First solder paste printing :
First check the solder paste printing machine parameter settings are correct, board of the solder paste should be on the pad, solder paste height whether a device or appear "ladder like". Paste the edge should not be rounded or collapse into a bunch of shape, but allow some due to out of the steel pull some paste caused by peak shape, if solder paste no uniform distribution is check the scraper on the solder paste, is insufficient and uneven distribution also need to be check printing plate and other parameters.
Second, the component placement
First has solder paste onto the board component placement before, you should first confirm rack is placed properly, components are correct errors and machine placement location is correct.
After the completion of the first piece of the board should be examined in detail, each part are correctly placed and light pressure in the center of the solder paste, rather than only "put on top of the solder paste. If the microscope can be seen in the solder paste slightly like sag said placed correctly. So it can avoid the element in the back of the welding have "sliding" phenomenon.
Three, reflow soldering
Reflow soldering temperature curve set (is said to have been used thermocouple measuring many board and make sure there is no disadvantage), only in quantity is big change or major deletions occur, will adjust row reflow profile. The so-called "perfect" solder joint is the appearance of bright smooth and in the next leg also has a complete solder coating.
Nearby the spot weld can also see some oxides mixed rosin residues, which represents the flux has the function of cleaning. This oxide is normal and is usually by the PCB detachment, but are also more likely to is from elements of the pin, due to flux cleaning off, this also means that the element may have been stored for a long time, even more than the PCB even longer.
Old or not completely mixed solder paste, may be because the welding and welding pad or element feet condition (wetting) and produce small beads (solder ball) (note: small beads have probably because process defect such as moisture in the solder paste or green paint (soldermask) caused by defective).But bad welding, and may also be because of bad management, makes some boards are staff contact with the hand, and by the hands of oil residue in welding mat was responsible for the poor., of course, this phenomenon may also be because the welding pad or at the foot of the components on the tin is too thin.
Finally, for an inspector, slightly gray solder joints may be due to the solder paste is too old, back to the welding temperature is too low, welding time is too short, or back to the welding curve setting is not correct, or reflow soldering.While small beads could be because the board did not finish baked or roasted for too long, too hot or place component or component, before someone adjusting component in welding into the furnace, and put the solder paste extrusion welding pad.