1、http:/ What is a flux?What does it do?Different types of fluxes?Flux testing&classification What is Solderability?What can effect solderability?How do we measure solderability?http:/ is soldering?In order to get a solder joint,the surfaces to be soldered must be free from dirt and oxides.Oxides are
2、formed during the hot soldering process.Therefore a FLUX must be used.The flux cleans and protects the joint during the soldering process.http:/ A fluxs job is to remove the oxide and other surface contaminants from the materials to be soldered and the solder itself.If this is not done the solder ca
3、nnot 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:/ of fluxes Fluxes may be found in various forms:Solid flux within a solder wire Liquid flux for flow sol
4、der machines Thick flux medium within a solder paste Gels or pens for rework The same principles apply for all the abovehttp:/ Wireshttp:/ Fluxeshttp:/ Fluxes Usually based on alcohol(typically 90-98%)May be rosin free or rosin containing Usually sprayed onto the board The board is then heated(to ev
5、aporate the solvent)then passed over a“wave”of molten solder Newer fluxes are water based.http:/ fluxer http:/ spray nozzle http:/ operating conditionshttp:/ Fluxeshttp:/ Retro-Flow Nozzlecirca 1973Stationary AreaSolder BathUni Directional Laminar Wavehttp:/ Wave Technologyhttp:/ Paste0.5mmPowder Pa
6、rticlesFlux Mediumhttp:/ medium A flux medium imparts certain characteristics to a solder paste activity rheology screen&tack life residue characteristicshttp:/ medium Usually rosin based(50-70%)Activators(usually halide)Solvents Viscosity modifiershttp:/ Usually extracted from pine trees May be sol
7、id or a thick liquid at room temperature Chemical composition may vary from year to year Natural product that has been used for soldering since Roman times It has a natural fluxing action May be chemically treated to form Modified Rosinshttp:/ Properties of Rosinl Melts(softens)at 80C esters&polymer
8、s slightly differentl Water solubility lowl Resistant to moisturel Electrical resistance highl Absorbs oxygen turns yellow,then brown reaction products catalyse processhttp:/ of Modified Rosin Higher softening point as high as120o C(248 o F)Increased thermal stability Clear residuesBetter chemical s
9、tabilityConsistency(Lot to lot performance)Residues tack free and dryhttp:/ Reactions of Rosin ll llIn solvent and by fusion RC-OH+MX-RC-OM+HX M=Sn,Pb,Cu X=Oxide,Hydroxide,CarbonateMetal Rosinates*Are soluble in Rosin (Lead 40 W/W%,Copper 8 W/W%)*Are invisible in Rosin (Lead salt-clear Copper salt-r
10、ed/brown)*Neutralize acidity of Rosin*Increase softening temperature of RosinOOhttp:/ For RosinCarboxylic Acids-stronger complexants -more reactive to metal oxides -include:Long chain fatty acids(palmitic,stearic)Polycarboxylic acids(adipic,succinic)Monocarboxylic acids(acetic acid)Amine Hydrohalide
11、s -rupture oxide filmsActivators soluble in RosinMetal salts soluble in Rosin http:/ ActivatorsCaboxylic acids Rosin Monocarboxylic acid (R-C02H)Dicarboxylic acid (H02C-R-CO2HHalides Fluoride F-Chloride Cl-Bromide Br-Iodide I-Astatide At-http:/ acid activation(R-CO2H)2+MO=(R-CO2)2M+H2O Metal salts t
12、hus formed disolve safely in rosinhttp:/ activation R3-NHCl Equilibrium between R3-N+HCl 2HCL+MO=MCl2+H2O Metal salts thus formed disolve safely in resinhttp:/ Activators Very effective-high yields High halide pastes must be cleaned No-clean materials should pass appropriate tests(eg.IPC,Bellcore,J-
13、STD)http:/ Industry ProtocolsFor example.J-STD-004(January 2019)replaced IPC-SF-818(January 1988)Bellcore GR-78-CORE Issue 1(September 2019)replaced TR-NWT-000078 Issue 3(December 1991)http:/ Industry ProtocolsThese test.Aspects of composition Interaction with copper(corrosion?)Surface Insulation Re
14、sistance(how conducting are flux residues?)Electromigration(do flux residues give rise to metallic dendrite formation?)http:/ Industry ProtocolsOther tests.Corrosion tests outside of these protocols Resistivity of water extract(an indication of composition)Ionic contamination on soldered PCBhttp:/ F
15、lux induced corrosion(copper mirror)Halide(Cl-and Br-)test with AgCrO4 paper Fluoride test Quantitative halides Flux solids(non-volatiles)Flux corrosion SIRhttp:/ Mirror Thin copper coating(50 nm)on glass 23C,50%RH,24 hrs Removal of copper film?http:/ Corrosion Flux solids Used to reflow alloy on co
16、pper coupon 40C,93%RH,10 days Corrosion?http:/ SIR Comb pattern printed Reflowed using standard profile 85C,85%RH,7 days,50V DC bias Surface resistance measured with 100 V DC reverse polarity 24,96,168 hours 1 x 108 passmarkhttp:/ SIR Combhttp:/ propensity of flux residue to absorb water and create
17、a conducting pathway between conductor tracks Governed by residue,pattern,temperature,RH and bias voltagehttp:/ Electromigration Comb pattern printed Reflowed using standard profile 65C,85%RH,21 days,10V DC bias SIR measured with 100 V DC same polarity 96(“pre-soak),500 hours If the surface resistan
18、ce falls by more than a decade,the flux fails Also,there must be no dendritic growthhttp:/ are designed to promote dendrite formation Eg copper ions diffuse towards cathode where they are reduced to elemental copper.http:/ Growthhttp:/ no-clean fluxes should be tested to a suitable protocol which wi
19、ll indicate the long term reliability of the residues under adverse conditions.All Multicore fluxes are test under elevated temperature&humidity to the IPC and Bellcore SIR and Electromigration tests.Other protocols may be used on request.http:/ A blend of solvents is often used to maximise the solu
20、bility of the flux components and give the best balance between printing and reflow characteristics A high boiling solvent will prolong tack&stencil life A low boiling solvent will give good reflow results with little slump Much research goes into the optimum blend of solventshttp:/ Agents Impart Su
21、itable Rheology Contribute to slump resistanceThe metal content of the paste will also effect these characteristics.http:/ Cleans substrate,component and powder to allow soldering to take place May use acid,halide or a combination of the two as the“active ingredient”Must be tested to the appropriate
22、 protocol if used as a no-clean fluxhttp:/ SubstrateSolderhttp:/ Solderability relates to how well a component will solder under production conditions.There are three aspects to be considered Wettability of metallisation Resistance of metallisation to dissolution Resistance to soldering heathttp:/ o
23、f Metallisation Mainly determined by the condition of the surface of the metallisation Effected by oxidation,surface contaminants and films Heat sink effects will contribute Will effect the spread or wicking of solderhttp:/ of Metallisation to Dissolution During soldering part of the substrate surfa
24、ce will be dissolved The rate of dissolution will depend on temperature,time,quantity of molten solder,composition of solder and composition of metallisation Low dissolution rates will give slow soldering High dissolution rates may result in de-wetting or solder contaminationhttp:/ DissolutionSolder
25、 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:/ RATES AuVery fast Agfast Cu,Ele
26、ctroless NiModerate PdSlow Ni,PtVery slowhttp:/ 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:/ testing The simplest solderability test is th
27、e“dip&look”test This is non-quantitative test Flux component with a test flux,dip in molten solder&inspect Only able to recognise very poor solderability componentshttp:/ Balance In order to obtain an accurate idea of how solderable a component(or board)is,you must have control of all parameters all
28、oy type alloy temperature pre-heat speed of immersion dwell time speed of withdrawalhttp:/ BalanceThe wetting balance should be capable of controlling all these parameters whilst making accurate measurements throughout the duration of the test in order to determinehow well the solder wets(wetting fo
29、rce)how fast the solder wets(wetting speed)if there is any evidence of de-wettinghttp:/ BalanceVery sensitive micro-balanceCopper coupon(fluxed)Solder bathhttp:/ balanceFThttp:/ balanceFThttp:/ balanceFThttp:/ balanceFThttp:/ balanceFThttp:/ balanceFThttp:/ balanceFTMax wetting forceTime to 2/3 max
30、forceEvidence of de-wettinghttp:/ De-wetting is the withdrawal of solder from the surface after initial wetting has occurred A thin layer of solder will be evident over most of the surface with irregular shaped“droplets”of solder here and there De-wetting will be caused by contamination of either th
31、e solder,substrate or component De-wetting will be influenced by soldering temperatures and timeshttp:/ BalanceFT The graphs are often shown this way up You can clearly see the speed and force of wetting Evidence of de-wetting can also be clearly seenhttp:/ Fluxes Any flux can be used for a solderab
32、ility test in order to get comparative result Test flux can be used to obtain results in accordance with specifications such as IEC 68-2-58 Common test fluxes are SM/NA(un-activated 25%rosin),Actiec2(25%rosin,0.2%chloride)or Actiec5(0.5%chloride)http:/ Testinghttp:/ Testing SMD components are often
33、difficult to test using a plain bath due to the small forces involved Considerable improvement has been found by use of a solder globule This amplifies the forces involved and will also enable“step&repeat”to be used on multi-leaded componentshttp:/ TestingBalanceSolder Globule(fluxed)Component(fluxe
34、d)http:/ TestingBalancehttp:/ TestingBalancehttp:/ A measure of how suitable a component or substrate is for the soldering process It involves:Wettability of metallisation Resistance of metallisation to dissolution Resistance to soldering heat It will be effected by age,handling,surface finish,solder type,contamination,storage conditions.
