 to 6/1 distributed in a semi-block manner.
3. The process of claim I wherein electrophoretic deposition is accomplished by applying said electrical current at a voltage of from about 6.5 to 8.0.
4. A process of claim 3 wherein said current is applied for from about 55 to about 65 seconds.
5. The process of claim 1 further comprising, after electrophoretic deposition, heating said polymeric coating, whereby said coating is fused and exhibits greater adherence to said metal electrical contacts.
6. The process of claim 5 wherein heating is at from about 102 to about C for from about 1 to about 2 hours.
7. The process of claim 1 wherein said coating is less than 0.0007l inch thick.