Odd form components

Hand soldering may be achieved in an assembly (or slide ) line process for larger volumes. In this case, each one of several operators solders only a few of the total components of the circuit board. In the case of single-person work cells, the operator may solder all of the components to the circuit board. Alternatively, the work cell may be used to add odd-form components as the final assembly step on a nearly complete circuit board. 

Odd-Form Components. The odd-form cximponents are the last devices to be assembled on the circuit board. Odd-form components, by definition, are those packages that are not readily addressed by automated assembly because (a) they are used at insufficient volumes on the product to justify machine space or (b) they are of a geometry (shape or size) that lacks the customer demand to justify the machine manufacturer to provide off-the-shelf tooling for them. 

FIGURE 40.13 Odd-form components as exemplified by surface-mount inductors. (Courtesy of Sandia National Laboratories.)... 

A number of assembly-related issues must be addressed with odd-form components. First, it is necessary that correct pad dimensions be designed on the circuit board. Also, the stencil must have the correct aperture size to print an adequate quantity of solder paste. The pick-and-place machine may require custom tooling in order to handle these components. Lastly, odd-form parts are typically larger and heavier. Therefore, it is possible that they wiU not readily self-align while the solder is molten during the reflow process. 

Part range Part capacity Placement speed Capabilities Full SMT (SOD, SOT, SOIC, PLCC, CCGA, BGA), direct-chip attach (DCA), and odd-form components. Several hundreds 5,000-15,000 CPH 1. Oversized and odd-form parts placed with very high accuracy. 2. Moving gantry, fixed PCB and feeders. 3. Flexibility of using tape, tube, bulk, and tray part supplies. 

Therefore, in the case of some very-high-mix products, it may be necessary to solder larger packages in a separate operation (e.g., hand soldering or selective soldering). Under this circumstance, the latter packages would be considered as odd-form components. 

Temperature sensitivity of devices will have to be documented in terms of surviving the Pb-free reflow profile. In fact, some devices that were previously processed on a Sn-Pb assembly line may have to be set aside as odd-form components because they cannot be exposed to the Pb-free process temperatures. Another risk of the higher-processing Pb-free reflow temperatures is the damage to moistirre-sensitive components. The typical Pb-free time-temperature profile adds one or two MSLs to a component.Iherefore, devices at MSL levels 5 or 6 per a Sn-Pb profile may have to be assembled to the circuit board in a separate operation to avoid damage, thus causing them to be odd-form components. 

Connectors comprise approximately 75 to 80 percent of odd-form components. See Fig. 40.6. For example, there are universal serial bus (USB) connectors, dual inline memory module (DIMM) connectors, phone jacks, and zero-insertion-force (ZIF) connectors, as well as a variety of surface-mount connectors. Other odd-form components include sockets, electrolytic capacitors, transformers or large inductors, LEDs, relays, switches, as well as speakers and vibrators for handheld communications devices. In surface-mount technology, odd-form components are most often through-hole packages, including both passive and active devices as well as connectors. When addressing such odd-form components, the first step is to check the availability of an equivalent surface-mount version since there has been a shift toward surface-mount versions of many of these devices. Of course, the materials used to construct the surface-mount replacement must survive the reflow furnace environment, and especially the Pb-free reflow temperatures, particularly connectors because of their complex internal construction. In some cases, it has been possible to replace the through-hole connector with one of a variety of solderless products. 

In the least complicated format, only an operator places the odd-form components on the circuit board. The soldering process is automated in that the operator picks components from bulk bins and repeatedly positions them onto the circuit board as it is carried along the conveyor Une. For paste-in-hole assembly, the solder paste and other components (surface-mount or through-hole) may already be on the board, being held in place only by the tackiness of the paste. Therefore, the operator must not inadvertently displace other components off of then-sites during handling. 

A large number of applications require the operator to place and then manually solder the odd-form component into place, often after the other components have been soldered into place. The operator must be cognizant of handling and ESD damage as well as prevent thermal damage or the inadvertent reflow of solder joints for neighboring components. The impact of... 

The automated assembly of odd-form components uses the same placement equipment as is used for other components, but with special tooling or a dedicated machine (cell), which is already set up with the required tooling and component feeder systems, that can be introduced... 

There are both advantages and disadvantages to the automated assembly of odd-form components. Advantages include the following ... 

FIGURE 40.25 Odd-form component placement machine having a fiexibie servo-driven gripper tooling that can place a wide range of different component configurations. (Courtesy of Universal Instruments.)... 

Component Packaging Formats. Odd-form components come in a variety of sizes, geometries, and I/O configurations that determine the ease with which the component can be automated into the assembly process line. Another important variable is the component placement machine. Typical component packaging formats for the placement machine include tape and reel (radial and axial leaded), extruded tube, tray, continuous strip, and bulk containers. Each of these formats has benefits and drawbacks, as noted in the following subsections. Industry specifications have standardized nearly all of these packaging schemes. For example, there is EIA-468-B for radial deviees and EIA-296-E for axial components. 

Typically, automated odd-form component placement machines use an overhead gantry-... 

Because odd-form components are often larger and/or have more complex constructions (geometry and materials), there is an increased likelihood of flux residues becoming trapped in confined areas. Therefore, the cleaning process must be sufficiently thorough to remove residues from those locations on the component. Similarly, it is necessary to verify that the cleaning solution has likewise been removed from those locations as part of the process. 

It may be preferred that, on the detection of a dispensing defect, the board is cleaned of aU adhesive or solder paste and the dispensing process repeated again. However, in other cases, it may be possible to correct isolated dispensing defects, such as the manual assembly of odd-form components or when as many or more defects would be created by a redispensing step. The latter case is particularly pertinent for very dense circuit boards. 

Successful control of the placement process, whether a highly automated chip shooter step or the manual placement of odd-form components, must meet the following objectives ... 

Generally, with the increased thickness and/or complexity of the circuit board as well as with the increased mix of components, the gap widens between the process requirements of the product and the process capabilities of the equipment or operator. There are two possible solutions to this discrepancy Either a capital investment can be made to upgrade the equipment (if available) to assemble the product successfully, or the primary assembly process must be limited to those components having similar geometries and thermal characteristics. The remaining devices are then treated as odd-form components and are soldered to the circuit board using secondary processes (e.g., selective soldering or manual assembly). 

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