Is this lead free?

275C for 4 sec is a short selective soldering time for a non thermal
relieved 2.1mm thick PCB for lead-free.
If this is point to point soldering how cool has the PCA become during the
soldering process?
Check the solder temp at the top of the fountain to see how much heat loss
there is compared with the machine controls.

Check the top side pre heat with the top side heater turned off.
There may be some inner layer connections that are not heating up as much as
the topside temp shows with the topside heater on.

Flux is important. Check for pattern on bottom of PCA and topside of pca.
The time is based on flux, amount of coverage and method used.

Flux is not the primary suspect, heat is.
Sounds like good hole fill on other components.


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