CA2063847A1 - Apparatus entailing adhesive bonding - Google Patents

Abstract

APPARATUS ENTAILING ADHESIVE BONDING
Abstract Cost and performance advantage are realized in a class of magnetic core transformers and inductors in which fabrication entails joining of core portionspartially enclosed within windings. Bonding is accomplished by use of thermo-setting adhesive which is introduced in the uncured state by capillarity betweenprepositioned aleady-mated surfaces to be bonded, followed by curing.

Description

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APP~RAT~lS ENTAILING ADHESlVE BONDING
Back~round of the In~en~on Technlc~ ~Yeld The invention is concerned with apparatus and fabrication dependent 5 upon high-strength adhesive bonding to produce and maintain a continuous magnetic path. A particularly significant category is that of wire wound transforrners and inductors dependent upon in situ bonding of magnetically soft ferrite sur~aces to complete core s~uctures.
Descriptiorl of the Prior Art The prior art discussion, in common with that of the detailed description, is primarily in terrns of the comraercial problem which provoked the effort and led to the solution of the inventive approach. Manufacture of a category of wire-wound devices - including transfolmers and inductors - commonly entails first winding a bobbin or other swpporting structure, and subsequently forming 15 magnetic core loop/s in part within the winding by joinder of preforrned coreportions. A particularly significant category is that class of devices which depends upon magnetically soft ferrite-core members. See, for example, E. C. Snelling and C.
Eng, "Soft Ferrites, Properties and Applications", Second Edition, Butterworths (1988).
A prevalent manufacturing approach depends upon permanent mecbanical clamping to keep mating surfaces in intimate contact (and thereby to maintain magnetic reluctance in the functioning device at the desired level). This approach continues in use despite high expense in tenns of cost, weight and space relative to adhesive bonding. See, for example, pages 160-162 of "Soft Ferrites,2s Properties and Applicativns", as cited above.
A category of ferrite-core devices depends on adhesive "bridge bonding"
in lieu of clamping. In accordance with this approach, temporarily clamped suufaces are joined by coating the outside of the joint with an epoxy or other thermo-setting resin which cures to leave an adherent encircling strength member, af~er which the 30 clamp is removed. Strength requirement gives rise to a need for a fairly thick enc~rcling adhesive layer. l~e expen~e of a mold is avoided by use of high viscosity/thixotropic material ~o minimize flow prior to and during cure. Tlle approach is usefully applied to fabrication of devices in which strength requ~ement is small - likely to joinder of core sections of relatively large cross-section - of 3s devices not likely LO encounter severe environmental conditions in use.
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The bridging adhesive method is costly - mal~imum strength afforded requires careful application of adhesive to the entirety of the peripheral surface to be wetted.
Under demanding space/strength needs where still further increased 5 application costs can be justified, adhesive bonding has taken the form of interfacial bonding - of coa~ing individual surfaces to be mated, followed by mating and rubbing to assure wetting and to drive out excess adhesive be~ore clamping.
~ esplte extensive effort to compensate for the various deficiencies ofadhesive bonding, the approach continues to be limited in many terms -10 performance/reliability under demanding circumstances is generally considered torequire mechanical fixturing ~e.g. clamping). lFor in~erfacial adhesive bonding an added complication arises in that removal of e~cess adhesive material by compressing the joint after mating and prior to curing, imposes a limit on pennitted viscosity. Furlher under many conditions, e.g. due to dissolved air and/or moisture, 15 voids may Çorm during elevated temperature curing, thus impairing initial strength and aggravating env~ronmentally-induced strength loss. Added constraint~s restrict adhesive composition and may impact performance needs.
Summary of ~he In~enffon l'he inventive teaching overcomes the obstacles to adhesive bonding as 20 outlined in the previous section. The thrust depends on capillary ~ow of uncured thermosetting adhesive as applied to properly-dimensioned, prepositioned mating surfaces, thereafter followed by curing to secure the wetted surfaces of magnetic mem~ers to result in a continuous magnetic flux path including such sur~aces. The usual objective is minimization or near-minimization of reluctance asso iated with 2S the joint so as to approach performance of a condnuous (unjointed) member.
Accommodation of a wide variety of epo~y and other adhesive materials is broadened by variation in temperature to satisfy flow and curing needs. Consequent freedom in adhesive and processing permits economies in ~erms of ease of application and high yield. Both are consistent with desired perforrnance properties -30 initial and as retamed under adverse conditions likely to be encountered in use. Withregard to the latter, maintenance of protective atmosphere, perhaps by device çncapsulation as wel} as other pracdced precautions, may often be avoided by theinventive approach.
The many limitadons associated with bridge bonding are avoiàRd.
3s Disadvantages of prior art interacial bonding are also overcorne. ~ irnportant part such disadvantages are due to need for fastidious co~ing of the enti~ety of surfaces ''.

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to ~e mated. In general, in the practice of the invention, application of uncured adhesive a~ but a single posi~ion per joint may suffice for adequate wetting of the prepositioncd surfaces, although for larger joints there may be some time advantage gain in multiple spot or stripe application - however, still depending on the thorough s wetting implicit in capillarity flow-distribution.
Bond strength realized by the inventive methodl contributes further to design freedom. An e~ample is that of device fabrication e~tailing mating E-coresections (F1~3S. 2 and 3) in which relia~le joinder has ~een accomplished by adhesive bonding of but two of the three matmg surfaces. Reference is here made to 0 the E-core soft ferrite structure, in which the already-wound bobbin conceals the center joint. This functionally desirable design is described, for exarnple, in "Soft Ferrites, Properties and Applications" cited above, at p. 281.
Inventive processing invaIiably depends upon capillarity to bring about wetting of already-positioned mating surfaces wbich are essentially in contact with 15 each other. It is this aspect which assuredly brings about many of the advantages associated with the invention - regartling both ease of application and effective performance. The "Energetic Considerations" section in the Detailed Description considers the various factors concerned with effective application - factors including:
spacing between mating surfaces; viscosity of the adhesive as affecting capiIlarity 20 and particularly viscous drag; contact angle; and temperature as affecting any of the foregoing. A major objective of the invention - that of magnetic continuity consistent with desirable physical properties (strength, resistance to adverse conditions in use, etc.) depends upon inherent wetting as provided by the capillarity mechanism. Forces inducing capillary ~ow for otherwise suitable mateAals - for a2s broad category of uncured thermosefflng resins in conjunction with contemplated surfaces ~o ~e joined - are considerable. Desired level of continuity in a preferred aspect of the invention is assured by maintaining surfaces to be joined in intimate contact as by clamping during fabrication. The magne~ically soft ferrites as used in devices fabricated in accordance with experimental work, including that of the 30 Fxamples, present surfaces suitable to such capillary flow. Experimentally, surfaces produced by simple abrasion, as by grinding, as well as those entailing polishing to near-milror surface, have all been joined by the inventive techniques. Clamping pressures to maintain minimal spacing between mating surfaces - to maintain intima~e conlact before capillary introduction - have been found insufficient to s prevent the capillary flow-wetting of the invention.
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Various means for initial introduction of the uncured resin are appropriate. Examples which have served experimentally inclllde (1) application of adhesive at the pe~pheral outer sur~ace of joints of an already-heated mating pair, and (2) hea~ing of a mating pair after room temperalure adhesive application.
5 Heating may serve a variety of purposes including either or both of - reducingviscosity of the uncured adhesive to assure timely wetting of mated surfaces, and to acceleIate subsequent curing. These differing objectives may be addressed suf~iciently by main~enance at constant temperature, or, alte rnatively, temperature may be ramped to most effectively satisfy the two. Alternalively, a variety of 10 considerations may dictate flow-wetting and/or curing without heating.
Brief Descripffoll of the Drawin~
FIG. 1 is a diagrammatic view of surfaces to be mated to which refere~nce is made in the general prc~ess description.
FIG. 2 is a schematic view of an, as yet unassembled, inductor with a 15 magnetic E-core struc~ure.
FIG. 3 is a view of ~he same E-core inductor as assembled.
lDetailed Descr~ption General Considera~ions set forth in this section are useful in identification of the 20 various parameters - composition, processing conditions - suited to the needs at hand. In more general terms, operation of the invention is assured by inhe~ency of suitable pararneters for a broad range of choices with only broad common sense restriction. For example, choice of adhesive on the basis of adhesion and bond strength necessarily entails wetting of magnitude sufficient for assuring capillary 2s flow. The additional requirement for application concerns viscosity - a requirement generally satisfied by use of unfilled therrnosefflng resins prior to curing. The fimctional mechanism of capillary flow, required for all aspects of the invention, is well-known as are the various considerations yielding timely flow (~iscosity, in tum as affected by temperature, molecular weight, etc.).
While section 5, in setting forth equations determinative of application, is usefully employed in optimization, the artisan is well~quipped to identi~y both materials and process conditions to reliably practiGe the invention. Specifica~on of differential pressures as well as values of surface tension, etc. concerns parameters to be optimized in usual terms. Operability of the invention does not depend upon such 35 considerations. For e~ample, while spacing between sur~aces and s~ace smoothness are of consequence for performance optimization, experiment .: ~
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-s-establishes suitability of a spacing as large as 10 mils for capillary~ e~70ver the indicated viscosity range of up to 500 cen~ipoise and higher. This value of nominal spacin~g certainly represents a maximum likely value from the performance standpoint - it is unlikely that desired vallles of inductance will suggest larger s spacing between bonded madng surfaces. In fact since surfaces involved in thise~cperiment were produced by simple grinding, the 1() mil spacers used assured only this minimum value with variations likely resulting in regions within which spacing was increased by up to 2 mils in regions ~tween protrusions engaged by Ihe spacers.
All such experiments, as ~supplemented by those involving joints clamped ~unspaced) lo under 50 psi pressure, support assurance of operability of the inventive mechanism for joints to be encountered in device design.
A number of magnetic devices require a non-magnetic gap ("air gap") in the magnetic flux pa~ is is typically accomplished by grinding down the central leg(s) of three-leg core parts. Required tolerances on ~e length of the gap (and, 15 therefore, on the total reluctance of the magnetic path) may be maintained in the mated structure by ensuring a minimal spacing between the mating surfaces of theouler legs, by clamping during bonding. Considerations pertaining to spacing between, and magnetic path continuity at, bonded mating sur~aces are, therefore,generally equivalent for such "ungapped" and "gapped" core structures.
Relevant considerations with regard to choice of composition of the adhesive as well as processing depend upon a variety of factors including: time needed for application; demands resulting from configuration and siæ of surfaces to ~e bonded; demands resul~ing from performance requirements; design life with attention to conditions to be provided for; and overall cost considerations which may 2s result in compromise of one or more of the foregoing. Such considerations are discussed, largely in exemplary terms.
Inventive Outline It is convenient to introduce relevant factors, in terms of an outline. The outline presented considers various factors - adhesive charac~er, application 30 procedure, overall performance. While the outline is primarily in terms of neeessary factors, variations including both optional procedures and permitted ~ariation in order may be useful. While some variations are discussed, others are inappropriate to Ihis disclosure and are left to the practitioner. In common with lhe remainder of the description, specific discussion is at least initially in terms of u~sual core 3s construction entailing joining of co~e portions to yield a completed loop. Certain considerations, e.g with regard to provision of deliberately reduced inductan~e, may , 2~384 ~
translate into specified small spacing in the loop.
1. Surface Characteristics ~ - ag~in! considerationx are fundamental and entail e.g. surface energetics on the basis of which adhesive composition is chosen.
s Physical - whether flat or o~her conforming geometry, surface roughness is of concern. From the device-functioning standpoint, some minimal smoothness is likely desirable to assure requisite continuity of the magnetic ?
path. F}om the adhesive How standpoint, ~surface topography of otherwise suitable s~rfaces is not cri~cal. ~imely wettirlg of an adequate por~ion of the 0 joint has becn attained for all surfaces otherwise acceptable from the functioning standpoint.
Size - for purposes of particular consequence to the invention - for "linear" devices such as inductors and transformers in a communications circuit - mating surfaces are likely to be srnall e.g. fractions of a square inch.
For so-called "power" devices, mating surfaces are often larger - may range to a square inch or more.
Positionin~ - mating surfaces are likely to be similar in size and shape.
Minimal spacing, of consequence for most contemplated purposes, is generally achieved by pressure as by clamping or by other forms of mechanical fixturing. Examples of the latter may depend upon: magnetic attraction, which may conveniently make use of the inherent soft magnetic propertie~ of commonly osed eores, by application of an inhomogeneous magnetic field; or simply gravity, perhaps as aided by additional weights.
Core structures, fabricated in feasiibility studies consistently showed maximum attainable inductance for the various surface topographics used at pressures within the 50 to 200 psi range.
2 Other .
~, whether with or without increased eemperatLIre; ~ means, whether for use before, during, or after application; te n~ whether of all or selective produc~, are among the many considerations familiar to those responsible for manufacturing specifications. (See "Hiandbook of Adhesivesl', ed. Irving Skeist, ~1977) New York). They are of concern to the inYention only insofar as they affect criteria set forth above. ;~
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Adverse Conditions - reliabili~y, largely in terms of aging, is assured by appropriate choice of the noted parameters. Introduction of a~lhesive by capillarity in accordance with the inven~ion pennits ma;ximization of properties inherent ~o both the adhesive and the surfaces to ~e bonded.
s Available materials and processes are sufficient to accommodate: temperature cycling both in fabrication and use; humidity aging; and mechanical conditions to be encountered - e.g. shock, nbration.

3. TheAdhesive The central thnust of the inventive teaching depends upon flow of the lo adhesive as induced by capillarity. Timely, flow of adhesive is, in turn, dependent upon spacing, surface regularity, needed path length and surface energetics. Such considerations translate into needed adhesive characte~istics for meeting such requirements. Adhesive characteristics of concern from this standpoint are viscosity and surface tension under temperature and other conditions during flow.
All such needs with regard to application are satisfied for a wide range of epoxy and other adhesives so that choice is not significantly limited due to such considerations.
Viscosity - a significant physical characteristic corlcerns this property.
2~ Always in terms of temperature during flow, timely flow for likely flow path length (e.g. centimeters per second) for most demanding use is realiæd for viscosities of less than about 5ûO centipoise (about 500 cps). Greater viscosities, not generally preferred -~rom standpoint of ~ow, may be tolerated in the interest of accornmodating adhesive materials of otherwise desired 2s characteristics and/or cost. Relevant viscosity may be as measured dunng application, or at the temperature to which the mated surfaces are heated after lower tempera~ure application (e.g. after room temperature application). For most rnagnetic core assemblies, choice of ~emperature is simply to assure flow before the onset of significant rdow-~npairing curing. For others, heat susceptibility may impose a maximum. For many otherwise suitable adhesives, e.g. for epo~y adhesives as used in examples herein, suitable ~ow is reali~ed for temperatures below about 200C. On occasion, heat susceptibility may suggest choice of adhesive from a somewhat more restricted class.
Altematively, this consideration may suggest redesign of the assembly being fabricated.

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Composition - detailed dcscription of suitable adhesive compositions is not appropriate. 1~ is fundamen~ that the invenfion c~itically depends upon availability of ads~esives of appropriate adhesion as well as s~ength properties.
Beyond such considerations, suitability depends upon inherent demands as isnposed by the invention. Con~empla~ed compositiorls are thermoset~ing (as desired to yield both the initial low viscosity required for flow as well as adhesive and strength properties yielded upon curing~. Flow propesties in the uncured state, as discussed in detail under "Application - Theory", entail such physical properties as YiSCoSit)T, surface tension, con~act ang1e, and dependence of such properties on temperature. There is a broad category of adhesive compositions (containing curing agent7 any modifier, arld the adhesive polysner itself) from which mateAals may be chosen to satisfy the requirements of the invention. ~ariant~s concern both choice of ing~edients in the generic terms set forth and characteristics of particular conæquence to the ls invention - e.g. choice of uncured polymer of molecular weight suitable to desired viscosity.
The "Handbook of Adhesives" (cited above~ identifies and characterizes several categories from which suitable adhesives may be selected. These include epoxies7 anaerobics (e.g. acrylates and diacrylates - either containing dispersed cu~ing agent), acrylics, urethanes7 polyes~ ~7j as well as other ~ -materials of requisite properties as now available or to become available in thefuture. Curing agents7 too, are chosen with regard to effect on invention requirements - e.g. effect on flow rate7 time to initiabon of curing to permit dis~ibution p~ior to significant flow-impeding curing. Required curing 2s temperature is a factor in such choice as well. Useful adhesive compositions may desirably include one or more modifiers7 for e~ample, to reduce viscosity.
O~her ingredients may serve: to promo~e adhesion (e.g. organofunctional silanes a~7 may b~ incorporated in some epoxies); to vary surface tension ("surfactants"); as well as to serve a variet y of ancillary purposes, as colorant7 ~o etc. In general7~ particulate filler malerials7 t7nixotropes, and other non-essential ingredients tending to increase viscosity are not usefully included. Even here, special circumstances may dictate such inclusions. While undesirable in the us1lal situation7 where the objective includes surface-to-surface con7~inui~T ornear-continuity (in tenns of magnetic reluctance), they may serve ~o restric~
~ow-loss, e.g. for vertically deposedj larger spacings between surfaces as desired to tailor inductance to some value below the ma~cimum attainable.

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Adhesives used in the e~amples were epoxies. Compositionally> they were based on diglycidyl ethers of bisphenol-A ~epoxy equivalent weight - ;
180) and included a heterocyclic amine curing agent.

4. Processing Application - the uncured adhesive may be applied in any convenient maMer - by syringe, eye dropper, nozzle, toothpick, etc. Quantity applied is sufficient to wet at least a major part of ~e mated surf aces - preferably to wet their entirety. Unlike pAor ar~ interfacial bonding, excess adhesive, in the preferred instance oï clamping, is kept from entering the joint in the first place and, accordingly, cannot result in unwanted su~face-to-su~face spacing.
Depending upon size criticality and other considerations, excess material may be permitted to remain outside the jointO

5. Applica~ion - Theory This section deals with factors relevant to introduction of the uncured lS thermosetting adhesive.
Consistent with common usage, flow, assuring wetting of mated surfaces is refelTed to as "wicking". The term is used as alternative to, and synonymous with "capillary flow".
Capillary forces are responsible for adhesive ~ow between~mated surfaces, and are resisted by viscous drag. The^time, t, required ~o wick between surfaces over a nOw path distance L is given by the equation:
t=
where g = gap spacing between mated surfaces 2s 11 = vi~cosity - sorface tension = dynamic (advancing) contact angle of adhesive to surface, all in compatible onits.
It is ~en that: increasing viscosity and fiow pa~ increase time ~quired, while increasing spacing size, sur~ace tension and cosine of the contact angle decrease time required.

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While it is common lo measure contact angle ~ statically, wicking rate is, in actuality, dependent upon wetting kinetics (upon the instantaneous value of ~).
As expec~ed, wicking is slowed by kinetic effects.
Refercnce is made to ~IG. 1 in fur~her consideration of the ~ow 5 mechanism. The figure schematically depicts bodies lû and 11 presenting prepositioned mating surfaces 12 and 13 defining gap, g. Overall path length, L~ is to be filled by advancing meniscus surface l S as originating from adhesive composition as initially applied at 14. The designation, 1 (t) represents the instan~neous length of the path defined by the advancing lmeniscus 15 at time The positive ~orce causing ~ow ("wicking'7 is due to the pressure differential, ~p, across meniscus IS in the direction of movement, ~p. This differential e.g. p liquid ~ Pa~ iS of the value:
2~ cos ~ (2) in which parameters are as defined above.
The instantaneous velocity, V, within a region near the entrance position at 14 (remote from meniscus lS at the position shown) is calculated as a balance~etween this positive force and viscous drag:
V= 2~ cos a((g/2) -Y ) - - ~ (3) - ~- ~ ~ ~~ ~
in which -y = distance measured from the center of the gap spacing The velocity of fiow at the advancing meniscus 15 ~the veloci~hy of the advancing front as represented by meniscus lS itself) is:
V = ~ (4) 2s Approximations made in development of the above equation a~e g< < L
and in which g2=gravitationalacceleration Both assumptions ~e justified for usually contemplated geometries.

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$ ~ 7 6. L, snits The general nature of the inventive advance is clear. Implications are most meaningfully in terms of economy realiæd in tne attainment of product excellence - largely as measured in te ms of bond streng!ht both initial and s during needed life. For many purposes, excellence must take pe.~ormancecharacterisdcs into account - for most purposes, e.g. in terms of magnetic reluctance, this requires preæribed spacing between bonded surfaces. This latter is generally optimized by minimal surface-to-surface spacing as assured by mechanical elamping.
The tnrust of the invention concerns the thorvugh surface wetting which is inherent in the capillary ~ow mechanism. It is expected ~at commercial advantage will be in terms of optimization of ~his approach. Bridge bondmg as practiced is premised upon sufficient viscosity prior to and during CDg -generally assured by deliberate addition of thixotrope - as to inherently minimize any capillary flow as well as viscous flow. PMctice change to follow disclosure of the inventive teaching will generally take the form of avoidance of thixotrope and of such other considerations - regarding composition and heating - as to assure the lessened viscosity which is botn necessa~ for practice of the presen~ invention and which is disadvantageous from the standpoint of bridge bonding.
The invention represents a distinct departure from prior art bridge oonding. Viscosity - for many purposes described as below about 500 centipoise (under temperature and other conditions during capillary ~ow wetting~ compares with values of many thousands, perhaps in t'ne range of 2s 50,000 centipoise or higher for bridge bonding.

7. The Drawin~
Reference has 'oeen mads to FIG. I in a discussion of the "wicking"
mechanism which constitutes a major thrust of the invention. The remaining figures involve fabrica~ion of an illustrative class of magnetic devices. It hasbeen noted that devices of concern generally depend upon const~uction of mechanically reliable low magnetic reluctance paths. FIGS. 2 and 3 are consistent with the rema~nder of this description in which emphasis is on wire-wound devices in which function entails inductive coupling via a core loop, e.g. of a magnetically soft fe~rite composition. l~e par~icular configuration depicted was used in examples included in section 8. This .. . - - ~ . . .

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, 2~ 7 device is an inductor with a magnetic "E-core" structure, as commonly uæd in cornmunications and power conversion devices. A varie~y of magnetic struc~ures desirably fabricated by pracdce of the invention is welli-known.
See~ for example, "Soft Ferrites", cited above e.g. at pp. 162, 281-284 and 288 describlng suitable standard core structures inclu!ding U, Pot, RM, PM, PQ, ETD, EC, El, LP and others as wel] as the E-core.
FIG. 2 depicts an unassembled E-core structure including E-shaped core portions 20 and 21 each containing two outer legs and one center leg, 2~, 23, 24, and 25, 26, 27, respectively. At the stage of fabrication shown, bobbin 2 lo has been wire wound to yield inductor winding 29.
Structures of the type shown were among those fabricated in accordance with examples in the following section. In example 1 ~he structure is ;
assembled and maintained in position by clamp 30 as shown in FIG. 3. As there depic~d, the wound bobbin 28 encompasses the interface forrned by center legs 24 and 27 (interface within and hidden by the bobbin 28 and not shown). (The particular structure shown is an inductor, and, so, has bu~ two terminals 34, 35.) With clarnp 30 in position, mating surfaces of leg pair 22 and 25 ~forming interface 32) and of leg pair 23 and 26 (forming interface 33) are adhesively bonded in accordance with the inventi~e ~teaehi}~g (se~i for examplei discussion of FIG. 1). Clamp 30 is generally removed foUowing curing of the thennosefflng resin.

8. Examples A considerable body of expenmental work ~serves as basis for description as well as lirnits set forth. Devices cons~ructed may serve a variety ~;
2s of magnetic functions. Construction of such devices entails inventive joining in the magnetic path, e.g. in the core loop in the instanee o~ common inductors and transforrners. In all structures, advantzge is gained from reliability in the various terms: notably joint iniegrily both initially and under various conditions to be encountered in life. In some instances, appropriate adhesive composition consistent with other requirements was such as to resis~ attack by humidity and provide long-term resistance to vapor transmission as verified by accelerated li~e-testing - by imme~sion testing.
Applicability of t~e inventive process in the ~erms described is justified on the basis of the hundreds of experiments conducted to qualify for 3s manufacture. The examples were selected as likely representative of near .

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terrn fabrication - of induc~ors and ~ransfQrmers of characteristics typical forsuch devices presently in use in ~elephony as well as for similar devices ~sed in power conversion.
E~c~nple I
s This example descr~bes fabrication of an E-core inductor as depicted inFIGS. 2 and 3. Overall dimensions uf the completed dlevice were approxirnately one inch by one inch in the major plane of the core. Leg surfaces joined were approxima~ely one quarter inch squa~. Fabrication entailed clamping with a total forGe of about ~en pounds (~ S0 psi). The 0 clarnped assembly was preheated in an oven to a temperaturs of approximately 150C, and a drop of adhesive composition was applied to one side of each of ~ -the exposed joints by uss of a syringe (the center leg joint was not accessible).
The particular adhesiYe composition was based on an epoxy resin - diglycidyl ether of bisphenol-A ("DGEBA") having an epoxy equivalent of 180-190. The composition con~ained ~ 10 phr ~pa~s per hundred resin by weight) curing agent - 2 ethyl-4 methyl imidizole (2,4-EMI). After permitting sufficient curu~g time (< S min.), the structure was removed from the oven, the clarnp was removed, and the resulting structure was tested. Mechanically, both tension and torsion testing resulted in failure of core material prior to adhesive joint failure. Similar result~s were realized after accelerated life testing -~
including one hour immersion in boiling water. Per~ormance, tooj easily;me~
usu~l specifications - inductance was equal to or superior to that of prior ar~ -stluctures which were bridge bonded or inter~acially bonded, as well as ~o pennanently clamped structures. Performance characteristics were essentially 2s unchanged following life testing.
Example 2 An inductor of the size and characteristics of that of Example 1 was fabricated with the same adhesive composition by a procedure which varied in but one respect - initial application of adhesive was at room temperature, following which the clamped assembly was placed in the oven, and was removed within five minutes after attaining the tempera~ure of 150C. Test results were unchanged.

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Claims (12)

1. Fabrication yielding a device dependent for its operation on a magnetic path, such fabrication entailing completion of such path by adhesive bonding of mating surfaces of magnetic elements which as bonded contribute to such path, adhesive bonding being due to wetting of at least portions of such surfaces with substantially uncured thermosetting adhesive followed by curing, characterized in that such wetting results from capillary flow of said uncured adhesive between such surfaces as prepositioned so as to approximate themating relationship.

2. Fabrication of claim 1 in which the said device comprises an electrically conductive coil and the said path is substantially defined within amagnetic core providing for inductive coupling in operation, and in which the said mating surfaces are those of core parts.

3. Fabrication of claim 2 in which the said coil is fabricated by wire winding a hollow member and in which the said path consists essentially of at least one loop partially contained within such member.

4. Fabrication of claim 3 in which the core parts consist essentially of magnetically soft material.

5. Fabrication of claim 4 in which the said core parts consist essentially of a ferrite material.

6. Fabrication of claim 5 in which any spacing between prepositioned surfaces is of minimum dimension having a maximum value of 10 mils.

7. Fabrication of claim 6 in which the said mating surfaces are prepositioned so as to be in physical contact so that the said minimum dimension is numerically 0.

8. Fabrication of claim 7 in which prepositioned mating surfaces are in clamped contact during capillary flow.

9. Fabrication of claim 8 in which the said uncured adhesive is maintained at elevated temperature for a period during which capillary flow is favored by decreased viscosity of the said adhesive.

10. Fabrication of claim 9 in which uncured adhesive is heated subsequent to introduction and prior to a substantial part of capillary flow.

11. Fabrication of claim 6 in which at least one of the said core parts has at least three legs, in which the said hollow member encompasses the joint formed between the center legs of the parts and in which the joints formed between the outer legs are bonded.

12. Product produced by any of claims 1 through 11.