1. Purpose and scope
Assembly turns a set of tested dies, an interposer or bridge and a substrate into one package. For multi-die products it is a development activity in its own right, not only a production step. Fine-pitch bumps, several dies placed with tight alignment, thin and large interposers, stacked dies joined through solder micro-bumps or direct copper bonding, and mixed materials all bring process windows that must be learned on real hardware.
The stage covers wafer bumping or bonding surface preparation, die thinning where stacking needs it, die attach to the interposer or substrate, interposer attach to the substrate where the technology uses two levels, reflow or bonding, underfill, encapsulation or moulding, lid or heat spreader attach, ball attach, and the inspection and electrical checks that confirm the build. Process development runs from the Package design freeze through to the Known-good-die release; the first builds run from then until the First assembled units gate.
Assembly is usually done by an outsourced assembly and test supplier, so the stage also covers line readiness at that supplier: equipment, materials, process documentation, handling of known-good dies and traceability from die bank to finished unit.
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