APPENDIX E: GUIDELINES FOR REFLOW SOLDERING
UNIFLOW
®
4 PULSED HEAT CONTROL
E-10 990-908
Preheat
It takes approximately two seconds to heat a modern designed thermode of up to 2” in length to
soldering temperature. During this time, the flux activates and starts to promote wetting by removal of
the oxide layer. Preheat is only used where there are excessive heat sinks affecting the thermode or
where the application has delicate substrates, like ceramic that need to be heated in a more controlled
fashion to avoid cracking.
Rise2
Rise time to soldering temperature is also programmable and allows precise heating rate control. This
again is particularly useful where delicate substrates can be easily damaged by too fast a heating rate.
Normal rise time for most thermodes is 1.5 – 2 seconds.
Reflow
The actual time and temperature can be programmed for this stage of the process. Time is
programmable in 0.1-second increments and temperature in 1-degree increments. Typically, the
temperature set point for an open solder joint with direct thermode contact to parts will be between 280 -
330C. Although normal solder will reflow at 180C the thermode must be set higher due to the thermal
transfer losses. A typical single sided flex will require between 330 - 400C due to the thermal losses in
the Kapton material. It is preferred to use the minimum time and temperature to achieve the desired
joint, so as to minimize the parts exposure to heat and chance of damage.
Cool1
Cool1, in a process without a Postheat process cycle, is a programmable temperature at which the
control unit will actuate the head to the up position. This temperature is set to just below the solder
solidification temperature. Therefore as soon as the solder becomes solid the process is ended and a
joint is formed. The cooling process can be shortened by the use of forced air cooling. The power
supply can be programmed to switch a relay that controls the flow of air at the end of the reflow period
and cool the joint and thermode rapidly. Because most connections have a relatively high heat sink, the
temperature in the solder is lower than the measured thermode temperature, even when using cooling
air. Therefore the release temperature can be set to 180º C in most cases without the chance of
encountering a dry joint.
Cool1, in a process with a Postheat process cycle, is a programmable temperature at which the Postheat
process cycle will start.
Postheat
The Postheat process cycle is a step useful for keeping the thermode in good clean condition. The
reflow head is typically lifted during the middle of this process cycle. The Control’s Head Up Delay
parameter is used to control this timing.
Cool2
Cool2 in a process without a Postheat process cycle is a programmable temperature at which the Control
will move to the End of Reflow process state.
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