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1、Reflow soldering for Fine-Pitch ComponentsCritical factors in reflowing soldering 1 The heat transfer from a heating source.2.The heat absorption rate and uniformity by assembly.3.The solderability of the metals to be joined.4.The melting point of the solder.5.The heat tolerance of the material on t
2、he assembly.6.The flux chemistry and the cleanliness of the metals to joined.Dynamics of the Reflow Process Phase One-Solvent Evaporation Phase Two-Flux Activation(Oxide Reduction)Phase Three-Solder Melt Phase Four-Complete Solder Melt Phase Five-Cool Down Temperature and time profileTin/Lead solder
3、 oxide growth rates-thickness vs timeIncreasing Flux RateProbability of IC package crackTemperature and time profileThe Heat Sources 1.Infra-red(including IR and IR mixed with convection.2.Vapor phase 3.Convection-hot gas 4.Laser 5.Thermosonic Infra-red reflow The infra-red heat transfer is done by
4、emitting light from the infra-red region of the spectrum.IR emission is that having wavelength of 0.72 microns to 1000 microns 0.72-2.5 micron,short IR 0.25-5.0 micron,medium IR longer than 5.0 micron,long wave IR The IR emitter temperature determines the emitted wavelength of IR radiation The highe
5、r the temperature,the shorter the emitted wavelength.The temperature of the PCBA is a function of the absorbitivity of the assembly.The absorbitivity is IR wavelength and material mass dependent if the assembly surface is nonlinear and has multi-colour bodies.q=o A FE Fa(TS-TA)q=The average radiativ
6、e heat transfer from the source to the assembly FE=The emissivity factor for the source and product Fa=The configuration or geometric view factor of the pcba o=The Stefan-Boltzman constant A=Area being heated TA=Temperature of the area being heated TS=Temperature of the source Infra-red reflow In th
7、e commonly used IR system,the PCBS absorbs heat radiated from the surrounding heated air.This thermal energy is absorbed by the PCBAs before soldering occurs.Advantages:Precise control of reflow temperature Faster energy transfer than convection Better radiated energy penetrationIR-Reflow Disadvanta
8、ges:Temperature is thermal mass dependent Colour selectivity Shadowing effect IR reflow optimization Component are placed with uniform mass distribution.Locate larger components near the corners or edges Adjust the belt speedSchematic view of an in-line IR reflow soldering systemTypical temperature
9、profile for area-source IR reflow systemT-3 tungsten lampT-3 quartz lamp at full and half rated powerDual-emitter T-3 lamp systemEmission spectra of dual-emitter T-3 lamp systemPanel IR emitterComparison of emitter types heat transfer Temperature adjust Colour sensitivity shadow effect temp uniformi
10、ty time to stabilize after change heat transfer by radiation convection Lamp Panel high low fast slow yes less yes less fair better shorter longer 80-90%20-40%10-20%60-80%Light Absorption and Reflection Properties for materials Vapour phase reflow soldering When most fluids are heated to their boili
11、ng point,they exist in two phase,vapor and liquid.In the vapor phase,these exist as a boiling fluid and a saturated vapor at the temperature of the boiling fluid.If an assembly at a lower temperature is placed in the saturated vapor,the vapor immediately condense on the assembly and transfer latent
12、heat extremely rapidly to the assembly.The rate heat transfer is:q=h AA(TS-TA)where q=the heat transfer to the assembly h=the vapor heat transfer coefficient(varies)TS=the saturated vapor temperature(constant)TA=the assembly temperature(varies)AA=the assembly area.Vapour Phase Reflow Soldering Vapou
13、r phase soldering or condensation soldering is by transferring the latent heat of a boiling vapour to the surfaces upon which it condenses.It is a surface heating phenomena.Objects in the reflow environment are heated on the surface first.Advantages Rapid rate of heat transfer(typical 6 to 10 deg C/
14、sec Inert atmosphere well controlled maximum temperatureVapour Phase Reflow Soldering Advantages:Insensitive to variations to board thermal mass Insensitive to variation in board geometry Disadvantages High operation cost(FC-70)Long heat up time Little control over temperature profile Unfavourable t
15、o low thermal mass part(solder wickingDual-vapour reflow systemBatch type productionIn-line single-vapor soldering systemConvection reflow soldering The process of heat transfer from a hot gas flowing over a cooler surface is called convection heating.If the gas circulation is caused by different de
16、nsities of masses being heated then the term free convection is applied.If gas circulation is done by mechanical means such as a fan or pump then it is termed forced convection q=hc AS(T G-TS)q=the heat transfer rate from the gas to the surface hc=thermal convection conductance based on fluid dynami
17、cs As=surface(the assembly)area TG=the gas temperature near the surface TS=the surface(assembly)temperatureConvection oven classification SMEMA classifies the convection system s class III and the mixed convection and IR systems as class II Air flow in a convection dominant reflow system Comparison
18、of heat flow transferNitrogen Reflow Soldering Reasons to use nitrogen in reflow soldering:1)More fine pitch package used 2)the increasing in use of low activity fluxes Advantage:can reduce solder ball avoid the flux from hardening in the absence of oxygenHot bar and thermode bonding Hot bar bonding
19、 uses metal bars that contact all the leads on a package simultaneously.The choice of bar metal and its shape are critical.The bars must resist heavy oxidation.The bars should be easily cleaned to assure optimum and uniform heat transfer q=kA(TB-TA)/e q=heat flow through the material being heated k=
20、the average thermal conductivity at the heater and material interface,A=the contact area TB=temp of the bar at the interface TA=temp of the assembly at the interface e=thickness of the region to be heated Heating profile-hot barHot-bar thermode heating unitLasers beams soldering Laser heat sources o
21、perate on the same principle as infrared radiation soldering except the heated area is only the area exposed to the laser light Common lasers used for soldering are neodymin-doped yttrium aluminum garnet(Nd:YAG)and carbon dioxide The potential advantages of laser reflow are precise placement of the
22、reflow heat in very dense areas A fiber optic of 0.1 or 0.15 mm diameter is sufficient to transmit enough thermal energy to reflow a typical FPT lead jointTemperature Profiling To achieve the ideal temperature profile by adjusting:specific emitters temperature better speed Method of getting feedback from temperature:thermocouples reflow temperature profiles谢谢观看/欢迎下载BY FAITH I MEAN A VISION OF GOOD ONE CHERISHES AND THE ENTHUSIASM THAT PUSHES ONE TO SEEK ITS FULFILLMENT REGARDLESS OF OBSTACLES.BY FAITH I BY FAITH