STM Based custom PCB having corrosion issue
I am using a STM32H723ZGT6 based custom PCB. The PCB is being soldered using ChipQuick’s TS391LT solder paste and CQ4LF-0.5 no clean liquid flux it is then cleaned using isopropynol (IPA) and an anti-static brush. We are using the hot air gun at 350 C. These are our findings so far with respect to the corrosion issue:
- There is a pale yellowish gunk deposit near the pins of the micro-controller which i suspect is the flux residue carried by the IPA and getting removed due to small pitch of the IC.
- The solder mask above the copper layer (specially the ground) underneath the STM chip getting pealed off.
- The exposed copper getting corroded (as shown in the picture - bluish part under the removed chip)
- There is an off-white residue under the chip surface in shape of a dried out liquid spot.
Since the average humidity of my area is 77% with a peat of about 90% we suspect that the gunk is trapping the moisture and a electrolytic cell is getting completed and hence the corrosion.
These Boards were stored in a relatively low humid environment for about an year and have less than 48 hours of operational time.
We plan to clean the board using an ultrasonic cleaner to rectify the issue.
Should we look at some other possibility as well?