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account the other components that are already on the circuit board, particularly potential handling damage. It may be preferred to use a no-clean or low-residue flux to minimize or eliminate the added cleaning step required for the later placement of odd-form components. 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.
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Process control refers to the capability of repeatedly making product units that meet performance specifications (including long-term reliability) within an allowable frequency of repairable and nonrepairable defects. There are two underpinning premises of process control, which can be described specifically with regards to circuit board assembly: (1) The assembly process is nominally capable of making a circuit board that meets the performance specifications; (2) the equipment and materials set are capable of making those acceptable circuit boards repeatedly. The types of defects, and the frequency of their occurrence, are the metrics for establishing and monitoring the process using statistical process control (SPC). Defects can be detected by visual or machine inspection, or are identified by electrical performance of the assembly (referred to as in-circuit testing). It is beyond the scope of this chapter to provide a detailed explanation of SPC. Rather, there will be a qualitative discussion of process control that addresses those factors affecting circuit board assembly, beginning with defect types in electronic solder joints. Then the discussion will turn to process control as it pertains to the basic assembly steps (e.g., dispensing, pick-and-place, etc.). The various material sets (e.g., circuit board, solder paste, flux, etc.) will be incorporated in the discussion. Also, the use of Pb-free soldering technology will be addressed where applicable. The yield drops that result from an out-of-control process impose significant cost penalties. High production volumes can potentially generate large quantities of defective printed circuit boards before a defect trend is discovered. Therefore, it is critical, first of all, to establish process control and maintain it throughout production and, second, be able to identify product defects, determine the responsible process parameter, and quickly correct the faulty equipment, material, or operation.
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Defects Defects are defined as those artifacts that are outside the window of acceptable attributes for the finished circuit board assembly. Thus, defects are not limited to the solder joints, specifically, but can also include damage to the circuit board material as well as degradation to component structures (e.g., molding compound, leads or terminations, etc.). Defect types and their allowable frequencies (often expressed in parts-per-million solder joints or product units) vary with the different assembly processes and applications. Therefore, product drawings, in conjunction with industry standards (e.g., IPC-610), are used to establish accountable defect types for printed circuit boards. Defects are most often detected by visual inspection or automated optical inspection (AOI). Other means of nondestructive evaluation (NDE) include electrical testing, x-ray inspection, and ultrasonic inspection. The preferred NDE inspection technique for BGA and CSP solder joints is x-ray inspection. Automated x-ray inspection equipment is often placed directly into the assembly process line for circuit board products having a large number of area-array components.
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There are destructive evaluation techniques that can also be used to find defects. Metallographic cross sections provide a means to inspect the internal structures of the solder joints, components, and circuit boards, albeit in only a single plane at a time. Mechanical tests (die shear, pull tests, etc.) can also be used to identify defects. However, because these approaches are destructive, and because they require long lead times to obtain results, they are generally inapplicable for real-time process control. They are better suited for process development or failure analysis activities. Defects serve two functions for the process engineer. First, defects provide the quantitative metric with which to ascertain process control, using SPC techniques. Second, the microstructural details of the defect(s) can be used in a failure analysis study to identify the root cause of an out-of-control process. There is very little difference between the types of defects that occur during a Pb-free assembly process and those that have been documented for Sn-Pb assemblies. The two exceptions are grainy solder fillets and fillet lifting of Pb-free, through-hole interconnections. The grainy fillets resulting from the use of some Pb-free alloys are intrinsic properties of their solidification behavior. Because they do not necessarily represent a cold solder joint that is, one that is only partially melted the appropriate revisions have been made to industry specifications to account for this effect. The second phenomenon, fillet lifting, occurs on some Pb-free plated through holes. Although several studies have concluded that fillet lifting does not necessarily impact either the short-term performance or long-term reliability of these interconnections, they may still be categorized as defects, depending on the particular product requirements. Finally, the frequencies of defects on a Pb-free product may differ from those for the same Sn-Pb assembly. Of course, proper process optimization will minimize the frequency of such defects. However, slightly different solder paste printing characteristics at finer pitches, as well as overall poorer wetting and spreading behavior of Pb-free solders, may result in an optimized process that simply cannot achieve the low defect rates of a Sn-Pb process under the capabilities of the equipment set. Additional mitigation steps that can be taken by the design and process engineers include altering the stencil design (e.g., slightly wider apertures); using alternative circuit board and component I/O surface finishes that enhance Pb-free solderability (e.g., one with a Au protective finish); changing the solder alloy composition (Bi-containing solder); replacing existing equipment to provide wider process window capabilities; or taking no action and simply accepting the reduced process yields, based on an accurate cost/benefit analysis.
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Dispensing Dispensing processes include those for an adhesive, solder paste, or both (e.g., surface-mount wave or selective soldering). Because the adhesive is dispensed prior to the solder paste, it is possible to inspect for defects incorrect dot volume, run-out on nearby solder pads or lands, and stringers prior to dispensing solder paste. By and large, the greatest number of solder-joint defects can be traced to the solderdispensing process and, specifically, the stencil or screen printing step. Therefore, process control is critical here. An absence of the control of any one of these attributes can significantly impact process yields. Important factors include the following:
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Have the appropriate stencil or screen design and materials (e.g., thickness, aperture geometry, wall finish, etc.) for the circuit board product. Design considerations must also address the need to widen fine-pitch aperture openings slightly for printing Pb-free solder paste. Properly design the circuit board to provide recognizable fiducials and an absence of burrs or other artifacts that can interfere with the dispensing machine and/or stencil performance. Ensure that the stencil is free of particles and other contamination.
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