Rework and reliability of underfilled CSP assemblies

Rework processes were developed for four different packages in combination with four underfills. Thermal profiles for component removal and replacement were developed for all the underfills. Several automated sequences were attempted to remove components using an independent pick-up tube, but the vacuum force was insufficient to overcome the adhesion of the underfill between the component and the board. A modified removal sequence and manual twisting motion were required to remove the defective component. Site redressing was performed using a rotary tool and dental brush. The brushing was followed by removal of excess solder using the soldering iron and wick method. New components were placed on the redressed site and soldered. The reworked assembly was then underfilled. Reliability studies were carried out using accelerated testing methods on four different packages in combination with three underfills. The reliability data for non-underfilled assemblies, underfilled assemblies, and reworked assemblies are compared. Accelerated testing conditions were obtained using air-to-air thermal cycling (0 to 100°C, 20 minute cycle) and mechanical testing (torsion).