Wafer Bank Conversion Agreements for Agile Iso-Die Sensor Package Sourcing
Wafer bank conversion agreements convert raw iso-die into multi-form sensor packages, shifting supply risk while managing lead time, yield, and bus drift.
Saw Street Oxidation
Chemical Reaction
Atmospheric exposure of the exposed silicon edges during the dicing process leads to the formation of an unwanted oxide layer. Saw street oxidation occurs when the heat generated by the cutting blade reacts with the cooling water and ambient oxygen. This reaction creates a thin film along the channel where the saw passed.
The resulting layer can interfere with the subsequent placement of the die or the adhesion of the underfill material.
Growth Mechanism
Localized heating at the point of contact accelerates the rate at which oxygen atoms bond to the silicon surface. In saw street oxidation, the thickness of the oxide is proportional to the duration of the cut and the temperature of the blade. Using deionized water with a low oxygen content helps to mitigate this effect.
Carbon dioxide injection is sometimes used to control the conductivity of the water and reduce the electrochemical potential for oxidation.
Impact Analysis
Electrical leakage along the edges of the die is a common consequence of a poorly controlled dicing environment. Saw street oxidation provides a path for moisture to penetrate the package over time. This moisture causes the corrosion of the active circuitry and leads to premature device failure.
Control Measure
Adjusting the feed rate of the saw and the rotational speed of the spindle minimizes the thermal energy transferred to the wafer. Saw street oxidation is further managed by applying a protective coating to the wafer surface before dicing. This coating is washed away after the operation, taking the debris and the oxide precursors with it.
Vacuum drying the wafers immediately after the rinse prevents the further growth of the oxide during storage. Proper storage in a nitrogen-purged cabinet maintains the surface quality until the die is ready for assembly.