Abstract
The processing of solder paste starts with paste handling and storage; therefore, there are certain associated problems. This chapter discusses the major problems related to solder paste applications in surface mount technology (SMT) prior to reflow, with a primary emphasis on storage, deposition, and component placement. Solder paste should be a homogeneous mixture of flux and solder powder. However, sometimes it may display flux separation upon opening the container; the symptom typically is a yellow layer of flux on top of gray paste in either a jar or a syringe container. The possible causes of flux separation include very high shipping or storage temperature, leaving the paste on the shelf for too long, the paste being too low in viscosity, and the paste being too low in thixotropic property. The solutions for eliminating flux separation can be categorized as process and material. Solder paste also displays a layer of crust on its surface. This can be observed in either a newly opened container or a container with used paste.
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