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FIGURE 40.29 Schematic diagram illustrating the available process window for the Sn-Ag-Cu Pb-free solders (having melting temperatures of 217 C) referencing the typical reflow process time-temperature profile.The 230 C mark is the minimum process temperature to ensure adequate solderability. Temperatures exceeding 245 C increase the likelihood of thermal damage to larger plastic-molded area-array and flat-package devices. Temperatures above 260 C increase the chances of thermal damage to smaller passive devices (e.g., chip resistors, capacitors, inductors, and filters) and the circuit board structures (e.g., vias) and laminate material.
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This point is illustrated by the schematic diagram in Fig. 40.29, which pertains primarily to the reflow process and typical time-temperature profiles. A minimum temperature of 230 C is required to ensure adequate wetting and spreading by the Pb-free solder on circuit board pads as well as on component leads and terminations. Temperatures exceeding 245 C increase the likelihood of thermal damage to larger, plasticmolded packages (e.g., BGA and QFP devices). When temperatures exceed 260 C, there is the potential for thermal degradation of passive chip components (chip capacitors, inductors, or filters) as well as to circuit board structures (e.g., vias) and laminate materials. In the case of Sn-Pb eutectic solder that melts at 183 C, the minimum process temperature of 215 C ensures adequate solderability, which is 15 C lower than that of a nominal Pb-free process. The available Sn-Pb process window is 215 C to 260 C, or a T equal to 45 C, rather than the T of 30 C for the Sn-Ag-Cu Pb-free solders (Tmelt = 217 C). The important consequence of the smaller process window is that, as the mix of components increases on the circuit board, the likelihood increases that the solder joints on some of the larger components will not reach the minimum 230 C required for good solderability when the maximum circuit board temperature is required to be less than 260 C. Similar process window scenarios can be evaluated for wave soldering and manual processes. For example, in the latter case of manual soldering, the operator is unable to use the same soldering tip or iron to assemble all of the components. On the other hand, a soldering iron that is found to be effective for soldering large components may be too hot and/or too large for smaller devices, causing thermal and/or physical damage, respectively, to them. The second premise of process control is the repeatability of making an acceptable solder joint on each assembled circuit board. With the exception of a major equipment failure that alters the time-temperature profile, the capability to solder the same joint repeatedly in an acceptable manner is more a function of the consistent solderability of the material set s properties. Those solderability properties include the efficacy of the flux, the properties of the solder metal (e.g., surface oxidation and particular to the case of solder paste, the solder powder particle size, and metal loading), and the solderability of the component I/O and associated circuit board feature. Moreover, the solderability of component I/Os and circuit board features is often identified as the primary factor in repeatedly making acceptable interconnections. Unfortunately, because poor solderability is often the leading cause of poor reproducibility of acceptable solder joints, this does not bode well for Pb-free solders. Intrinsically, the Pbfree solders generally show poorer solderability than the eutectic Sn-Pb composition.
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Therefore, to minimize assembly (solderability) defects, process engineers must be particularly vigilant to ensure that the surfaces of component I/Os and circuit board pads have optimum solderability.
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Cleaning The use of no-clean and low-solids fluxes has reduced the need to clean a greater number of circuit boards. Nevertheless, high-reliability electronics, as well as those used in harsh environments, may still require the removal of flux residues. The primary metric used to control the cleaning process is the quantity of residues that remain on the circuit board or components. Visual inspection (white light or ultraviolet light) and ionic testing are the two most commonly used methods to assess a cleaning step. In addition, in-circuit testing (ICT) provides a secondary means to indicate excessive residus presence. The residues prevent the electrical probe from reaching the conductor, which is registered as an open in the data collection routine. Unexpectedly large numbers of open defects may indicate excessive flux residues on the circuit boards as well as signify an actual electrical performance defect. Also, in the case of high-frequency circuit boards (radio frequency and microwave), ICT can identify flux residues as indicated by signal leakage and other parasitics when inspecting for electrical performance. The cleaning step must be carefully considered for a Pb-free soldering process. The higher processing temperatures can increase the tenacity of residues, particularly those of fluxes designed for Sn-Pb processes. This point addresses fluxes used separately, as in manual or wave soldering, as well as the flux in solder pastes. The cleaning step may need to be enhanced to ensure that these residues are removed satisfactorily from the circuit board surface. On the other hand, flux formulations that have been developed specifically for the Pb-free soldering processes may create residues that are not compatible with cleaning processed developed specifically for Sn-Pb technology.
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Network Communications The generation, transmittal, and storage of process information is critical for real-time control of a circuit board assembly process, whether that process is for building consumer electronics in high volume or involves a low-volume assembly line for high-reliability military, space, or satellite electronics. The information that is generated by operators or automated machines on the shop floor typically includes commodity usage (i.e., circuit boards, components, etc.) as well as defects and machine malfunctions. Commodity usage information can then be made available to inventory control in the plant stockroom or even to the bill-of-material (BOM) suppliers across town and around the world, identifying the need for additional components, flux, and such at that particular site. Process information is also accessed by the manufacturing engineer, who is monitoring the overall assembly flow, as well as by the process engineers or technicians who are directly responsible for equipment operations (e.g., furnace zone temperatures, flux levels, etc.). In addition, information may be shared between the different machines on an assembly line to prevent slowdowns in process flow due to equipment outage. Such slowdowns and stoppages can impact upstream steps. For example, there can be shelf-life issues with solder paste that is left out in a printer too long due to a stoppage created at a subsequent component placement machine or reflow oven. It is often necessary to archive process-related information. Archived data are used to track defects as a means of process control. These data can assist in quickly determining the root cause of an out-of-control process. Process information can also be used to monitor long-term equipment performance, indicating the need for periodic preventive maintenance or machine replacement. Easy access to historical process information can also facilitate the reintroduction of a particular product line with a minimum investment in process development efforts.
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