5 Rework

The WLCSP parts removed during PCB rework should not be reused for final assemblies unless the part needs to be used for further Failure Analysis (FA) work.

The WLCSP rework process is similar to the BGA-type package rework process. The rework process should use the following steps:

• Part Removal: The failed part should be removed from the board by applying hot air on the top of the component and the bottom heating the PCB. Discard the removed failed part if it is not needed for FA work.
• PCB Cleaning: The PCB land site must be cleaned and dressed for the attachment of the new component. Either a de-soldering system or iron with solder wick method can be used to effectively remove the residual solder without damaging the solder mask material and the pads. Note that the applied temperature should not be more than 245°C, otherwise, the copper pads on the PCB may peel off.
• Solder Paste Deposition: A mini stencil with the same thickness and aperture openings as the production stencil should be used to deposit the solder paste. The printed solder paste must be inspected to ensure uniform and sufficient volume is deposited before the part placement.
• Part Placement: A WLCSP reworking station with a vacuum nozzle and vision system should be used to pick the new WLCSP part and accurately align and place it on the corresponding footprint
• Rework Reflow: The replaced WLCSP component is then soldered to the PCB using a temperature profile similar to the production reflow profile. The WLCSP reworking station typically has a programmable reflow profile to be selected for a given part.