TN1171 Datasheet, PDF (19/26 Pages) STMicroelectronics – Description of UFDFPN5, UFDFPN8 and WFDFPN8 for STMicroelectronics EEPROMs and recommendations for use

TN1171
10 Reflow soldering
Reflow soldering
The purpose of the reflow process is to melt the solder particles, wet the surfaces to be
joined, and solidify the solder into a stronger metallurgical bond.
Temperature profile is the most important control in reflow soldering and it must be fine-
tuned to establish a robust process. DFN components probably would be among the
smallest devices on any PCB. In such cases, place thermocouples under the heaviest
thermal mass device on the PCB to monitor the reflow profile. Generally, when the heaviest
thermal mass device reaches reflow temperatures, all other components on the PCB will
reach reflow temperatures as well.
For all devices on the PCB, the solder paste needs to be taken into account for the reflow
profile. Every paste has a flux, and the flux dominates the reflow profile for steps like soak
time, soak temperature, and ramp rates. Peak reflow temperature is the melting
temperature of the metals in the paste, plus a “safety” margin to ensure that all solder paste
on the PCB reflows.
The reflow profile should follow the paste supplier’s “recommended” profile.
Since solder joints are not fully exposed in the case of DFN, any retouch is limited to the
side fillet. For defects underneath the package, the whole package has to be removed.
Rework of DFN packages can be a challenge due to their small size and is not
recommended. In most applications, DFN will be mounted on smaller, thinner, and denser
PCBs that introduce further challenges due to handling and heating issues. Since reflow of
adjacent parts is not desirable during rework, the proximity of other components may further
complicate this process.
DocID026092 Rev 3
19/26
26