Assembly process Automated soldering

In the case of automated assembly processes, the general sequence is part insertion followed by the soldering step. (Slightly different steps are used for the paste-in-hole process that is, the paste may be applied before or after part insertion.)... 

Of course, various levels of automation may be considered, depending on production volumes and capital expenditure costs. However, with continued miniaturization of surface-mount products as well as the strict requirements for the repeatable placement of specific solder paste volumes and accurate placement of components, will be a foregone necessity to assemble surface-mount technology with a fuUy automated assembly process. 

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. 

As noted previously, placement defects can be identified in the machine by automated inspection. Alternatively, such defects may be detected by a separate machine inspection station or through visual inspection. A trade-off must be made between the number of parts inspected and detail of that inspection on the one hand, and the assembly process flow on the other. The more information that must be processed by the inspection step, the slower the placement process. Finally, every attempt is made to correct placement defects prior to the soldering step. 

After placement of the component, the soldering process is performed to make the joints. Irrespective of the exact method whether it is hot gas or a soldering iron, or the extent of automation the overall objective should be that the time-temperature profile resemble as closely as possible the process used to make the original solder joints. The solder paste is the same as that used in the initial assembly process. As a result, each step, from the preheat stage... 

This chapter covers why manufacturers inspect printed circuit assemblies, how they have implemented and enhanced visual inspection, what automated inspection systems they are using, and how they have implemented these antomated systems. The scope of this chapter includes only inspection of printed circuit assemblies during the assembly process, as typically shown in Fig. 53.1. Thus, it includes inspection of solder paste after the paste printing process step, components after the component placement process step, and solder joints after the solder reflow process step. Not included, however, is incoming inspection of components and the bare printed circuit board (PCB). The focus of this chapter is on prodnction nse of inspection, not the collection of measurements dnring process development in a research and development (R D) environment. 

SPC requires reliable data that can be analyzed either in real time or historically. Visual inspection collects defect data, such as the number of solder joint defects per assembly right after the solder reflow process (either reflow or wave soldering). Some manual and automated inspection techniques also take quantitative measurements of key assembly parameters, such as solder paste volume or solder joint fillet height. To the extent that these data are repeatable, manufacturers use defect data or measurements to characterize the amount of process variation from assembly to assembly or from solder joint to solder joint. When the amount of variation starts to drift outside its normal range or outside its control limits, manufacturers can assess the assembly process and monitor or choose to take action until the process is adjusted to eliminate this drift. Historical analysis of the defect or measurement... 

During PCB assembly and transportation, forces imposed on a component can cause the device to move from its original position on a PCB. Solder paste and/or flux is used to retain components in place during the assembly and transportation of the PCBs. The tackiness required of a flux or paste depends on the assembly process utilized. In-line automated assembly requires less retention force than manual batch assembly. Additionally, placement equipment utilizing table movement rather than head movement requires much greater retention forces due to high accelerations imposed on the PCBs and components. The retention force can be calculated by simply multiplying the acceleration imposed on a component by its mass. A tack test can be performed on the solder paste or flux as described in section 2.3.5 of this chapter to determine its retention capability. 

Surface-mount technology replaces previous methods of inserting component leads into plated through-holes of PWBs, wave soldering from the back side to flow solder into the holes, and simultaneously forming both mechanical and electrical connections. SMT is highly automated and currently the most widely used production process for the assembly of single-layer, double-sided, and multilayer circuit boards. 

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. 

Dispensing defects can be detected by visual inspection or automated inspection techniques. Such automated techniques include those based on visible light images as well as laser profilometry that determines the actual volume of the adhesive or solder paste deposit. However, inspection slows the process assembly line. The more joints that are selected for inspection (that is, not aU joints need to be inspected) and the greater the information detail required from of the inspection results (referring to the height profilometry data collection), the longer the delay in the process flow. 

Lead-free solder has a rougher surface finish and generates a different-shaped fillet. It also is more prone to voids and tombstoning. These and other deviations can require adjustments to commonly used inspection techniques, such as automated optical inspection (AOI). While the results of a National Physical Laboratory (NPL) study confirm that AOI systems can be used to inspect lead-free surface mount assemblies, many defects created by lead-free processes are not visible. The added loss of visual and electrical access due to the growing complexity of PCBAs compounds the problem. 

Implementation of the MID application in series production necessitated the development of a fully automated assembly solution by a manufacturer of special-purpose machines. The 3D placement of the SMDs, the switch elements, and the contact pins is only one of the functions discharged by the system. Others include incoming-goods inspection, electrical testing of the conductor tracks, dispensing the solder paste with optical process monitoring, and final inspection. [116]... 

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