Die Overcoat

Die Overcoat is the process of applying a pliant but moisture-resistive material over the surface of the die for the purpose of minimizing package stresses on the surface of the die and providing additional protection against corrosion. Silicone materials are very effective for this purpose.

Die coating may be selective or non-selective. Selective die coating, as the name implies, dispenses overcoat material on certain areas of the die only. Non-selective or full die coating covers the entire surface of the die with overcoat material. The amount of overcoat material dispensed on the die surface should be calculated properly, as excess overcoat material that rise above the ball bond may exert tremendous shearing stresses on the wire, resulting in neck breaks.

Die coating entails assembly expenses, so it must be avoided whenever possible. Better die lay-outing, fab processing methods, and circuit designs can make a device less sensitive to package stresses, allowing the elimination of die coating during assembly.

Common Die Overcoat-related Failure Mechanisms:

Die Stressing - generation of excessive package stresses on the die which may result in electrical failure; this mechanism is alleviated by die overcoating

Die Scratch - inducement of any mechanical damage on the die, as when an operator scratches a die with tweezers due to mishandling. Common Causes: insufficient operator training, disorderly workplace, use of improper tools

Die Metallization Smearing - depression or deformation of any metal line on the die surface. Common Causes: dirty or worn-out die attach pick-and-place tool, wafer mishandling

Die Corrosion - corrosion of the metal lines of the die, often due to the presence of corrosive contaminants and moisture on the die surface

Neck Breaking - breakage of the bond wire at the ball bond neck. In the context of die coating, it is usually caused by excessive overcoat material which can exert tremendous shearing stresses at the neck.

Front-End Assembly Links: Wafer Backgrind; Die Preparation; Die Attach; Wirebonding

Back-End Assembly Links: Molding; Sealing; Marking; DTFS; Leadfinish

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