wire wrapped joints reviewwrapping. the tensile strain whichis caused remains in the wire after...

Electrocomponent Science and Technology1974, Vol. 1, pp. 17-25

Gordon and Breach Science Publishers Ltd.Printed in Great Britain

WIRE WRAPPED JOINTS REVIEW

P. M. A. SOLLARS"[he Plessey Company Limited, Allen Clark Research Centre, Northants, U.K.

(Received October 27, 1973; in final form December 6, 1973)The basic technique of wire wrapping is now well established as a simple and reliable method for joining conductorwires in a permanent, though easily replaceable, manner to more massive terminals. This paper describes themechanical and the metallurgical basis of the joining technique and outlines the advantages. The scope of thetechnique as it has developed up to the present day is discussed in terms of the ranges of wrapping terminals andwrapping wires used and the types of wrapped joint that have been evolved. Finally the tools available for wirewrapping are briefly reviewed with particular emphasis on the wrapping bit.

THE MECHANICAL BASIS OF THE WRAPPEDJOINT

The mechanical basis of the wire wrapped joint wasinvestigated very extensively by workers at BellTelephone Laboratories 20 years ago. Their very fullanalysis of the joining system is still considered to beessentially correct. The work included photoelasticobservations on a wrapped joint model to investigatestrain patterns produced by wrapping, and the studyof stress relaxation as a function of time and tem-perature.

The wire wrapped joint consists of a wire which istightly wrapped around a sharp cornered terminal.Sufficient deformation is engendered in the manynotches created by the terminal in the wrapping wireto create metal to metal interfaces with a high level ofintegrity. The wrapping wire, which is bent severaltimes during wrapping before final positioning in thewrap, is under a high level of tensile stress duringwrapping. The tensile strain which is caused remainsin the wire after wrapping because the stretched wireis locked by the notches formed in it. The bulk of thewrapped wire will remain under a tensile stress withthe wrapped terminal under a compressive stress: thezones, in the wrapped wire, immediately adjacent tothe notches will also be under compressive stress.Since the wrapped wire is a helix, rather than a seriesof hoops the stresses in the wire have components incertain directions and produce a bending momentabout the long terminal axis. The terminal will twistslightly until the relevant stress components in thewire are balanced by those in the terminal. The twistinduced in the terminal is a useful indication of thelevel of elastic stress in the joint and it provides amonitor for following the process of stress relaxation

17

at various temperatures. Extrapolation techniques canbe used to predict the long term stress relaxationbehaviour at room temperature using results obtainedby accelerating the process at elevated temperature.The elastic stress level in the joint is of importancesince it indicates the pressure with which the wrap-ping wire and terminal are being pressed together tomaintain electrical contact.

2 THE METALLURGICAL BASIS OF THEWRAPPED JOINT

Work at this laboratory2 has suggested that theconcept of the wrapped joint as essentially a mechan-ical joint, with residual elastic stress keeping therelevant metal components in close contact, is notsufficiently comprehensive. The stresses involved inthe notch zones are apparently sufficiently large topenetrate oxide and tarnish films and promote coldwelding. Evidence of such cold welding has beenobtained by unwrapping joints and examining themusing scanning electron microscope and electronmicroprobe analysis techniques. The latter techniquewas used to show the transfer of metal from onecomponent to the surface of the other, indicative ofwelding phenomena. Thus tin from a tinned copperwire wrap has been found to be present on thesurface of a brass terminal after unwrapping. Othermetal systems have provided similar evidence and thescanning electron microscope has shown examples ofcold welded metal debris. The original notch pro-duced during wrapping is modified when the adjacentnotch is formed; the wire is effectively pulled towardsthe second notch so that while one side experiencesan increased compressive stress the other experiences

18 P.M.A. SOLLARS

a shear stress. The increased compression will tend toextrude any soft terminal coating and increase theintegrity of the metal to metal contact locally; suchan area will frequently be one in which pronouncedmetal transfer can be observed.

The occurrence of pressure welding in the notchesof wrapped joints provides connections which aremore resistant to corrosive attack; even when thewhole joint is visibly corroded electrical quality canstill be high. The maintenance of satisfactory elec-trical quality at low levels of residual elastic stress(for example after accelerated stress relaxation) indi-cates the benefits of this high level of joint integrity.Because of the increased importance of the metallurg-ical micromechanisms in the notch the coating chosenon the terminal attains greater importance. It issuggested that it is particularly important in the caseof micro-wrapped joints, where the contact areas arevery small and therefore required to be of highintegrity if general electrical quality is to be good.

3 THE ADVANTAGES OF WIRE WRAPPEDCONNECTIONS

Whenever component leads or wire connections are tobe jointed to a panel or base in an electrical assembly,soldering is usually considered as the technique to beused. However against this prevalent acceptance ofsoldering can be laid several disadvantages. Primarilythe hand soldered joint is not of reproducible quality;the joint quality will depend very largely on operatorexperience and expertise. The wrapped joint qualityis much more consistent, the operator can be trainedvery quickly to produce consistent, good qualityjoints and with some experience these joints can bemade at a very fast rate. Most soldered joints can bemade to a suitable quality in many cases, with handsoldering. If the joint does not wet immediatelyprolonged attention will usually achieve the goal.Because the quality of the soldered joint is dependenton operator technique, however, a certain percentageof joints will be of poor and unacceptable quality.This percentage of faulty joints may be quite low, butfor assemblies which must give very high reliability,especially where large numbers of joints are used, itcan be unacceptable. The percentage of unacceptablejoints produced by correct wrapping techniques isextremely low, in fact it is very difficult to find anyfailures even when relatively severe accelerated testprocedures are employed. This high level of con-sistency is essentially due to the inherent ’over-design’of the wrapped joint. The typical joint might involve

six wrapped turns and, except for the outer wraps,there will be notches on four terminal corners foreach turn. Therefore about twenty successful ter-minal/wire notches are normally produced, each withintimate metallic contact.

The wrapped joint need not be regarded as apermanent connection. It can be removed relativelyeasily when required, by unwinding the wire in theopposite direction to that used during wrapping.Fresh wrapped joints can be produced on a terminalfrom which previous wrapped joints have beenremoved. This can be performed many times beforethe terminal corners are sufficiently rounded to affectjoint integrity. The number of rewraps will depend onthe terminal material but typically the numberallowed is thirty, or in the case of brass, ten

Another advantage which wire wrapping holdsover soldering is the absence of extraneous elements,such as solder and flux, that are required to producethe joint. The active tlux which is frequently neces-sary to ensure good joint integrity during solderingmust be carefully removed when the joint procedureis finished in order to minimise the possiblity ofcorrosion. Wire wrapping also avoids damage toadjacent insulation and heat sensitive components bythe soldering bit, and damage caused by excess soldersplashes falling into the circuitry. An additionaladvantage claimed for wrapped joints is that theyoften show less tendency to fail under vibration thansoldered joints; this is though to be due to the ’gradedstiffness’ of the joint. The change from the terminalto the wire dimensions is less sudden, especially as thewrap does not maintain close contact on the first twoterminal corners, and there is thus a lower stressconcentration effect.

4 THE RANGE OF TERMINALS USED IN WIREWRAPPED JOINTS

An appreciation of the range of terminals which areused to produce wire wrapped joints can be gained bya study of Figure 1. Many of the terminals on the righthand side of the photograph are relatively massive,.072" x .048" cross section being typical, and they areused extensively in panels in telephone exchangeracks. An example of an array of wrapped jointsmade on such terminals is shown in Figure 2. This panelcontains 210 double-sided terminals and some ofthese can be seen to have two wrapped joints on theone side. Many of the terminals on the left hand sideof Fig. are relay springs stamped out of suitablehard grades of spring alloys; one end of the stamping

WIRE WRAPPED JOINTS 19

FIGURE A representative selection of terminals on to which wire wrapped connections are made.

FIGURE 2 A typical example of an array of wire wrapped connections taken from a telephone exchange rack.

20 P.M.A. SOLLARS

acts as a wrapping terminal and the central portion islocated in a suitable plastic moulding. Normally themechanical requirements of the spring end are themost critical, and to obtain suitable rigidity in thewrapping terminal, such that it will not simply distortwhen it is wrapped, it is generally embossed. Theembossing procedure adds to the compliance of thecross section, and helps to maintain the level ofelastic stress existing between the terminal andwrapped wire. This level will already tend to be lowbecause the thinner terminal has a low torsionalrigidity and cannot maintain a high level of storedelastic stress. The use of wire wrapping on the ends ofclose packed relay springs was the original use forwhich Bell Telephone Laboratories developed thetechnique. An example of the close packing of wirewrapped connections on a stack of relay springs isshown in Figure 3. Some terminals used for wrappingwill be joined, for example by spot welding, to springdevices so that the optimum dimensions of each partcan be chosen separately; two such terminals areshown at the bottom of Figure 1. Other types ofwrapping terminal can be found; for example V-section and double V-section (where two V-sections

FIGURE 3 Wire wrapped connections made to the ends ofeach of a stack of relay springs.

are folded together apex to apex) exist. These twotypes do not find universal favour because it isconsidered that the pressure exerted by the wrappingwire can cause plastic distortion and reduction of thewrapped terminal dimensions.

Efforts have been made to standardise terminalcross section sizes .045" square is a particularlywidely used size, but each manufacturer tends to havehis own particular sizes. A terminal length is chosensuch that more than one wrapped joint can be madeon it. A inaximum of three joints is normal and inthis way the density of connections can be greatlyincreased. The terminal length is frequently increasedto provide a double sided wrapping facility, i.e.portions that can be used for wrapping exist on eitherside of a central mounting portion. The terminal isinvariably radiused or chamfered at the tip to aid leadin of the terminal into th.e aperture of the wrappingtool.

All the terminals discussed above are relativelymassive, but a whole range of smaller terminals arenow used for wrapping. Typical cross sections ofthese ’micro-wrap’ terminals are .025" square and.030" x .015". It has also been shown4 that wrappedjoints can be made with 36 s.w.g, wire on.010" x .015" terminals, which enables very closedensity to be achieved (.050" centres). The smallerterminals will frequently be staked into printedcircuit board laminates; relatively hard beryllium-copper terminals find use here. These can be puncheddirectly into the laminate and retained by heading theend of the terminal with a die. In another system, thesmall terminals are retained in metal eyelets let intoholes drilled into the laminate, with interconnection.finally achieved by soldering to printed circuit boardlands (this system is illustrated in the centre ofFigure 1).

4.1 The Materials Used to Produce WrappingTerminals

Wrapping terminals are generally manufactured fromrelatively high strength copper-based alloys; phosphorbronze, beryllium copper and hard brass are mostfrequently used. Nickel silver is also encountered butgenerally when the terminal is one end of a one-partspring component. Monel has also been recommendedwhere high strength, and ability to withstand plasticdistortion are required. It is comnon to find many ofthese materials electrotinned or solder coated. Theoriginal reason for such coatings was that if anysoldering was found to be necessary subsequently itwas facilitated by the terminal being already tinned.


It has been shown (as discussed above) that theserelatively soft coatings in fact play a very significantrole in increasing joint integrity and electricalquality2 Gold plating of terminals is frequentlyencountered where high reliability and thereforegeneral tarnish resistance, is required. Whatever ter-minal material is chosen, a temper designation isselected such that the terminal is significantly harderthan the wrapping wire. Thus the notch formed on theterminal edge is much less severe than that in thewrapping wire, and the terminal is capable of reuse.

5 THE WIRE USED TO PRODUCE WRAPPEDJOINTS

The most common wrapping material is insulatedtinned copper wire which is prestripped before beingfed into the wrapping tool. This is the same type ofwire used to make other types of connections, such assoldered or screwed joints, and it is normally per-fectly suitable for producing wrapped joints. 23 and25 s.w.g, sizes are most commonly used on the largerwrapping terminals. Larger and smaller gauges are alsoencountered, the smaller gauges being increasinglyimportant with the trend towards component mini-aturisation. Specifications generally allow fewerwrapped turns in joints using larger gauge wires. If awrapping wire is required that will not relax appre-ciably at the elevated temperature which the jointmust withstand, other materials are suggested. Beryl-lium-copper, copper-cadmium, copper-zirconium5 orsteel-cored copper have all been used. Where micro-wrap joints are concerned the wire used is very thin,or the order .008" to .012", and thus susceptible tobreakage. In this instance the use of a higher strengthcopper alloy, 135, is common. Though aluminiumwire has been thought to relax too quickly to achievedurable high quality wrapped joints, work has beencarried out6 which shows that joints can be madesuccessfully with aluminium alloy wire. Aluminiumalloys containing small additions of silicon andmagnesium have sufficiently high strength to be usedto wrap terminals, and provide joints which pass thenormal qualification approval tests up to relativelyhigh temperatures. For the best general corrosionresistance wrapped joints using aluminium alloy wireshould be made on aluminium alloy terminals.

The wire used for wrapping is most likely to beprestripped for the required distance, and thestripped end fed into the normal wrapping bit. Themost common joint produced is of the ’modified’type with the stripped length deliberately reduced to

achieve a wrapped turn of insulated wire. This joint isfavoured since the first notch in the wrapped wire,which is the critical zone from a vibration viewpoint,is protected and the joint can survive more severetests.

5.1 The Use ofInsulated Wire in the Production ofWrapped Joints

A development of the basic wire wrapping technique,which enables wire not prestripped of insulation to beutilised, is termed the ’Cut Strip and Wrap’ process. Toproduce wrapped joints from such wire a specialwrapping tool bit has been developed, a generalpicture of this bit is given in Figure 4. The basis ofthe technique can be followed by examination ofFigure 4A.

The wire is fed along the tool as illustrated and asthe tool tip revolves, one end of the wire is cut byflange C1, which contains a slot with a knife edge,rotating in relation to the slot in flange C2. Thelength of wire between A and C1, which is guided bythe slot in flange B, is the wire which is eventuallywrapped around terminal T. Insulated wire will be

FIGURE 4 A Cut-Strip and Wrap tool tip used to producewrapped joints from insulated wire.

22 P.M.A. SOLLARS

TO A CONNECTION

MADE PREVIOUSLY

END OF WIRE CUT OFF

Cl

FIGURE 4a. Diagram of a Cut-Strip and Wrap tool tip.

wrapped around the terminal and moulded tightlyover tip A until the pressure exerted by the cuttingedge of the tip A (i.e. Ac in Figure 4A) on the wireinsulation is sufficient to sever the insulation. Theremaining wire between A and C1 is then drawn outof its insulation, the latter being retained behind partA, and wrapped around the terminal as bare wire. Asthe wraps are formed on terminal T the tool tip ispushed towards the right (in Figure 4A) and over-lapping of the wraps is avoided. The process ofcutting, stripping and wrapping is thus carried outusing a single light hand tool and obviously offerssignificant economies in assembly. The process hasnot, however, gained the same level of generalacceptance as the pre-strip and wrap procedure. Thisis probably because the properties which define theease of cutting and stripping the insulation are verydifficult to specify, and are found to be inconsistent.The temperature of the workshop, the lubricationthat exists between insulation and wire, and thegeometry of the cutting edge all profoundly affectthe ease of stripping the insulation. Therefore, a widevariation in the force required to strip the insulationexists and different reels of wire, nominally similar,can exhibit different behaviour. Because of this forcevariation, different severities of ’snatch’ are impartedto the wire when the insulation is initially severed.This snatch results from a momentary retardation ofthe wrapping bit when the severing occurs, followedby a momentary acceleration. Any notch which isimparted to the wrapping wire at the same moment as

the snatch tends to be relatively severe and can containincipient cracking. This severe notch will tend to bethe weakest portion of the joint, but the effect can bereduced by having it formed under the insulation ofthe single insulated first turn. It is possible for thesnatch to be strong enought to break the wrappingwire, especially when it is thin.

The angle at which the insulated wire is ledtowards the cut, strip and wrap tool tip is particularlyimportant. Ideally the wire should be led in at rightangles to the bit axis; at other angles the wire can bemoulded across the relatively large face of the bit,creating additional frictional forces. This can cause anincrease in the incidence of wire breakage, the breaksoccurring a short distance along the wire beyond thewrapping.

6 SECONDARY WRAPPED JOINTS

A modification to the basic procedure of wrapping aconductor around a terminal to produce a primarywrapped joint is the procedure of binding a con-ductor, laid along the terminal, with a separatewrapping wire. This produces a secondary wrapped orbound joint, a type of joint which is very much lesscommon than the primary joint. It is possible toproduce such a joint with the bound wire andwrapping wire both being conductors, thus effectivelyproviding two connections at one operation, but thesystem is precluded in some specifications.


The basic well-proven geometry of the primarywrapped joint is departed from in the secondarywrap, and the variation in size of the bound wire,which must be laid along the widest face of theterminal, has an obvious effect on geometric inte-grity. If the bound wire is too small, it will encountera very low level of stress exerted by the wrappingwire because the wrap inevitably contains a ’naturalgap’ between the wrapping wire and the terminal (thegap when no bound wire is present). With very largebound wire sizes, the wrapping wire is pushed awayfrom the terminal and the number or severity ofnotches in the wrapping wire is reduced. 7 Theseverity of the two notches which may be producedin the wrapping wire in the position closest to thebound wire will be modified by differences in the layof the bound wire on the terminal face. The boundwire will not necessarily be positioned centrally,parallel to the axis, and this factor also affects thelength of wire used to complete a wrapped turn. If thewire is offset from the central position, then redun-dancy occurs in the length of wire used to make awrapped turn (that is wire additional to the minimumrequirements). This redundancy, which could affectstress relaxation effects in secondary wrapped joints,can also accrue if the wrapped coil is distorted in thedirection along which pressure is exerted by thewrapping tool. The number of deep notches in thesecondary wrapped joint is frequently half that of aprimary joint but a joint of satisfactory quality can

usually be made because of the inherent overdesign ofwrapped joints (as discussed above).

The bound wire used in making secondarywrapped joints will frequently be a normal circularsection copper wire, particularly when the secondarywrap is used as a repair procedure. Thus if the normalprimary wrap is attempted and the wrapping wire isbroken in the process, the remaining wire, especiallyin the instance of a complex cable lay, may haveinsufficient slack to produce a second wrap. The endof the wire is therefore smoothed out, laid along theterminal, and bound with a suitable wrapping wire. Inother instances the bound wire could be a componentlead, circular or even rectangular, in various materials(Kovar or nickel are typical possibilities) which arenot amenable to wrapping. The design parameters ofsecondary wrapped joints are at present empirical andrather broad. To rectify this situation a government.sponsored programme of work is in progress evaluat-ing secondary wrapped joints made to the outlinesgiven in DEF STAN 59-49/1 and investigating theeffect of various joint parameters on stress relaxationbehaviour.A variation of the secondary wrapped joint is the

twin bound joint. In this joint two similar terminalsare bound together. Typically the dimensions of theterminals chosen for this work are such that the twotogether produce a square cross section. Some men-tion has been made in the literature of the use ofstranded wire in making secondary wrapped joints

FIGURE 5tool.

A typical electric-motor driven wire wrapping

24 P.M.A. SOLLARS

this type of joint does not, however, find universalapproval.

7. THE TOOLS USED TO PRODUCE WRAPPEDJOINTS

There is at present a large range of tools available forproducing wire wrapped Joints. The simplest tools areoperated entirely manually. These are relatively inex-pensive and provided the wrapping is completed inone operation, without stopping, they are capable ofproducing good quality joints. The most commontools are pistol type hand tools operated electrically(mains or battery) or by compressed air. The lattertool offers the advantage of a steadier application oftorque to the wrapping bit and is the lighter tool tohandle. It can be more expensive initially however, ifcompressed air lines have to be installed. A typicalelectric motor driven wrapping tool is shown in Figure5; the compactness of the complete system can beappreciated. In some wrapping tools, the one impor-tant parameter which is still dependent on theoperator, the lateral pressure exerted by the toolalong the wrap, is controlled by a spring systemsupporting the wrapping bit. The reaction to thelateral force applied by the tool is ideally strongenough to provide a series of wrapped turns which arecompressed tightly together, without overlapping,with very few gaps in between.

Wire wrapping procedures are now available withvarious levels of automatic operation. The semi-automatic machine has been used for many years; thismachine indicates the terminals to be wrapped using a’sight’ driven by a numerically controlled X-Ysystem. The operator simply wraps the terminalindicated; the search for the correct terminal nolonger depends on the operator and the percentage ofwrongly positioned and missing joints is significantlyreduced. Because the operator carries out the wrap-ping operation, a much larger error in correctterminal positioning can be tolerated; even witherrors up to 30% of the connector pitch, the operatoris still able to select the correct terminal. Thesemi-automatic system enables the number of connec-tions made in unit time to be increased compared tothe manual system. The capital outlay involved, beingsignificantly lower than the fully automatic machine,means that this transitional system can often beshown to be the most economic.

The fully automatic machine is a recent additionto the wire wrapping machine range.8 The connectionwire is stripped at both ends and held by two separate

carriages; the pattern required in the wire is thenprepared by routing around a series of dressingfingers. The pattern is brought down on to theterminals and the two wire ends are wrapped by twowrapping heads. The carriages are capable of move-ment in the XY plane, under numerical controlcommands, and movement in the Z plan can beprogrammed into the operation to enable up to threewraps to be made on any terminal. The tolerancerequired on the correct positioning of the wrappingterminal tip is very tight; typically the terminal mustbe located within a radius of .005" of the trueposition. The fully automatic wrapping systemenables connections to be made at very high rates andthese connections are of very high and consistentquality. The system is however expensive and onlyjustified if large numbers of the same complexassembly are being wired up.

7.1 Wrapping Tool Bits

The normal wrapping tool tip consists simply of acylinder with a central bore which accommodates theterminal and a slit of smaller dimensions on thecircumference which holds the length of the pre-stripped wrapping wire. The bit revolves inside anappropriate sleeve feeding the wire around the ter-minal in a number of close wraps. The tensionimparted to the wire during wrapping is importantand is determined by certain bit dimensions. Theseare the wall thickness between the central apertureand the slit, and the radius around which the wire isfed out of the slit. The wire is wrapped under atension which is not so large that it producesexcessive notch severity, ’embrittling’ the wire andmaking it prone to fracture. The tip over which thewire is led should be smoothly contoured since thiswill avoid scouring the surface of the wrapping wire.The aperture for the terminal in the tool tip shouldideally be situated concentrically. If the aperture iseccentric the wrapping process will tend to createlateral oscillations of the terminal and strain theterminal mounting. The cross sectional area of the tipof the wrapping bit is kept to a minimum to reducethe space that must be provided between terminals; inthis way the closest pitch of connection is obtained.

8. CONCLUSIONS

The technique of wire wrapping is well establishedand there is generally a good understanding of thebasic mechanical and metallurgical factors involved.


The continued success of various wrapped jointsystems in use and under accelerated test conditionsconfirms the quality of this connection procedure.The most common use of the wire wrapping proce.dure has been in the telecommunications industry butnow a whole range of wrapped joints, of various sizesand types, are being used throughout the electronicsindustry. This connection system is used whereverhigh volume production of inexpensive, reliable con-nections is required. Thus the procedure is used in theproduction of computers, business machines, air-bourne navigational equipment and machine toolcontrol systems.

REFERENCES

1. R. F. Mallina et al., SoMerless lCrapped Joints (BellTelephone System Technical Publication Monograph2085, 1953).

2. D.G. Davies, ’Micro-mechanisms of solderless wrappedjoint formation’ Proceedings of the Holm Seminar onElectric Contact Phenomena, Chicago 1971, p. 103-117.

3. DEF STAN 59-49/1 November 1972.4. R.A. Leather, ’Wrapped joint connection systems’, Elec-

tron Packaging and Production (UK/European ed.)28-34, 36. (Aug. 1969).

5. A. Fox and J.H. Swisher, ’Superior hook-up wires forminiaturised solderless wrapped connections,’ J./.M. 100,30-32. (1972).

6. D. W. Halley, ’The evaluation of D50S aluminium alloywire wrapped joints’ (Ferranti Ltd. report Sept. 1969).

7. D. G. Davies, Permanent Electrical Pressure ConnectorsReport No. 2, ’Secondary wrapped joint’ (Internal Plesseyreport, 1971).

8. P.M. Blogg, ’Wire wrapping and wire-wrapped electronicconnections’ Proc. 4th lnternepcon Connector Sym-posium, Brighton Oct. 1973 pp. 26-34 (Kiver Com-munications Ltd.).

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