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1、Fluxes&Solder Ability,http:/,Fluxes&Solderability,What is a flux?What does it do?Different types of fluxes?Flux testing&classificationWhat is Solderability?What can effect solderability?How do we measure solderability?,http:/,What is soldering?,In order to get a solder joint,the surfaces to be solde
2、red must be free from dirt and oxides.Oxides are formed during the hot soldering process.Therefore a FLUX must be used.The flux cleans and protects the joint during the soldering process.,http:/,Flux,A fluxs job is to remove the oxide and other surface contaminants from the materials to be soldered
3、and the solder itself.If this is not done the solder cannot wet the surfaces to be soldered.The flux will also form a protective layer over the joint,preventing further oxidation during the soldering process.The flux will also aid heat transfer.,http:/,Flux,Substrate,Solder,Flux,Oxide,Intermetallic,
4、http:/,Types of fluxes,Fluxes may be found in various forms:Solid flux within a solder wireLiquid flux for flow solder machinesThick flux medium within a solder pasteGels or pens for reworkThe same principles apply for all the above,http:/,Solder Wires,5 Core(sometimes called“Multicore”),2 Core,1 Co
5、re,http:/,Liquid Fluxes,Pre-heat,Direction of travel,Fluxer,PCB,SolderWave,http:/,Liquid Fluxes,Usually based on alcohol(typically 90-98%)May be rosin free or rosin containingUsually sprayed onto the boardThe board is then heated(to evaporate the solvent)then passed over a“wave”of molten solderNewer
6、 fluxes are water based.,http:/,http:/,http:/,Pre-heat,Direction of travel,Fluxer,PCB,Solder wave,http:/,http:/,Foam fluxer,http:/,Flux spray nozzle,http:/,Pre-heat,Direction of travel,Fluxer,PCB,SolderWave,http:/,http:/,Ideal operating conditions,http:/,Liquid Fluxes,Pre-heat,Direction of travel,Fl
7、uxer,SolderWave,http:/,http:/,http:/,Pioneer Retro-Flow Nozzlecirca 1973,http:/,Dual Wave Technology,http:/,Solder Paste,0.5mm,Powder Particles,Flux Medium,http:/,What does the flux medium do?,Flux ensures successful solder joint is formed between component and pad.Suspends powderProvides correct rh
8、eology and tackiness Cleans surfacesRemoves oxide from solder powderProtects surfacesLeaves safe(or removable)residues,http:/,Flux medium,A flux medium imparts certain characteristics to a solder pasteactivityrheologyscreen&tack liferesidue characteristics,http:/,Flux medium,Usually rosin based(50-7
9、0%)Activators(usually halide)SolventsViscosity modifiers,http:/,Rosin,Usually extracted from pine treesMay be solid or a thick liquid at room temperatureChemical composition may vary from year to yearNatural product that has been used for soldering since Roman timesIt has a natural fluxing actionMay
10、 be chemically treated to form Modified Rosins,http:/,Physical Properties of Rosin,Melts(softens)at 80Cesters&polymers slightly differentWater solubility lowResistant to moistureElectrical resistance highAbsorbs oxygenturns yellow,then brown reaction products catalyse process,http:/,Advantages of Mo
11、dified Rosin,Higher softening point as high as120o C(248 o F)Increased thermal stability Clear residuesBetter chemical stabilityConsistency(Lot to lot performance)Residues tack free and dry,http:/,Acid Reactions of Rosin,ll llIn solvent and by fusion RC-OH+MX-RC-OM+HX M=Sn,Pb,Cu X=Oxide,Hydroxide,Ca
12、rbonate,Metal Rosinates*Are soluble in Rosin(Lead 40 W/W%,Copper 8 W/W%)*Are invisible in Rosin(Lead salt-clear Copper salt-red/brown)*Neutralize acidity of Rosin*Increase softening temperature of Rosin,O,O,http:/,Activators For Rosin,Carboxylic Acids-stronger complexants-more reactive to metal oxid
13、es-include:Long chain fatty acids(palmitic,stearic)Polycarboxylic acids(adipic,succinic)Monocarboxylic acids(acetic acid)Amine Hydrohalides-rupture oxide filmsActivators soluble in RosinMetal salts soluble in Rosin,http:/,Flux Activators,Caboxylic acidsRosinMonocarboxylic acid(R-C02H)Dicarboxylic ac
14、id(H02C-R-CO2H,HalidesFluoride F-Chloride Cl-Bromide Br-Iodide I-Astatide At-,http:/,Carboxylic acid activation,(R-CO2H)2+MO=(R-CO2)2M+H2OMetal salts thus formed disolve safely in rosin,http:/,Halide activation,R3-NHCl Equilibrium between R3-N+HCl 2HCL+MO=MCl2+H2OMetal salts thus formed disolve safe
15、ly in resin,http:/,Halide Activators,Very effective-high yieldsHigh halide pastes must be cleanedNo-clean materials should pass appropriate tests(eg.IPC,Bellcore,J-STD),http:/,How are fluxes Classified?,Federal specification QQ-S-571EInstitute for Interconnecting and Packaging Electronic Circuits-IP
16、C-SF-818J-Std 005,http:/,Various Industry Protocols,For example.J-STD-004(January 1995)replaced IPC-SF-818(January 1988)Bellcore GR-78-CORE Issue 1(September 1997)replaced TR-NWT-000078 Issue 3(December 1991),http:/,Various Industry Protocols,These test.Aspects of compositionInteraction with copper(
17、corrosion?)Surface Insulation Resistance(how conducting are flux residues?)Electromigration(do flux residues give rise to metallic dendrite formation?),http:/,Various Industry Protocols,Other tests.Corrosion tests outside of these protocolsResistivity of water extract(an indication of composition)Io
18、nic contamination on soldered PCB,http:/,J-STD-004,Flux induced corrosion(copper mirror)Halide(Cl-and Br-)test with AgCrO4 paper Fluoride testQuantitative halidesFlux solids(non-volatiles)Flux corrosionSIR,http:/,Copper Mirror,Thin copper coating(50 nm)on glass23C,50%RH,24 hrs Removal of copper film
19、?,http:/,Copper Corrosion,Flux solidsUsed to reflow alloy on copper coupon 40C,93%RH,10 daysCorrosion?,http:/,J-STD-004 SIR,Comb pattern printedReflowed using standard profile85C,85%RH,7 days,50V DC bias Surface resistance measured with 100 V DC reverse polarity 24,96,168 hours1 x 108 passmark,http:
20、/,J-STD-004 SIR Comb,http:/,SIR?,Assesses propensity of flux residue to absorb water and create a conducting pathway between conductor tracksGoverned by residue,pattern,temperature,RH and bias voltage,http:/,Bellcore Electromigration,Comb pattern printedReflowed using standard profile65C,85%RH,21 da
21、ys,10V DC bias SIR measured with 100 V DC same polarity 96(“pre-soak),500 hoursIf the surface resistance falls by more than a decade,the flux fails Also,there must be no dendritic growth,http:/,Electromigration?,Conditions are designed to promote dendrite formationEg copper ions diffuse towards cath
22、ode where they are reduced to elemental copper.,http:/,Dendritic Growth,Cathodes,Dendrites,Direction of flow,Direction of growth,http:/,All no-clean fluxes should be tested to a suitable protocol which will indicate the long term reliability of the residues under adverse conditions.All Multicore flu
23、xes are test under elevated temperature&humidity to the IPC and Bellcore SIR and Electromigration tests.Other protocols may be used on request.,http:/,Solvents,A blend of solvents is often used to maximise the solubility of the flux components and give the best balance between printing and reflow ch
24、aracteristicsA high boiling solvent will prolong tack&stencil lifeA low boiling solvent will give good reflow results with little slumpMuch research goes into the optimum blend of solvents,http:/,Gelling Agents,Impart Suitable RheologyContribute to slump resistanceThe metal content of the paste will
25、 also effect these characteristics.,http:/,Flux:Summary,Cleans substrate,component and powder to allow soldering to take placeMay use acid,halide or a combination of the two as the“active ingredient”Must be tested to the appropriate protocol if used as a no-clean flux,http:/,Solderability,Intermetal
26、lic,Component,Substrate,Solder,http:/,Solderability,Solderability relates to how well a component will solder under production conditions.There are three aspects to be consideredWettability of metallisationResistance of metallisation to dissolutionResistance to soldering heat,http:/,Wettability of M
27、etallisation,Mainly determined by the condition of the surface of the metallisationEffected by oxidation,surface contaminants and filmsHeat sink effects will contributeWill effect the spread or wicking of solder,http:/,Resistance of Metallisation to Dissolution,During soldering part of the substrate
28、 surface will be dissolvedThe rate of dissolution will depend on temperature,time,quantity of molten solder,composition of solder and composition of metallisationLow dissolution rates will give slow solderingHigh dissolution rates may result in de-wetting or solder contamination,http:/,Substrate Dis
29、solution,Solder partly dissolves the surface being solderedToo much dissolution can contaminate the solder or remove the solderable coatingRate can be reduced by loading the solder with constituent of substrate.eg.Cu when soldering to Cu(SAVBIT)Ag When soldering to Ag/Pd(SN62),http:/,DISSOLUTION RAT
30、ES,AuVery fastAgfastCu,Electroless NiModeratePdSlowNi,PtVery slow,http:/,Resistance to Soldering Heat,Will the component withstand the soldering process?(e.g thermal shock)If a component is incapable of seeing the temperatures require to obtain a good solder joint it is unsolderable.,http:/,Solderab
31、ility testing,The simplest solderability test is the“dip&look”testThis is non-quantitative testFlux component with a test flux,dip in molten solder&inspectOnly able to recognise very poor solderability components,http:/,Wetting Balance,In order to obtain an accurate idea of how solderable a componen
32、t(or board)is,you must have control of all parametersalloy typealloy temperaturepre-heatspeed of immersiondwell timespeed of withdrawal,http:/,Wetting Balance,The wetting balance should be capable of controlling all these parameters whilst making accurate measurements throughout the duration of the
33、test in order to determinehow well the solder wets(wetting force)how fast the solder wets(wetting speed)if there is any evidence of de-wetting,http:/,Wetting Balance,Very sensitive micro-balance,Copper coupon(fluxed),Solder bath,http:/,Wetting balance,F,T,http:/,Wetting balance,F,T,http:/,Wetting ba
34、lance,F,T,http:/,Wetting balance,F,T,http:/,Wetting balance,F,T,http:/,Wetting balance,http:/,Wetting balance,F,T,Max wetting force,Time to 2/3 max force,Evidence of de-wetting,http:/,De-Wetting,De-wetting is the withdrawal of solder from the surface after initial wetting has occurredA thin layer of
35、 solder will be evident over most of the surface with irregular shaped“droplets”of solder here and thereDe-wetting will be caused by contamination of either the solder,substrate or componentDe-wetting will be influenced by soldering temperatures and times,http:/,Wetting Balance,F,T,The graphs are of
36、ten shown this way upYou can clearly see the speed and force of wettingEvidence of de-wetting can also be clearly seen,http:/,Test Fluxes,Any flux can be used for a solderability test in order to get comparative resultTest flux can be used to obtain results in accordance with specifications such as
37、IEC 68-2-58Common test fluxes are SM/NA(un-activated 25%rosin),Actiec2(25%rosin,0.2%chloride)or Actiec5(0.5%chloride),http:/,Globule Testing,http:/,Globule Testing,SMD components are often difficult to test using a plain bath due to the small forces involvedConsiderable improvement has been found by
38、 use of a solder globuleThis amplifies the forces involved and will also enable“step&repeat”to be used on multi-leaded components,http:/,Globule Testing,Solder Globule(fluxed),Component(fluxed),http:/,Globule Testing,Balance,http:/,Globule Testing,http:/,Solderability:Summary,A measure of how suitable a component or substrate is for the soldering processIt involves:Wettability of metallisationResistance of metallisation to dissolutionResistance to soldering heatIt will be effected by age,handling,surface finish,solder type,contamination,storage conditions.,http:/,
