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Leaded Surface Mount Technology (SMT) 7 7.1 Introduction Traditional through-hole Dual In-Line Package assemblies reached their limits in terms of improvements in cost, weight, volume, and reliability at approximately 68L. SMT allows production of more reliable assemblies with higher I/O, increased board density, and reduced weight, volume, and cost. The weight of printed board assemblies (PBAs) using SMT is reduced because surface mount components (SMCs) can weigh up to 10 times less than their conventional counterparts and occupy about one-half to one-third the space on the printed board (PB) surface. SMT also provides improved shock and vibration resistance due to the lower mass of components. The smaller lead lengths of surface mount components reduce parasitic losses and provide more effective decoupling The smaller size of SMCs and the option of mounting them on either or both sides of the PB can reduce board real estate by four times. A cost savings of 30% or better can also be realized through a reduction in material and labor costs associated with automated assembly. 7.2 Types Of Surface Mount Technology SMT replaces DIPs with surface mount components. The assembly is soldered by reflow and/or wave soldering processes depending on the mix of surface mount and through-hole mount components. When attached to PBs, both active and passive SMCs form three major types of SMT assemblies, commonly referred to as Type 1, Type II, and Type III (see Figure 7-1). Type I is a full SMT board with parts on one or both sides of the board. Type II is probably the most common type of SMT board. It has a combination of through-hole components and SMT components. Often, surface mount chip components are located on the secondary side of the Printed Board (PB). Active SMCs and DIPs are then found on the primary side. Multiple soldering processes are required. Type III assemblies are similar to Type II. They also use passive chip SMCs on the secondary side, but on the primary side only DIPs are used. 2000 Packaging Databook 7-1 Leaded Surface Mount Technology (SMT) Figure 7-1. Surface Mount Technology Board Types PLCC Passive Components SO Solder Paste Type I