Part 1 1 1. Introduction 1 2. What is SMD compared to TMD (Through Mounted Device) 2 3. What are the reasons for the fast spread of this technology? 5 4. Components 7 4.1 Passive components 7 4.1.1 Resistors 7 Chip-resistor 7 MELF resistors 8 4.1.2 Capacitors 9 Ceramic multilayer chip capacitors 9 Film chip capacitor 10 Tantalum chip capacitor 10 Aluminium chip electrolyte capacitor 11 4.1.3 Structural shapes of resistors and capacitors 12 Shapes of chip components and tantalum chip capacitors 15 Aluminium chip electrolyte capacitor 17 Aluminium electrolyte capacitor / cup shape 18 4.1.3 Other passive components in SMD technology 19 Case and pad dimensions of some passive components 19 4.2 Active components 20 4.2.1 Discrete semiconductor components 20 Diodes and Transistors 20 Case and pad dimensions of diodes 21 Case and pad dimensions of transistors 21
4.2.2 Integrated circuits 22 Circuits with SO (small outline) and VSO (very small outline) case 22 Established members of SO/VSO cases 23 New IC cases 24 Plastic leaded chip carrier case (PLCC) 24 Quad flat pack case 25 Overview of the usual SMD types 27 5. Assembly 28 5.1 Fundamentals of SMD assembly 28 5.2 Automatic placement machines 29 5.2.1 Pick and place - small system 29 5.2.2 Fine- pitch pick and place 30 5.2.3 Chip shooter (high performance placement system) 32 5.2.4 Collect and place machine 33 5.2.5 Collect and pick and place machine 33 5.3 Manual assembly systems 35
Part 1 1. Introduction generation of interconnection technologies for electronic compo- SMD stands for the 4 th nents. Four generations of interconnection technologies in electronics The 1 st generation of interconnections used screw terminals and clipped connections with insulating wires. The 2 nd generation was mainly based on cable harnesses and insulating wires. The 3 rd generation used printed circuit boards. Initially one-sided with printed conductor pattern, later double-sided plated-through and finally multilayer boards. 1
PCB performance SMD originally addressed the single components, but now the whole technology of surface mounting is named SMD. 2. What is SMD compared to TMD (Through Mounted Device) SMD is an electrical or electromechanical component with contact platings, suited for electrical interconnection on the surface of a printed board or a ceramic substrate. The connection points are either solderable areas or component leads.. 2
There are differences between SMD and TMD concerning geometry, functions and production. Geometrically, SMD shows no connecting wires. In the case of TMD the connecting wires are fed through a hole in the board and are soldered at the printed circuit board solder side. Mechanical and thermal stress is carried by the wires. Components with leads thus allow some deformations. The forces of the deformation are carried by the connecting wires. SMD components have to carry all the deformation stress by the solder joint itself. Various pin forms of SMD components From a functional point of view the compact shape of SMDs results in a higher gate density and therefore an increased heat generation per unit area. This is to be considered during the layout. The production of SMD boards by automatic assembly is of great advantage. 3
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3. What are the reasons for the fast spread of this technology? Constant innovation, increasing compactness and more and more automation in the mass production of electrical and electronic units and systems make tough demands on the devices designed into them. So, in passive components, too, you see an unbroken trend away from leaded to surfacemount solutions. The requirements in modern electrical and electronic engineering are changing fast, calling for increasingly complex and powerful circuits, implemented on ever smaller circuit boards. Surface-mount technology has been the major change in production methods of recent years and is the real mover behind the trends we see. It allows independent design of both sides of a board, without having to worry about the holes required for the placement of leaded components. So components can be placed much tighter. And this technology is also promoted by the use of increasingly smaller components. Smaller designs are necessary because they further enhance the attractive features that we know from the original SMDs, i.e.: higher packing density, fewer parasitic electrical characteristics (capacitive and inductive), less sensitive to mechanical stress, high resistance of the solder joint to cyclic temperature stress, small space requirement in transport and storage. But there is no overlooking the drawbacks, namely: greater placement effort, feeding to automatic placement systems is more difficult, soldering problems can increase, adhesion is more difficult, the electrical loadability is poorer. SMD is one of the leading technologies for electronics. About two thirds of the flat-pack assemblies are produced with SMD components sometimes combined with leaded components. 5
Example of mixed assembly Because SMDs fulfil the high requirements of conformity and reliability, miniaturisation and quality could be improved. Miniaturisation is still in progress. Increasing the functional density results in smaller and lighter devices. First SMD was used for consumer electronic devices (walkman, camcorder, mobile phones etc.). Printed circuit assemblies soldered at both sides could be realised. The number of gates per chip doubles every two years and thus the number of connections. This high connection density cannot be realised with leaded components economically. The pin grid is 2.54 mm. Complex SMD chips meanwhile use a grid of 0.5 mm. Make-up of the board with SMD 6
4. Components 4.1 Passive components In describing exemplary components, only those features of the components will be addressed which are of interest for interconnection technology (i.e. component shape, terminal pad, contact shape, metallisation of connection etc.), and not their function. 4.1.1 Resistors Resistors show two shapes: chip and MELF (metal electrode face bonding). Chip resistors are rectangular, MELF resistors are cylinders. Thick-film resistors are basic components used in all kinds of electrical and electronic equipment, from watches through communication and data systems up to automotive electronics.. Chip-resistor The resistive film of a chip resistor is fixed to a ceramic substrate by screening methods. Laser trimming is used for compensation. Afterwards the resistive film is covered by enamel painting or a lacquer film. The contact pads are multilayered and are covered by a tin layer. Construction of a chip resistor 7
MELF resistors MELF resistors are produced like metal-leaded film resistors. Under vacuum a metallic layer is evaporated. After laser trimming the attachment points are tinned and the resistive film is covered by a lacquer film. Construction of a metal film resistor 8
4.1.2 Capacitors For surface mounting, there is an extensive range of chip capacitors with rated voltages from 16 V through 500 V. The 16 V and 25 V type series feature especially high volumetric efficiency. Multilayer capacitors offer maximum capacitance within the smallest space. They are found in many areas of advanced microelectronics, for instance in telecommunications, entertainment and automotive electronics, and in PCs. Ceramic multilayer chip capacitors The notion ceramic capacitor comprises different classes of capacitors with distinct features. The common feature is the use of oxide ceramic as a dielectric. Special procedures allow the construction of thin ceramic layers which are the base of capacitors. Multilayer capacitors consist of a monolithically ceramic block with comb-like electrodes. These are visible at the surface of the block where they are contacted by burnt-in metallic layers. 9
Film chip capacitor The transition from through-hole to surface-mount technology is now also feasible in the field of metallised film capacitors. The new SMD series has all the technical benefits known from self-healing stacked-film capacitors. SMD film capacitors are typically used in applications requiring high linearity and capacitance stability, as for example in filters and oscillators. The basis for film chip capacitors is a temperature-stable plastic film. Layered structures result in a stack capacitor. Tantalum chip capacitor Tantalum capacitors offer the ideal combination of very high capacitance with the smallest dimensions, low leakage current and low dissipation factor. 10
They are firmly established in important products of automotive electronics, data processing and telecommunications, in medical appliances and measuring and control systems. A tantalum chip capacitor consists of a pure tantalum anode inside a dielectric layer. Aluminium chip electrolyte capacitor These capacitors consist of a roll containing electrolytic material inside a cylindrical aluminium container closed by a rubber plug. 11
4.1.3 Structural shapes of resistors and capacitors Let us first look at the soldering terminal of resistor and capacitor shapes. As already mentioned in connection with the resistors, the contact areas are multilayered. Contacting example of chip and MELF components Beispiel für Kontaktierung von Chip- und Melf Bauelementen nickel (diffusion stoppage 1µm) (Diffusionssperre 1µm) tin Zinn (solder layer 7-10µm) (Lötschicht 7-10µm) steel cap Stahlkappe silver Silber (ground layer >20µm) (Grundschicht >20µm) tin Zinn (solder layer 3-8µm) (Lötschicht 3-8µm) Nickel (diffusion stoppage 5µm) (Diffusionssperre 5µm) copper Kupfer (adhesive layer > 2-3µm) (Haftschicht > 2-3µm) resistive film Widerstandsschicht nickel-phosphorus Nickel-Phosphor (thermal insulation) (Wärmesperre) chipkontakt.dwg As an example of many different contact constructions, two constructions are shown here which correspond to the majority of all applications and which show the features that are decisive for selecting a multilayer construction. Important for the user is a readily solderable outer contact layer. This is therefore a tin layer. But tin greatly tends to form alloys with other metals so as to change the composition and reduce the solderability or even make soldering impossible. Furthermore, intermetallic phases are formed between background and tin through long-term diffusion processes. They reduce the thickness of the tin layer and deteriorate the physical and mechanical properties of the soldered connection and shorten the durability of the soldered joint. We will discuss intermetallic phases in more detail in the chapter on soldering. A reliable soldering terminal therefore needs an effective diffusion barrier as the background. 12
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Nickel is in all respects a superior material for this purpose. Only nickel makes it possible to effectively prevent the fatal alloying through of tin and the migration of underlying material. Underneath the solderable tin layer there is thus nearly always a nickel layer as a diffusion barrier. Underneath the nickel layer, in the simplest case, an easy to contact and highly conductive layer is now applied for contacting the component. This may be silver. Depending on the application and quality requirements, other layers may follow, such as a copper layer to accommodate thermal stresses for increasing the reliability of the component. Furthermore, the picture of the MELF resistor shows a nickel-phosphorus layer for contacting to the cap of the resistor and preventing the resistor material from overheating during soldering due to its poor heat transfer value. In the recent past, the interconnection of components by means of conductive adhesive has frequently been discussed. The components industry offers silver-nickel contacting for such applications. Soldering is not possible with this contact area. The shape of chip components is specified by their size of length x width in 100/inch. 14
Shapes of chip components and tantalum chip capacitors CHIP 0402 1.0mm x 0.5 mm C to 25V R 60mW CHIP 0603 1.5mm x 0.75mm C to 50V R 100mW CHIP 0805 2.0mm x 1.25mm C to100v R 125mW CHIP 1206 3.2mm x 1,6 mm C to100v R 250mW CHIP 1210 3.2mm x 2.5 mm C to 50V CHIP 1812 4.5mm x 3.2 mm C to 50V CHIP 2220 5.7mm x 5.0 mm C to 50V ChIP 2512 6.5mm x 3.2 mm R 1 W The size of Mini MELF resistor is as large as shape 1206 The size of Micro MELF resistor is as large as shape 0805 The shape of tantalum chip capacitors is designated by alphabetic characters A.B.C.D.E 15
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Shape A 3.2mm x 1.6 mm (approximately 1206) Shape B 3.5mm x 2.8mm Shape C 6.0mm x 3.2mm Shape D 7.3mm x 4.3mm x 2.9mm Shape E 7.3mm x 4.3mm x 4.1mm The black bar at the top is the plus marker, just as the split connection. Aluminium electrolyte capacitors are available as chip capacitors and as cup-shaped capacitor Aluminium chip electrolyte capacitor smpadaluchip.dwg Shapes of aluminium chip capacitors are 1a 9.0mm x 4.0mm 1 12.0mm x 4.0mm The marginal notch at the shape is the plus marking. 17
Aluminium electrolyte capacitor / cup shape shape: B shapes: E and F shape: C shape: G shape: D smpadbecher.dwg The shape of aluminium cup capacitors is designated analogically to tantal chip capacitors by alphabetic characters B.C.D.E.F.G. but the dimensions are different. Shape B is equal to 4.3mm x 4.3mm Shape C is equal to 5.3mm x 5.3mm Shape D is equal to 6.6mm x 6.6mm Shape E is equal to 8.3mm x 8.3mm Shape F is equal to 8.3mm x 8.3mm Shape G is equal to 10.3mm x 10.3mm 18
4.1.3 Other passive components in SMD technology Trimming resistors, Trimming capacitors Coils Switches Connectors Sockets Relays All of these components have to withstand the soldering temperatures used for SMD components. Case and pad dimensions of some passive components 8-pattern DIL-Switch/SMD Trimming resistor Resistor array Chip choke SIMID 03 passiv.bauel.dwg 19
4.2 Active components 4.2.1 Discrete semiconductor components Diodes and Transistors In general SMD diodes and SMD transistors show the same internal structures as their leaded component counterparts because they house the same crystals. But the housing forms are different. Firstly they show contacting surfaces and secondly because of higher thermal resistance. The dissipation power of SMD semiconductors must be away via the printed circuit board. Therefore, the thermal limit values (i.e. barrier layer temperature and soldering joint temperature) must be observed especially when power transistors are used. Diodes show cylindrical forms like MELF resistors but the case is made of glass. There are also diodes with rectangular cases and two connector pins at the faces or at the longitudinal sides similar to the standard transistor case. 20
Case and pad dimensions of diodes SOD 80 cathode PAD-areas as chip 1206 SOD 123 cathode SOD 87 cathode PAD-areas as chip 1206 SOT 23 Diode Diode Dual diode Diode Diodenpad.dwg For transistors a variety of rectangular cases are used depending on the type and dissipation power. For a dissipation power up to 400 mw case types SOT-23 and SOT- 143 are used. Up to 1 watt SOT-89 is used. SOT-223 is used for a dissipation power exceeding 1 Watt and currents up to 3 amps. Case and pad dimensions of transistors 21
4.2.2 Integrated circuits Integrated circuits for surface mounting were designed in the early 70 for hybrid technologies. Square shaped plastic miniature cases with pins at the sides have been used. Two different connections exist: "gull wing" forms bent outwards and "J" forms bent to the inner side. There are several case types which will be described in the next chapter. Circuits with SO (small outline) and VSO (very small outline) case Construction of an SO case 22
For integrated circuits with 8 to 32 pins the SO case is used. The dimensions of this package family are standardised (IEDEC MS 012/013). The pitch of the connections is 1.27 mm (SO). Established members of SO/VSO cases Three housing widths are used: 4 mm for cases with 8, 14, 16 pins (SO-small), 7.6 mm for cases with 8, 16, 20, 24 and 28 pins (SO-large) and 8.5 mm (SO-XL) for large crystals. 23
New IC cases In addition there is a subminiature case SSO-20 with 4.5 mm width and a pitch of 0.65 mm. For high connection numbers, compact cases with lead pitches of 0.76 mm (0.03") up to 0.63 mm (0.025") are used (VSO = Very Small Outline). Plastic leaded chip carrier case (PLCC) For integrated circuits with high pin counts PLCC cases have been standardised. The pins of these square shaped plastic cases are out at all sides and are bent under the case. The grid size is 1.27 mm (0.05"). All types of PLCC cases are standardised (IEDEC MO-47-AC-AF). Connection 1 is indicated by an oblique edge. Construction of a PLCC case The forms of their connections - renders the PLCC cases insensitive to mechanical stress and therefore they are well suited for socket mounting. Another advantage is the low thermal resistance value. Disadvantages are the height of the cases and the position of the soldering joints, which aggravates soldering and visual inspection. 24
Complete PLCC case family Quad flat pack case For integrated circuits with many pins various quad flat packages have been developed. These square or rectangular cases use gull wing connections at all sides. Construction of a QFP case Pitches of 1 mm, 0.8mm, 0.75 mm, 0.65 mm and 0.5 mm are used. 25
This leads to compact cases with a high number of pins (44 to 208) avoiding some of the disadvantages of PLCC packs. QFP case forms with differing pitch Because of the challenging requirements of ICs the development of the case form is not finished. Insensitivity to mechanical stress and simple and safe mounting are top priorities. 26
Overview of the usual SMD types 27
5. Assembly Before the placement of the components - coating of the solder paste is performed for reflow soldering. In case wave soldering is used, adhesive dots must be applied which fixes the components before soldering. Attributes and handling of solder pastes are treated in chapter 6 "Soldering". 5.1 Fundamentals of SMD assembly The term "pick and place" characterises the assembly process of SMDs quite well. What is to be done: The right component must be fetched from the readiness place and then placed on the printed circuit board at the right position with the required precision and orientation. Manual pick and place system The assembly process with SMDs is simpler compared to leaded components. It can be automated and nearly all component forms can be placed by a single placing machine. Leaded components require distinct insertion machines for the forms "Axial" / "Radial" or "DIL". The SMD components are delivered in tapes, trays or bulk. Chip components: in paper or plastic tapes, or bulk Melfs: in paper or plastic tapes, or bulk SO ICs : in plastic tape, or bar trays PLCC ICs: in bar trays QFP ICs: in flat trays TAP ICs: on film 28
In order to accommodate the different components in tapes, standard tape widths have been agreed upon. The tape widths are: 8, 12, 16 and 24 mm. The most widely used tapes are those of 8 mm width because practically all chip resistors, about 90 % of the chip capacitors used as well as semiconductors in SOT-23, SOT-43 and SOD-80 cases fit into these tapes. Recently, even larger tape widths, namely 32, 44 and 56 mm, have been put on the market. The delivery of components in this form is ideally suited for automatic assembly. These delivery forms are well suited for automatic insertion. But manual assembly facilities are still used. For development purposes only small numbers of boards are built, such that automatic insertion is not cost effective. Small automatic insertion devices handle 2000 to 3000 components per hour. In reality the assembly performance drops to 50 % of these features. 5.2 Automatic placement machines For laboratories automated inserters are not required because of: Small numbers of boards Frequent reorganisation of the component feeders Short insertion times related to preparation Problems of availability of special components Quality of components (oxidation, wrong orientation) All these problems can be corrected manually. Therefore I am not going to present the details of such systems. Just the principles are mentioned. SMD insertion systems having to fulfil high requirements are getting smaller, the pitches are decreasing, the pin count increasing and the variety of components is increasing. 5.2.1 Pick and place - small system The pick and place machine always works with fixed feeders and a fixed printed circuit board. The mechanic is similar to a XY plotter with a vacuum nozzle for placement. The placement head can be rotated and is equipped with pliers. 25 to 80 feeder positions in the form of tapeware magazines, bar magazines and vibration magazines are used. 29
Pick and place principle 5.2.2 Fine- pitch pick and place With fine-pitch Inserters ICs with lead pitches of 15 mils (0.381 mm) and even down to 12 mils (0.305 mm) and with side lengths of 50 mm can be placed. These devices can also be used for chip components with a size of 1 mm x 0.5 mm (0402). Fine-pitch machines can only work efficiently on the pick and place principle. Fine pitch assembly head 30
This results from the necessity to handle components individually (because of the different packages) with respect to positioning and special movements during optoelectrical correction and test of coplanarity. Fine-pitch machines -based on the pick and place principle- are very flexible but cannot reach optimal speed. 31
Coplanarity 5.2.3 Chip shooter (high performance placement system) The classical shooter optimised for high placement performance uses a fixed horizontal revolver head with a number of vacuum tweezers performing fetching and on-setting simultaneously. Component feeders and PC board are mounted on a movable desk. Chip shooter with revolver head and moving feeder The board positioning and the rotation of the revolver head are performed concurrently. The theoretical performance of such a device is 40000 components per hour. This can only be reached with optimised co-ordination of fetching and on-setting. Chip shooters are only used for mass production. 32
5.2.4 Collect and place machine The collect and place principle combines the advantages of the flexible pick and place and the shooter. In contrast to pick and place a revolver head with up to 12 vacuum tweezers is installed on a X/Y gantry system. The time between fetching and onsetting is minimal. In order to increase the performance, a second revolver head can be mounted. This leads to a performance of 12000 components per hour. Collect and place principle 5.2.5 Collect and pick and place machine Optimal flexibility and high assembly performance with a single machine can be achieved by combining a revolver head and an IC-head (pick and place) on one X/Y system. This principle covers all essential SMD on-setting criteria. The normal components are placed by the revolver head, whereas the pick and place head is used for the fine-pitch ICs. 33
Collect and pick and place machine 34
5.3 Manual assembly systems Manual assembly like automatic assembly requires two tasks: 1.) Fetching the component from the readiness place 2.) Placing the component correctly. In no case should one should touch the connection areas of an SMD component. When touched the solderability of the SMD pads degreases significantly. SMD high-precision hand tools Components are manipulated with special tweezers or vacuum tweezers. Components can be easily placed by hand under normal eye control down to a 1.27 mm grid or chip forms 1206. Reflow soldering improves the placement by self-alignment of the components (depending on the layout). In case a component is not placed correctly, the solder paste application should not be disturbed by moving the component. There is the danger of solder ball formation or badly filled soldering joints. For tight grid spacing (smaller than 1.27 mm) a magnifying glass (magnification 2-3) is useful. Devices for electrical hand-placing are offered by a number of vendors in various versions. They include vacuum tweezers, hot air blowers, special soldering equipment up to manipulators that can be moved in three axes. 35
SMD electrical hand tools Manual assembly work station 36
The simplest case is to keep the bulk components in containers. This works with chips, Melfs and SOs as long as the component leads do not mix up. For larger hand insertion systems, component feeders for tapeware, bar magazines, flat magazines and integrated storage systems are used. Modern hand insertion systems consist of up to 40 tape feeders, carousels and mass storage magazines. In addition optical, mechanical and automated aiding devices are available. Fine pitch system With a pitch less than 0.65 mm it is difficult to place components correctly without optical devices. Usually microscopes or camera systems are used. But even with a camera the manual placing is difficult. Mechanical devices are required for sinking and adjustment by means of micrometer controlled board shifting. With computer control, the feeder which holds the next component to be placed - can be optically indicated - and carousels can be turned. For placing the component onto the board systems are offered which co-ordinate the CAD data of the PCB and the insertion table. All the leading vendors offer such systems (i.e. Fritsch etc.). End of Part 1 33
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