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By David Bernard, Ph.D., Dage Precision Industries, a Nordson Company
With PCB assemblies increasing in complexity, test access for ICT and functional test is reduced. Along with AOI, 2D and 3D X-ray is moving beyond hidden-joint inspection into the high-density PCBA inspection area, including ASICs, memory, thru-hole joints, and more. X-ray inspection needs vary with each application, but a combination approach with 2D and 3D systems provides comprehensive coverage.
The ongoing increase in the complexity of PCB assemblies (PCBA) has resulted in reduced test access by traditional means such as in-circuit test (ICT) and functional test (FT). Because of this, 2D and 3D X-ray inspection is used as a fundamental component of an overlapping and complementary test-and-inspection process together with automated optical inspection (AOI).
Non-visible solder joints for BGA devices have long been the traditional focus of X-ray inspection. However, the defect spectrum is now shifting to include the inspection of application specific integrated circuits (ASIC), memory devices, BGA connectors, and plated thru-hole (PTH) solder joints as the demand for increased product reliability emphasizes minimal test escapes and corresponding field failure exposure.
Much can be gained by using good quality visual inspection systems including AOI and manual visual inspection. This type of fault coverage is used by many board-level assemblers in their drive to find the optimum test strategy with continuous feedback of inspection data considered by many as the "holy grail" of process improvement and refinement.1 AOI systems have undergone considerable development, many of them now using 3D laser or camera systems and offering lower false-call rates than previously exhibited. There is a perception that a top-grade in-line AOI, tuned to allow a few possibly good products to fail but never to allow bad ones to pass, can be used in combination with a top-of-the-line off-line 2D X-ray, or together with an in-line automated X-ray inspection (AXI) 3D system. The advantage being that an off-line 2D X-ray can offer the best micro-focus tube technology, have enough time to properly evaluate the products in batch mode, return the few good products back to the production line, and leave the manufacturing staff safe in the knowledge that the system detects products with known defects.
Inspection StrategiesBoard level assemblers use test and inspection for defect coverage and process control. The primary function of AOI and AXI systems are to provide overlapping and complementary inspection coverage in combination with ICT. While some defects will be identified by several of these test techniques, in many integrated AOI/AXI/ICT case studies it has been seen that it is precisely by having this complementary, and overlapping, test and inspection methodology that any gaps in the coverage are minimized.2 This increases the assurance of overall product quality relative to the ICT and functional test process.
X-ray Methodologies In-line automated 3D AXI systems are capable of providing fast detection of defects within complex assemblies at line speed and with a structured image review process. Manual or off-axis 2D X-ray inspection systems, on the other hand, are an off-line failure analysis tool providing detailed analysis of defects with high-resolution imaging and oblique angle viewing capabilities. While there are choices between these methodologies, the complementary nature of these X-ray systems ensures that, in most manufacturing environments, the need for both sets of functionality exists.
Manual or off-axis 2D X-ray inspection systems generally provide non-destructive defect analysis and process control and improvement. The basic premise of an off-axis 2D X-ray inspection is to function as a feedback tool for refining the assembly process. An advantage of off-line 2D X-ray is the ability to tilt the product through high angles, allowing users to look deep into microvias or PTH joint barrels or any suspect joint or material issue. As opposed to this type of off-line 2D X-ray, an in-line AXI system cannot tilt the product and is also restricted to a standard conveyor width of a production line.
Today's board-level assemblers face the challenge of detecting, analyzing and repairing defects associated with hidden joints of BGAs, column grid arrays (CGAs), quad flat pack no lead (QFN), and paste-in-hole (PIH) through-hole components. Automated 3D AXI systems inspect both sides of the entire board to identify potential defects; off-axis 2D X-ray confirms a defect prior to rework and repair. An example of this complementary test-and-inspection strategy is the detection and analysis of insufficient BGA solder joints (Figure 1).
Another example of in-line 3D AXI and off-axis 2D X-ray systems complementing each other is in the detection of incomplete PIH thru-hole solder joints. Once a potential defect is identified by the automated 3D X-ray system, the off-axis 2D X-ray machine is used to measure the fill percentage of the thru-hole joints with partially filled joints highlighted against an inspection scale.
Communication InterfaceFine-pitch area array packages continue to drive demand for X-ray inspection, as it is the only inspection technique that can inspect, verify, and aid the repair of hidden solder joints, as well as confirm the quality of any necessary rework. Complementary X-ray inspection techniques enabling fast in-line defect detection, off-line confirmation, and diminished repair difficulties gains importance as devices evolve and challenge the assembly status quo.
Integrated repair and quality tools are available that provide actionable information directly from an automated 3D AXI system to an off-line high-resolution off-axis 2D system for direct analysis and confirmation of the most complicated, and expensive to repair, hidden joint defects.3 Direct communication between these systems bridges the gap between inspection technologies and provides critical information to be passed along to repair stations and process experts to facilitate effective repair and process improvement (Figure 2).
This communication between the two systems provides a non-destructive way to verify non-visible defects and quickly identify suspect solder joints without a time-consuming manual search. It indicates the results of the 3D system test, together with CAD files, to the off-axis 2D system as well as providing alignment points, board orientation, and naming conventions. Automatic transfer of critical information allows full point-and-click access by the off-axis 2D system to areas highlighted by the 3D system as needing further review. The result is a significant increase in the speed and accuracy of the manual inspection process, an enhanced quality regime, and use of production line personnel instead of diverting process engineering resources.
ConclusionThe choice between an automated 3D AXI system and an off-axis 2D X-ray system is dependent upon the requirements of the specific test-and-inspection strategy into which it is incorporated. Selecting the optimal X-ray inspection methodology for a given application is a function of the role for which it is tasked. A critical issue that should be considered is the complementary set of strengths that both X-ray inspection methodologies possess and the benefits that can be derived from employing both technologies in unison by means of a direct communication protocol.
references:1. Mendez, David, "Deployment of an Integrated and Overlapping ICT and X-ray Inspection Process," Electronics Assembly Technology Forum, May 2001.2. Schreiber, Steve, "Study Compares Effectiveness of Test & Inspection," Test & Measurement World, August 2007.3.Bernard, David and Pippert, Tami, "X-ray Choices for SMT Manufacturing," SMT, November 2007.
David Bernard, Ph.D., product manager x-ray systems, Dage Precision Industries, a Nordson Company, may be contacted at firstname.lastname@example.org.