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SMT & Inspection | July 01, 2010

Integrated Verification enables IPC-Compliant Inspection

During manufacture of electronic assemblies, production defects are unavoidable. Systems for automatic optical inspection (AOI) have proven themselves in securing product quality.

The guidelines in IPC-A-610D are frequently drawn upon to evaluate and classify potential production defects. When these guidelines are used on an AOI system, several aspects need to be considered in order to assure IPC-compliant inspection. Production defects during the automated assembly and soldering of electronic assemblies are unavoidable. AOI systems are deployed more and more to assure these defects are detected and to further optimize the process. Frequently, these systems are situated after the solder oven for post-reflow inspection, because this inspection stage presents the greatest potential for improving quality. In addition to AOI defect rates (pseudo-defects and slip), recurring themes such as inspection depth, or inspection features and their definition, come to the fore when acceptance criteria are specified. When specifications are described, exact definition of which defects are to be inspected and what exactly a defect is are crucial in preventing later discussion. Overview: IPC-A-610D IPC-A-610D (IPC), as stated above, is a "collection of visual quality acceptability requirements for electronic assemblies." It subdivides assemblies into three classes, each with its own specific criteria (IPC, 1.4.1 Classification): Class 1 – General Electronic Products Class 2 – Dedicated Service Electronic Products Class 3 – High Performance Electronic Products The criteria for each class are then presented in four stages (IPC, 1.4.2 Acceptance Criteria): Target Condition (ideal, to aim for) Acceptable Condition (permissible) Defect Condition (impermissible) Process Indicator Condition The described ideal state cannot always be achieved, making the acceptable condition sufficient. Process indicators comprise a special condition. They are not actual defects, yet do not fully satisfy the acceptance criteria. If an increase in process indicators is observed, the process should be analyzed to further improve production. Illus. 1: IPC-A-610D, 8-20: End joint width/displacement, chip component (Describes the permissible boundaries of end width for Class 3: "The end width of the solder joint (C) is at least 75% of the component connection width (W) or the connection surface width (P), whichever is less.") It should be noted that the IPC-A-610D describes criteria for visual inspection and presents standards for "visual inspection" (IPC, 1.2 Purpose). "Automatic inspection technology (Automatic Optical Inspection (AOI) represents a practical alternative to visual inspection... " (IPC, 1.6 Inspection Methodology). This means, the IPC was explicitly conceived for human visual inspection. Transferability of the criteria to AOI Thus the question arises, to what extent IPC acceptance criteria for electronic assemblies can be directly transferred to AOI. The ideal prospect in many cases is to adopt the dimensions for permissible acceptance conditions displayed in the illustrations as threshold values for AOI. Insofar as an AOI records proportions on the assembly geometrically, comparison with the adopted thresholds delivers the inspection result. However, one aspect deserves consideration here. As explained above, IPC is conceived as a tool for human visual inspection ("AOI as practical alternative"); therefore the threshold values are approximately quantified, as in the 25% increments for evaluation of chip components (Table 8-2). This permits a subjective comparison of the schematic depiction with the actual assembly and the qualitative evaluation of visible features; finer gradations could hardly be implemented. As well, not all dimensions are provided with thresholds, such as in the definition of solder joint length D on cylindrical end cap connections (Melfs). Here the remark: "Visually good wetting" (8.2.3.4) is listed. Certain components, like QFN/MLFs (Table 8-13), receive an even more rudimentary description. Illus. 2: IPC-A-610D, 8-50: Solder joint length on a Melf Illus.3: IPC-A-610D, 8-51, Soldering on a Melf A further example is the handling of "combined conditions." This term refers to the simultaneous occurrence of deviations for at least two features. IPC acceptance criteria are defined individually for each single feature; the large number of possible feature combinations prevent a complete definition. When defect features occur in combination – e.g. insufficient solder on a displaced component – a defect state can be present even though the individual features are within tolerances. This can be detected by a human visual inspection, implying that the human factor is critical in implementing IPC. This discussion points out, simple transfer of available IPC threshold values to AOI is insufficient for inspection of electronic assemblies. The IPC is a tool, yet does not replace the electronic assembly manufacturer's responsibility (nor that of the AOI manufacturer!) for the quality of the finished product. Concept for applying IPC-A-610D to AOI The Viscom concept for using IPC-A-610D together with AOI optimally incorporates IPC, so it is used according to its original intentions. The basic idea is to draw upon IPC for final evaluation of the inspection results at the classification station (review of the inspection results provided by AOI), so a random sampling to verify good and bad examples for the AOI library can result. The following illustration explains this correlation. Illus. 4: Concept for applying IPC to Viscom AOI The AOI delivers inspection results in the form of image data for review at the classification station. Generally there a zero-escape policy is already in place, meaning no defect may be overlooked (1). In actual practice, escape will sometimes occur, such as when defect conditions that were not previously considered crop up. A good AOI must be in the position to provide technology and methodology for such cases, to verifiably preclude repetition of a particular escaped defect. The IPC acceptance criteria are taken into account by IPC-qualified personnel during classification of inspection results and their ultimate evaluation (2). To guarantee the requisite level of knowledge, employees should receive regular training on the basis of pre-classified images, so the quality of evaluations is attained and documented. The software evaluates classified AOI results to automatically sort the defect images. This provides a valuable verification data base containing good and bad templates (3). Based on 25 years of AOI experience, Viscom delivers a corresponding verification data base with its systems, which can then be automatically supplemented by the customer. Feedback into the AOI is provided by reliance on this IPC-compliant verification data base to assess the AOI inspection library (4, 5). Corresponding offline software is employed to automatically verify whether the current parameter settings and thresholds correctly evaluate all template examples in the data base (separation of classes). Illus. 5: Separation of classes during Integrated Verification This verification also produces explicit documents, so IPC-compliant inspection and zero defect escape can be proven during audits as well. Conclusion IPC-A-610D has established and proven itself as a valuable tool in the manual visual inspection of electronic assemblies. A flexible concept for the application of IPC to AOI, like the Integrated Verification from Viscom, is required to fully exploit the performance capabilities of modern AOIs. The assumption that available IPC thresholds need only be uploaded to the AOI and numerically compared with the geometric actual state is inadequate to support a zero-escape approach. Integrated Verification provides an image-based, classified verification data base for the manual review of inspection results, which can consider all influences and criteria for the good/bad decision and deliver optimal flexibility: in addition to IPC, individual manufacturer requirements can also be incorporated. Good software tools working automatically and offline are decisive in benefiting from this exceptionally valuable data base in connection with an IPC-compliant inspection. Without additional time expenditure, conformance of the AOI inspection library (and ultimately, the AOI itself) to IPC stipulations is attained to secure a zero-escape inspection. Of course, since the IPC also describes many criteria that are not optically visible, this concept can also be fully applied to X-ray assembly inspection. ----- Author: Peter Krippner, Vice President Assembly Inspection, Viscom AG
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