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Moisture Sensitivity/Desiccant Packaging/Handling of PSMCs 8 8.1 Introduction This chapter examines surface mount assembly processes and establishes preconditioning flows which encompass moisture absorption, thermal stress and chemical environments typical in the variety of surface mount assembly methods currently in use. Also discussed are the standardized moisture sensitivity levels which control the floor life of moisture/reflow sensitive PSMCs along with the handling, packing and shipping requirements necessary to avoid moisture/reflow related failures. Baking to reduce package moisture level and its potential effect on lead finish solderability is described. In addition, drying, shipping, and storage procedures are included. 8.2 Moisture Sensitivity of PSMCs This section addresses technical issues related to maintaining package integrity during board level assembly processing using Plastic Surface Mount Components (PSMC). Surface mount processing subjects the component body to high temperature and chemicals (from solder fluxes and cleaning fluids) during board mount assembly. In through-hole technology the board assembly process uses wave soldering which primarily heats the component leads. The printed circuit board acts as a barrier to protect the through-hole package body from solder heat and flux exposure. Note: No component body should ever be immersed directly in the solder during the wave solder operation. To ensure PSMC package integrity throughout the surface mount process, precautions must be taken by both supplier and user to minimize the effects of reflow solder stress on the component. Plastic molding compounds used for integrated circuit encapsulation are hygroscopic and absorb moisture dependent on time and the storage environment. Absorbed moisture will vaporize during rapid heating in the solder reflow process, generating pressure at various interfaces in the package, which is followed by swelling, delamination and, in some cases, cracking of the plastic as illustrated in Figure 8-1 and Figure 8-2. Cracks can propagate either through the body of the plastic or along the lead frame (delamination). Subsequent high temperature and moisture exposure to the package can induce the transport of ionic contaminants through these openings to the die surface increasing the potential for circuit failure due to corrosion. Components that do not exhibit external cracking can have internal delamination or cracking which impacts yield and reliability.