Laser soldering of small pitch BGAs on Plexiglas substrate PCBs
The paper presents the results of the study and the procedure of soldering 200µm pitch BGAs on circuits on PMMA (Poly-methyl-methacrylate or Plexiglas) substrate, starting from the paste deposition process, fine placement of the BGAs and finally soldering. The novelty in this research is that it had to face two restrictions. The combination of PMMA substrate, transparent to laser, and SnBi, ecologic solder paste, which required a low and very well-controlled soldering temperature, and the small size of the BGAs which needed a punctual soldering. These particularities led the team to use laser for soldering, due to the possibility of highly accurate energy calculation and very small size of the focused beam.
Diode laser soldering: a lumped parameter mathematical model and comparison of different optical soldering technologies
A lumped-parameter mathematical model has been developed for the laser soldering process and validated experimentally by measuring the temperature of solder preforms placed on a lead on a printed circuit board when heated with laser. This model results in four first-ordered nonlinear differential equations, which can be solved with MATLAB. Further, a comparison has been made between diode laser, and Soft Beam (xenon-arc white light) soldering with respect to the solder joint quality, cycle time and input optical energy requirements. In addition, scanning electron microscopic studies have been done to compare the microstructure of the solder and the formation of intermetallics, when the solder is melted with laser, Soft Beam and a conventional soldering iron. It is found that the diode laser soldered joint has a thinner intermetallic and finer grain size resulting in a higher mechanical strength.
SR topography of plastically deformed Fe-Si bicrystals - observation in situ and post mortem
SR reflection topography was applied to study the interaction of slip dislocations with the grain boundaries in Fe-4at%Si bicrystals deformed by compression. Three experimental settings with either a white or monochromatic beam were used and the corresponding orientation contrast analysed. Dislocation transmission through the grain boundary was observed in bicrystals with favourable geometry of the slip systems.