Fracture of Underfilled and Edge-Bonded BGA-PCB Assemblies: Effects of Joint Geometry, Strain Rate and Stiffness

The reliability of the solder joints connecting microelectronic components to printed circuit boards (PCBs) is a serious concern in the design of many electronic devices. The focus of the present research was the fracture performance of component-solder-PCB assemblies with and without underfill adhesives under bending loading conditions. Fracture tests of copperâ solderâ copper double-cantilever-beam (DCB) specimens with a single continuous solder layer showed that for joints shorter than a characteristic length the fracture load increased with increasing solder joint length. Also, the fracture tests of DCBs with two discrete solder joints showed the strength was maximum at an optimal joint spacing. An analytical solution was developed for the characteristic length of mode I DCB specimens with a single continuous solder layer as well as the optimal spacing of double-joint DCBs, taking into account the mechanical properties and dimensions of the substrates and solder. Fracture experiments with underfilled ball grid array (BGA)- PCB specimens in a DCB configuration showed that the specimens always failed in the PCB. The strength of the underfilled BGAs was solely a function of the fillet size, and independent of the underfill thermal and mechanical properties. Edge-bonding is a less expensive alternative to underfilling, in which only the edges or corners of the component are bonded to the PCB using a viscous adhesive. Edge-bonded and underfilled specimens with fillets of the same size and shape tested using DCB specimens failed at the same load, because the stress distribution in the PCB near the PCB-fillet interface was only a function of the adhesive fillet size and shape, and independent of the extent of the adhesive layer between the PCB and the BGA. Finally, a cohesive zone model (CZM) was developed to predict the PCB delamination bending loads in underfilled BGA-PCB assemblies. The cohesive parameters were obtained from fracture tests of bending test specimens consisting of PCB substrates bonded with the underfill adhesive. The model provided relatively accurate predictions of the failure mode and fracture load of the underfilled BGA-PCB assemblies for a range of PCB stiffnesses, adhesive fillet sizes, and strain rates.Ph.D