The purpose of encapsulation is the protection of die and wire bond assembly, or components on a substrate such as a PCB board. The encapsulation failure mechanisms are usually physical defects in the encapsulation which expose the die/wire bonds to the environment. The defects are identified by visual inspection of the encapsulation layer. The visual inspection can be carried out by naked eye, microscopes including SEM and optical profilers. Specific type of defects can point to specific issues in the fabrication process. Common failure mechanisms include non-uniform thickness, inadequate coverage, bubbles, voids and delamination.

Non-uniform thickness
It refers to an encapsulation layer producing non-uniform thickness. It specifically becomes a concern when the non-uniform thickness exposes one or more parts of a die/wire bond assembly or a component on a substrate.

The failure mechanism can be rectified by adjusting the deposition process, for higher surfaces the encapsulant volume can be increased to achieve a uniform deposition. Varying amounts of surface energy owing to either variation in the constituent materials or different surface finishes can also affect the uniformity of an encapsulation deposition. Lower surface energy parts are more likely to be difficult to wet by an encapsulant and may lead to a non-uniform deposition. More than once encapsulant can be used to cover different areas of the substrate.

Inadequate coverage
It refers to an encapsulant deposition that is unable to completely cover the required components/area. The causes of inadequate coverage include insufficient deposition volume, surface contamination, low surface energy and choice of encapsulant.

Inadequate coverage can be rectified by using one or more of the following. The volume of the encapsulant can be increased by increasing the pressure and decreasing the printing speed. The surfaces to be encapsulated must be thoroughly cleaned using standard cleaning solvents or plasma. An encapsulant must be compatible with the substrate to provide complete coverage.

Bubbles and voids
Bubbles and voids in an encapsulation deposition are mainly caused by either trapped air or entrapped solvent which is unable to escape the deposition before it cures. They can be avoided ensuring that there is no trapped air in the encapsulant before it is deposited. An encapsulant can be degassed prior to its deposition on the package/substrate. The assembly to be encapsulated can be heated prior to the encapsulation deposition to get rid of any solvent present on its surface. The curing regime of the encapsulant must be strictly followed to ensure any solvent which may be part of the encapsulant has completely evaporated.

It refers to when parts of the encapsulation layer start to lose adhesion to the substrate/assembly. The causes of delamination are contamination, low surface energy and incomplete curing.

This failure can be rectified by either cleaning the surface using plasma or by selecting an encapsulant which is compatible with the low energy surface. An encapsulant if not cured properly can also delaminate due to poor adhesion.

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Testing the quality of Die encapsulation