Profiles should be established for all new board designs using thermocouples placed at multiple
locations on the component. In addition, if there is a mixture of devices on the board, then the
profile should be checked at various board locations to ensure that the minimum reflow
temperature is reached on the larger components, and at the same time the temperature does
not exceed the threshold that might damage the smaller, heat sensitive components.
The following tables and figures provide the recommended guidelines for Pb-free solder PCB
assembly reflow. In general, a gradual-linear ramp into a spike is proven, by various sources, to be
the optimal reflow profile for Pb-free solder. This profile results in a better wetting yield and less
thermal shock than the conventional ramp-soak-spike profile. It is a known fact that SnAgCu
(SAC) alloy reaches its full liquidus at a temperature of 235°C. In the reflow profiling, the coldest
solder joints need to be identified and to ensure that they reach a minimum peak temperature of
235°C for at least 10 seconds. Reflowing at high-peak temperatures of 260°C or above can
damage the heat sensitive components and cause board warpage. Refer to the latest IPC/JEDEC
J-STD-020 standard for allowable peak temperature on the components. The allowable
component peak temperature is determined by the component size. The following table lists the
reflow soldering temperature profile information. In any case, use as low of a peak temperature
reflow profile as possible. The following image shows an example of the reflow temperature
profile.
Table 16: Recommended Reflow Soldering Temperature Profile
Profile Feature
Convection, IR/Convection
Preheat ramp-up rate 30°–150°C
3°C/s maximum
Preheat temperature soak time 150°–200°C
60–120 seconds
Temperature maintained above 217°C
60–120 seconds (60–90 seconds typical)
Time within 5°C of actual peak temperature
30 seconds maximum
Peak temperature (lead/ball)
235°C–245°C typical (depends on solder paste, board size,
component mixture)
Ramp-down rate
4°C/s maximum
Figure 10: Recommended Reflow Soldering Temperature Profile
Chapter 3: Mechanical Design Considerations
UG1091 (v1.0) April 20, 2021
Carrier Card Design for Kria SOM
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