Ball Grid Array Assembly is flip-chip package with solder balls on underside enabling high-density I/O and superior thermal properties — dominates modern packaging. Layout grid of ball pads on bottom (0.5-1.5 mm pitch typical). Solder Balls lead-free (SAC, SnAg, SnCu); lead-based (SnPb legacy). Ball Placement pick-and-place onto substrate pads. Precise (~±0.1 mm). Reflow controlled thermal cycle melts solder, bonds balls. Joint Quality good: shiny smooth surface, correct height. Defects: cold solder, voids, bridges. Coplanarity all balls at same height (±0.1 mm). Non-coplanar: opens. Thermal excellent heat path: die → substrate → balls → PCB. Mechanical solder joints absorb vibration/shock stress via fatigue. Underfill optional potting protects; CTE stress mitigation. Thermal Cycling −40 to +125°C fatigue solder. Creep, low-cycle failures. Lead-Free SAC higher Tg (217°C); processing hotter. Less ductile. Whiskers tin whiskers risk; coating mitigation. Inspection X-ray detects voids, bridges, placement; non-destructive. Rework thermal reflow removes; new balls placed, reflowed. Yield micro-bump placement challenging; yields ~99%. BGA achieves maximum I/O density and superior thermal properties.