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Completely Inspecting New Packages
December 31, 1969 |Estimated reading time: 7 minutes
By Carsten Salewski, Viscom
In Summary
This article addresses questions about the optimal inspection solutions for new packaging technologies. Where in the production process is automated optical inspection (AOI) best applied? Which components require automatic X-ray inspection (AXI)? What hardware and software requirements must be met?
The latest packaging technologies, as well as higher quality in electronic components, require high-performance and flexible inspection solutions. AOI and AXI not only assure defect-free products, but also secure a competitive edge for the manufacturer or EMS provider. Intelligent inspection strategies can prevent the drift of production to Asia, and sometimes regain lost orders.
AOI makes a significant contribution wherever productivity and quality are required to be at highest possible levels. Whether it involves inspecting visible solder joints, checking component position and presence, void formation in the case of wave soldering in open solder joints, or inspection of rounded solder menisci, the AOI application spectrum is multifaceted and covers the majority of situations. AXI, on the other hand, becomes necessary in the inspection of hidden solder joints, as with BGAs, quad flat pack no lead packages (QFNs), etc., and in void recognition on flat-soldered components.
Packaging Trends
As electronics constantly penetrate deeper into all spheres of life, electronic assemblies get smaller and assume wider functions. This increases population density on PCBs, which, along with shrinking components and rising performance requirements, means more connection leads and smaller pitches. Thus, component types such as standard chipscale packages (CSPs) have been developed into CSPs with a 0.4-mm raster; small quad flat packs (QFPs) often are replaced by QFNs; CSP memory components are stacked one atop another as package-on-package (PoP) in mobile phones and flash drives.
Increasing BGA use, where connecting joints simply are not visible from above, makes X-ray analysis indispensable. Coverage on such components is limited to presence and placement accuracy; only X-ray inspection can guarantee a reliable quality inspection of the solder joints. Assembly on both sides of the board sometimes leads to concealment and shadowing of the solder joint, requiring 3D X-ray inspection.
In principle, assembly houses can accommodate all these developments in components. One remaining challenge, however, is to reach peak quality consistently without slowing down production speed. A succession of new potential defects, all of which must be mastered in the production process, accompanies these new components. Frequent defects include open solder joints, short circuits, displacement, insufficient solder, and entrapped air.
Intelligent AOI and AXI Use
PCB production and assembly is a process of utmost complexity, with numerous parameters that influence final product quality. These include board layout, paste thickness, temperatures in the solder oven, and more. All these influencing factors generate costs, and reducing these costs is crucial to a manufacturer’s competitiveness. A poorly produced assembly increases rework costs. Two essential inspection processes AOI and AXI can help prevent rework. These accomplish 100% inspection of the PCB, ideally. At what point in the production process is their deployment most beneficial?
High-precision AOI is one of the most important modern processes to attain quality; modern automated production is inconceivable in its absence. The challenges of electronics manufacturing frequent product changes, tiny components, and short inspection times could not be managed without it. Today’s AOI systems provide high performance and are implemented at various places in the production process after paste print, placement, and soldering according to the assembler’s needs. A post-reflow AOI system, however, covers 90% of all typically occurring defect types and contributes most significantly to cost reductions. Prompt, reliable information about production quality provides process indicators that allow manufacturers to respond in sufficient time and prevent costly repair work.
Purely optical inspection is difficult when dense component placement blocks optical and electrical access. Gullwing or J-lead solder joints such as those on QFPs and PLCCs can be inspected with AOI because they are located outside the component. Here, reliable inspection of lifted leads (Figure 1) is possible with an oblique viewing angle.
Figure 1. Packages with lifted leads are detected via oblique-angle AOI.
When comparatively tall components are packed closely on the board, shadowing or concealment blocks angled optical inspection. In this case, a 100% inspection strategy needs to be based on X-ray inspection, which, due to the principle of irradiation, delivers essential information about non-visible solder joints. What all this means is that X-ray inspection makes sense in cases where AOI runs up against its limits. However, it must be kept in mind that X-ray inspection usually is slower and more expensive than AOI, and therefore the following rule of thumb applies: Optical inspection should be used wherever defects can be optically recognized. Only where there is no other way to spot defects does X-ray inspection make sense. Moreover, AOI can recognize defect features such as mechanical damage, displacement, and reversed polarity, which remain hidden from X-ray inspection.
Efficient deployment of various test systems helps keep costs low while achieving the best possible inspection coverage. Another method to be considered at this point is in-circuit test (ICT). Due to higher placement density, covering all test sites has become more difficult for ICT. Lot size also plays a crucial role. Small lot sizes require ongoing and costly investment in new adapters. An AOI system, in contrast, can be adapted quickly and flexibly with a justifiable personnel effort. Another criterion is process control. Because ICT and functional test (FT) usually are situated at the end of the production line, ongoing process control is no longer possible, generally. AOI also is an advantage here.
AOI Requirements
The fundamental core of an AOI system is its cameras (Figure 2). They determine throughput and resolution. Modern AOI systems’ camera technology must guarantee sufficient inspection depth, even under extreme cycle-time demands.
Figure 2. The core of AOI inspection comes from a system’s cameras.
The field of view (FOV) is the observable area of the PCB that is acquired by one camera or an array of cameras in one position. The number of pixels used to digitize the image of the FOV determines the pixel resolution, one of the most important features of any AOI system. A higher resolution makes more details visible and thus enables higher inspection depth. A resolution of 25 µm/pixel can reliably inspect standard components down to 0402. For optical inspection of today’s smaller components, such as 01005s or fine-pitch components down to 0.3 mm or less, a higher resolution of about 12 µm/pixel is required.
Higher resolutions can be achieved with a more powerful lens; this results in a smaller FOV and leads to longer inspection times, since the camera needs to stop in more positions to cover the same PCB area.
Using a camera with a higher pixel count while maintaining the same FOV can require longer exposure times, due to reduced light sensitivity of the camera chip, and often leads to longer image-upload times.
Modern AOI cameras feature resolutions in the tens of microns/pixel, allowing users to inspect 01005 components. On-demand resolution allows users to switch from standard to high resolutions and back, while maintaining the same FOV. This allows reliable inspection of standard and fine-pitch components without investment in more cameras. Camera chips have both low- and high-resolution capability to enable this close analysis.
The latest AOI camera technology offers the option of color evaluation. Here, additional defect features such as copper exposure, e.g., non-wetted copper pads, are recognizable (Figure 3).
Figure 3. Color analysis shows copper exposure.
An important question for any AOI operator is how to ensure quality of an inspection program. The answer is integrated defect verification, which simplifies reducing false calls while ensuring virtually zero defect escapes. Verifying each parameter change against all real defects detected by AOI and previously validated at the repair station allows quality of the inspection program to be confirmed at any time, whether for production needs or for documentation during customer audits.
Statistical process control (SPC) continuously accepts processed data from the AOI system and displays deviations in paste print, component placement, and soldering. With this tool, changes in the production process can be determined promptly, serial defects prevented, and the process adapted virtually in real time. It contributes to optimized product quality and improves process performance.
Conclusion
New packaging technologies present inspection equipment with new challenges. These are not insurmountable hurdles. Those who recognize them in time and act correspondingly can position themselves more strongly in the market and remain competitive or even gain a step ahead of other PCB assemblers.
High-performance post-reflow solder joint inspection with AOI helps maintain quality in electronics production at reliable and cost-effective metrics. Where hidden defects occur, X-ray inspection is a sensible addition that contributes to further process optimization.
Carsten Salewski, CEO, Viscom Inc., may be contacted at Carsten.Salewski@viscomUSA.com.