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Lead-free: AOI in High-volume Production Assemblies
December 31, 1969 |Estimated reading time: 5 minutes
In accordance with the EU Guideline 2002/95/EG, PCB assemblies must comply with a lead-free process beginning July 1, 2006. The effects of this conversion will result in minor changes to automatic optical inspection (AOI) systems. This article uses a practical example to explain some expected changes.
By Detlef Beer
Developments in electronics manufacturing can be characterized by increasing miniaturization, growing demands on quality, and high throughput rates. In the search for suitable yield-management concepts, automatic optical inspection (AOI) has manifested itself as a necessity with regard to the need for zero-defect manufacturing. Implementing a lead-free process confronts manufacturers with new challenges, causing concern with respect to additional manufacturing problems. This article examines the entire conversion to a lead-free process, particularly the introduction of lead-free 0402 components in high-volume production.
In 2001, the reflow process in high-volume production of PC hardware was converted to lead-free. From this point, only lead-free paste and lead-free bare boards were used. The conversion to lead-free components occurred in stages due to the lack of availability. Increasing miniaturization requirements in 2004 forced the widespread use of 0402 components, replacing 0603s and 0805s.
Process Conditions
Despite universal use of 0402 components, a first pass yield (FPY) of 95% had to be achieved for the PCBs, and defect detection had to be carried out in accordance with IPC Class 2. In one example, 168 components with 608 solder joints would be equivalent to a false-call rate of 65 ppm. To achieve FPY, the following conditions must be taken into consideration for defect detection: tolerance of the component length, component suppliers, placement tolerance, the global inspection library on 25 AOI systems, the global inspection library for 80 unique products, lead-free solder, varying bare-board suppliers, and the inspection quality of IPC Class 2 (Figure 1).
Figure 1. Process conditions that should be taken into consideration for defect detection.
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Lead-free Soldering: Changes
There are three identifiable lead-free influences: increased gray-scale values, altered flow behavior, and aggressive flux. The brightness of a lead-free solder joint is, on average, 2.5% higher. This is equivalent to a gray-scale increase of about five scales.1 Solder joints visually appear to have a matte and coarsely granular finish. This can be eliminated or filtered out using characteristics-based extraction (Figures 2 and 3). The somewhat sluggish flow behavior prevents sinking in or floating up in the melted paste, especially with lighter components. This means component self-centering occurs to a lesser degree. The resulting lack of effect means that light 0402 components have an increased tendency to tilt up lengthwise, resulting in components caps that are not fully visible.
Figure 2. Standard eutectic Sn62Pb36Ag2 solder paste.
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Figure 3. Lead-free (SAC) solder paste. Photo courtesy of W.C. Heraeus GmbH.
Higher reflow temperatures and more aggressive flux also lead to a situation in which lower-profile components that come into direct contact with the flux are etched, and the light reflection of component caps disappears. The influences of the altered flow behavior and aggressive flux affect R0402-type components more than C0402-type as they are lighter and have a lower profile. In the case of R0603 components, this also was less frequent. These conditions demonstrate the increased requirements expected of AOI systems when managing a global inspection library.
The Inspection Library
Negative influences of conditions surrounding the process must be minimized in several ways to measure up to the task at hand.
- AOI global inspection libraries - Adjusting the calibration characteristics on the part of the AOI manufacturer is crucial so these changes can be transferable to several cameras and lighting modules.
- AOI global inspection libraries for different products, with the possibility of performing adjustments locally - This is a necessary characteristic of AOI software.
- Placement tolerance - Regular maintenance and calibration of the feeder.
- Determining inspection quality: IPC Class 2 standards - Face-down resistors must be permissible. Tombstones and co-planarity must be detected reliably.
Regarding component-length tolerance, a variety of component suppliers, bare board suppliers, and lead-free solders should not have any direct influence. Any good AOI program should be able to cope with these influences. If the variability of these individual points could be kept constant, it would simplify AOI programming considerably. The result of this investigation concluded that with these lead-free influences, image-comparison systems could not provide suitable inspection results because good samples vary too much. A more feasible approach is the extraction of characteristics defining each individual process and component variants. These variants are separated into individually distinct classes. Should a new variant be introduced into the currently running process, it is appended as an additional class to ensure inspection accuracy. All recognized and known defects are stored, and their types or images can be applied to AOI systems and inspection programs located within the global library. It is not necessary to archive PCBs with distinctive defects for use as a counter check.
Genuine Defect Verification of AOI Software
An integrated verification function in AOI software makes it possible to reduce false calls while ensuring a zero-defect capability of inspection programs. It is used to check stored bad samples, for example, by the repair station and empty, soldered PCBs. The reason to spend time on this during the optimization phase is to prevent any defects from slipping through. All known defects must be detected and, at the same time, maintain a minimum number of permissible false calls. When adapting any program aimed at reducing false calls, a check must be made to see if any previously genuine defects were detected and validated at the repair station. With integrated verification, it is possible to ensure the quality of inspection programs for proprietary manufacturing and audits while keeping track of false calls.
Lead-free and the Inspection Process
Adaptive procedures found no influence of the conversion to lead-free on inspection results of solder joint quality. Defects continue to look the same. It is clear that minor library adaptations are sufficient to exclude influences that might create additional false calls. In the case of component-cap changes, extensive threshold-determination procedures are required. These can be integrated into a standard library. With tilted components, an additional linked inspection can be activated so a reliable analysis is performed. The widespread opinion with increased formation of bridges or higher incidences of tombstones also was not proven to occur frequently. Experience shows that bridge formation remains constant, and tombstone formation has decreased.1 Converting to lead-free solder does not require an investment in new systems or equipment as long as the AOI system involved is equipped with flexible sensor modules, lighting, and software to adapt to these changes.
REFERENCES
- “AOI in the Lead-free Age,” Detlef Beer. For more information, e-mail: angs@viscom.de.
Detlef Beer, head of applications, SP division, Viscom AG, may be contacted at +49-511-94996-689; e-mail: detlef.beer@viscom.de.