Assembly inspection Solder joint

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. 

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... 

Visual inspection is the visnal comparison by a human of some attribute of the printed circuit assembly with specified standards that describe the acceptable range for that attribute. The inspector normally picks np the printed circuit assembly or places it under a microscope and carefully observes particular attributes, such as the condition or bend radius of component leads or the wetting of solder joints to a lead. Visual inspection always involves human judgment in comparing the attribnte to its conditions of conformance to standards. 

Visual inspection often occurs at the end of the assembly process or even after all of the electrical tests on the assembly have been completed. It is not possible, however, for invisible or hidden joints. The visual inspector looks for scratches, partial delaminations, solder splashes or solder balls away from solder joints, and any other condition that does not affect assembly performance but does make the assembly look as though it might not be a high-quality product. 

Repeatability Limitations. Several studies have documented the low repeatability rate of visual inspection of solder joints. One snch stndy was condncted by AT T at its Federal Systems Division. This study showed that even the same inspector inspecting the same assembly twice had a defect call repeatability rate of only about 50 percent. Two different inspectors inspecting the same assembly had a defect call repeatability rate of only about 28 percent. This study did not include any very-fine-pitch SMT solder joints or 0603 passive components, which are more difficult to inspect visually. 

Test throughput is not always fast enough to allow inspection of all solder joints within the manufacturing cycle time for the printed circuit assembly. 

Application. Cross-sectional x-ray automated inspection systems work well for all types of printed circuit assemblies, including single-sided and double-sided, surface-mount, through-hole, and mixed-technology assemblies. These systems accurately detect the same solder joint and component defects as do transmission x-ray systems, but, in addition, the cross-sectional x-ray systems accurately detect insufficient solder conditions for BGA and pin-through-hole solder joints. 

The inspection system s ability to identify, measure, and analyze defect data after assembly is also critical. Inspecfion of BGA solder joint integrity is important but cannot be effectively achieved by visual inspecfion because of hidden solder joints. For these types of packages, in addition to process control during manufacturing, nondestructive techniques such as x-ray are needed to determine the integrity of an attachment... 

Examples of optical and SEM inspection of tin-lead and lead-free solder joints are presented before and after exposure to environmental conditions. Assemblies include both conventional leaded and leadless packages, as well as CSP leadless packages. 

The 2D x-ray systems are very effective in testing single-sided assemblies. With the use of a sample manipulator, an oblique view angle enhances inspection of both single- and doublesided assemblies with some loss of magnification due to increase in distance between source and detector. Experience is needed in discerning between bottom-side board elements and actual solder and component defects. This can be very difficult or impossible on extremely dense assemblies. As discussed previously, certain solder-related defects such as voids, misalignments, solder shorts, etc. are easily identified by transmission systems. However, even an experienced operator can miss other anomalies such as insufficient solder, apparent open connections, and cold solder joints. 

Visual inspection, refined over the years to identify defects and faults that have been correlated to mechanical property behavior and reliability in the field, is a very important aspect of the electronic assembly process. Accordingly, a set of visual inspection criteria has been identified and generally practiced across the industry for eutectic Sn-Pb solder joints. However, the mechanical properties, visual appearance, and solder joint geometry of lead-free solders, in combination with lead-free and lead-containing terminations, are markedly different from those of PbASn, as listed in Table 28. Therefore, new visual inspection guidelines must be developed [19]. 

A disadvantage of the BGA package is that solder connections cannot be visually inspected and removed parts cannot be reused without the parts being re-baUed. A major manufacturer however, reports a joint failure rate of only 6 ppm. Problems generally occur only from defective components, bad boards, setup errors, or worn/damaged machines in the assembly hne. 

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. 

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