Laser Assisted Bonding

Laser-assisted bonding heats only the bond site itself, a localized spot measured in microseconds, instead of putting the whole package through a furnace cycle the way conventional thermocompression bonding does. A pulsed or continuous-wave laser, typically a near-infrared diode laser in the 900–980nm range, is focused through a transparent or reflective bond-head onto the solder microbumps joining a chip to its substrate or to another die; the laser energy melts just the solder, the surrounding silicon and package materials stay close to ambient temperature, and the whole joint reflows and solidifies in well under a second per site. That changes what kind of process window is available. Conventional reflow thermocompression bonding (the broader, more established process this site already covers in depth) heats the entire assembly, which means every material in the stack — the die, the substrate, the underfill, any already-bonded neighboring dies — has to tolerate the full thermal cycle and its coefficient-of-thermal-expansion mismatch every single time. Laser-assisted bonding, because the heat stays confined to the bond site, lets engineers attach dies sequentially onto a substrate that already has other components mounted and reflowed, without re-melting or thermally stressing those earlier joints — a real practical advantage in 2.5D and 3D heterogeneous packages where dozens of chiplets land on one interposer and nobody wants the fifth die's bonding cycle to loosen the first die's joints. Besi and PacTech are among the production equipment vendors building laser-assisted bonders for this exact high-mix chiplet assembly use case, and the technique is gaining attention specifically because fine-pitch microbump arrays, now routinely below 40 microns, leave very little thermal and mechanical margin for error; localized, fast heating reduces the warpage and bump-collapse risk that a slow, full-package thermal ramp can introduce. The tradeoff is throughput per unit and equipment cost: laser-assisted bonding is inherently a sequential, site-by-site process, so it has to compete on precision and selective-heating advantages rather than on raw parallel throughput against a batch reflow oven that processes an entire panel at once. In practice this makes it most attractive for the highest-value, highest-density bonding steps in a package — fine-pitch chiplet attach and hybrid-bonding-adjacent steps — rather than as a wholesale replacement for conventional reflow everywhere in the assembly flow.

Laser-assisted bonding is not trying to out-throughput a batch reflow oven; it earns its place specifically in the highest-density, highest-value bond steps of a heterogeneous package, where confining the heat to one joint at a time is worth more than processing an entire panel in one pass.