10
Lead Free Reow Prole General Guidelines
i. Ramp 1
Ramp to 100°C. Maximum slope for this zone is limited to
2°C/sec. Faster heating with ramp higher than 2°C may
result in excessive solder balling and slump.
ii. Preheat
Preheat setting should range from 100 to 150°C over a pe-
riod of 60 to 120 seconds depending on the characteristics
of the PCB components and the thermal characteristics
of the oven. If possible, do not prolong preheat as it will
cause excessive oxidation to occur to the solder powder
surface.
iii. Ramp 2
The time in this zone should be kept below 35 seconds to
reduce the risk of ux exhaustion. The ramp up rate should
be 2°C/sec from 150°C to re-ow at 217°C. It is important
that the ux medium retains its activity during this phase
to ensure the complete coalescence of the solder particles
during re-ow.
iv. Reow
The peak reow temperature is calculated by adding
~32°C to the melting point of the alloy. Lead free solder
paste melts at 218°C and peak reow temperature is 218°C
+ 32°C = 250°C (±5°C). Note that total time over 218°C
is critical and should typically be 60 – 150 seconds. This
period determines the appearance of the solder joints.
Excessive time above reow may cause a dull nish and
charred of ux residues. Insucient time above reow
may lead to poor wetting and improperly fused (cloudy)
ux residues.
v. Cooling
Maximum slope for cooling is limited to 3°C/sec. More
rapid cooling may cause solder joints crack while cooling
at a slower rate will increase the likelihood of a crystalline
appearance on the solder joints (dull nish).
Solder Reow Prole
The most commonly used solder reow method is accom-
plished in a belt furnace using convection heat transfer.
This prole is designed to ensure reliable nished joints.
However, the prole indicated will vary among dierent
solder pastes from dierent manufacturers and is shown
here for reference only.
Other factors that can aect the prole include the density
and types of components on the board, type of solder
used and type of board or substrate material being used.
The prole shows the actual temperature that should
occur on the surface of a test board at or near the central
of the solder joint. For this type of reow soldering, the
circuit board and solder joints are rst to get heated up.
The components on the board are then heated by con-
duction. The circuit board, because it has a large surface
area, absorbs thermal energy eciently and distributes
this heat to the components.
Reow temperature proles designed for tin/lead alloys
will need to be revised accordingly to cater for the melt-
ing point of the lead free solder being 34°C (54°F) higher
than that of tin/lead eutectic or near-eutectic alloys. In
addition, the surface tension of molten lead free solder
alloys is signicantly higher than the surface tension for
tin/lead alloys and this can reduce the spread of lead free
solder during reow.
250
200
150
100
Melting point = 218˚C
Suggested Lead Free Reflow Profile For SnAgCu Solder Paste
0
50
100
150 200
250
Seconds
Ramp 1
Preheat Ramp 2
Reflow
Cooling
50
Peak = 250 ± 5˚C
