Most defects in circuit-board assembly are due to the solder paste or the process of printing solder paste. Since the transition of to lead free soldering, a whole new spectra of problems have emerged in older paste printing and its due processes. Incomplete coalescence of ball grid array (BGA) spheres and solder paste deposits, are failure modes that has increased in frequency since the transition to lead-free soldering.
The solder paste deposition process continues to be the leading source of end of line defects in SMT assembly process. Solder paste printing (even though the printing equipment and process has become much more reliable over the past 10 years) is still a complex process where many process variables, materials, environmental influences (temperature and humidity), and human factors, meet.
It is consistently reported that this process step continues to be responsible for the greatest number of end of line defects. Changing the process to accommodate smaller packages, new stencil designs and paste formulations will make it more difficult to maintain high yields.