3D Package-on-Package

PoP stacking integrates logic and memory dies vertically. SMT places bottom BGA (0.4mm pitch) followed by top package alignment within ±15μm. Dual reflow process: First reflow at 235°C for base, second at 220°C for top package. Underfill capillary flow fills 50μm gaps. X-ray verifies joint integrity between layers. Thermal management requires thermal interface materials with >5W/mK conductivity. Yield challenges: Warpage control below 0.1mm/m.