ASSEMBLY PROCESS:ELECTRONIC PRODUCTION

ELECTRONIC PRODUCTION

Process Chain in Electronic Production

Production of electronic devices and systems has developed into a key technology that affects almost all areas of products. In the automotive sector, for example, the number of used electronic components has increased about 200,000% during the last 20 years, today accounting for more than 30% of the total value added.

The process chain of electronics production can be divided into three process steps: solder paste application, component placement, and soldering (Figure 28). Solder paste application can be realized by different technologies. The industry mainly uses stencil printing to apply the solder paste. Solder paste is pressed on the printed circuit board (PCB) by a squeegee through form openings on a metal stencil, allowing short cycle times. Several other technologies are on the market, such as single-point dispensing, which is very flexible to changes of layout of PCBs, but leads to long cycle times due to its sequential character.

In the next step the components of surface-mounted devices (SMDs) and / or through-hole devices (THDs) are placed on the PCB. Finally the PCB passes through a soldering process. Traditionally the soldering process can be classified into two groups: reflow and wave soldering. The purpose of reflow processing is to heat the assembly to specified temperatures for a definite period of time so that the solder paste melts and realizes a mechanical and electrical connection between the compo- nents and the PCB. Today three reflow methods are used in soldering: infrared (IR), convection, and condensation. A different method of molten soldering is wave or flow soldering, which brings the solder to the board. Liquid solder is then forced through a chimney into a nozzle arrangement and returned to the reservoir. For wave soldering, the SMDs must be glued to the board one step before. The quality of the electronic devices is checked at the end of the process chain by optical and electrical inspection systems (IPC 1996).

The special challenges in electronics production result from the comparatively rapid innovation of microelectronics. The continuing trend toward further integration at the component level leads to permanently decreasing structure sizes at the board level.

Electronic Components and Substrate Materials

Two main focuses can be distinguished concerning the development of electronic components. With the continuing trend toward function enhancement in electronic assembly, both miniaturization and integration are leading to new and innovative component and package forms. In the field of passive components, the variety of packages goes down to chip size 0201. This means that resistor or ca- pacitor components with dimensions of 0.6 mm X 0.3 mm have to be processed. The advantages of

such components are obvious: smaller components lead to smaller and more concentrated assemblies. On the other hand, processing of such miniaturized parts is often a problem for small and medium- sized companies because special and cost-intensive equipment is needed (high-precision placement units with component specific feeders, nozzles, or vision systems).

Regarding highly integrated components two basic technologies should be mentioned. First, leaded components such as quad flat pack (QFP) are used within a wide lead pitch from 1.27 mm down to 300 f.Lm. But these packages with a high pin count and very small pitch are difficult to process because of their damageable pins and the processing of very small amounts of solder. Therefore, area array packages have been developed. Instead of leaded pins at all four sides of the package area, array packages use solder balls under the whole component body. This easily allows more connections within the same area or a smaller package within the same or even a larger pitch. Ball grid arrays (BGAs) or their miniaturized version, chip scale packages (CSPs), are among the other packages used in electronics production (Lau 1993).

Further miniaturization is enabled by direct assembly of bare dies onto circuit carriers. This kind of component is electrically connected by wire bonding. Other methods for direct chip attachment are flip chip and tape automated bonding. All three methods require special equipment for processing and inspection.

In addition to planar substrates, many new circuit carrier materials are being investigated. Besides MID (see Section 4), flexible circuit technology has proved to be a market driver in the field of PCB production. Today, flexible circuits can be found in nearly every type of electronic product, from simple entertainment electronics right up to the highly sophisticated electronic equipment found in space hardware. With growth expected to continue at 10–15% per year, this is one of the fastest- growing interconnection sectors and is now at close to $2 billion in sales worldwide.

However, up to now, most flexible circuit boards have been based on either polyester or polyimide. While polyester (PET) is cheaper and offers lower thermal resistance (in most cases reflow soldering with standard alloys is not possible), polyimide (PI) is favored where assemblies have to be wave or reflow soldered (with standard alloys). On the other side, the relative costs for polyimide are 10 times higher than for polyester. Therefore, a wide gap between these two dominant materials has existed for a long time, prohibiting broad use of flexible circuits for extremely cost-sensitive, high-reliability applications like automotive electronics. Current developments in the field of flexible-base materials as well as the development of alternative solder alloys seem to offer a potential solution for this dilemma.

Application of Interconnection Materials

The mounting of SMD components onto PCBs or flexible substrates demands a certain amount of interconnection material (solder or conductive adhesive) to form a correct joint. In contrast to wave soldering, where heat and material (molten solder) are provided simultaneously during the soldering process, reflow soldering of SMDs necessitates the application of solder in the form of paste as the first process step in the assembly line. Also, for interconnections established by conductive adhesive, the application of interconnection material is necessary. This process is a decisive step in electronic device assembly because as failures caused here can cause difficulties during the following process steps, such as component placement and reflow soldering. In modern high-volume assembly lines, stencil printing is the predominant method for applying solder paste to the substrates. Other methods that are used industrially to a minor extent are automatic dispensing and screen printing. Each method has its specific advantages and drawbacks, depending on, for example, batch sizes or technological boundary conditions like component pitch.

The main advantage of stencil printing over screen printing occurs in applications where very small areas of paste have to be deposited. For components with pitches equal to or smaller than 0.65 mm, the stencil printing process is the only viable way for printing solder paste. Therefore, stencil printing has replaced screen printing in most cases.

Dispensing has the advantage over screen and stencil of being highly flexible. For small batches, dispensing may be an economical alternative to printing. On the other hand, dispensing is a sequential process and therefore quite slow. Additionally, paste dispensing for fine-pitch components is limited.

The principle of a stencil printer is shown in Figure 30(a). The major components of an automatic screen printer are squeegee blades (material stainless steel or polyurethane), the screen itself, the work nest (which holds the substrate during printing), an automatic vision system for PCB alignment, and sometimes a printing inspection system. During the printing process, solder paste is pressed by a squeegee through the apertures in a sheet of metal foil (attached in frame, stencil) onto the substrate.

Dispensing is an application of solder paste in which separate small dots of paste are deposited onto the substrate sequentially. Dispensing is a good way for applying solder paste when small batches have to be produced. The main advantages are short changeover times for new boards (due to loading only a new dispensing program) and low cost (no different screens are used). Several dispensing methods are realized in industrial dispensing systems. The two dominant principles are the time– pressure dispensing method and the rotary pump method (positive displacement). The principle of dispensing by the time–pressure method is shown in Figure 30(b). A certain amount of solder paste is dispensed by moving down the piston of a filled syringe by pressure air for a certain time. The paste flows from the syringe through the needle onto the PCB pad. The amount of solder can be varied by changing the dispensing time and the air pressure.

Component Placement

Traditional THD technology has nearly been replaced by SMD technology in recent years. Only SMD technology permits cost-efficient, high-volume, high-precision mounting of miniaturized and complex components. The components are picked up with a vacuum nozzle, checked, and placed in the correct position on the PCB.

Kinematic Concepts

Within the area of SMD technology, three different concepts have found a place in electronics pro- duction (Figure 31) (Siemens AG 1999). The starting point was the pick-and-place principle. The machine takes one component from a fixed feeder table at the pick-up position, identifies and controls the component, transports it to the placement position on the fixed PCB, and sets it down. New concepts have been developed because pick-and-place machines have not been able to keep pace with the increasing demands on placement rates accompanying higher unit quantities. The original pick-and-place principle process is now only used when high accuracy is required.

The first variant is the collect-and-place principle based on a revolver head (horizontal rotational axis) on a two-axis gantry system. Several components are collected within one placement circle and the positioning time per component is reduced. Additionally, operations such as component centering are carried out while the nozzle at the bottom (pick-up) position is collecting the next component, and the centering does not influence the placement time.

The highest placement rate per module can be obtained with the carousel principle. The most widely used version of this principle uses a movable changeover feeder table and a moving PCB. It is analogous to the revolver principle in that the testing and controlling stations are arranged around the carousel head and the cycle time primarily depends on the collecting and setting time of the components.

High-performance placement systems are available for the highest placement rates, especially for telecommunications products. These systems take advantage of several individual parallel pick-and- place systems. A placement rate of up to 140,000 components per hour can be attained.

Classification of Placement Systems

The classification and benchmarking of placement machines depend on different influence coeffi- cients. In addition to accuracy and placement rate, placement rate per investment, placement rate per needed area, operation, maintenance, and flexibility have to be considered. In the future, flexible

machines with changeable placement heads will be configurable to the current mounted component mix within a short time. The whole line can thus be optimized to the currently produced board in order to achieve higher throughput and reduce the placement costs.

Component and PCB Feeding

An important factor for the throughput and the availability of a placement machine is the component feeding. Depending on package forms, the user can choose between different types of packages (Figure 32). For packages with low or middle complexity, taped components are favored.

With the development of improved feeders, the use of components in bulk cases is becoming more important. Compared to tapes, bulk cases have reduced packaging volume (lower transport and

stock costs) and higher component capacity (less replenishing) and are changeable during the place- ment process. The components can be disposed with higher accuracy, and the cassette can be reused and recycled. The disadvantages of the bulk cases are that they are unsuitable for directional com- ponents and the changeover to other components takes more time and requires greater accuracy.

For feeding complex components, often in low volumes, waffle pack trays are used. Automatic exchangers, which can take up to 30 waffle packs, reduce the space requirement.

The trend towards more highly integrated components is reducing placement loads for PCBs. As a direct result, the nonproductive idle time (PCB transport) has a greater influence on the processing time and leads to increased placement costs. With the use of a double transport (Figure 32) in asynchronous mode, the transport time can be eliminated. During component placement onto the first PCB, the next PCB is transported into the placing area.

For the optimal placement rate to be achieved, high-performance placement machines have to assemble more than 300 components per board. To reach this optimal operating point of the machine with an acceptable cycle time, the machines are connected in parallel instead of in a serial structure. Parallel machines place the same components. Depending on the product, the placement rate can be raised up to 30%.

Measures to Enhance Placement Accuracy

The trend in component packaging towards miniaturization of standard components and the devel- opment of new packages for components with high pin account (fine pitch, J..BGA, flip-chip) are increasing requirements for placement accuracy.

For standard components, an accuracy of 100 J..m is sufficient. Complex components must be placed with an accuracy better than 50 J..m. For special applications, high-precision machines are used with an accuracy of 10 J..m related to a normal distribution and at a standard deviation of 4u. This means, for example, that only 60 of 1 million components will be outside a range of ±10 J..m. Due to the necessity for using a small amount of solder paste with fine-pitch components (down to 200 J..m pitch), minimal vertical bending of the component leads (about 70 J..m) causes faulty solder points and requires cost-intensive manual repairs.

Each lead is optically scanned by a laser beam during the coplanarity check after the pick-up of the component (Figure 33), and the measured data are compared with a default component-specific interval. Components outside the tolerance are automatically rejected.

Given the increasing requirements for placement accuracy, mechanical centering of PCBs in the placement process will not suffice. The accuracy needed is possible only if gray-scale CCD camera systems are used to locate the position of the PCB marks (fiducials) in order to get the position of the PCB proportional to the placement head and the twist of the PCB. Additionally, local fiducials are necessary for mounting fine-pitch components. Furthermore, vision systems are used to recognize the position of each component before placement (visual component centering) and to correct the positions. Instead of CCD cameras, some placement systems are fitted with a CCD line. The com- ponent is rotated in a laser beam and the CCD line detects the resulting shadow.

Component-specific illumination parameters are necessary for ball / bump inspection and centering of area array packages.

Direct optoelectronic scanning units are used for the positioning control of the axis to minimize positioning offsets. The glass scales have a resolution up to 1 increment per J..m. The influence of the temperature drifts of, for example, the drives can be partly compensated for.

Despite the preventive measures and the high quality of the single systems in placement machines, reproducible errors caused by manufacturing, transport, or, for example, crashes of the placement head during the process must be compensated for. Several manufacturers offer different calibration tools that make inline mapping of the machines possible. Highly accurate glass components are positioned on a calibration board (also glass) with special position marks. The fiducial camera scans the position of the component proportional to the board (resolution about 2–3 J..m). Extrapolating a correction value allows the placing accuracy of the machine to be improved.

Fine-pitch components must be placed within a close interval to guarantee sufficient contact with the solder paste but avoid deformations of the leads. Adapted driving profiles are necessary to reach the optimal positioning speed with accelerations up to 4 g, but the last millimeters of the placing process must be done under sensor control to take the positioning power down to a few newtons.

Interconnection Technology

In electronics production, two main principles of interconnection are used: soldering using metal- based alloys and adhesive bonding with electrically conductive adhesives (Rahn 1993).

Soldering is a process in which two metals, each having a relatively high melting point, are joined together by means of an alloy having a lower melting point. The molten solder material undergoes a chemical reaction with both base materials during the soldering process. To accomplish a proper solder connection, a certain temperature has to be achieved. Most solder connections in electronics are made with conventional mass soldering systems, in which many components are soldered si- multaneously onto printed circuit boards. Two different mass soldering methods are used in today’s electronics production. Wave or flow soldering is based on the principle of simultaneous supply of solder and soldering heat in one operation. The components on a PCB are moved through a wave of melted solder. In contrast, during reflow soldering process solder preforms, solid solder deposits or solder paste attached in a first operation are melted in a second step by transferred energy.

Used in more than 80% of the electronic components processed worldwide, reflow soldering technology is the predominant joining technology in electronics production. Reflow soldering works satisfactorily with surface mount technology and can keep up with the constantly growing demands for productivity and quality. Current developments such as new packages or the issue of lead-free soldering, however, are creating great challenges for reflow soldering technology.

The reflow soldering methods of electronics production can be divided according to the type of energy transfer into infrared (IR), forced convection (FC), and condensation soldering. In contrast to the other versions of energy transfer, reflow soldering using radiant heating is not bound to a medium (gas, vapor), but is subject to electromagnetic waves. For IR reflow soldering, short- to long-wave infrared emitters are used as energy sources. Although heat transfer established with this method is very efficient, it is strongly influenced by the basic physical conditions. Depending on the absorptivity, reflectivity, and transmissivity of the individual surface, large temperature differences are induced

across the electronic modules. The problem of uniform heat distribution is increased by intricate geometries, low thermal conductivity, and variable specific heat and mass properties of the individual components. Therefore, this soldering method leads to very inhomogeneous temperature distributions on complex boards.

The transfer of energy by means of forced air convection induces a more uniform heat distribution than in IR soldering. The heating up of the workpiece is determined by, in addition to the gas temperature, the mass of gas transferred per time unit, the material data of the gas medium, and by the geometry and thermal capacity of the respective PCB and its components.

Reflow soldering in a saturated vapor (vapor phase or condensation soldering) utilizes the latent heat of a condensing, saturated vapor, whose temperature corresponds to the process temperature, to heat the workpiece. Due to the phase change from vapor to liquid during condensation, the heat transfer is very rapid, resulting in very uniform heating that is relatively independent of different specific heat and mass properties or geometric influences. An overheating of even complex surfaces is impossible due to the nature of vapor phase heating.

Despite the optimization of these mass soldering methods, an increasing number of solder joints are incompatible with conventional soldering methods, so microsoldering (selective soldering) meth- ods have to be used. In the last few years there has been a steady flow of requests for selective soldering. Several systems have been designed and introduced into the market. Depending on the kind of heat transfer, different systems using radiation, convection, or conduction can be distin- guished. The most popular selective soldering in modern assembly lines seems to be fountain sol- dering, a selective process using molten solder. The process is similar to the conventional wave soldering process, with the difference that only a few joints on a PCB are soldered simultaneously.

In contrast to soldering, conductive adhesives are used for special electronic applications. Con- ductive adhesives simultaneously establish mechanical and electrical joints between PCBs and com- ponents by means of a particle-filled resin matrix. Whereas the polymer matrix is responsible for the mechanical interconnection, the filling particles (silver, palladium, or gold particles) provide the electrical contact between PCB and component. Therefore, in contrast to solder joints, conductive adhesive joints have a heterogeneous structure.

Quality Assurance in Electronics Production

Quality assurance has become a major task in electronics production. Due to the complex process chains and the huge variety of used materials, components, and process steps, the quality of the final product has to be assured not only by tests of the finished subassemblies but also by integrated inspection steps during processing. It is common knowledge that the solder application process causes about two thirds of process-related failures but is responsible for only about 20% of process and inspection costs (Lau 1997). This means that the first step of processing leads to a huge amount of potential failures but is not or cannot be inspected sufficiently. Therefore, a combined strategy for the assembly of complex products such as automotive electronic or communication devices is useful in electronics production (Figure 38). Both capable processes and intelligent inspection tools are needed.

Several optical inspection systems are available that measure, for example, shape and height of the applied solder paste or placement positions of components. These systems, based on image

processing with CCD cameras, capture 2D images for analysis. Advanced systems work with 3D images for volume analysis or with x-ray vision for visualization of hidden solder joints (as are typical for area arrays). The main problem with complex inspection strategies is inline capability and the long duration of the inspection itself. As a result, 3D inspection is used almost entirely for spot testing or in scientific labs. On the other hand, inline strategies are often combined with and linked to external systems for quality assurance. These systems are suitable for collecting, handling, and analyzing all relevant data during processing. They allow both short-term control loops within a single step and long-term coordination strategies for optimizing assembly yields. The vital advantage is the direct link between a real product and its database information.

Further tasks of quality assurance systems occur in diagnosis and quality cost analysis. Machine diagnosis is necessary to ensure and enhance machine or process capabilities. Especially, systematic errors such as constant placement offsets can only be detected and regulated by integrated diagnosis sensors. Further, telediagnosis and defect databases are possible. Both support quick and direct re- moval of problems with the assistance of the machine supplier, who remains distant. Another goal is to calculate process- and defect-related costs in electronics production. With an integrated quality cost-evaluation system, it should be possible to optimize not only quality, but costs as well within one evaluation step. This will finally lead to a process-accompanying system that focuses on tech- nically and economically attractive electronics production.