Considerations in the Implementation of SMT Main reasons to consider implementation of SMT include: reduction in circuit board weight reduction in number of layers in the circuit board reduction in trace lengths on the circuit board, with DISTANCE orrespondingly shorter signal transit times and sOLDER LAND However, not all these reductions may occur in any given FIGURE 26.1 Placement misalignment of an SMT product redesign from through-hole technology(THT)to chip resistor( Source: Phillips Semiconductors, Surface SMT. Mount Process and Application Notes, Sunnyvale, Calif: Most companies that have not converted to SMT are Phillips Semiconductors, 1991. With permission. considering doing so. All is of course not golden in SMT and. During the assembly of a through-hole board, either the component leads go through the holes do not, and the component placement machines can typically detect the difference in force involved SMT board assembly, the placement machine does not have such direct feedback, and accuracy of final placement becomes a stochastic(probability-based)process, dependent on such items as component pad design, ccuracy of the PCB artwork and fabrication which affects the accuracy of trace location, accuracy of solder paste deposition location and deposition volume, accuracy of adhesive deposition location and volume if adhesive is used, accuracy of placement machine vision system(s), variations in component sizes from the assumed sizes, and thermal issues in the solder reflow process. In THT test, there is a through-hole at every potential test point, making it easy to align a bed-of-nails tester. In Smt designs, there are not holes corre- sponding to every device lead. The design team must consider form, fit and function, time-to-market, existing capabilities, testing, rework capabilities, and the cost and time to characterize a new process when deciding on a change of technologies 26.3 SMT Design, Assembly, and Test Overview Circuit design(not covered in this chapter) Substrate [typically Printed Circuit Board(PCB)) design Thermal design considerations Bare PCB fabrication and tests(not covered in this chapter) Application of adhesive, if necessary Application of solder 1 Placement of components in solder paste Reflowing of solder pa Cleaning, if necessary Testing of populated PCB(not covered in this chapter) Once circuit design is complete, substrate design and fabrication, most commonly of a printed circuit board (PCB),enters the process. Generally, PCB ass mbly configurations using surface mount devices(SMDs)are classified as shown in Fig. 26.2. Type I-only SMDs are used, typically on both sides of the board. No through-hole components are used. Top and bottom may contain both large and small active and passive SMDs. This type board uses refo Type II-a double-sided board, with SMDs on both sides. The top side may have all sizes of active and passive SMDs, as well as through-hole components, while the bottom side carries passive SMDs and© 2000 by CRC Press LLC Considerations in the Implementation of SMT Main reasons to consider implementation of SMT include: • reduction in circuit board size • reduction in circuit board weight • reduction in number of layers in the circuit board • reduction in trace lengths on the circuit board, with correspondingly shorter signal transit times and potentially higher-speed operation However, not all these reductions may occur in any given product redesign from through-hole technology (THT) to SMT. Most companies that have not converted to SMT are considering doing so. All is of course not golden in SMT Land. During the assembly of a through-hole board, either the component leads go through the holes or they do not, and the component placement machines can typically detect the difference in force involved. During SMT board assembly, the placement machine does not have such direct feedback, and accuracy of final soldered placement becomes a stochastic (probability-based) process, dependent on such items as component pad design, accuracy of the PCB artwork and fabrication which affects the accuracy of trace location, accuracy of solder paste deposition location and deposition volume, accuracy of adhesive deposition location and volume if adhesive is used, accuracy of placement machine vision system(s), variations in component sizes from the assumed sizes, and thermal issues in the solder reflow process. In THT test, there is a through-hole at every potential test point, making it easy to align a bed-of-nails tester. In SMT designs, there are not holes corre￾sponding to every device lead. The design team must consider form, fit and function, time-to-market, existing capabilities, testing, rework capabilities, and the cost and time to characterize a new process when deciding on a change of technologies. 26.3 SMT Design, Assembly, and Test Overview • Circuit design (not covered in this chapter) • Substrate [typically Printed Circuit Board (PCB)] design • Thermal design considerations • Bare PCB fabrication and tests (not covered in this chapter) • Application of adhesive, if necessary • Application of solder paste • Placement of components in solder paste • Reflowing of solder paste • Cleaning, if necessary • Testing of populated PCB (not covered in this chapter) Once circuit design is complete, substrate design and fabrication, most commonly of a printed circuit board (PCB), enters the process. Generally, PCB assembly configurations using surface mount devices (SMDs) are classified as shown in Fig. 26.2. Type I — only SMDs are used, typically on both sides of the board. No through-hole components are used. Top and bottom may contain both large and small active and passive SMDs. This type board uses reflow soldering only. Type II — a double-sided board, with SMDs on both sides. The top side may have all sizes of active and passive SMDs, as well as through-hole components, while the bottom side carries passive SMDs and FIGURE 26.1 Placement misalignment of an SMT chip resistor. (Source: Phillips Semiconductors, Surface Mount Process and Application Notes, Sunnyvale, Calif.: Phillips Semiconductors, 1991. With permission.)