NOVEL APPLICATION METHODS OF SOLDER JOINT ENCAPSULANT MATERIALS FOR SnAgCu FCBGA SOLDER JOINTS

摘要

The use of polymeric materials to enhance the solder joint reliability of flip chip ball grid arrays (FCBGA) packages is now common in the computer and server products manufacturing environment. Examples of such polymeric reinforcement applications at the individual package level include corner glue (CG), edge bonding, and board level underfill (BLUF) application. One inherent drawback in these package level polymeric reinforcement strategies is that they require additional time consuming steps in the surface mount technology (SMT) board assembly process. These steps include material dispensing and curing, which slows down the board throughput rate on a high volume manufacturing (HVM) line. BLUF application has an added drawback caused by very long flow times under the FCBGA, especially as the FCBGA ball size and pitch decreases, and body size increases. As a result, solder joint encapsulant materials (SJEM) that are applied and cured during the SMT process have been developed to provide polymeric reinforcement of the individual solder joints. Applying the SJEM during the SMT process eliminates the time consuming extra process steps associated with corner glue (CG), edge bonding, and board level underfill (BLUF) applications post reflow. However, SJEMs have their own drawbacks. The standard SJEM application process consists of dipping the component into a tray containing the liquid resin SJEM. When applying the dipping process to larger form factor FCBGAs, physical limitations arise that result in significant SJEM solder joint encapsulation variation. Having sufficient and uniform encapsulation by the cured resin of the solder joints after reflow is a critical requirement to attain required levels of mechanical shock and thermo-mechanical-induced fatigue reliability for FCBGA components. This paper addresses various methods of overcoming such SJEM application process limitations by modifications during the dipping process as well as other SJEM application processes to enable larger applied volumes of SJEMs as needed for larger BGA sizes. The two experimental package dipping techniques that will be evaluated are force dipping and a compliant dipping plate. SJEM jetting is evaluated as an alternative processing solution. A high density FCBGA with 1310 439 micron diameter balls on mixed pitches with a 34 mm × 28mm body size is used as a test vehicle. A new SJEM volume metrology consisting of offline optical measurement techniques and cross sectioning along with package separation and inspection using UV microscopy has been developed. These techniques are used to measure and correlate pre-reflow solder ball level SJEM volume to post-reflow solder joint level SJEM encapsulation and to analyze and compare processes. The capability of each process to address package characteristics and achieve sufficient and uniform solder joint encapsulation is discussed. The experimental findings are used to extrapolate SJEM process requirements and processing recommendations.