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measuring the temperature on the top of the package it is best to use a
preinstalled component.
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 is best accomplished by drilling through the
bottom of the PCB into a solder joint and attaching the thermocouple or
by sliding the thermocouple under the package. When sliding a
thermo couple under a component, it is critical that the thermo couple
be in contact with a solder ball.
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 and Maximum Temperatures
Acceptable ramp rates and maximum temperatures should be obtained
from the package manufacturer. It is recommended to select a
maximum temperature below the manufacturer’s specification to
provide for a margin of safety. Typically, 20
°
C below maximum is
selected.
Pre-Heat Phase
1. In a “step profile”, the top of the PCB and package should reach a
stable temperature of 95-105
°
C. If plotting the temperature curve,
the trace should level off within this temperature range.
2. If a “linear slope” is desired, pre-heat and soak phases are
combined. Both the package and the PCB are warmed at a
constant ramp rate (usually 1-3
°
C/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
stable temperature of 145-170
°
C (determined by the activation
temperature of the flux used) should be maintained for approximately
40-60 seconds. This allows for uniform ramping across the entire
package and PCB during reflow.
Reflow Phase
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