SMT培训资料05

• Advantages : – Precise control of reflow temperature – Faster energy transfer than convection – Better radiated energy penetration
IR-Reflow
• Disadvantages : – Temperature is thermal mass dependent – Colour selectivity – Shadowing effect
•
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)
Schematic view of an in-line IR reflow soldering system
Typical temperature profile for area-source IR reflow system
T-3 tungsten lamp
T-3 quartz lamp at full and half rated power
• Advantages
– Rapid rate of heat transfer( typical 6 to 10 deg C/sec
– Inert atmosphere – well controlled maximum temperature
Vapour Phase Reflow Soldering
• shorter longer
• heat transfer by
– radiation – convection
• 80 -90% 20-40% • 10-20% 60-80%
Light Absorption and Reflection Properties for materials
• Vapour phase reflow soldering
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
assembly. • 6. The flux chemistry and the cleanliness of the
metals to joined.
Dynamics of the Reflow Process
• Phase One • Phase Two -
• Phase Three • Phase Four • Phase Five -
• 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
•
TA = the assembly temperature(varies)
•
AA = the assembly area.
Vapour Phase Reflow Soldering
• Vapour 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.
SMT培训资料05
Critical 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 the
• If the gas circulation is caused by different densities of masses being heated then the term free convection is applied.
• Infra-red reflow – The 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 microns – 0.72 -2.5 micron , short IR – 0.25 - 5.0 micron , medium IR – longer than 5.0 micron, long wave IR
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
Dual-emitter T-3 lamp system
Emission spectra of dual-emitter T-3 lamp system
Panel IR emitter
Comparison of emitter types
• Lamp Panel
• heat transfer
• When 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 :
– Unfavourable to low thermal mass part(solder wicking
Dual vapour reflow system
Batch type production
In-line single -vapor soldering system
B oiling Tem p. C degree 174 215 215 230 253 260
• The IR emitter temperature determines the emitted wavelength of IR radiation
• The higher the temperature , the shorter the emitted wavelength. • The temperature of the PCBA is a function of the absorbitivity of
D en sity M fg r’s
M anufacturer
g/m l D esignation
1 .8 8 F C -4 3
3M
2 .0 3 F C -5 3 1 1 3 M
2 .0 1 A P F – 2 1 5 A ir p ro d u cts
1 .8 2 L S /2 3 0
G alden
IncreasiΒιβλιοθήκη Baidug Flux Rate
Probability of IC package crack
Temperature and time profile
The Heat Sources
• 1. Infra-red (including IR and IR mixed with convection. • 2. Vapor phase • 3. Convection -hot gas • 4. Laser • 5. Thermosonic
• IR reflow optimization – Component are placed with uniform mass distribution. – Locate larger components near the corners or edges – Adjust the belt speed
1 .9 0 F C -7 1
3M
1 .8 4 H S ./2 6 0
G alden
Convection reflow soldering
• The process of heat transfer from a hot gas flowing over a cooler surface is called convection heating.
Solvent Evaporation Flux Activation (Oxide Reduction) Solder Melt Complete Solder Melt Cool Down
Temperature and time profile
Tin/Lead solder oxide growth rates - thickness vs time
• high low
• Temperature adjust
• fast
slow
• Colour sensitivity
• yes
less
• shadow effect
• yes
less
• temp uniformity
• fair
better
• time to stabilize after change
Infra-red reflow
• In 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.