Organic coatings are used directly on semiconductor surfaces prior to plastic molding or glob-top encapsulation to stabilize semiconductor junctions and dissipate stresses. High-purity silicones, polyimides, and BCBs are used because of their wide operating temperatures (-65° to 275°C for silicones), high breakdown voltages (500 to 1,000 V/mil), electrical stabilities minimizing leakage currents and surface effects, and compatibility with epoxy molding or encapsulation.
Epoxy molding compounds or encapsulants may impart stresses of several thousand pounds per square inch, especially if they are rigid. Stresses may be reduced by fifty percent or more by using flexible molding resins, appropriate fillers, or by applying a stress-relief coating prior to encapsulation. Silicone and polyimide coatings are especially suited for this function because of their high thermal stabilities and compatibility with both the active devices and the encapsulant. Polyimides can be applied at the wafer stage not only as buffer coatings, but also to facilitate processing of the chips. Buffer coats provide protection to the finished die during backgrinding, singulation, and assembly resulting in improved yields. Buffer coatings can be applied by spin coating, then photodelineated, and cured in a manner similar to other semiconductor fabrication batch processes. They may also be applied by automated selective dispensing. Polyimide stress buffers have been successfully applied at the wafer stage in fabricating memory chips, logic chips, and ASIC devices.[13][14]
High-purity soft silicones and silicone gels have also been used to relieve stresses induced by subsequent transfer molding or encapsulating with epoxies. Silicones are also effective in protecting electronic components from vibration and shock. Stress-relief coatings must be flexible and possess a low modulus of elasticity. Silicones meet these requirements and have found many applications in component packaging. For example, transistors and ICs may be coated with 10 to 20 mils of a silicone, then encapsulated, glob topped, or transfer molded with an epoxy or another silicone. The stress-relief coatings applied to semiconductor devices should be localized so that they cover only the active surface, metallization, and a portion of the internal leads. The coating should not be allowed to spread to the edges of the device or onto the leads where it might affect the adhesion and moisture permeability of the subsequently applied molding compound or the solderability of the leads. Because of these restrictions, a practical method is to dispense the coating in small drops from a microsyringe that can be either manual or automated. The location of the barrier coating relative to the encapsulant is shown in Fig. 4.10 for a widely used dual-in-line IC package.
The effectiveness of flexible dip coatings to relieve stresses is shown in Fig. 4.11 where strain gages coated with silicone, then potted in a polyamide-cured epoxy, were measured and compared with those potted in the epoxy only.[15][16]
In other studies, stresses of 5,000 psi at -40°C initially measured for components embedded in a silica-filled epoxy were greatly reduced when the components were precoated with an RTV silicone coating. At -40°C, a