Meaning
A small interconnect structure enables the physical and electrical joining of stacked integrated circuits in advanced packaging. Each microbump functions as a tiny solder pillar that bridges the gap between two silicon surfaces. Their compact size allows for thousands of connections in a space that previously held only hundreds.
They are restricted to applications where precision alignment can be maintained throughout the assembly process.
Solder Architecture
Composition typically involves a copper base followed by a thin cap of tin or solder alloy. The microbump undergoes a transformation into a solid metal joint during the furnace reflow step. Careful control of the alloy mix prevents the formation of brittle phases that could crack later.
Stack Alignment
Placement of top and bottom wafers must occur with sub-micron accuracy to ensure contact. If the microbump misses its target pad, the entire electronic assembly remains non-functional. Optical sensors confirm successful positioning before the bonding force is applied.
Metal Intersection
Interaction between different metals at high temperatures creates intermetallic layers near the bond line. The stability of the microbump depends on keeping these layers thin enough to avoid mechanical failure. Excessive growth of these compounds reduces the ductility of the join and causes failure during thermal expansion.
