Surface-mount technology

The English term surface- mounted device (SMD, German: surface mounted device ) is a technical term from the electronics. SMD components, in contrast to components through installation (English Through Hole Technology, THT ), the " lead type devices ," any wire connections, but by means of solderable pads on a printed circuit board soldered ( printed circuit board ) directly. The associated technique is surface mounting (English: surface - mounting technology, SMT).

• 6.1 production lines for SMD assembly (principle)
• 6.2 applying the solder paste or glue
• 6.3 loading of components
• 6.4 curing of the adhesive and soldering
• 6.5 Typical errors

Survey

During the connection wires of conventional components can be performed by fitting holes and on the back of the circuit board (or the inner layer ) to be soldered ( via hole ), this does not apply to surface mount components. This very dense configurations and especially a two-sided fitting the circuit board are possible, which is especially positive influence on the electrical characteristics of the circuits at higher frequencies and reduces the space required for the components. This makes the devices can be smaller and at the same time produced much lower cost.

SMD components are transported to the production in belts, rod magazines or on blister trays and fitted with machine on printed circuit boards. A manual assembly is self- development patterns in the industry because of the smallness of the components and the small spacing of the conductors from each other nowadays no longer makes sense because the large positioning differences in manual loading may well have an influence on the high frequency response of the circuit. The pads of the SMD components on the circuit boards are printed prior to loading using screen printing or stencil printing solder paste. After assembly, the SMD components are soldered with suitable method. For the top of a board reflow process has advocated enforced. SMD components on the underside of a printed circuit board are glued and soldered in wave or wave soldering.

Prerequisite for a high quality of a circuit produced in SMD technology is a perfect soldering of SMD components. Therefore, all the solder pads are controlled in the manufacturing process. The continuing miniaturization of SMD components however, makes it now impossible with SMD components to control PCBs with the naked eye or with the aid of a microscope. For this purpose, now were industrial image processing systems ( AOI systems, English: automated optical inspection) developed the check with high precision and high speed, the prescribed important parameters.

For electronics hobbyists results from the SMD technology of disadvantage that can be SMD components without soldering machines or the relevant know -how is very difficult. With a suitable forceps, a fine soldering tip and 0.5 mm Solder, a steady hand and possibly a stereo microscope but can already achieve good results. The use of classical breadboard cards and strips raster maps is limited for experimental set-ups or prototypes possible. However, suitable laboratory cards and adapters are commercially available, although comparatively expensive, or even manufactured. Therefore, the use of SMD components as a hobby by many is - if possible - avoid viewed by others as a challenge, as in trade magazines and on the internet many -to-find building instructions with SMD components show. The limited availability of used components from waste equipment by partially ambiguous markings is another problem for the hobbyists.

History

The surface mount technology was developed in the 1960s by IBM. The first need to semiconductor devices came from the leading manufacturers such as Fairchild, where in 1965 the dual in - line package ( DIL ) has been developed. In Germany Valvo, the components subsidiary of Philips was, from the beginning of 1980 pioneer for the SMD technology with their devices. But other German companies ( like Siemens, Roeder stone) participated instrumental in the development and improvement of these novel components. Also the time of his coming Japanese components industry has accelerated this development, especially the development in the field of passive components.

Trigger for the surface mount technology was the progressive integration in the semiconductor technology. The number of circuit functions that have been integrated on a semiconductor crystal, rose with minimum component cost increases from year to year ( Moore's Law ). At the same time decreased, the operating voltages of the circuits. Thinner conductor paths could thus be easily realized. In parallel, the operating frequencies of processors and circuits and paths have been sought to improve the characteristics of the circuits at high frequencies. It was necessary to avoid long line guides to reduce lead inductance, which is why the components had to be smaller. In addition to the integration in the semiconductor industry in particular bleeder resistors and filter capacitors were needed in smaller dimensions. With the thick-film resistors on ceramic substrate and the multilayer chip ceramic capacitors ( MLCC) have been developed in the 1970s, this then novel, miniaturized components.

The basic idea for the surface mount technology was then eliminating the connections which have been contacted and held by the holes in the printed circuit board. By avoiding the holes significant reductions of the layout of the circuit board could be obtained, but there was accounted for in a borehole Lötkreis solder necessary to holes, which was significantly greater than the 1.27 mm, on which the conductor tracks have been reduced in the meantime. Characterized the peripheral devices could be mounted closer to the semiconductor, resulting in the desired reduction of the impedance. At the same time production costs could be significantly reduced, because each hole causes costs.

From mid- 1980 SMD circuits have already been used on a large scale, especially for circuits that were operated at higher frequencies. These included circuits of emerging digital technology, but also the tuners for televisions.

Advantages and disadvantages in the use of SMDs

SMD components have replaced in many applications, components with through-plated lead wires. They have over those following, among other advantages and disadvantages:

Advantages:

• Miniaturization, significant reduction of circuits and devices with smaller device dimensions (results in higher component density), the narrower trace spacing and thinner traces on the SMD board. Ideally suited for flexible printed circuit boards, such as cameras or LED light strips.
• Cost reduction, holes in the PCB accounts, the board is more cost effective, especially if it can be used by the SMD technology on single-layer printed circuit boards without holes.
• Weight reduction through elimination of lead wires and smaller components.
• Improvement of high-frequency characteristics to each other by offering a low- distance and shortening of conductors (smaller ohmic losses, lower inductance of the shorter conductor paths ). Components can also be fitted directly above one another on both sides of the printed circuit board (important for high-frequency components ).
• Faster devices manufacturing by rapid automatic assembly ( Collect & Place / Pick & Place / Chip Shooter), thereby results in lower production costs.
• Increase the quality of production with automatic placement.
• Increase the production quality through automatic optical inspection ( AOI) of all critical, visually verifiable factors in most of SMDs possible.
• Small positioning errors in the component mounting can be automatically corrected by the surface tension of the liquid solder during soldering.
• Boards with a smooth back to produce, for example, for remote controls and keyboards - a hobby or as a housing part.

Cons:

• For components with connectors on the component base (such as BGA ) solder joints can be checked only by X-ray.
• Through the reflow soldering SMD components are briefly exposed to a high temperature. ( > 200-250 ° C). Some traditional designs are therefore not suitable for this technique. Examples are aluminum electrolytic capacitors, or plastic film capacitors.
• Lower mechanical strength, especially with large, heavy components and connectors; requires at least additional fixations, in turn, require holes

SMD connector types

Passive components such as resistors, capacitors, crystals or inductive components such as chokes are produced predominantly in cuboidal shapes. Two or more sides of this design are designed for making electrical contact solderable. Proper soldering of these "chips" can be recognized by a well-trained solder meniscus.

Transistors and integrated circuits formed at the beginning of the SMD components of the ( wired ) dual in - line package ( DIP package), a case in which the solder terminals of the component are led out at both side surfaces. The vertical solder connections of the case are simply for SMD soldering either bent outwards (English Gull - wing) or inward (English J- leads) side. With the progressive integration density especially in the processors with its many connections other ways were sought in connection technology. This led to the development of the grid array connector technology. Here are the solder connections as small metallized connecting pads under the housing of the circuit. When conventional ball grid array (BGA ) solder balls already present on the contact surfaces, which are melted during the soldering process only. In the Land Grid Array ( LGA) on the other hand solder must be applied to the board to be fitted, so LGA components are rarely used for soldering. However, you can also, in contrast to BGAs, be operated on a matching pin array. When appropriately designed locking LGA components can then be replaced easily, which is exploited in microprocessors, for example.

• SMD connector types

Gull-wing solder connections on the sides of the housing an integrated circuit

J-Lead solder connections on the sides of the housing of an integrated circuit ( obsolescent )

Solder balls under a microprocessor with BGA - contact system

Quarter turn arranged LGA pads under the case of a microprocessor

SMD types, SMD package

For the types of SMD components there are in the industry Two different preambles. In the passive components (resistors, capacitors, inductors, crystals, etc.), the concept design for the different construction methods is in use. The particular design may then differ even in different dimensions and the nature of the connection form. In the field of semiconductor technology, the active components, however, the different designs are by the preamble summarizes housing ( chip package ). The active cell, the which is installed in a housing which is the reason for the different name. Each transistor or IC package has its own name, which is derived from the first letter of the English description. Variations within the same housing terms that result by the number and arrangement of connections and the shape of the terminals are identified by a name to the numbers.

SMD types of passive components, diodes and transistors

Passive components, and optionally also the diodes and transistors are fabricated is supplied and processed in the following forms:

• SMDs

V-chip, vertical - cylindrical SMD series aluminum electrolytic capacitors

Lying horizontally SMD series for resistors ( MELF ) and diode (SOD )

SMD transistor (possibly BC817 ); Typical Wiring: base / collector / emitter.

Chip, a cuboid design, is the typical design for MLCC and tantalum capacitors, inductors, and nonlinear and linear resistors (R ) chips. The term "chip" can be easy with the homonymous term from the semiconductor technology, the " chip " ("The " english ) a semiconductor device to be confused. Cuboid -shaped special designs for eg crystals or oscillators may provide additional solder pads on the housing, either for reverse battery protection or for better and stronger vibration mechanical fastening included.

The chip design of passive components differs mainly by the size that is specified with a code such as " 1206 ". Where " 12" and the length "06" of the width of the component in the unit Zoll/100. Meanwhile, however, is more and more by a metric encoding the size.

Extended tables of the available chip sizes and the dimensions of which are also found in the technical articles of the components ceramic capacitors, tantalum electrolytic capacitors and resistors.

• V-Chip (vertical chip ) is a cylindrical design with shims, mounted vertically. In particular, aluminum electrolytic capacitors are delivered and processed in this design. The sizes of this design are not standardized. V -chip aluminum electrolytic capacitors can reach quite large sizes; it is the most exotic of the SMD components.
• MELF (Metal Electrode Faces ), is the name for a cylindrical design of passive components, which are mounted horizontally. She is the typical design for metal film resistors and non-linear resistors.
• SOD ( Small Outline diode ), the cylindrical housing for the semiconductor component diode which is as MELF design also mounted lying called. Also, the SOD is supplied in various sizes.

In addition, numerous passive components such as potentiometers, trimmers, transformers, transformer, quartz crystals, oscillators possess special SMD types, which are derived from the geometry and the ports of the components. A special challenge is the electro-mechanical components such as switches, sockets, plugs and sockets, their designs do not fit into conventional schemes.

SMD chip packages for semiconductors (transistors, integrated circuits )

Soldered connections on two sides of the housing

SMD transistor package with solder terminals at two sides of the case are marked with the following chassis name:

• SOT ( Small Outline Transistor): design with three or four connections for transistors, the fourth terminal is often carried out as a heat dissipating (English heatsink ), connection spacing typically 1.27 mm

IC package with solder terminals at two sides of the case are marked with the following chassis name:

• SOIC ( small-outline integrated circuit): IC SMD package with the same row spacing as the through-hole version, the connection distance is typically 1.27 mm
• SOP (Small Outline Package ): smaller version of the SOIC package, provides the basis for a whole series of modifications which are vendor-specific part, such as: PSOP (Plastic Small - Outline Package )
• TSOP ( Thin Small - Outline Package ): Connections on the narrow side of the housing,
• SSOP (Shrink Small - Outline Package )
• TSSOP (Thin Shrink Small - Outline Package )
• QSOP (quarter -size small-outline package)
• VSOP ( Very Small Outline Package )

Solder connections at four sides of the housing

IC package with solder terminals at the four sides of the case are marked with the following chassis name:

• PLCC (plastic leaded chip carrier): IC SMD package with 20-84 " J -lead " connections, housing with connection distance of typically 1.27 mm.
• QFP (Quad Flat Package ): IC SMD package with 32 to 200 terminals, forms the basis for a whole series of modifications that are specific to the part of manufacturers, such as: LQFP ( low-profile Quad Flat Package): IC SMD package height of 1.4 mm
• PQFP (Plastic Quad Flat Package ): IC SMD package which has been generally replaced by the thinner TQFP package.
• CQFP (Ceramic Quad Flat Package '' ): IC SMD package comparable to the PQFP package but with ceramic cladding.
• MQFP ( Metric Quad Flat Package ): IC SMD package comparable to the QFP package but with metric distance between the connectors from each other.
• TQFP (Thin Quad Flat Package): A thinner IC SMD package version of PQFP package with a height of either 1.0 mm or 1.4 mm.

Solder lands underneath the body, " lead frame " and " grid arrays "

• SMD connector types

LGA, view the contacts (English: lands ) on the bottom of the housing

BGA, the upper side view picture shows two opposing BGAs soldered

• Quad Flat No leads package ( QFN ), micro leadframe package (MLP), and micro leadframe MLF The terms include a family of integrated circuit packages, the electrical terminals are arranged like a frame around the lateral surfaces of the housing and to be metallized contact areas located under the housing. This housing sometimes having a more metallized contact surface below the housing, which is connected with in order to enable better heat dissipation. A variant MLPD ("D" stands for " dual" ) is pin compatible with the DIP SOIC package, with MLPD (Dual ) and MLPQ ( Quad) the connection configurations can be distinguished, MLPM (Micro) is a miniaturized version of the case. The QFN IC SMD package is a special MLPQ - housing, the pin assignment and connection distances correspond to the leaded QFP package, the metallized terminal areas, however, are attached as contact surfaces below the housing.

Highly integrated semiconductor devices such as microprocessors have so many electrical connections that they are no longer to accommodate the circumference of the housing. Therefore, these terminals are in the form of metallized pads checkerboard or grid-like manner (English Grid Array ) is mounted under the housing. This semiconductor package checkerboard pattern under the housing be marked with the following chassis name with solder lands:

• LGA ( Land Grid Array): An LGA is a checkerboard or grid -like arrangement of electrical connection pads ( english country ) on the bottom of a housing for ICs with very many connections such as microprocessors. When soldering this many contacts under the LGA package, however on rare occasions can solder defects occur which are only quite difficult to recognize ( X ). Since repair is costly and error prone, LGA ICs are often put on base. These sockets with the same pinout as the IC are soldered in the SMD process to the board and can be tested for contact reliability pretty easy. The processor is then pressed for electrical connection by means of a clamp with its pads to the contacts located at the top of the base. The socket has resilient contact pins, so that a reliable electrical contact can be made.
• BGA ( ball grid array, ball grid array dt ): a BGA as the LGA package, a housing form of integrated circuits, in which the electrical connections checkerboard or grid-like arrangement of the bottom surface are fitted. However, the connections are in the form of small solder balls ( engl. balls ). These beads are melted during reflow soldering in a furnace and combine with the copper of the PCB.

Processing

Since the SMD components are fitted on a printed circuit board, this processing is called assembly, even though the assembly also includes other steps than the placement of the components on the circuit card. These steps are:

• Applying solder paste (a mixture of tin balls and flux ) or adhesive to the printed circuit board
• Loading of components
• Soldering the circuit board or curing of the adhesive

After each step, the quality of the product is visually checked before it is passed on to the next step. The manufacturing steps are carried out automatically in a rule for single pieces or in prototype but is occasionally omitted on machines or individual steps are performed manually. The equipment and processes required for the processing are referred to as surface mount technology. The area of ​​an electronics plant will be involved in the processing of SMDs therefore referred to as SMT range or SMT Department.

Production lines for SMD assembly (principle)

The versions shown here can also be mixed with each other, so that certain processes are controlled manually, other optical automatic inspection. In some cases, a test step is completely eliminated. So can account for about a final visual check if the product is very easy as there is anyway subjected to a function test. Often, individual tests are already integrated into the production step. How have modern paste printer via an optical system for controlling the pressure and the cleanliness of the template.

Applying the solder paste or glue

Solder paste or adhesive can be applied in several ways: Either it is applied by screen printing method or metered in small portions. The latter is mainly practiced manually in prototype and small-. However, machines are used for the adhesive application, the meter the adhesive. The adhesive is applied by a thin tube to the desired location or contact- sprayed ( Jetten ).

When applying the paste to the screen printing method has prevailed. Where later component connections come to lie on the board, has this copper areas - so-called pads - which are either gold-plated or tinned. The printing screen is positioned on the circuit board, that the holes of the screen centered resting on the pads. Usually they are less than a few hundredths of a millimeter, in order to prevent that the paste is printed in addition to the pad.

Circuit board and screen are pressed against each other, and a doctor blade forces the paste through the screen, so that this passes through the holes in the pads. The thickness of the wire determines the amount of tin in this case ( solder paste ) per area. In some cases, however, it is necessary that certain ports gotten more tin - this can not be achieved by a larger pad area, and later additional solder paste must be metered.

The screens are now largely replaced by lasered metal stencils. Thus, smaller structures could be printed. To achieve the required accuracy in the printing, printing can be used to enable a precise alignment of the mask to the printed circuit board. This can be done either manually or automatically both aligns by a camera system which can recognize marks on the template and the circuit board prior to printing and to each other.

Loading of components

The smaller parts are packed in cardboard or plastic belts. In the straps are pockets in which are the components. The top of the bag is closed by a film which is drawn off to remove the component, like a blister pack. The straps themselves are wound onto a roll. On at least one side of the belt to transport holes are located at a distance of 4 mm, over which the belt is moved by the placement machine. These roles are delivered with the aid of feeder, so-called feeders, the placement machine.

ICs and other large components are often in plastic poles (English sticks ) or in small pallets, known as trays, packed. While the trays can be inserted directly into the machine, for the rods are also feeder (English stick feeder ) is required. By vibration of the drive reach of the sampling feeder, the parts further forward so that the placement head can take out the next component.

The components are removed with vacuum pipettes (English nozzle ) or grippers and then placed on the target position ( X, Y - coordinates) of the printed circuit board. This process is repeated for all components. After the PCB is fully populated, it is replaced by the next.

Curing of the adhesive and soldering

When affixed SMD components, the adhesive is cured by heat. This is done in a furnace which is suitable for reflow soldering. In contrast to the reflow soldering, the curing of the adhesive at a lower temperature. After curing, the modules must be wave soldered. However, this happens after the THT assembly, provided that such THT components are also available.

The assembly is provided for reflow soldering, it is brought to the required process temperature in an appropriate reflow. The solder balls in the solder paste to melt and thereby provide both a mechanical and electrical connection between the component and the circuit board.

Typical errors

Some of the following errors only occur during reflow soldering, others only when flow soldering. In addition to classic solder problems, such as non-wetting and solder bridges, which also occur in through -hole components, are to be mentioned here:

In addition to unfavorable Lötparametern to long storage of components is a cause if the connection surfaces ( different) are oxidized. Already desoldered SMD components are particularly prone to the grave stone effect and are not suitable for reflow.

For slow-moving or become wet components can be described by so-called " baking" at about 110 ° C in 24 hours again dry inside, so they are suitable for loading or destructive desoldering.

By appropriate placement of the components already in the PCB design, the problem is avoided. The design also specifies the direction of passage determined by the soldering.