Blackwell, G.R. " Surface Mount Technology The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Blackwell, G.R. “Surface Mount Technology” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
26 Surface mount Technology 26.1 Introduction 26.2 Definition and Considerations Considerations in the Implementation of SMT 26.3 SMT Design, Assembly, and Test Overview 26.4 Surface Mount Device(SMD)Definitions 26.5 Substrate Design Guidelines 26.6 Thermal Design Considerations 26.7 Adhesive 26.8 Solder Paste and Joint Formation 26.9 Parts Inspection and Placement 6.10 Reflow Soldering Glenn r. blackwell 26.11 Cleanin Purdue University .6. 12 Prototype Systems 26.1 Introduction This section on surface mount technology(SMT)will familiarize the reader with the process steps in a successful SMT design. The new user of SMT is referred to Mims [1987] and Leibson [1987] for introductory material. Being successful with the implementation of SMT means the engineers involved must commit to the principles of concurrent engineering. It also means that a continuing commitment to a quality techniques is necessary, whether that is Taguchi, TQM, SPC, DOE, another technique, or a combination of several quality techniques, lest you too have quality problems with SMT(Fig. 26.1) 26.2 Definition and Considerations SMT is a collection of scientific and engineering methods needed to design, build, and test products made with electronic components that mount to the surface of the printed circuit board without holes for leads [ Higgins 1991]. This definition notes the breadth of topics necessary to understand SMT, and also clearly says that the successful implementation of SMT will require the use of concurrent engineering [ Classon, 1993; Shina, 1991] Concurrent engineering means that a team of design, manufacturing, test, and marketing people will concern hemselves with board layout, parts and parts placement issues, soldering, cleaning, test, rework, and packaging before any product is made. The careful control of all these issues improves both yield and reliability of the final product. In fact, SMT cannot be reasonably implemented without the use of concurrent engineering, and/or the principles contained in Design for Manufacturability(DFM)and Design for Testability(DFT), and therefore any facility that has not embraced these principles should do so if implementation of SmT is its goal c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 26 Surface Mount Technology 26.1 Introduction 26.2 Definition and Considerations Considerations in the Implementation of SMT 26.3 SMT Design, Assembly, and Test Overview 26.4 Surface Mount Device (SMD) Definitions 26.5 Substrate Design Guidelines 26.6 Thermal Design Considerations 26.7 Adhesives 26.8 Solder Paste and Joint Formation 26.9 Parts Inspection and Placement Parts Placement 26.10 Reflow Soldering Post-Reflow Inspection 26.11 Cleaning 26.12 Prototype Systems 26.1 Introduction This section on surface mount technology (SMT) will familiarize the reader with the process steps in a successful SMT design. The new user of SMT is referred to Mims [1987] and Leibson [1987] for introductory material. Being successful with the implementation of SMT means the engineers involved must commit to the principles of concurrent engineering. It also means that a continuing commitment to a quality techniques is necessary, whether that is Taguchi, TQM, SPC, DOE, another technique, or a combination of several quality techniques, lest you too have quality problems with SMT (Fig. 26.1). 26.2 Definition and Considerations SMT is a collection of scientific and engineering methods needed to design, build, and test products made with electronic components that mount to the surface of the printed circuit board without holes for leads [Higgins, 1991]. This definition notes the breadth of topics necessary to understand SMT, and also clearly says that the successful implementation of SMT will require the use of concurrent engineering [Classon, 1993; Shina, 1991]. Concurrent engineering means that a team of design, manufacturing, test, and marketing people will concern themselves with board layout, parts and parts placement issues, soldering, cleaning, test, rework, and packaging, before any product is made. The careful control of all these issues improves both yield and reliability of the final product. In fact, SMT cannot be reasonably implemented without the use of concurrent engineering, and/or the principles contained in Design for Manufacturability (DFM) and Design for Testability (DFT), and therefore any facility that has not embraced these principles should do so if implementation of SMT is its goal. Glenn R. Blackwell Purdue University
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 corresponding 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.)
Type I H Hh CommeNts Type I H Type Ill Intel does not recommond active devices be im in solder wan FIGURE 26. 2 Type I, Il, and III SMT circuit boards. Source: Intel Corporation, Packaging, Santa Clara, Calif: Intel Corporation, 1994. with permission. small active components such as transistors. This type board requires both reflow and wave soldering, and will require placement of bottom-side SMDs in adhesive Type Ill- top side has only through-hole components, which may be active and/or passive, while the bottom side has passive and small active SMDs. This type board uses wave soldering only, and also requires placement of the bottom-side SMDs in adhesive. It should be noted that with the ongoing increase in usage of various techniques to place IC dice directly on circuit boards, Type III in some articles means a mix of packaged SMT ICs and bare die on the same board. A Type I bare board will first have solder paste applied component pads on the board paste has been deposited, active and passive parts are placed in the paste For prototype and low-volume lines this can be done with manually guided X-Y tables using vacuum needles to hold the components, while in medium and high-volume lines automated placement equipment is used. This equipment will pick parts from c 2000 by CRC Press LLC
© 2000 by CRC Press LLC small active components such as transistors. This type board requires both reflow and wave soldering, and will require placement of bottom-side SMDs in adhesive. Type III — top side has only through-hole components, which may be active and/or passive, while the bottom side has passive and small active SMDs. This type board uses wave soldering only, and also requires placement of the bottom-side SMDs in adhesive. It should be noted that with the ongoing increase in usage of various techniques to place IC dice directly on circuit boards, Type III in some articles means a mix of packaged SMT ICs and bare die on the same board. A Type I bare board will first have solder paste applied to the component pads on the board. Once solder paste has been deposited, active and passive parts are placed in the paste. For prototype and low-volume lines this can be done with manually guided X–Y tables using vacuum needles to hold the components, while in medium and high-volume lines automated placement equipment is used. This equipment will pick parts from FIGURE 26.2 Type I, II, and III SMT circuit boards. (Source: Intel Corporation, Packaging, Santa Clara, Calif.: Intel Corporation, 1994. With permission.)
reels, sticks, or trays, then place the components at the appropriate pad locations on the board, hence the term pick and place"equipment. After all parts are placed in the solder paste, the entire assembly enters a reflow oven to raise the temperature of the assembly high enough to reflow the solder paste and create acceptable solder joints at the component d/pad transitions Reflow ovens most commonly use convection and Ir heat to heat the assembly above the point of solder liquidus, which for 63/37 tin-lead eutectic solder is 183.C. Due to the much higher thermal conductivity of the solder paste compared to the IC body, reflow soldering temperatures are reached at the leads/pads before the IC chip itself reaches damaging temperatures. The board is inverted and the process repeated. If mixed-technology Type II is being produced, the board will then be inverted, an adhesive will be dispensed at the centroid of each SMD, parts placed, the adhesive cured, the assembly re-righted, through-hole components mounted, and the circuit assembly will then be wave-soldered which will create acceptable solder joints for both the through-hole components and bottom-side SMDs A Type Ill board will first be inverted, adhesive dispensed, SMDs placed on the bottom-side of the board, the adhesive cured, the board re-righted, through-hole components placed, and the entire assembly wave- soldered. It is imperative to note that only passive components and small active SMDs can be successfully bottom-side wave-soldered without considerable experience on the part of the design team and the board assembly facility. It must also be noted that successful wave soldering of SMDs requires a dual-wave machine with one turbulent wave and one laminar wave It is common for a manufacturer of through-hole boards to convert first to a Type II or Type III substrate design before going to an all-SMD Type I design. This is especially true if amortization of through-hole insertion and wave-soldering equipment is necessary. Many factors contribute to the reality that most boards are mixed- technology Type II or Type III boards. While most components are available in SMT packages, through-hole onnectors are still commonly used for the additional strength the through-hole soldering process provides, and high-power devices such as three-terminal regulators are still commonly through-hole due to off-board heat-sinking demands. Both of these issues are actively being addressed by manufacturers and solutions exist which allow Type I boards with connectors and power devices [Holmes, 1993] Again, it is imperative that all members of the design, build, and test teams be involved from the design stage. Today's complex board designs mean that it is entirely possible to exceed the ability to adequately test a board if the test is not designed-in, or to robustly manufacture the board if in-line inspections and handling Or not adequately considered. Robustness of both test and manufacturing are only assured with full involvement of all parties to overall board design and productio It cannot be overemphasized that the speed with which packaging issues are moving requires anyone involved in SMT board or assembly issues to stay current and continue to learn about the processes. Subscribe to one or more of the industry-oriented journals noted in the"Further Information"section at the end of this Chapter, obtain any IC industry references, and purchase several SMT reference books. 26.4 Surface Mount Device(SMD)Definitions The new user of SMDs must rapidly learn the packaging sizes and types for SMDs Resistors, capacitors, and most other passive devices come in two-terminal packages which have end-terminations designed to rest on P SMD ICs come in a wide variety of packages, from 8-pin Small Outline Packages(SOLs)to 1000+ connection packages in a variety of sizes and lead configurations, as shown in Fig. 26.4. The most common commercial packages currently include Plastic Leaded Chip Carriers(PLCCs), Small Outline packages(SOs), Quad Flat Packs(QFPs), and Plastic Quad Flat Packs(PQFPs)also know as Bumpered Quad Flat Packs(BQFPs). Add in Tape Automated Bonding(TAB), Ball Grid Array(BGA)and other newer technologies, and the IC possibilities become overwhelming. Space prevents examples of all these technologies from being included here. The reader is referred to the standards of the Institute for Interconnecting and Packaging Electronic Circuits(IPc)to find the latest package standards, and to the proceedings of the most recent National Electronics Production and 'IPC, 7380 N. Lincoln Ave, Lincolnwood, IL 60646-1705, 708-677-2850. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC reels, sticks, or trays, then place the components at the appropriate pad locations on the board, hence the term “pick and place” equipment. After all parts are placed in the solder paste, the entire assembly enters a reflow oven to raise the temperature of the assembly high enough to reflow the solder paste and create acceptable solder joints at the component lead/pad transitions. Reflow ovens most commonly use convection and IR heat sources to heat the assembly above the point of solder liquidus, which for 63/37 tin-lead eutectic solder is 183°C. Due to the much higher thermal conductivity of the solder paste compared to the IC body, reflow soldering temperatures are reached at the leads/pads before the IC chip itself reaches damaging temperatures. The board is inverted and the process repeated. If mixed-technology Type II is being produced, the board will then be inverted, an adhesive will be dispensed at the centroid of each SMD, parts placed, the adhesive cured, the assembly re-righted, through-hole components mounted, and the circuit assembly will then be wave-soldered which will create acceptable solder joints for both the through-hole components and bottom-side SMDs. A Type III board will first be inverted, adhesive dispensed, SMDs placed on the bottom-side of the board, the adhesive cured, the board re-righted, through-hole components placed, and the entire assembly wavesoldered. It is imperative to note that only passive components and small active SMDs can be successfully bottom-side wave-soldered without considerable experience on the part of the design team and the board assembly facility. It must also be noted that successful wave soldering of SMDs requires a dual-wave machine with one turbulent wave and one laminar wave. It is common for a manufacturer of through-hole boards to convert first to a Type II or Type III substrate design before going to an all-SMD Type I design. This is especially true if amortization of through-hole insertion and wave-soldering equipment is necessary. Many factors contribute to the reality that most boards are mixedtechnology Type II or Type III boards. While most components are available in SMT packages, through-hole connectors are still commonly used for the additional strength the through-hole soldering process provides, and high-power devices such as three-terminal regulators are still commonly through-hole due to off-board heat-sinking demands. Both of these issues are actively being addressed by manufacturers and solutions exist which allow Type I boards with connectors and power devices [Holmes, 1993]. Again, it is imperative that all members of the design, build, and test teams be involved from the design stage. Today’s complex board designs mean that it is entirely possible to exceed the ability to adequately test a board if the test is not designed-in, or to robustly manufacture the board if in-line inspections and handling are not adequately considered.Robustness of both test and manufacturing are only assured with full involvement of all parties to overall board design and production. It cannot be overemphasized that the speed with which packaging issues are moving requires anyone involved in SMT board or assembly issues to stay current and continue to learn about the processes. Subscribe to one or more of the industry-oriented journals noted in the “Further Information” section at the end of this Chapter, obtain any IC industry references, and purchase several SMT reference books. 26.4 Surface Mount Device (SMD) Definitions The new user of SMDs must rapidly learn the packaging sizes and types for SMDs. Resistors, capacitors, and most other passive devices come in two-terminal packages which have end-terminations designed to rest on substrate pads/lands (Fig. 26.3). SMD ICs come in a wide variety of packages, from 8-pin Small Outline Packages (SOLs) to 1000+ connection packages in a variety of sizes and lead configurations, as shown in Fig. 26.4. The most common commercial packages currently include Plastic Leaded Chip Carriers (PLCCs), Small Outline packages (SOs), Quad Flat Packs (QFPs), and Plastic Quad Flat Packs (PQFPs) also know as Bumpered Quad Flat Packs (BQFPs). Add in Tape Automated Bonding (TAB), Ball Grid Array (BGA) and other newer technologies, and the IC possibilities become overwhelming. Space prevents examples of all these technologies from being included here. The reader is referred to the standards of the Institute for Interconnecting and Packaging Electronic Circuits (IPC)1 to find the latest package standards, and to the proceedings of the most recent National Electronics Production and 1 IPC, 7380 N. Lincoln Ave, Lincolnwood, IL 60646-1705, 708-677-2850