System Operations Manual
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Page 31 of 50
a preinstalled component, as the thermocouple wire will typically cause the
component to not lay flat on the PCB.
When using a previously installed package, the placement of the thermocouples is
important. They must be in contact with the existing solder joints. This task can be
accomplished by either (1) drilling through the bottom of the PCB into a solder joint
and attaching the thermocouple or by (2) sliding the thermocouple under the
package. When sliding a thermocouple under a component, it is critical that the
thermocouple be in contact with the solder. Information from the thermocouples will
assist in determining the proper time and temperature parameters. In general, the
following guidelines should be adhered to when developing profiles.
Ramp Rates
Acceptable ramp rates and maximum temperatures should be obtained from the
component manufacturer. Typical ramp rates are 2-5 ºC/s (4-9 ºF/s) for plastic parts
and 1 ºC/s (2 ºF/s) for ceramic parts. It is recommended to select a maximum
temperature below the manufacturer’s specification to provide for a margin of safety.
Typically, 20 ºC below maximum specified temperature is selected.
Pre-Heat Phase
1. In a “step profile”, the PCB and package should reach a stable temperature of 95-
105 ºC. If plotting the temperature curve, the trace will usually level off within this
temperature range.
2. If a “linear slope” profile is desired, pre-heat and soak phases are combined. Both
the package and the PCB are warmed at a constant ramp rate (usually 2-4
ºC/second) until the desired soak temperature is reached.
Soak Phase
The soak phase is a crucial part of the reflow process. During this period, the flux
activates and drives off volatiles and excess flux. A temperature of 145-165 ºC
(determined by the activation temperature of the flux used) should be maintained for
approximately 20-40 seconds. This allows for uniform ramping across the entire
package and PCB during reflow.
Ramp Phase
The ramp phase is variation to the soak phase. When using lead free solders, it is
sometimes desirable to add a second “step” to the process as to not thermally shock
the PCB or component while trying to reach the solder melt temps for lead free
solders. If this is not desired, simply uncheck the “Use Ramp” check box on the
profile manager on the profile development page of the PC software.
Reflow Phase
During this phase, the solder reaches solder melt and forms joints between the
package and the lands. It is critical for all areas of the component to reach solder
melt together and all solder joints remain in a liquid state for at least 10-20 seconds.
Generally, plastic packages should not be exposed to temperatures higher than
230 ºC. Always consult the device specifications for maximum temperature
recommendations. As a rule of thumb, a safe “maximum temperature” is the
maximum temp specified by the manufacturer minus 20ºC. Lower temperatures and
shorter times are common in CSPs and FCs. The lowest heater set temperatures
possible should always be used to ensure safety of the device and PCB.
Cool Down Phase
The cool down phase is necessary to bring the temperature of the package, solder
joints and PCB under the package below solder melt temperatures. Cooling should
be controlled. A good reference is to use the same cool down rate as for ramp up.
The cooling fan on the TF 1700 and TF 2700 will remain on for a minimum of 50
seconds from the start of the cool down cycle. Some types of components (like
CBGAs) should be allowed to cool without external assistance from the cooling fan.
When installing these packages, turn the fan away from the PCB so the air doesn’t
blow on it.
General
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