Mold compound

Mold compound is the hard black material that makes up the body of most chips. It surrounds the delicate die and its wires, sealing out moisture and dirt, blocking light and holding everything firmly in place.

It's a blend of three main ingredients. Epoxy resin is the binder: with a hardener mixed in, it sets into a tough solid when heated. Very fine silica powder, often as tiny melted spheres, is the filler, and makes up most of the weight, often 70 to 90%. Silica matters because it expands with heat far less than plastic does. It keeps the package's expansion closer to silicon's, so temperature swings don't crack the die or break the wires. A dash of carbon black makes it black and opaque. The ingredients are mixed hot, cooled, ground and pressed into pellets.

For molding, strips of wired-up dies go into a steel mold. A pellet is heated to around 175 °C until it flows, and a plunger pushes it into the cavities, where it cures in a minute or two. It forms the body of integrated circuits, diodes, power transistors, and the system-on-chip, memory and storage chips. The markings on top are usually etched by laser.