FluxSolderability(英文)

1、Fluxes & Solder Ability,Fluxes & Solderability,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?,What is soldering?,In order to get a solder joint, the surfaces to be solder

2、ed 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.,Flux,A fluxs job is to remove the oxide and other surface contaminants from the materials to be soldered and t

3、he 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.,Flux,Substrate,Solder,Flux,Oxide,Intermetallic,Types of

4、 fluxes,Fluxes may be found in various forms: Solid flux within a solder wire Liquid flux for flow solder machines Thick flux medium within a solder paste Gels or pens for rework The same principles apply for all the above,Solder Wires,5 Core (sometimes called “Multicore”),2 Core,1 Core,Liquid Fluxe

5、s,Pre-heat,Direction of travel,Fluxer,PCB,Solder Wave,Liquid 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 evaporate the solvent) then passed over a “wave” of molten solder Newer fluxes are water b

6、ased.,Pre-heat,Direction of travel,Fluxer,PCB,Solder wave,Foam fluxer,Flux spray nozzle,Pre-heat,Direction of travel,Fluxer,PCB,Solder Wave,Ideal operating conditions,Liquid Fluxes,Pre-heat,Direction of travel,Fluxer,Solder Wave,Pioneer Retro-Flow Nozzlecirca 1973,Dual Wave Technology,Solder Paste,0

7、.5mm,Powder Particles,Flux Medium,What does the flux medium do ?,Flux ensures successful solder joint is formed between component and pad. Suspends powder Provides correct rheology and tackiness Cleans surfaces Removes oxide from solder powder Protects surfaces Leaves safe (or removable) residues,Fl

8、ux medium,A flux medium imparts certain characteristics to a solder paste activity rheology screen & tack life residue characteristics,Flux medium,Usually rosin based (50-70%) Activators (usually halide) Solvents Viscosity modifiers,Rosin,Usually extracted from pine trees May be solid or a thick liq

9、uid 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 Rosins,Physical Properties of Rosin,Melts (softens) at 80C esters & polymers slightly

10、different Water solubility low Resistant to moisture Electrical resistance high Absorbs oxygen turns yellow, then brown reaction products catalyse process,Advantages of Modified Rosin,Higher softening point as high as120o C (248 o F) Increased thermal stability Clear residues Better chemical stabili

11、ty Consistency (Lot to lot performance) Residues tack free and dry,Acid Reactions of Rosin,ll ll In solvent and by fusion RC-OH + MX - RC-OM + HX M = Sn, Pb, Cu X = Oxide, Hydroxide, Carbonate,Metal Rosinates * Are soluble in Rosin (Lead 40 W/W %, Copper 8 W/W %) * Are invisible in Rosin (Lead salt

12、- clear Copper salt - red/brown) * Neutralize acidity of Rosin * Increase softening temperature of Rosin,O,O,Activators For Rosin,Carboxylic Acids - stronger complexants - more reactive to metal oxides - include: Long chain fatty acids (palmitic, stearic) Polycarboxylic acids (adipic, succinic) Mono

13、carboxylic acids (acetic acid) Amine Hydrohalides - rupture oxide films Activators soluble in Rosin Metal salts soluble in Rosin,Flux Activators,Caboxylic acids Rosin Monocarboxylic acid ( R-C02H) Dicarboxylic acid ( H02C-R-CO2H,Halides Fluoride F- Chloride Cl- Bromide Br- Iodide I- Astatide At-,Car

14、boxylic acid activation,(R-CO2H)2 + MO = (R-CO2)2M + H2O Metal salts thus formed disolve safely in rosin,Halide activation,R3-NHCl Equilibrium between R3-N + HCl 2HCL + MO = MCl2 + H2O Metal salts thus formed disolve safely in resin,Halide Activators,Very effective - high yields High halide pastes m

15、ust be cleaned No-clean materials should pass appropriate tests (eg. IPC, Bellcore, J-STD),How are fluxes Classified ?,Federal specification QQ-S-571E Institute for Interconnecting and Packaging Electronic Circuits - IPC -SF- 818 J-Std 005,Various Industry Protocols,For example . J-STD-004 (January

16、1995) replaced IPC-SF-818 (January 1988) Bellcore GR-78-CORE Issue 1 (September 1997) replaced TR-NWT-000078 Issue 3 (December 1991),Various Industry Protocols,These test . Aspects of composition Interaction with copper (corrosion?) Surface Insulation Resistance (how conducting are flux residues?) E

17、lectromigration (do flux residues give rise to metallic dendrite formation?),Various Industry Protocols,Other tests . Corrosion tests outside of these protocols Resistivity of water extract (an indication of composition) Ionic contamination on soldered PCB,J-STD-004,Flux induced corrosion (copper mi

18、rror) Halide (Cl- and Br-) test with AgCrO4 paper Fluoride test Quantitative halides Flux solids (non-volatiles) Flux corrosion SIR,Copper Mirror,Thin copper coating (50 nm) on glass 23C, 50% RH, 24 hrs Removal of copper film?,Copper Corrosion,Flux solids Used to reflow alloy on copper coupon 40C, 9

19、3% RH, 10 days Corrosion?,J-STD-004 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 passmark,J-STD-004 SIR Comb,SIR ?,Assesses propensity of flux residue to absorb water and

20、 create a conducting pathway between conductor tracks Governed by residue, pattern, temperature, RH and bias voltage,Bellcore 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

21、the surface resistance falls by more than a decade, the flux fails Also, there must be no dendritic growth,Electromigration ?,Conditions are designed to promote dendrite formation Eg copper ions diffuse towards cathode where they are reduced to elemental copper.,Dendritic Growth,Cathodes,Dendrites,D

22、irection of flow,Direction of growth,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 fluxes are test under elevated temperature & humidity to the IPC and Bellcore SIR and Electromigratio

23、n tests. Other protocols may be used on request.,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 characteristics A high boiling solvent will prolong tack & stencil life A low boiling solvent will give

24、good reflow results with little slump Much research goes into the optimum blend of solvents,Gelling Agents,Impart Suitable Rheology Contribute to slump resistance The metal content of the paste will also effect these characteristics.,Flux: Summary,Cleans substrate, component and powder to allow sold

25、ering to take place May 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,Solderability,Intermetallic,Component,Substrate,Solder,Solderability,Solderability relates to how well a component will solder under p

26、roduction conditions. There are three aspects to be considered Wettability of metallisation Resistance of metallisation to dissolution Resistance to soldering heat,Wettability of Metallisation,Mainly determined by the condition of the surface of the metallisation Effected by oxidation, surface conta

27、minants and films Heat sink effects will contribute Will effect the spread or wicking of solder,Resistance of Metallisation to Dissolution,During soldering part of the substrate surface will be dissolved The rate of dissolution will depend on temperature, time, quantity of molten solder, composition

28、 of solder and composition of metallisation Low dissolution rates will give slow soldering High dissolution rates may result in de-wetting or solder contamination,Substrate Dissolution,Solder partly dissolves the surface being soldered Too much dissolution can contaminate the solder or remove the so

29、lderable coating Rate 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),DISSOLUTION RATES,AuVery fast Agfast Cu, Electroless NiModerate PdSlow Ni, PtVery slow,Resistance to Soldering Heat,Will the component withs

30、tand 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.,Solderability testing,The simplest solderability test is the “dip & look” test This is non-quantitative test Flux component with a test flu

31、x, dip in molten solder & inspect Only able to recognise very poor solderability components,Wetting Balance,In order to obtain an accurate idea of how solderable a component (or board) is, you must have control of all parameters alloy type alloy temperature pre-heat speed of immersion dwell time spe

32、ed of withdrawal,Wetting Balance,The wetting balance should be capable of controlling all these parameters whilst making accurate measurements throughout the duration of the test in order to determine how well the solder wets (wetting force) how fast the solder wets (wetting speed) if there is any e

33、vidence of de-wetting,Wetting Balance,Very sensitive micro-balance,Copper coupon (fluxed),Solder bath,Wetting balance,F,T,Wetting balance,F,T,Wetting balance,F,T,Wetting balance,F,T,Wetting balance,F,T,Wetting balance,Wetting balance,F,T,Max wetting force,Time to 2/3 max force,Evidence of de-wetting

34、,De-Wetting,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 the solder, substrate or component De-wetting will be influenced by soldering temperatures and times,Wetting Balance,F,T,The graphs are oft