Here are some basic guidelines for laser soldering with Lead Free alloys:
It sounds like you are seeing a reflective bounce of the laser beam to other surrounding areas.
A higher rated Nm diode laser tends to be more penetrative and less reflective vs a lower rated Nm system.
When soldering with an 810 Nm diode more energy is applied onto the pad vs the higher frequencies, however this lower frequency tends to reflect off shinny surfaces more, thus the potential for affecting surrounds components is greater.
In contrast, a 980 Nm ~ 1080 Nm diode is more penetrative but often requires a slightly greater power output setting. Since this frequency penetrates the solder/pad more, less actual power is applied to the pad at the same setting of an 810 Nm rating. As a result you will need to increase the laser output to achieve the desired soldering. However at this frequency rating, the beam will not reflect as much, creating a more stable soldering environment.
If a thermal plane exists on the circuit to be soldered, we suggest localized hot air assist or pre heating the PCB assembly first. By raising the profile temperature (preheating the area or joint to be soldered) it allows the laser to operate to its maximum potential, reducing cycle rate and.minimizing thermal shock.
A few final notes:
Most diode laser's solder best with a slightly higher flux content, using special formulated paste or wire for laser soldering.
The spot size should match the pad size (when possible) If the system you are using has a fixed focal spot, select the smaller spot size that fits most joints/pads. Over shotting the pad can burn mask and create other issues.
Angle of the laser beam is also critical. Most laser soldering manufacturers would suggest pointing the beam at a 1~2 degree angle only.
