Encapsulated component stress testing

1. Adverse effects on embedded components appear to be mainly due to localized stresses on portions of the component and are not due to hydraulic forces; therefore, true effects of embedment stresses can only be determined on the components themselves. 2. Embedment stresses can be obtained directly from embedded components such as capacitors and ferrite cores. 3. Component embedment stress measurements can be used to directly select the best embedment materials, stress relief coatings and processing techniques and physical orientation of component. 4. Capacitor component embedment stress measurements were us ed to determine the following parameters: (a) Stresses from embedment material are reduced by many factors—among them are law modulus down to the lowest operating temperature, law coefficient of thermal expansion, law cure temperature, poor bonding to the substrate, and low compressive strength. (b) Among embedment materials producing the lowest stresses on components are urethane and silicone elastomers, foam-in-place resins, and syntactic epoxy foams. (c) Many factors are involved in the selection of conformal coatings which relieve embedment stresses. Among these factors are high coefficient of thermal expansion, law modulus of the coating down to the lowest operating temperature and poor bond of the coating to at least one interface. With a law modulus, bond to the interfaces is not detrimental. (d) Many types of conformal coatings can be used to reduce embedded component stresses to an acceptable level. Among the best coatings materials are silicone, urethane and polysulfide elastomers. Rigid conformal coatings exhibiting good bond properties increase stress effects on embedded capacitors. (e) The position of a component relative to its assembly boards is an important factor of protection from embedment stresses. The worst orientation tested was capacitors mounted perpendicular to a single board.