1. Basic Concepts in Reliability Design - nl3prc

[2] Designing for Reliability28), 29)Figure 3.24 Structure of resistor TEGY−14700−15680−16660−17640−18620−19600−20580−22540−21560−20580−19600−18620−17640294019609800−980−1960−2940−2940−1960−9800−15680−16660−17640−18620−19600−19600−18620−17640−16660−15680−15680−16660−2940XσyσyτxyFigure 3.25 Stress distribution after encapsulation 28)(3) Top Passivation Crack Caused by Fillers in Mold Resin 30)Mold resins contain SiO 2 fillers. When these fillers exist in the interface between the die and theresin, cracks can be formed in the passivation due to the stress from TCT and other thermalfactors. As a result, the Al beneath the crack deforms, and the crack can extend to the interlayerfilm beneath the Al. This can form a leakage path in the crack and cause a leak between the Aland the poly-Si beneath the Al, resulting in device failure. A countermeasure is to apply apolyimide coating to the die surface.(4) Effect on Reliability of Soldering Stress in Surface-Mounted DevicesPlastic-encapsulated packages, which are easily molded, are fabricated in various shapes.Consequently, a wide variety of surface-mount products have been developed so as to increase theIC density on circuit boards.Compared to board-insertion packages such as DIPs and SIPs, surface-mount devices are prone topackage cracks and degradation of moisture resistance. This is because there is a greater chancefor the mold resin to be exposed directly to heat during mounting. Recently, the trend towardsusing slim-profile packages and increased chip sizes make surfac-mount devices (SMDs) evenmore susceptible to thermal stress during soldering.The reliability of surface-mount packages is, in most cases, determined primarily by the solderingconditions. Therefore, when devices are mounted on a circuit board, moisture absorption controland soldering conditions must be carefully considered.2-37