SMT Soldering Handbook surface mount technology 2nd phần 10 pdf

Searching investigations have shownthat they do not affect the life expectancy of joints or their reliability, in any way.Corrective soldering can only mask, but notfill, a porous hole, an

Figure 9.3 The yes/no nature of soldering success

Bridges and solderballs

A circuit board cannot function if it contains a short circuit, i.e a solder bridge.Wavesoldering without bridging demands special techniques, such as optimizingthe configuration of the wave (Section 4.4.4) and the board layout (Section 6.4.1).Boards with a pitch below 1 mm40/mil are difficult to wavesolder without faults,unless soldered in a nitrogen atmosphere

With reflowsoldering, especially of fine-pitch boards, the type of paste and itsquality and the precision of the printing of it are key factors in achieving solderingsuccess (Section 5.2.3)

Solderballs need not necessarily be classed as soldering faults If a solderball sitsbetween two neighbouring footprints on afine-pitch board, it can constitute ashortcircuit, and prevents the board from functioning Elsewhere, solderballs repre-sent potential shortcircuits, and as such reduce the reliability to an extent which is

difficult to quantify How solderballs are to be regarded is very much a matter ofindividual company policy

Even a single soldering fault on a board prevents it from functioning, and thereare only two options: correct it or scrap the board The choice between themdepends on several factors, which will be discussed in Section 10.1 What must bestressed here is the following

The nature of the soldering fault

The existence of a soldering fault is an objective fact A joint is either soldered or it isnot soldered A bridge is either there or else there is none The soldering faultpresents a ‘yes/no situation’ (Figure 9.3) To pronounce upon it is in the nature of averdict upon an observed fact, and two or more inspectors must necessarily reachthe same verdict

Because of its objective ‘yes/no’ nature, the success/fault verdict can be entrusted

to an automatic quality assessment system, which may be based on opto-electronicinspection or functional electronic testing (Section 9.5.5)

Quality control and inspection 329

Figure 9.4 Some soldering imperfections

9.2.2 Soldering perfection and soldering imperfections

Assessing soldering perfection presents an inspector with a fundamentally differentsituation: imperfect soldering does not prevent the affected circuit board fromfunctioning, but it can be seen as endangering or reducing its reliability It may also

affect its saleability where the buyer has specified precise criteria

Criteria for perfection may include the following features (Figure 9.4):

Wetting angleJoint profile and amount of solder on a jointAlignment or displacement of components

If an imperfection disqualifies the product in the eyes of the customer, it becomes

a soldering fault, because it makes the product unsaleable A product which isunsaleable does not work as far as the vendor is concerned

Being saleable is the first function any manufactured product must fulfil Aproduct which is not saleable in the market for which it has been made does notfunction from the point of view of its maker (unless it is still saleable elsewhere forless profit or at a loss) The offending feature must be corrected, or else the productmust be scrapped

In contrast to the unequivocal yes/no verdict upon the verifiable fact of solderingsuccess or fault, a pronouncement upon the soldering perfection of a joint represents

a judgement, which is necessarily subjective The judgements arrived at by differentinspectors represent points along a scale, which separate the ‘perfect’ or ‘acceptable’from the ‘imperfect’ or ‘non-acceptable’ (Figure 9.5) On either side of theaccept/reject divide are areas of doubtful acceptability and false alarm

It has been found that only 44% of the quality judgements on the same set ofsoldered boards, made on two different days by the same inspector, agree with oneanother The quality judgements of the same boards made by two different inspec-tors overlap by only 25%, while those made by three inspectors overlap by 14%(A T & T Bell, Burlington, N Carolina)

To sum up: deciding between soldering success and a soldering fault amounts to averdict Deciding whether soldering perfection has been approached sufficiently is amatter of judgement, and the making of this decision can be automated only withgreat difficulty

The blowhole problem

Blowholes in wavesoldered throughplated joints, caused by ‘gassing’ of the walls ofthe hole, are a special form of imperfection (Figure 9.6)

330 Quality control and inspection

Figure 9.5 The perfect/imperfect judgement scale

Figure 9.6 Blowholes in a throughplated wavesoldered joint

The causes of gassing of throughplated holes, and the measures which are needed

in order to avoid it, are by now well understood Gassing can be prevented byensuring the smoothness of the drilled holes and the continuity and adequatethickness of the copper plating on their walls It can be cured by a suitable heattreatment of boards which are liable to form blowholes before using them

Quality control and inspection 331

Because blowholes do not interfere with the functioning of a circuit board, theyare soldering imperfections rather than soldering faults, though their presence orabsence is an unequivocal yes/no situation Searching investigations have shownthat they do not affect the life expectancy of joints or their reliability, in any way.Corrective soldering can only mask, but notfill, a porous hole, and it is bound toshorten the life expectancy of the joint.

9.3 Practical examples of soldering faults

The nature of a soldering fault means that a circuit board is faulty and cannotfunction until every single fault on it has been corrected Therefore, the mostimportant task of any quality-control system is tofind every one of them In Tables9.1–9.3, the various types of soldering faults are listed and illustrated For complete-ness’ sake, faulty throughplated joints are included

9.4 The ideal and the imperfect joint

The criteria of perfection in a soldered joint go back to the days of handsoldering.They have to do with two parameters:first, the wetting angle between the solderand the substrate; and secondly the amount of solder in or on the joint Together,they determine the so-called joint profile The ideal handsoldered joint has a ‘lean’profile: the solder meniscus has a concave shape, so that the sharp wetting angle can

be seen clearly Also, the contours of the ends of the joint members must be visible,

so that an inspector can be sure that, in the case of the leadwires of insertedcomponents, the wires do in fact project through the hole and that all leads havebeen properly tinned (Figure 9.7)

The criteria of perfection of wavesoldered and reflowsoldered joints on circuitboards go back to these early days They deal with surface contours, surface areas,the relationships between distances It is possible to base judgements like good/bad,acceptable/unacceptable or beautiful/ugly on these criteria, provided every inspec-tor can refer to a set of pictures or samples of ‘perfect’ and ‘imperfect’ joints It isdifficult and certainly expensive to derive a clear yes/no verdict unless precise,time-consuming and therefore expensive measurements of individual joints aremade It is equally difficult, if not impossible, to base an automatic, opto-electronicinspection system on a ‘good/bad’ or ‘beautiful/ugly’ situation instead of a ‘yes/no’one Tables 9.4 and 9.5 illustrate practical examples of perfect and imperfect joints.There is one instance where an imperfection can become a fault: ‘fat’ joints withtoo much solder at the ends of a melf or chip-capacitor can cause the ceramic body

of the component to crack under the mechanical stresses caused by temperaturefluctuations during service Fat joints hold the component as in a vice, lean jointscan yield

Table 9.1 Soldering faults I: Open joints

method (C, if correction is possible)

and wire or lead (C)

Solderwave did not reach the joint (If the joint was not fluxed, it is covered by lumps of solder); or the solderwave did not overcome the shadow e ffect, because either layout or waveshape was unsuitable Remedy: improve waveshape or layout, or change orientation of the board towards the direction of travel.

or stencil for blockage

Wave Solder on wire or lead, but

not on land or footprint (C,

but di fficult and costly)

With wired components: land unsolderable through faulty solermask or misplaced marking With SMDs, as above or misplaced adhesive

to lack of coplanarity of leads Remedy: quality control before placement.

Unsuitable soldering parameters Remedy: put right, or choose paste with higher-melting soldermore than ‘just having a look’ To be meaningful and cost effective, every inspectionprocedure must have a set of well-defined targets or criteria, preferably in the form ofwritten, and sometimes illustrated, lists, or explicit software when automatic imageanalysis is used

The distinction between ‘soldering success’ and ‘soldering perfection’ (Section9.2) simplifies the task of inspection, in the same way that it is easier to umpire a horserace than a beauty contest

Without inspection, a manufacturing process like the soldering of electronic circuitboards is incomplete

There is a basic difference between the inspection of engineering products like acrankshaft and the inspection of a soldered circuit board The dimensions of the

Quality control and inspection 333

Table 9.2 Soldering faults II: Bridging and solderballs

multileads

As above, or add solderthieves to layout Change direction of travel by 90°

Remedy: carry out solderballing test, and if necessary use fresh paste Paste-printdown too thick: check and if necessary, correct

Wave Solderballs near footprints (C) Can happen with controlled

atmosphere machines Remedy: change to di fferent or slightly thicker flux Slow down conveyor

Remedy: if solderballing test con firms, use fresh paste Solderballs under melfs or

chips (C: de-solder and

resolder a ffected SMDs)

Printdown too large or too thick Remedy: check and correct screen

or stencil

Wave Scattered solderballs, or

‘spider’s webs’ (C: pick up

with tip of soldering iron)

Flux too thin, or insu fficiently pre-dried Remedy: check and, if necessary, adjust

Re flow Scattered solderballs (C: as

above)

Paste spits because it has picked up moisture, or temperature pro file too steep Remedy: check and, if necessary, use fresh paste or adjust temperature pro file of reflow oven

Note: As pointed out above,

solderballs do not necessarily constitute a soldering fault, unless they are loose, or are liable to become loose, and can roll about on the board

334 Quality control and inspection

Table 9.3 Soldering faults III: Displaced components

method

Re flow One or more leads or faces fail

to connect with their

footprints (C: desolder and

resolder by hand)

Serious misplacement or floating Remedies: check pick-and-place equipment; with chips or melfs, lack

of solderability at one end, or lack of symmetry between footprints

Floating or tombstoning (C):

desolder and resolder

One end less solderable than the other One end solders later than the other because of asymmetry of size

or thermal behaviour of footprints Remedy: check quality of components or correct layout fault

Figure 9.7 The ideal handsoldered joint

former can be expressed numerically, and readily and automatically comparedagainst prescribed standards, within a given set of tolerances It is very difficult, andcertainly expensive, to ascribe numerical values to a soldered joint Hence the needfor visual, optical or functional inspection of soldered circuit boards

9.5.1 When to inspect

Inspecting every soldered board when it has reached the end of the production line

is certainly necessary, but it is not enough Unless intermediate inspections arecarried out after various stages of production, errors or defects which are carriedover into the next production stage can be very expensive to correct later.The printdown of the adhesive whichfixes SMDs to the board before wavesol-dering must be checked for completeness and correct placement Missing adhesivemeans a lost SMD; misplaced adhesive can make adjacent footprints unsolderable.Mistakes in adhesive printdown are very difficult and expensive to correct oncethe joints have been cured Most adhesives are given conspicuous, sometimesluminescent, colouring to make visual or opto-electronic automatic inspectioneasier

Similarly, the correct printdown of solder paste must be checked before the

Quality control and inspection 335

Table 9.4 Too much or too little solder

Wave Too little solder

Re flow H1: 30%(H! +H%)

Acceptability as above Remedy: raise wave, slow down conveyor, more paste in case of

re flow Wave Too little solder

Re flow W1:75%W!

Acceptability as above Remedy: check solderability of footprint and component

Re flow Too little solder on PLCC

J-leg W1 :50%W!

Acceptability as above Remedy: see ‘wicking’

Table 9.5 Unsatisfactory wetting angle; displaced components

customer

with fine-pitch layout.

In that case, count as soldering fault

SMDs are placed on the board Faults are easily corrected at this stage If detectedafter soldering in the form of empty joints or bridges, the cost of correction rises by

at least one order of magnitude

With hand-placed components, a final check before the soldering stage isadvisable Much mechanized pick-and-place equipment is equipped with integ-rated checks for correct identity, polarity and placement of the components(Section 7.4) If it is not, afinal visual or optical check before soldering is worthwhile

336 Quality control and inspection

9.5.2 Visual inspection

It is useful to distinguish between two basic types of visual inspection, which onecould call the ‘general picture’ and the ‘detailed inspection’

The general picture

In small-scale production, where boards are soldered individually, by hand or onbenchtop equipment, the operator will naturally look at every single board before itleaves his workstation If there is an obvious fault due to a malfunctioning of hisequipment, or a defective board, he or she will put matters right before carrying onsoldering

With in-line soldering, the operator, or supervisor in charge of the soldering line,whether wave or reflow, should have a brief look at the boards leaving the line atregular intervals, perhaps at one board in every ten, in order to ensure that the line isrunning normally Obvious major faults might be unfluxed areas or uneven solder-ing because of an unsteady solderwave, or reflowed boards which are scorched, ordid not get hot enough for the paste to melt on all joints Unless such disasters arespotted before many boards reach the next inspection station, the line may havebeen producing a good deal of expensive scrap

The detailed inspection

The manner of the detailed visual inspection and the equipment used for it dependvery much on the type and volume of production and the size of the boards Thetype and specification of optical inspection equipment ranges from simple orilluminated magnifiers with a power of about five times, to sophisticated apparatuswith zoom optics, binocular operation, stereoscopic vision, and facilities to look atthe J-legs of PLCCs at an angle The advent of low-priced, small, readily manipu-lated video systems has added a new dimension to visual inspection

As a general rule, optical systems where the operator has to look into a singleeyepiece, or a binocular, which forces him to keep his head in afixed position, aremore fatiguing to operate than systems which show the object of observation on ascreen A good and flexible system of illumination is essential with all opticalinspection methods An easily operated handling system of the boards under test is

equally important With a number of systems, boards are mounted on a movable xy

table, which allows for overall scanning, or indexing into preset positions wherecertain recurring faults tend to occur

Recent studies pinpoint the problems of visual inspection: operators, oftenfemale, are under increasing stress, mental rather than physical, as boards get morecomplex and the pitch getsfiner They rate their stress factors in descending order asintense concentration, burden of responsibility and time pressure Faulty ergonom-ics and noise can be additional problems Withfine-pitch layouts and components,the rate of inspection falls dramatically, and the stress is greater

The solution is seen in systems where automated opto-electronic inspectionprecedes, and is linked with, inspection by a closely integrated team of about threeoperators, who visually inspect and manually correct faulty joints at the same time

Quality control and inspection 337

The value of linking visual inspection with corrective soldering is increasinglyrecognized and practised (Section 10.1).

9.5.3 Automated opto-electronic inspection

Unless the distinction between ‘soldering success’ and ‘soldering perfection’ ismade, automatic inspection must recognize both of them and be able to evaluatefeatures like joint contours This demands expensive systems of great complexity Ifthe judgement on soldering perfection is omitted, existing technology, which isconstantly being refined, permits relatively straightforward practical solutions forautomatically recognizing footprints without solder paste, empty joints or thepresence of bridges or solderballs These systems are based on video scanning of aboard surface, combined with an automatic comparison between the actual imageand the ideal image of a faultless board Equipment which operates fast enough tokeep up with in-line soldering machines and reflow installations is commerciallyavailable

Opto-electronic systems are able to recognize the following soldering failures:Missing, misplaced or defective printdown of solder paste

Missing or misplaced adhesiveMissing, misplaced or displaced componentsBridges, ‘spider’s webs’ and solderballs

A recently developed automatic opto-electronic inspection system does not quire a pre-programmed ‘ideal’ image of a faultless board, but creates its own

re-‘learning curve’ by evaluating parameters such as shape of solder-fillet, identity anddislocation of components, bridges etc

tion equipment is expected to become commercially available before the end of1997

9.5.4 X-ray inspection

X-ray inspection represents an optical system, which operates at two levels Theboard with its joints is scanned by penetrating X-ray radiation, which is absorbedmost strongly by the lead-containing solder in either joints or paste printdown, less

so by metallic conductors, ICs and other semiconductor devices, and least of all byorganic substances like FR4 and ceramic or plastic component housings Theresulting X-ray image is converted to a monochrome image in the visible range,which can be evaluated visually by an operator, or processed photo-electronically asdescribed above, and compared with the image of a faultless board One of theproblems which might perhaps be encountered in this context with lead-free solders(Section 3.2.3) is the reduced contrast between such a solder and its surroundings in

an X-ray image, unless the solder contains bismuth

X-ray images are shadowgraphs In the last decade, X-ray sources have beendeveloped with emitter-spots small enough to provide shadows of sufficient sharp-ness to allow even micrographic evaluation By controlling the voltage applied to

338 Quality control and inspection

Figure 9.8 X-ray images of capillary joints (a) Innocuous cavities caused by trapped air or vapour; (b) pattern of unsoundness indicating a wetting or solderability problem.

the X-ray tube, the penetrating characteristics of the radiation can be adjusted to suitthe features which need to be examined

X-rays show up the solder in the joint itself, which is a great advantage, but theimage must be sensibly interpreted: as has been explained in Section 3.6.3 (Figure3.19), almost all capillary joints contain voids A distinction must be made herebetween two kinds of joint porosity If the porosity appears as voids which aresurrounded by solder, it is safe to conclude that the solder has wetted the jointsurfaces with a satisfactory, sharp wetting angle This type of porous joint will be asreliable as a completelyfilled one If, however, the solder in the joint gap appears as

an archipelago of separate islands, one of the joint surfaces is likely to have dewetted(Figure 9.8), and reliability of the joint is impaired Image processing software mightwell be devised to be able to distinguish between these two types ofporosity, where the solder forms either a continuous or a discontinuous phase, andraise the alarm with dewetted capillary joints

X-ray inspection is the only option for inspecting joints underneath BGAs andflip-chips (see Section 2.2) To spot an open joint between a solderbump and thepaste print-down on the board is difficult, because the bump itself will produce acircular shadow A strategy to overcome this problem has been proposed by N.Manson

diamond-shaped or square outlines A square shadow means that the paste has beenmelted, but has not been drawn into the joint A round X-ray shadow indicates acorrectly formed joint It should not be difficult to program an automatic opto-electronic X-ray inspection system to recognize this type of defect The discovery of

an open joint underneath aflip-chip poses a problem: once the underfill underneaththe chip has set, the chip cannot be removed without damaging the board, and thecost of scrapping the board must be balanced against the costly and delicatemicrosurgery needed to clear away the underfill

There is no reason why the adhesive forfixing SMDs prior to wavesolderingshould not be made opaque to X-rays by the addition of a filler like a bariumcompound Thus, X-ray scanning could be used for the intermediate scanning ofboards for the correct presence of either adhesive or solder paste before placing thecomponents X-rays are equally suitable for checking the correct placement ofcomponents before the boards enter their soldering stage The virtues of suchintermediate inspections between successive manufacturing stages are obvious

Quality control and inspection 339

9.5.5 Electronic inspection

Testing the correct functioning of a circuit board by electronic means has developedinto a complex technology which is outside the scope of this book ATE testingwith an array of probes, not so long ago a cause of concern for solder pasteformulators, is still being practised but on a declining scale The growing number offunctions, crowded not only on to a board, but also into individual components andMCMs, demand that the board itself is designed for testability – for example bymethods such as ‘boundary scanning’ Dedicated contact points are designed intothe board circuitry, from which the board can be interrogated for its functions, aswell as for the existence and location of soldering faults Functional testing ofsoldered boards has become an important capital- and cost-intensive part of elec-tronic production It is still in a state of constant further development

9.5.6 Thermographic inspection

For the sake of completeness, the so-called thermographic methods of sequentiallychecking all the joints on a board for their quality by measuring their thermalcapacity must be mentioned here The basic idea is tofind out whether there is toomuch or too little solder on any given joint, or whether the joint is open, bymeasuring its thermal capacity and comparing it with an ideal value

In order to do this, a short, accurately dosed pulse of Nd: YAG laser energy isfocused onto the joint, and its temperature rise is picked up by a fast-responsepyrometer and recorded

which the laser beam is focused also enter into the result and the evaluation of thetest The method requires a laser scan of great accuracy and, for a start, a board withevery joint of an adequate degree of perfection – a so-called golden board – so thatfor every type of board, a library of ideal thermal capacities can be established.The Vanzetti method was and still is used, especially with military and spaceelectronics Other, similar, thermographic testing systems have been developed inEurope,

9.6 References

1 Birolini, A (1992) Guidelines for the development and design for quality,

reliability and maintainability Report Z4, 10 January 1992, Swiss Fed Inst.

Technology, Zurich

2 Lea, C et al (1987) The Scientific Framework leading to the recommendations

for the elimination of Blowholing in PTH Solder Fillets Circuit World, 13, No.

3, pp 11–20

3 Lea, C (1990) The harmfulness of reworking cosmetically defective joints

Soldering and SMT, No 5, pp 4–9.

4 Strauss, R (1992) The Difference between ‘Soldering Success’ and ‘SolderingQuality’; Its Significance for Quality Control and Corrective Soldering Proc 6th Intern Conf Interconnection Technol in Electronics, Fellbach, DVS Report 141,

Duesseldorf, Germany (in German)

340 Quality control and inspection

5 Vanzetti, R (1984) Automatic Laser Inspection System for Solder-Joint

Integ-rity Evaluation Proc 3rd PC World Convention, Washington DC, Paper WC

7 Graumueller, B et al (1997) Non-destructive Examination of Soldered Joints,

VTE (3) June, pp 145–149 (in German)

8 Manson, N (1996) Qualification Requires Keen Vision Electronic Production,

July/August, pp 15–16

Quality control and inspection 341

10 Rework

10.1 The unavoidability of rework

10.1.1 Rework in the production process

In our imperfect world, zero-fault soldering does not exist Soldering faults willoccur, and because even one single fault makes a board unusable, each must becorrected by rework or corrective soldering

It would be a mistake to regard rework as an unavoidable, tedious adjunct toelectronic production On the contrary, it is an essential link in the productionchain Unless it is taken seriously, properly organized, managed, monitored andintegrated into production, reworking the soldered boards may well cost more thansoldering them in thefirst place

On the other hand, if rework is monitored systematically, so as to lead to alearning curve and a fault catalogue for each type of board, its cost can be reduced toits unavoidable minimum At the same time, the fault catalogue will form a valuabletool, for use by management, designers, buying departments and quality managers.This applies equally to a large organization or manufacturing unit with manysoldering lines as to a small manufacturer with a handful of employees

Every rework operation must involve three steps:

1 Diagnosis Having located a fault which must be put right, try tofind out firstwhy it has occurred Don’t start working on it, until you have satisfied yourselfthat you have found the answer, and have made a record of it Otherwise, youmay destroy vital evidence, which could have helped to prevent the faultrecurring again

2 Remedy Put the fault right

3 Prevention Make sure that whoever in the organization could or should haveprevented the fault from occurring, knows what you have found and doneabout it, and if possible that this information is recorded Procedures andinformation technologies to take care of that are commercially available (Sec-tion 10.5.2) The rework rate can be regarded as the fever thermometer of amanufacturing line If nobody cares to read it, the patient may well bemoribund before anybody has noticed that he is sick

10.1.2 Desoldering and resoldering

Rework itself often involves two closely linked operations: if the correction of afault requires the removal of a faulty, misplaced or dislodged component and itsreplacement, the desoldering and resoldering operations which this implies shouldfollow closely upon one another

Removing bridges and, if necessary, solderballs, are simple desoldering ations Desoldering leaded components from throughplated holes requires solderingirons which can suck the solder out of the hole With SMDs, the removal of adefective component and its replacement with a new one can often be carried outwith one and the same tool With the desoldering of wavesoldered SMDs, separat-ing the glued joint underneath the component is an additional operation Openjoints are normallyfilled with a small temperature-controlled handsoldering iron

oper-10.2 Basic considerations

10.2.1 Metallurgical and mechanical consequences of rework

A reworked joint is never as good as thefirst joint would have been had it notneeded to be replaced or corrected The reasons are found principally in themetallurgy of joint formation Furthermore, the additional thermal, and sometimesmechanical, stresses of resoldering may easily weaken the bond between footprintand board A lifted footprint is one of the most serious types of damage, caused bygetting a joint too hot or heating it for too long during desoldering Repairing alifted footprint is possible, but it is expensive, takes time, and the board will never be

as good again as it was in the

Industrial experience con

did a cooperative research project carried out by a number of industrial companies

in the UK under the auspices of the National Physical Laboratory

The latter counted the number of through-hole joints on standardized sampleboards with 2000 joints each which showed visible cracks after they had beenreworked with a soldering iron under controlled conditions and then exposed to up

to 1000 temperature cycles between −20 °C/−4 °F and 100 °C/212 °F For parison, boards which had not been reworked were given the same thermaltreatment Some of the results are given in Table 10.1

com-Two conclusions are clear:

1 The longer the rework contact time of the soldering-iron bit, the moresoldered joints are visibly cracked after a given number of thermal fatiguecycles

2 The higher the rework temperature, the more heat is supplied to the solderjoint and the greater is the degradation of its performance under thermalfatigue, i.e the loss of reliability

The metallurgical reason for this degradation is the growth of the brittle lic layer between solder and substrate Its thickness is a function of the confrontationtime between the molten solder and the substrate, and of its temperature

intermetal-Rework 343

Table 10.1 The degradation of soldered jonts caused by rework

Rework time at soldering

Rework temperature using

rework time 4 sec

Number of cracked joints after cycles as above

10.2.2 The cost of rework

Rework is a joint-by-joint procedure, and necessarily time-consuming and sive Reworking a single joint costs on average as much as did the first-timesoldering of the complete circuit board on which it sits, and often considerablymore Depending on the type of product and its sale value, it may sometimes becheaper to scrap a faulty circuit than to rework it Some low-cost products likepocket calculators, cheap electronic watches and toys fall in that class

expen-In small-scale production in particular, where visual inspection and rework arecarried out by the same operator or operators, it is important to keep this cost aspect

in mind It is tempting to say ‘I might as well touch up this joint while I am looking

at it.’ Not only do joint quality and reliability suffer through this practice, but costsare liable to rise in an uncontrollable manner

10.2.3 Lessons to be learned

The lessons to be drawn from these considerations are plain:

1 Whatever the method used for rework, choose only the best equipmentavailable, and maintain it in top condition

2 Complete every joint quickly The solder should be molten for not longer than

a few seconds

3 Keep the working temperature as low as is compatible with this requirement

4 Preheating the board, either locally or overall, before carrying out any rework

on it brings two benefits First, the specific heat of the FR4 substrate is almost

344 Rework

four times that of copper or solder This means that four times as much heatenergy is needed to bring the board up to soldering temperature than to heatthe joint itself Preheating considerably shortens the heating time necessary tocomplete a joint, particularly with heavy multilayer boards.

Secondly, with wavesoldered boards where the SMDs are glued to the boardsurface, preheating softens the adhesive joint and makes it easier to break whendesoldering becomes necessary (Section 4.9.5) A preheating temperature ofabout 60 °C/140 °F to 100 °C/210 °F is usually enough for this purpose

A stream of warm air directed against the underside of the board is the usualmethod of preheating This avoids the danger of a sharply localized hot spot,which could distort the board

5 Rework for cosmetic reasons alone is an expensive and damaging luxury, and itshould only be carried out if the customer or the market demands it and pays forit

10.3 Rework equipment

10.3.1 Heat sources

Almost every heat source which is employed in production solderingfinds its use inrework: soldering irons in various forms, thermodes, solderwaves in miniatureform, infrared radiation, hot air or gas, and in recent years also the vapour of avapourphase soldering machine With all of them, efficiency of the heat transferfrom source to joint is of the essence, together with precise temperature control.Soldering irons

Molten solder is the ideal heat transfer medium: its heat conductivity is high and,being molten, it adapts perfectly to the surface contours of the joint and the jointmembers For this reason, a soldering iron used for rework must have a well tinnedtip, preferably with a drop of molten solder on it, to establish instant and goodthermal contact with the joint The best solder to use for rework, not only with aniron but with any method, is the silver-containing 63% tin solder (Section 3.3.1).The size of the iron and the shape of the tip must suit the type and theconfiguration of the joint, as will be shown in Section 10.3.2 Naturally, theergonomic aspects of the hand-held soldering iron must not be neglected

Precise control of the tip temperature, and a fast response to changes in theamount of heat demanded from it, are very important Recent years have seen rapidadvances in the technology of the soldering iron, which had remained static throughseveral decades Above all, methods of heating the soldering tip, and of shorteningthe path from the heat source to the end of the tip, have improved itgreatly Many handsoldering work stations include a fast response temperatureindicator of the soldering tip itself

Bearing in mind what is at risk if rework is carried out badly, it is worth stressingthe importance, not only of choosing a state-of-the-art soldering tool with antistatic

Rework 345

protection and a quick response to sudden heat-demands or changes in thermostatsetting, but also of maintaining it well and periodically checking the accuracy of itstemperature control.

Heated tweezers and thermodes

Resistance-heated tweezers are a convenient tool for desoldering the two joints of a melf or chip simultaneously PLCCs can be removed with a shapedtweezer, which grips the vertical shanks of the J-legs on all four sides of thecomponents simultaneously Tweezers are particularly useful for removing andresoldering these components on a crowded board, where the neighbours of the

end-affected component must be protected from the soldering heat of the site of therepair (see below) Tweezers, like thermodes, are made from an untinnable metallike tungsten

The thermodes used for desoldering are the same as those which are used for theimpulse-soldering individual multilead SMDs (Section 5.7) Because their solderingsurfaces are untinnable, they cannot be pretinned like a soldering iron, and moltensolder cannot be used as a thermal link between the tool and the joints A thincoating of a low-solids RMAflux, brushed over the row of leads on all four sides,will have to serve as a substitute for the molten solder in establishing a thermal linkbetween the soldering tool and the joint Once the thermode sitsfirmly on the flatleads, the heat transfer between the soldering tool and the joints takes place byconduction, as with a tinned soldering iron

Miniature solderwaves

Miniature solderwaves were developed in the sixties for the desoldering andresoldering of inserted multipin devices, like DILs With this method, the board isplaced over the nozzle of a dedicated small wavemachine, so that the leads whichhave to be desoldered sit closely above it As soon as the wave is switched on andtouches the underside of the board, the solder in the joints melts and the DIL can belifted off A replacement DIL can then immediately be soldered into the emptyholes, its legs having been fluxed first Alternatively, the molten solder can besucked out of the holes, an operation for which this type of machine is equipped,and the replacement part can be soldered in position, on the same wave, at a laterstage

Hot air or gas

Jets of hot air or gas, of a controlled temperature, are widely used for desolderingSMDs Because they transfer their heat to the joints by convection, which is muchless efficient than conduction through molten solder or contact with a thermode(Section 5.5), it takes longer to melt the solder in the joints, and this must be takeninto account when deciding on the reworking procedure

Single jets from hand-held nozzles can be used for desoldering and resoldering

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melfs, chips, SOs and multilead devices For the latter, however, many vendors offerequipment with interchangeable arrays of jet nozzles, which direct the hot air or gastowards the joints on all four sides of the SMD at the same time These jet arrays areuseful for the desoldering and resoldering of PLCCs, where the joints are notdirectly accessible to a plain thermode.

The hot air stream from a hand-held nozzle is usually controlled by the operatorvia a pedal, while multijet soldering heads are operated by a timer, the correctduration of heating having been determined empirically for a given board andcomponent With most hot-air desoldering machines, the board is preheated locallyfrom underneath, for reasons already mentioned Preheating is especially importantwith heavy multilayer boards

Infrared radiation

Beams of infrared radiation, focused on the joints of a multilead device, are anotheroption for supplying the heat for desoldering and resoldering Similar to the jetnozzles, the IR desoldering heads are interchangeable to suit specific componentdimensions Heating by radiation represents a non-equilibrium system (Section 5.8)where the temperature of the heat source is much higher than the target tempera-ture of the joint For this reason, such systems demand precise dosage of theradiation input, and ideally a feedback from the joint temperature to the heatsource.Infrared desoldering heads normally operate with medium wavelength emitters,operating in the 300 °C/550 °F to 500 °C/950 °F range

10.3.2 Rework stations

The workplace or work station for carrying out desoldering and resoldering must beuser-friendly Reworking can be a stressful task, and requires constant concentra-tion and decision making The workplace must therefore be well lit, provided with

an exhaust which removes theflux fumes, and the equipment must be cally designed so that it can be operated with a minimum of fatigue

ergonomi-For manual rework, adjustable inclined frames which hold the board are standard.Wide-field magnifiers with a power which need not exceed five, or at most ten,times are also standard Often, provision is made for a set of soldering irons of

different size to be readily at hand In addition to soldering irons, workstations mayprovide sets of hot air or gas jets, vacuum pipettes for handling components, andheated tweezers for desoldering and resoldering them Small mechanical tools likerotary drills and reamers to clean or remove footprints are also often part of a reworkstation

More elaborate manual stations arefitted with an overhead illuminator, whichdirect a pencil of light in sequence to the several joints or components whichrequire rework This sequence can be created by the inspection station and stored

on software in cases where visual inspection and manual rework are carried outseparately from one another

From simple manual workstations, the market offers a gradual succession of

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increasingly complex and sophisticated equipment for rework, particularly for usewith multilead SDMs Most of them use sets of quickly interchangeable hot-airnozzles or thermodes, together with placement systems for aligning the component

to be dealt with accurately with the soldering tool, visually or by opto-electronicmeans

Notwithstanding these electromechanical aids, the accuracy of feedback betweenthe human eye and hand must not be underestimated It has been shown that atrained operator, aided by a low-power magnifier, can place a multilead device with

a 0.5 mm/20 mil pitch on its footprints with sufficient accuracy by hand This means

a lateral accuracy of ±0.125 mm/5 mil, since the maximum permissible lateralmisplacement offine-pitch components is one quarter of the pitch distance (M.Cannon, Pace Europe Ltd, verbal communication)

10.4 Rework tasks and procedures

Most of the published information and know-how in thisfield can be found in thevoluminous vendors’ literature A few comprehensive accounts have appeared inthe technical press –

10.4.1 Removing bridges and solderballs

A solder bridge is removed by touching it briefly, for not more than a few seconds,with the well-tinned tip of a small soldering iron, with a rated output of about 40 W,and set to a temperature of about 250 °C/480 °F For de-bridgingfine-pitch leads, aconical 20 W soldering bit is best All that is necessary is to disrupt the bridge Donot try to ‘tidy up’ the joints on either side, and do not useflux A solder wick (seebelow) should not be used either, because it is likely to suck out too much solderfrom the joints themselves and thus weaken them

Solderballs are removed in the same way As soon as the tip of the iron touchesone, it will disappear The same will happen with ‘spiders’ webs’ Because a webcovers more ground than a solderball, a chisel-shaped soldering tip is more conveni-ent for removing them

Excess solder is removed from the tip of a soldering iron by wiping it with a piece

of linen or cotton, but never with fabric made from synthetic or mixedfibre Manyhandsoldering stations are equipped with a heat resistant sponge for cleaning thesoldering tip

10.4.2 Desoldering SMDs

Any of the following circumstances makes desoldering of an SMD obligatory:

E Melfs and chips, if one or both of their metallized faces are seen to be badlysolderable or unsolderable, or if the metallization has been leached off by themolten solder The desoldered components are discarded

E Chips which have ‘tombstoned’ The desoldered components are discarded,because their solderability is doubtful

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Figure 10.1 Desoldering a melf or chip with a solder-wick

E Melfs, chips and SOs which have floated away from their footprints Thedesoldered components are discarded, unless there is a desperate need for theirre-use

E Any SMD which is in the wrong place, or wrongly orientated If carefullydesoldered, the component can be reused

If X-ray inspection or functional testing reveal an open joint underneath a BGA, the

offending component must be desoldered, because the joints underneath are cessible, and cannot be corrected with the component in position The BGA itselfshould be discarded, unless it is irreplaceable To put fresh solderbumps on theunderside of a desoldered BGA is a delicate and laborious operation, though solderpre-forms for putting down a fresh array of bumps are on the market Unless the tem-perature is carefully controlled, the operation could easily ruin the IC inside the com-ponent, or cause the housing to crack due to the ‘popcorn’ effect (see Section 2.6).Flip-chips pose another problem: once the underfill has been applied andhardened, it is almost impossible to remove aflip-chip without damaging the board.Delicate microsurgery with special tools would be required to cut away thehardened underfill without damaging the board The cost of this operation, whichmay put the board at risk in any case, must be balanced against the cost of scrappingthe board (see Section 9.5.4)

inac-Desoldering melfs and chips with a solder-wick

Melfs and chips are joined to their footprints by butt joints The joints of the largerones contain too much solder to be sucked up by the tip of the hot soldering iron.Instead, a device called a ‘solder-wick’ is used to remove it A solder-wick is a tape

of closely braidedfine copper wire, a few millimetres wide, which is impregnatedwith an RMAflux Recently, solder-wick impregnated with a no-clean flux hasbecome available The wick acts towards molten solder like blotting paper againstink: for desoldering, it is pressed into the corner between the endface of thecomponent and the footprint (Figure 10.1) with the end of a chisel-shaped soldering

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