If making an investment in an automated eutectic bonding system, getting the right partners and the right process (the first time) will prove tremendously cost-effective in the short and long run. Control is imperative!
Contract manufacturers doing eutectic die attach applications usually specify process conditions, often gained from in-house experience, such as how much solder reflow is desirable to achieve sufficient mechanical bond strength to prevent voids between die and substrate. I should note that every application has its own specific set of requirements, with some machine parameters more critical than others - ultimately all need to be in proper porportion to obtain desired results.
Critical control parameters include 1) head force, 2) background temperature and incremented thermal energy, 3) soldering time, and 4) cover gas flow.
Head Force: Immediately after placing die on the substrate, the bond head pushes down on the die with a specified force. The amount of force applied must be sufficient to stimulate solder reflow and hold the die in place during reflow. Too much force can damage a component (by introducing "micro-cracks" that may affect electrical performance, for example). Small stress-sensitive die ranging up to 20 mils can take a fair amount of head force of typically up to 50 grams. Larger die generally require head forces of 100 grams or more.
Background Temperature and Incremental Thermal Energy: The substrate is heated to a background temperature, which is boosted during bonding till reflow occurs. The background temperature determines the amount of incremental thermal energy needed to initiate solder reflow - it indirectly controls soldering time.
Soldering Time: The time span of head force and incremental thermal energy largely determines the extent of solder reflow and also impacts throughput time. Actual soldering time varies from a few seconds to 10-12 seconds depending on die size.
Cover Gas: The flow of cover gas affects the rate at which the solder reflows and determines the quality of the intermetallic bond.