NPL defect database service PoP process defect

This month we feature one recent process control issue submitted to the database.

The image shows the bottom of a PoP (Package on Package) device, small BGA, prior to placement onto the top of the bottom package. The liquid flux should only ever be on the surface of the balls. In this case the part has been drowned with flux covering all of the balls and the surface of the package. Excess application of flux can result in component float during reflow, depending on the solvent in the flux the part could even jump off the board during reflow. Excess flux should be avoided, it shows poor control, may cause problems during cleaning and will impact successful underfill if conducted.

Flux or dip paste is used to assembly PoP components during the normal SMT process, as a guide the package balls would only be dipped to a depth of 50% of the height of the balls. Clearly in this example the flux tray was not flat, the amount of the flux was uneven on the tray or the doctor or control blade height was incorrectly set,

The NPL Database is available to readers to search for solutions 24/7 or submit your own problems/examples online to obtain a possible solution at http://defectsdatabase.npl.co.uk/.