Electrical degradation encapsulants

Because the silver fillers are encapsulated with an epoxy layer, however, Ag migration is not likely to occur in conductive adhesives under test conditions relevant in practice, e.g., 85 °C (185 °F) and 85% RH or 60 °C (140 °F) and 90% RH under 5 V bias (Ref 12). However, under more severe conditions, such as the presence of a liquid water film, higher bias, and smaller pitch spacing, Ag migration does occur. For example, short circuit between 0.20 mm (8 mil) spaced pads has been observed after 2000 h of 85 °C (185 T) and 85% RH test with 15 V bias (Fig. 6). In Ref 28, the migration of Ag particles was also observed in ICA joints subjected to the current-induced aging (10 30A) and the consequent electrical degradation was reported. 

Silicones, as a class, are rated among the highest temperature stable polymers. They can withstand temperatures of 200 °C, almost continuously, without degradation of physical or electrical properties and have been used at temperatures as high as 300 °C. Because of their high thermal stabilities, they are used as adhesives and encapsulants for electronic modules that are expected to perform in extreme temperature environments, such as near automotive engines and in deep-well sensors. Because of their low moduli of elasticity, silicones also fare well at very low temperatures. They are rated for continuous use at —80 °C, but may be used at even lower temperatures. 

To use Equation 19.52, it is important to employ high purity polymers with low trap concentrations, since the EA technique and Equation 19.49 through Equation 19.51 are based on the electric field (E) in the bulk of the polymer layer being uniform [4,111], This can be confirmed by capacitance measurements [75] in the reverse to small forward bias regime. It is also essential to protect the samples during measurement procedures from contact with the atmosphere, either by encapsulation or by placing them in a vacuum chamber, in order to avoid degradation/contamination [112]. 

Extrinsic degradation is attributed to chemical reactions of the active materials with its environment. The most common cause of degradation is the contact with oxygen and water in the presence of light, which leads to a rapid decrease of performance of the diodes, resulting from a modification of the structure of the active material, which is furthermore accelerated by electrical stress. The degradation onset corresponds to a formation of dark spots [33], which are nonemis-sive areas of the device surface. Most of the devices should therefore be protected by an encapsulation to prevent the contact with ambient air. A properly protected encapsulated diode will not develop chemical reactions that affect its lifetime. An alternative approach is the use of metal oxide layers combined with high work function and air-stable metals such as A1 or Au to replace the usual transport layers (PEDOTPSS or LiF) to fabricate diodes that can be used without encapsulation [34]. 

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