Solder paste inspection

THREE-DIMENSIONAL AUTOMATED SOLDER PASTE INSPECTION... 

System Throughput. The test speed for solder paste inspection systems varies anywhere from 2 to 22 cmVsec. The high end of this speed range can be fast enongh to keep... 

Automated optical inspection systems also image only a small portion, or view, of the assembly at a time. These systems normally can use somewhat bigger views than the 3-D solder paste systems because the features being extracted often do not require as much magnification as do measurements of solder paste depositions. However, inspection of components, such as 0402, 0201, and 01005 passive components, or very-fme-pitch deposits, can require the same level of magnification and therefore views as small as the 3-D solder paste inspection systems. 

The last general, board-independent issue is the effectiveness of different test and inspection systems at finding different types of defects. It is recommended that test effectiveness studies are performed with some regularity. A test effectiveness study is performed typically on a small set of boards (about 20 to 100 boards). All defects are categorized into true defects and false calls process indicators can also be identified. If solder paste inspection (SPI) and AOI prereflow are included in the study, potential defects will also be identified. During the test-effectiveness study, no defects should be repaired until the last test/inspection has had a chance to detect all defects. 

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... 

The solder paste volume calculation consists of two parts, (1) solder fill for the through-hole and (2) solder for fillet inspection. The second part may or may not be necessary, depending on whether the solder joints will undergo visual inspection or not. Lack of necessity for fillets reduces the amount of solder paste that must be deposited. This makes the deposition easier. The solder fill calculation is straightforward (volume of the through hole) — (volume of the part lead). Consideration must then be... 

The primary objective in each of the five dispensing techniques is to deposit consistently a specific quantity of adhesive or solder paste at each designated site. Too small of a quantity of adhesive, especially dot height, can fail to attach the part to the board. Too much adhesive causes it to run on to the solder pads, degrading solderability. In the case of solder paste, an insufficient quantity of paste will cause an incomplete joint or, in the worst case, an open circuit. An excess of solder paste results in a fillet that is difficult to inspect for solderability or risks formation of short circuits between neighboring interconnections. 

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. 

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. 

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... 

Automated inspection systems generate images of the item to be inspected (normally solder paste, components, or solder joints), digitally analyze the image to locate and measure key features, and, based on these measurements, automatically decide whether a defect exists or not. 

Inspection systems normally are dedicated to one type of measurement capabihty solder paste, pre-reflow, or post-reflow inspection. For example, systems for solder paste measurements do not normally also make component placement measurements. The cost of combining different measurement capabilities into one system would typically make that system prohibitively expensive. More importantly, to reduce manufacturing costs, manufacturers want to implement linear, sequential production lines where an assembly always flows in one direction and goes through each machine only once per assembly side. So automated inspection systems fall into three major categories ... 

For all three types, the automated inspection system compares the measurements taken against a specified conformance range to accept or reject autonaaticaUy a solder paste brick, component placement, or solder joint as being within specification. 

FIGURE 53.9 Schematic of automated inspection system for solder paste measurement. The camera or LED sensor obtains images with discontinuities in the laser line scan the image-processing software finds these discontinuities, measures them, and calibrates them to real physical dimensions. 

Automated 3-D inspection of solder paste depositions has the following major advantages ... 

In general, pre-reflow AOI systems are typically two to three times faster than 3-D systems and range in inspection speed between 10 and 40cm /sec. The prices of 2-D component and solder paste placement automated inspection systems are typically somewhat less than those of the 3-D solder paste systems with the fastest inspection speed capabihty. 

Pre-reflow AOI systems also have the ability to inspect 2-D solder paste however, this ability is utihzed only to inspect a small percentage of the solder paste deposits combined with the component misalignment measurements. Component misalignment measurements cover the passive components, whereas the solder paste measurements cover deposits for BGA, CSP, or fine-pitch QFP devices. Therefore, these systems are placed within production lines after the pick-and-place systems for passive devices but before the pick-and-place systems for the larger area-array and leaded devices. These systems serve the same purpose as those only meant for component placement measurement, both detecting defects and monitoring measurements within control limits to discover process drift as early as possible. 

In this test, multiple, stainless steel stencils and PCBs were prepared with the latest lead-free solder pastes available. After an hour of drying time, each stencil, in conjunction with a misprinted board, was cleaned at room temperature in spray-in-air equipment for three to six minutes (Table 2). Subsequently, cleaning tests were repeated at room temperature in ultrasonic equipment. Test substrates cleaned were visually inspected under a microscope (lOx) and tested for solder-paste residues. It was shown that all tested cleaning agents removed all lead-free solder pastes (Table 3). Differences were observed in the cleaning times for the different cleaning applications. 

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]... 

Using a test vehicle which included QFPs with pitches ranging from 0.3 to 0.5 mm, and CSPs with pitches ranging from 0.4 to 0.6 mm, a study was conducted to understand the manufacturing impact of three no-clean, lead-free solders from the Sn-Ag-Cu family compared to a standard no-clean eutectic Sn-Pb solder paste. The evaluation focused on printability, solder paste pot-life, wettability, reflow process window, and inspection [33]. 

The central issues were to develop pastes for lead-free soldering and to establish the time-temperature process window that would enable the production of joints that pass visual inspection and exhibit metallurgical integrity. Various solder pastes were developed and assessed. Only one simple daisy-chained board was designed and used as the test vehicle (described in Section 7.4.5) for all reflow trials. A systematic process parameter study was performed, but large-scale process verification with the preferred parameter set was not performed. 

This standard defines the classification of soldering materials through specifications of test methods and inspection criteria. These materials include liquid flux, paste flux, solderpaste... 

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