SMT Soldering Handbook surface mount technology 2nd phần 3 doc

Hence the need to have a drop of molten solder on the tip of a soldering iron or thermode, or at least someflux on the joint to bridge that gapSection 5.7... This is the virtue of the sol

efficiency of the flux is derived from the resulting wetting curve The solderbathcontains a 63% Sn solder (for instance to JSTD-006, Sn63Pb37C), held at atemperature of 250 °C/480 °F.

Corrosive action

The test for corrosive action is again confined to observing what a flux will do tocopper during soldering, or what the residue which is left on the copper will do in amoist atmosphere

In ISO 9455–13flux residue, left on a copper coupon after having melted a smallamount of 60% tin solder together with theflux under test, is stored in a humidatmosphere, at 40 °C/645 °F and 91% to 95% relative humidity, for three days.Corrosion is deemed to have occurred if theflux residue has changed colour, or ifwhite spots have appeared in it

In ISO 9455–5, a drop of theflux to be tested is placed on a flat glass slide, on towhich a thinfilm of copper, with a thickness of 0.05 m/0.002 mil (500 angstrom)has been deposited by an evaporation technique, a so-called ‘copper mirror’.Copper mirror slides are commercially available The slide with the drop offlux on

it is kept in a humidity chamber at 23 °C/73 °F and 50% relative humidity for 24hours, and then examined If the copper mirror has disappeared underneath theflux, it is deemed to have failed the test A flux which passes the copper mirror test is

an ‘L-type’ (low activity) flux, which group comprises all R-type fluxes, mostRMA, and some R If some of the copper mirror has gone, it is an ‘M-type’(medium activity) flux, which may still be an RMA, but is mostly RA andsometimes a watersoluble or a synthetic activatedflux If the copper mirror hasdisappeared completely, theflux is an ‘H-type’ (high activity) Watersoluble andsynthetic activatedfluxes fall in that group An important aspect of flux classificationrelates to the surface–insulation–resistance (SIR) properties of aflux (ISO 9455–17,not yet issued)

Halide content

Determination by analysis

If a halide-freeflux is specified, some standards give a detailed analytical procedure forquantitatively determining the halide content of theflux If this exceeds 0.05% byweight of the rosin content of theflux, calculated as Cl, the flux does not conform to,for example, a BS 5625 halide-freeflux If it exceeds 0.5% calculated Cl on the solidscontent of theflux, it does not conform to an ANSI/J-STD-004 flux of type LI

Silver-chromate test

This is a qualitative yes/no test, and does not indicate a specific halide percentage.Silver chromate (AgCrO) is a brick-red substance, which turns white or yellow inthe presence of a halide Silver-chromate impregnated testpaper is commerciallyavailable If such a piece of paper turns white or yellow when a drop of thefluxunder test is placed on it, halide is deemed to be present, and theflux cannot be

classed as ‘L0’ or ‘L1’ to J-STD-004 There is a problem, though: certain acids andamines (which may well be free of halide) are also capable of causing the colour ofsilver-chromate paper to change Because this test is relatively insensitive, afluxwith up to 0.05% halide will still pass it as ‘halide-free’.

Beilstein test

This test, which is mentioned in ANSI/J-STD-004, is more sensitive than thesilver-chromate test, but it is a qualitative test and gives no indication of the actualquantity of halogen present Its drawback is that it will also respond to any non-ionichalogen in a halogenated solvent, should any such be contained in theflux.The Beilstein test detects the presence of halogen in an organic compound Itrequires a small piece offine copperwire gauze, which is heated in an oxidizingflame (e.g the blue part of a bunsen-burner flame) until it ceases to turn the flamegreen It is withdrawn, allowed to cool, and a small amount of theflux under test isplaced on it It is then put back into theflame If the flame turns blue-green, the fluxcontains traces of halide If not, it is deemed to be halide-free The Beilstein effectdepends on the formation of a volatile copper halide (F K Beilstein, Russo-German chemist, 1838–1906.)

Solubility of flux residues

The averageflux user needs guidance on how to assess the ease with which theresidue of theflux he is using, or wants to use, responds to the cleaning method he isusing or intends to use The international standard ISO 9455–11: 1991 (E) isrelevant to this problem

This standard describes a method of heating a sample of theflux on a dish-shapedpiece of brass sheet up to 300 °C/570 °F for a given time, placing the sample in ahumidity chamber for 24 hours and then immersing it in the solvent which is to beused for cleaning The presence of any residualflux left after cleaning is indicated bythe ability of the cleaned test specimen to form an electrolytic cell

Surface insulation resistance (SIR) of the flux residue

By definition, the residue from a ‘no-clean’ flux remains on the board Obviously,not only must it cause no corrosion, but its presence must not interfere with thefunctioning of the circuitry by lowering the surface insulation resistance (SIR) ofthe board between adjacent conductors: a leakage current of 10\ A betweenneighbouring IOs of a high-impedance microprocessor is enough to cause it tomalfunction (see Section 8.1.1) A number of tests to measure the SIR after varioussoldering and cleaning procedures have been devised over the years They aredescribed in Section 8.6.3

J-STD-004 includes a method for testing theflux residue for its moisture- andsurface-insulation resistance The relevant ISO working group is expected tocomplete its deliberations on the same subject in about three years’ time (informa-tion from BSI, London, April 1997)

Tackiness of the flux residue

Finally, the residue from a no-cleanflux must be dry and not sticky or ‘tacky’ undernormal temperature and moisture conditions Tackiness is tested by applyingpowdered chalk to afluxed coupon which has undergone a specified temperatureregime If the powder can be removed with a soft brush, theflux has passed the test

215 °C/420 °F and is often much higher

3.5.1 Heat requirements and heat flow

Heat is a form of energy, which is usually measured in one of the following ways.One calorie (1 cal) raises the temperature of one gram of water by 1 K (which is thesame temperature difference as 1 °C, Section 5.4.2) One calorie equals 4.187 joule,

or in units which are meaningful in the context of soldering, 4.18 watt.seconds(W.sec)

Table 3.12 indicates the amounts of heat required in some common solderingsituations In this context, it is useful to know the heat conductivity of the variousmaterials involved, so as to be able to gauge the speed with which the heat inputspreads within an assembly (Table 3.13)

Thefigures given in Tables 3.12 and 3.13 are worth studying Table 3.12 showsthat organic substances like FR4 have a much higher specific heat than metals Thishas an important bearing on most soldering situations The greater part of thesoldering heat expended in making a joint is not used to heat the metallic jointpartners, but to heat the FR4 epoxy board on which the copper laminate sits Hencethe need to preheat the boards before they pass through the solderwave (Section4.3), but also the benefit of preheating the circuit board, at least locally, whensoldering single multilead components (Section 5.7), or before carrying out repairwork, i.e desoldering and resoldering single components (Section 10.3)

The list of heat conductivities is equally illuminating The heat conductivity ofepoxy is two orders of magnitude lower than that of the ceramic substrate of ahybrid assembly Hence the need for taking the thermal management of SMDs,which are mounted on an epoxy board, much more seriously than that of hybridconstructions, which were initially the beginnings of SMD technology

Thefigures also show how even the narrowest air gap prevents the flow of heatbetween two hot bodies Hence the need to have a drop of molten solder on the tip

of a soldering iron or thermode, or at least someflux on the joint to bridge that gap(Section 5.7)

Table 3.12 Heat required to raise the temperature of a substance from 20 °C/68 °F to a soldering heat of 250 °C/482 °F

1 g solder 102 watt sec (including heat of melting)

A soldered joint (volume 1 cub mm) 0.7 watt sec

Equilibrium and non-equilibrium situations

The basic aim of every heating process is the transfer of heat from a heat source tothe heat recipient, i.e from a hot body to a colder one via a heat transfer medium.There are two basic heating situations: equilibrium and non-equilibrium systems

In equilibrium situations, the temperature of the heat source is the same as thesoldering temperature which must be reached The time within which the jointreaches its soldering temperature depends on the efficiency of the thermal couplingbetween source and joint The joint cannot be overheated, i.e it cannot get too hot,but it can be ‘overcooked’, i.e it can be heated for too long a time The latter carriesthe risk of excessive growth of the brittle intermetallic compound, and thus anunsatisfactory joint structure and the risk of a shortened joint life-expectancy

In non-equilibrium situations, the temperature of the heat source is higher, oftenvery much so, than the soldering temperature itself Whether the correct solderingtemperature is reached or exceeded is a matter of timing the heat exposure Thehigher the temperature of the heat source, the steeper is the temperature rise of thesolder joint, and the more critical becomes the precise control of the duration of itsheat exposure Overheating may not only endanger the joint and its properties, but

in severe cases it can damage the assembly itself (Figure 3.14)

Wavesoldering, vapourphase soldering, hot air or gas convection soldering,impulse soldering and handsoldering with a soldering iron present equilibrium

Figure 3.14 Equilibrium and non-equilibrium heating situations

heating conditions Infrared soldering, laser soldering and flame soldering arenon-equilibrium systems

Heat sources

A thermostatically controlled electrical resistance heater is the most commonprimary heat source This transmits its heat to the heat-transfer medium, whether it

be the drop of solder on a soldering iron or the solderwave in a wavesolderingmachine The reader may be amused to learn, though, that thefirst few wavesolder-ing machines were gas heated.

Small, pointed butane- or propane-gasflames are used for soldering individualjoints in awkward locations Equipment using a very hot, needle-shaped hydrogen–oxygenflame is also commercially available These flames, which represent extremecases of non-equilibrium heating, may be hand-held, but more often are manipu-lated by programmed robots, and then of course equipped with controls whichprevent overheating

Laser beams present the ultimate in non-equilibrium heating To speak of the

‘temperature’ of a laser source makes no real sense; what matters is the extremeenergy density of the spot of laser light, which impinges on the joint surface, andwhich may reach 10 kw per square millimetre (Section 5.6) A very precise energydosage is of the essence, to avoid destruction of the joint and burning a hole into thesubstrate Exposure times are measured in milliseconds

Heat transfer mechanisms

The soldering heat can be transmitted from the heat source to the joint by any one

of three basic mechanisms: conduction, convection and radiation

Conduction relies on a direct physical contact between a hot solid body or liquidand the surface of one of the joint members The efficiency of heat transfer dependscritically on the closefit between the heating and the heated surface Any airgapsbetween them fatally affect the heat transfer Molten solder is the best heat-transfermedium available: being a liquid, it conforms perfectly to whatever surface it has toheat This is the virtue of the solderwave, as well as of the drop of molten solder onthe tip of a soldering iron, which will come in very useful with repair soldering(Sections 10.2 and 10.3) Strictly speaking, convection comes into the heat-transfermechanism of wavesoldering as well, because the solderwave consists of a body ofmoving solder By contrast, dipsoldering in a stationary bath relies on heat transfer

by conduction only, like a soldering iron

Figure 3.15 The wetting angle

Wetting is not a ‘yes or no’ situation; there is a scale of wetting quality betweentotal non-wetting and complete wetting The yardstick for measuring the quality ofwetting is the ‘wetting angle’, which is formed between the surface of the solid andthat of the liquid along the line where they meet (Fig 3.13 and 3.15)

A wetting angle of 180° is a sign of total non-wetting, while an angle towards zerodenotes complete wetting In the context of soldering, a wetting angle of less than60–75° is normally, but arbitrarily, considered acceptable; anything up to 90° isdoubtful, and beyond 90° de

be equated with ‘non-acceptable’ will be discussed in Section 9.3

The wetting or contact angle between the molten solder and the substrate is theresult of the opposing forces of the surface tension of the solder, which tries to pull ittogether into a globule (somewhatflattened by gravity), and the interfacial tensionbetween the solder and the substrate, which tries to pull the solder across its surface,

so that as much of the solder as possible can come in contact and react with it Thewetting angle can be interpreted in terms of the three surface energies involved: that

of the molten solder, of the solid, and of the interface between the two KleinWassink

In practical terms, the significance of wetting can be stated very simply Goodwetting helps the solder to get to all the places where it ought to be; doubtful and badwetting prevent the solder from entering a joint

Dewetting

‘Dewetting’ is not the same as ‘non-wetting’ As the term implies, in a dewettingsituation the molten solder did get to where it ought to be, but it does not stay there.Instead, it pulls back and forms separate islands of solder, with areas of exposedintermetallic compound in between This situation can occur in dip-tinning, e.g inthe hot-air levelling process for circuit boards (HAL), or in wavesoldering, but onlyrarely in reflowsoldering

Dewetting is caused by local, untinnable spots of surface contamination, such asoxide particles, or surface dirt like traces of silicones orfingerprints Non-metallicinclusions in galvanic coatings, for instance embedded colloids caused by unsuitable

or badly controlled plating baths for copper, nickel or gold, can cause dewettingtoo

Surfaces which dewet are atfirst completely covered with molten solder, which

Figure 3.16 Dewetting and non-wetting

bridges the untinnable spots Before it can solidify, its surface tension pulls the stillliquid soldercoating apart, and away from the discontinuities (Figure 3.16)

3.6.2 Capillarity and its effects

A capillary is a very thin hole or a narrow gap (from capillus, Latin for ‘hair’) If the

surfaces of the hole or gap are wettable, interfacial tension quickly pulls the liquidsolder into it with considerable force, often against the force of gravity On theother hand, if the inner walls of the gap are untinnable, the surface tension of thesolder prevents it from entering it

If one of the members forming the gap is movable, like the gullwing legs of anSMD during reflowsoldering, the interfacial tension pulls the walls of the gaptowards one another, which means it pulls theflat end of the leg into the middle ofits footprint If, on the other hand, one or both are untinnable, the surface tension ofthe solder pushes them apart (Figure 3.17)

The consequences of capillarity for soldering are important:

1 If the joint surfaces are wettable, capillarity pulls the solder into the joint, againstthe force of gravity if necessary If they wet badly or not at all, the solder cannot getinto the joint, even if gravity would tend to pull it into the gap

2 If one of the joint members is mobile, as is the case in reflowsoldering, and ifboth are wettable, interfacial and surface tension pull the joint memberstogether If one of them is unwettable, they are pushed apart

In reflowsoldering, capillary forces are the cause for the self-alignment of BGAs andsmall SMDs, but also for ‘tombstoning’ and thefloating of chips or melfs on badlydesigned layout patterns (Sections 6.4.2 and 11.2.2; also Figure 3.18)

3.6.3 Capillarity and joint configuration

Capillary joints and open joints

The way in which the solderflows into a wettable joint depends on the solderingmethod and on the shape of the joint itself Basically, there are two types of joint:

‘capillary joints’ and ‘open joints’ (Figure 3.19) With a capillary joint, or lap joint,two flat and essentially parallel surfaces face one another, and the joint forms a

Figure 3.17

two-dimensional gap Tubular joints, like through-plated holes, are a special form

of capillary joint, where the gap is cylindrical With an open joint, or butt joint, one

or both of the joint members are notflat, and they touch one another along a line orjust in one spot

With capillary joints, the escape route for the air andflux in the joint can getblocked if the molten solder closes all the edges around the joint gap before all theair andflux inside the gap have been pushed out by an orderly, frontal advance ofmolten solder into the gap from one (or at most two) sides only With an open joint,there is no such problem: from whichever direction the solder enters an open joint,the escape routes for air andflux cannot be blocked

With wavesoldering, all capillary joints,flat and tubular, fill from one side only.Both types will normally be sound, especially the latter, unless air or water vapourescape from the walls into the hole after the solder has entered it (blowholing),which is a matter dealt with in Sections 9.5.3 and 11.2.2

Figure 3.18 The effects of capillarity in reflowsoldering (a) Self-alignment of components; (b) tombstoning

Figure 3.19 (a) Capillary joints and (b) open joints

Figure 3.20 How molten solder fills a capillary joint

The penetrating speed of molten solder into aflat capillary gap between twocopper surfaces, 0.09 mm/3.5 mil apart, has been measured for various solders and atvarious temperatures by McKeown (see Reference 2) At 243 °C/470 °F, using a63% tin solder and a concentrated zinc–ammonium chloride flux, McKeownmeasured a penetration speed of 3.5 sec over a gap length of 10 cm/4 in, whichequals about 2 m/6 ft per minute, the average travelling speed of a circuit boardacross a solderwave

When reflowsoldering a capillary joint with solder paste, solder and a partiallyvolatileflux are already in the gap before soldering starts, and entrapment of gas andflux in the flat joint is almost impossible to avoid The same is true for a reflowedcapillary joint, where solid solder is preplaced on one of the joint members, becausethe molten solder tends to advance more quickly along the edges of a joint than inthe middle (Figure 3.20)

Only with impulse soldering, where the joint members are pressed togetherduring soldering, are joints less likely to be porous Internal porosity in a capillaryjoint is almost undetectable by external inspection and only shows up under X-rayexamination (Section 9.4.3) However, whether a porous capillary joint, even if itcontains up to 50% voids by volume, is really inferior to a completely solid one isvery debatable; this will be dealt with in Section 9.3

Open joints, as which one can count not only the end-joints of melfs and chips,but also the joints of all SOs, because their horizontal legs are short and the joint gap

is wedge-shaped, do not trap air The few airbubbles found in such joints, if they aresectioned, are no cause for worry On the contrary, they are more likely to arrestincipient internal cracks rather than starting them

Open joints versus capillary joints

Open joints have several advantages over capillary joints in both wavesoldering and

reflowsoldering The solder gets into them more easily, and they are much moreinspectable by optical means, unless they are partially or completely hidden underthe component housing like the J-legs of a PLCC, or underneath a BGA or aflip-chip The presence of solder in the joint and its wetting angle with the joint

members are both readily verified With capillary joints, the only external evidencefor adequate wetting and penetration is the continuity and the wetting angle of itscircumferential solderfillet.

Thus, a good case can be made for avoiding parallel capillary gaps and fordesigning wedge-shaped, open ones instead, unless the joints are to be soldered byimpulse soldering under the mechanical pressure of a thermode Here too, it may beworth while considering giving the faces of the thermodes a slant, thus making theimpulse-soldered joints wedge-shaped, with any trappedflux and vapour pushedtowards one end

3.6.4 The importance of solderability

The ease with which molten solder wets a metallic surface is called ‘solderability’ Ithas a decisive influence on the achievement of both soldering success and solderingquality, and thus on the fault rate of a soldering operation and its cost efficiency It istherefore important to choose joint surfaces which are inherently solderable and tomake sure that they remain in a solderable condition To this end, one must be able

to measure, or at least to assess, their solderability

The inherent solderability of a metal

The solderability of a metal depends on two factors:first, on the nature and thechemical stability of the oxide layer on its surface; secondly, on the chemical affinitybetween the metal and the solder, in other words on the readiness of the solder toform a diffusion zone at its interface with the substrate

Thefirst factor determines whether a mild flux will do, or whether an active,highly polar and corrosiveflux is needed to remove the oxide The chemical affinity

is based on the amount of energy set free by the reaction between the tin, morerarely by the lead in the solder, and the substrate The inherent solderability of anumber of common metals is shown in Table 3.14

This listing assumes clean, though not oxide-free, surfaces Solderability is erned by several factors, among them the following:

gov-1 The ease with which surface oxides or sulfides are dissolved by a flux

2 The surface energy of the metal surface (which means its readiness to react withwhatever comes in contact with it), metals with low surface energies beingmore difficult to solder

3 The metallurgical affinity between the metal to be soldered and the constituents

of the solder For example, lead is more compatible with nickel than tin,therefore lead-rich solders are better on nickel than pure tin or a eutectictin–lead solder

3.6.5 Oxide layers

The chemical stability of the various metal oxides differs widely Gold and platinum

do not form oxides under normal circumstances, so their chemical behaviour is

Table 3.14 The solderability of common metals, listed in order of descending solderability

A Readily solderable with mild fluxes (R and RMA)

Gold and its alloys Tin–lead solder Tin

B Solderable with mild fluxes (RMA)

Copper Silver Copper + 2% iron Silver/palladium (as thick- film on chips and melfs) Gold/platinum (as thick- film on LCCCs)

C Solderable with activated fluxes (RA)

Brass Nickel Cadmium

D Solderable with active fluxes (OA etc.)

Zinc Tin/bronze Nickel/copper alloys Nickel/iron alloys (Alloy 42) Nickel/iron/cobalt alloys (Kovar) Mild steel

Alloy steels Beryllium bronze

E Only solderable with special fluxes

Aluminium bronze ( fluxes based on phosphoric acid) Stainless steel (ditto)

Aluminium and its alloys ( fluxes containing zinc compounds) Cast iron (not solderable but tinnable with fluxes consisting of fused chlorides)

F Unsolderable

Chromium Silicon Titanium Manganese

irrelevant Silver does not oxidize at room temperature, but ozone, an ingredient ofurban smog, attacks and blackens it

present in our normal industrial atmosphere The familiar brown tarnish of silversulfide which results from this is resistant to mild fluxes, which on the other handdeal readily with copper oxide and zinc oxide Iron oxide is more difficult, and castiron, because of the non-metallic graphite particles on its surface, is untinnable andunsolderable by the methods admissible in electronic soldering Chromium and itsalloys owe their resistance against tarnish to a transparent, but stable and tough,oxide layer which makes them almost unsolderable The oxide layer on aluminium

Figure 3.21 The lift-off effect

and its alloys is equally transparent, but can be dealt with by special, thoughcorrosive, fluxes On the other hand, surface oxides and sulfides of copper arereadily removed, even by mildfluxes

A particularly obnoxious and almost unsolderable oxide layer forms if theintermetallic compound, which is the top layer of the diffusion zone between acopper surface and the molten solder (see Section 3.3.2, Figure 3.7), is exposed toair This can happen when desoldering a faulty joint or component, if the exposedfootprint is wiped clean of the solder remaining on it (Section 10.4.2) How toguard against this mishap, and what to do if it has happened, is discussed in therelevant section

3.6.6 Solderability-enhancing surface coatings

Uncoated soldering surfaces, like bare copper footprints or Alloy-42 gullwings,have become rare Most surfaces intended to be soldered are coated with tin or atin–lead alloy to improve and preserve their solderability Silver or gold are also usedsometimes, but their value as solderability preservers is doubtful Silver is liable totarnish, and the presence of gold in a soldered joint can lead to embrittlement ifthere is too much of it (see below)

Tin and tin–lead coatings

It is true to say that nothing is more solderable than solder itself It is even moresolderable than pure tin, because the lead in the solder makes it more resistant toatmospheric moisture and to corrosive environments For that reason, a coatingwith a 50% or even a 40% tin solder is often preferred to one with 60% tin

As soon as the molten solder encounters a tin or tin–lead coating, it melts anddissolves it Provided the coating is thick enough, above about 25m/0.1 mil, themolten solder will lift off any surface contamination which might sit on the surface

of the coating, andflow underneath it This is called the ‘lift-off effect’ (Figure3.21)

Galvanically deposited tin–lead coatings have their problems They consist ofdiscrete particles of tin and lead, and are therefore porous All the damagingingredients of the atmosphere, especially an industrial one, can and will slowly

penetrate between the particles down to the base metal The tin–lead depositdissolves on contact with the molten solder, which is then confronted with the barebase metal, which was originally quite clean of course, otherwise the galvanicdeposit would not have adhered to it But after a period of unsuitable storage,contamination might have penetrated down to it, and its solderability will have

suffered, if not disappeared

The remedy is to fuse, i.e to reflow, galvanic tin or tin–lead deposits in thepresence of aflux cover, and turn them into a coherent layer of tin or tin–lead solder(Section 6.3) As a bonus, this creates a layer of intermetallic compound between thecoating and the base metal

If the substrate base is not readily solderable with an RMAflux (like iron or aniron–nickel alloy), a thin galvanic coating of nickel, possibly with a topcoat ofcopper, must be provided between the base metal and the tin or solder topcoat.Nickel dissolves only slowly in molten solder While the topcoat disappears im-mediately in the molten solder once soldering starts, the nickel survives long enough

to protect the possibly badly solderable base underneath With a pre-fused coating,the solder willfind a ready-made intermetallic layer already in place when solderingstarts The alternative to a fused galvanic tin–lead coating is one produced by hottinning, by one of the processes of roller-tinning, immersion tinning or the HALhot-air-levelling process (Section 6.3.1)

Silver coatings

Once popular, silver is now used less frequently, for several reasons One is its lowresistance to unsuitable storage conditions, as described above Another is thedanger of migration of silver between neighbouring conductors under the influence

of an electrical DC potential between them This leads to the formation offibrousdendrites, which cause short circuits

Silver dissolves relatively quickly in molten solder If it has to serve as a solderablecoating on an otherwise unsolderable substrate, measures which are describedbelow must be taken in order to prevent it from disappearing before soldering iscompleted

Gold coatings

Gold as such is superbly solderable, with excellent storage properties In spite of this,

it is rarely used because it is associated with several problems, apart from its cost.Galvanic gold deposits, mostly1 m/0.04 mil thick, can be almost unsolder-able if they were produced by an unsuitable plating technique Shiny gold deposits,such as are used in jewellery, contain colloid additions and fall into that class Goldcoatings thinner than 1m/0.04 mil are liable to be porous and soon becomeunsolderable

Gold rapidly dissolves in molten solder and with3% Au present in a joint,large, brittleflakes of the intermetallic compound AuSnforminthejointgapwhenthe solder solidi

For this reason, gold-coated joint surfaces should only be considered if there is a

Table 3.15 Leaching rates of some substrates

Substrate Leaching rate in 60% Sn/40% Pb solder ( m/sec)

3.6.7 Leaching effect of molten solder

The rate at which a substrate dissolves in the molten solder is called the leaching rate,and it differs from metal to metal It depends on the rate at which substrate andsolder react with one another, on the solubility of the reaction products in thesolder, and of course on the soldering temperature Table 3.15 lists the leaching rates

of some substrates, in descending order Thefigures show that silver disappears inthe molten solder ten times and gold up to thirty times as fast as copper, and that theleaching rate rises quickly with the soldering temperature

The leaching effect can have serious consequences, for instance if the thick-filmsolderable metallized surfaces on chips, melfs and the (now obsolescent) LCCCsdisappear, making them unsolderable There are two ways of reducing a highleaching rate:

1 An alloying addition About one per cent of indium added to the solder slowsdown the solution of Au in the solder The 1.65% to 2% of Ag added toelectronic solders (Section 3.2.2) does the same for a silver substrate, but this

effect is small

2 Modifying the substrate An addition of up to 35% platinum to the thick-filmgold on LCCCs significantly slows down the leaching rate Up to 20%palladium does the same for thick-film Ag on resistors and condensers Cheaperand more effective, however, is a galvanically applied, leach-resistant nickellayer, topped with a tin or solder coating for optimal solderability

The leach resistance of components can be tested by the immersion solderabilitytest, described in the following section

3.6.8 Measuring solderability

The solderability of a surface is based on the speed and reliability with which it iswetted by the molten solder using a givenflux Naturally, the milder the flux whichone wants to or must use, the higher must be the solderability of all the surfacesinvolved Solderability is one of the most important parameters in all solderingprocesses, especially the mechanized and automated ones which are used in masssoldering The soldering success, and with it the reject rate and the economics of thewhole operation, depends on a consistently good solderability of all footprints,lands, throughplated holes, and component wires, leads, and sintered surfaces It istherefore essential to have a meaningful and reproducible method for measuring it

In the age of handsoldering, it was mainly the flux vendors who measuredsolderability in order to assess the ability of their activatedfluxes to cope with theoften indifferent solderability of component wires and soldering lands With theadvent of the mechanized soldering of large numbers of joints, where solderingtimes must be as short as possible, and with no-clean soldering, wherefluxes should

be as mild as possible, the emphasis shifted to the soldering surfaces involved in theprocess: verifying and monitoring their optimal and consistent solderability became

of paramount importance

Uses of solderability measurement

Solderability measurement has several uses The main one is the assessment of thesolderability of a substrate, using aflux of known and standard efficiency The otherone is the assessment of the efficiency of a flux, using a substrate of standard andreproducible solderability This latter requirement is not an easy one to fulfil and ithas engaged the attention offlux manufacturers and drafting committees of standardspecifications for quite some time (Section 3.4.7) Comparing the efficiency of analternativeflux with that of a flux with a proven performance, using components ofknown and consistent solderability for reference, is a simpler proposition, and isfrequently practised in the industry

Surfaces which need testing

Circuit boards

Given the present state of the art, the solderability of almost all commerciallyavailable circuit boards, that is their lands and footprints, can be assumed to be good,unless they are bare copper In that case, a simple dipping test, which is describedlater, will verify solderability With HAL pretinned boards and reflowed galvaniccoatings, smooth and fully tinned footprints are a safe indicator of perfect solderabil-ity Any defects in this respect can be assumed to have been spotted by the qualitycontrol of a competent board manufacturer

Component leads and metallized surfaces

The solderability of components is not necessarily visually obvious Only withAg/Pd sintered thick-film faces on melfs and chips, a dark grey or brown tarnish

Figure 3.22 The solder meniscus and its assessment

means that sulfide has formed on them because of unsuitable storage and that theyhave become unsolderable Should that have happened with a batch of loose, notbelted, components, they can be saved by a short immersion in a photographicfixing bath (known as ‘hypo’), or its equivalent, a 10% sodium thiosulfate solution

in water, followed by a rinsefirst in de-ionized water, then in clean isopropanol, anddrying off in air Hypo is an efficient solvent for silver compounds, including thebrown sulfide

Wetting tests

Observing the solder meniscus

The contour of the surface of the molten solder along the line where it touches animmersed metallic body is called the ‘meniscus’ The shape of the meniscus is anindicator of whether and how well the solder wets the metal (Figure 3.22) For avisual check of whether all is well with a doubtful leadwire or component, looking

at the meniscus is a quick, simple test The component lead or wire is dipped in anMRAflux, conveniently the one which is used in production, allowed to dry for ashort while, and dipped in a small bath of 63% Sn solder held at 250 °C/480 °F (it isuseful to have such a bath handy in the quality control or production department;see below) The meniscus formed by the solder against the immersed body isobserved visually Opto-electronic equipment for measuring the deflection of abeam of light focused on the meniscus has been described

The wetting balance

The wetting balance has become the standard instrument for measuring the bility of a metal surface, using aflux of standardized fluxing power, or the fluxingefficiency of a flux, using a copper specimen with a surface of standardized soldera-bility, coated with theflux under test (see Section 3.4.9) By now, it has reached a