SMT培训资料05.ppt

Reflow soldering for Fine-Pitch Components2021/9/171Critical factors in reflowing soldering1 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 the assembly.6.The flux chemistry and the cleanliness of the metals to joined.2021/9/172Dynamics of the Reflow ProcessPhase One -Solvent EvaporationPhase Two -Flux Activation(Oxide Reduction)Phase Three -Solder MeltPhase Four -Complete Solder MeltPhase Five -Cool Down 2021/9/173Temperature and time profile2021/9/174Tin/Lead solder oxide growth rates-thickness vs time2021/9/175Increasing Flux Rate2021/9/176Probability of IC package crack2021/9/177Temperature and time profile2021/9/178The Heat Sources1.Infra-red(including IR and IR mixed with convection.2.Vapor phase3.Convection-hot gas4.Laser5.ThermosonicInfra-red reflowThe infra-red heat transfer is done by emitting light from the infra-red region of the spectrum.IR emission is that having wavelength of 0.72 microns to 1000 microns0.72-2.5 micron ,short IR0.25-5.0 micron ,medium IRlonger than 5.0 micron,long wave IR2021/9/1792021/9/1710The IR emitter temperature determines the emitted wavelength of IR radiationThe higher 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 radiative 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 2021/9/1711Infra-red reflowIn the 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 temperatureFaster energy transfer than convectionBetter radiated energy penetration2021/9/1712IR-ReflowDisadvantages:Temperature is thermal mass dependentColour selectivityShadowing effect IR reflow optimizationComponent are placed with uniform mass distribution.Locate larger components near the corners or edgesAdjust the belt speed2021/9/1713Schematic view of an in-line IR reflow soldering system2021/9/1714Typical temperature profile for area-source IR reflow system2021/9/1715T-3 tungsten lamp2021/9/1716T-3 quartz lamp at full and half rated power2021/9/1717Dual-emitter T-3 lamp system2021/9/1718Emission spectra of dual-emitter T-3 lamp system2021/9/1719Panel IR emitter2021/9/1720Comparison of emitter typesheat transferTemperature adjustColour sensitivityshadow effecttemp uniformitytime to stabilize after changeheat transfer byradiationconvectionLamp Panelhighlowfastslowyeslessyeslessfairbettershorterlonger80-90%20-40%10-20%60-80%2021/9/1721Light Absorption and Reflection Properties for materials2021/9/1722Vapour phase reflow solderingWhen most fluids are heated to their boiling 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 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.2021/9/1723Vapour Phase Reflow SolderingVapour 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.AdvantagesRapid rate of heat transfer(typical 6 to 10 deg C/secInert atmospherewell controlled maximum temperature2021/9/1724Vapour Phase Reflow SolderingAdvantages:Insensitive to variations to board thermal massInsensitive to variation in board geometryDisadvantagesHigh operation cost(FC-70)Long heat up timeLittle control over temperature profileUnfavourable to low thermal mass part(solder wicking2021/9/1725Dual-vapour reflow systemBatch type production2021/9/1726In-line single-vapor soldering system2021/9/17272021/9/1728Convection reflow solderingThe 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 densities 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 dynamics As=surface(the assembly)area TG=the gas temperature near the surface TS =the surface(assembly)temperature2021/9/1729Convection oven classificationSMEMA classifies the convection system s class III and the mixed convection and IR systems as class II 2021/9/17302021/9/1731Air flow in a convection dominant reflow system 2021/9/1732Comparison of heat flow transfer2021/9/1733Nitrogen Reflow SolderingReasons to use nitrogen in reflow soldering:1)More fine pitch package used2)the increasing in use of low activity fluxesAdvantage:can reduce solder ballavoid the flux from hardening in the absence of oxygen2021/9/1734Hot bar and thermode bondingHot bar bonding 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)/eq=heat flow through the material being heatedk=the average thermal conductivity at the heater and material interface,A=the contact areaTB=temp of the bar at the interfaceTA=temp of the assembly at the interfacee=thickness of the region to be heated 2021/9/1735Heating profile-hot bar2021/9/1736Hot-bar thermode heating unit2021/9/17372021/9/17382021/9/1739Lasers beams solderingLaser heat sources operate on the same principle as infrared radiation soldering except the heated area is only the area exposed to the laser lightCommon lasers used for soldering are neodymin-doped yttrium aluminum garnet(Nd:YAG)and carbon dioxideThe potential advantages of laser reflow are precise placement of the reflow heat in very dense areasA fiber optic of 0.1 or 0.15 mm diameter is sufficient to transmit enough thermal energy to reflow a typical FPT lead joint2021/9/17402021/9/17412021/9/17422021/9/17432021/9/17442021/9/17452021/9/1746Temperature ProfilingTo achieve the ideal temperature profile by adjusting:specific emitters temperaturebetter speedMethod of getting feedback from temperature:thermocouplesreflow temperature profiles2021/9/1747