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Integration of X-ray Inspection and Rework improves SMT yields
December 31, 1969 |Estimated reading time: 7 minutes
X-RAY TESTING COMBINED WITH NEW STATE-OF-THE-ART REWORK SYSTEMS CAN LOWER COSTS AND REDUCE REPAIR TIME FOR PROCESS DEFECTS. BY FRED SCHLIEPER
Limiting a manufacturer's vulnerability to loss of yield and profitability requires a well thought out proactive strategy. As the market pushes for smaller component designs, connections occur in significantly less space. Using smaller components makes printed circuit board (PCB) assembly, testing and rework more difficult, and increases the likelihood of generating defects defects that decrease production yield but also can be prevented.
Unfortunately, most companies view assembly process quality testing and rework as an afterthought, addressing these problems reactively, instead of proactively planning for the inevitable. Solder joint quality verification and rework require an aggressive management strategy to prevent profitability loss because of the lack of process/solder joint quality feedback and rework optimization. The goal is to identify structural or component defects in the final assembled product. This requires the tools to identify process variations, pinpoint faults and accurately control environmental conditions during the repair process.
Figure 1. During an inspection, X-rays emitted from the source pass through the circuit board to an X-ray detector. Then the images are directed from the detector through a mirrored assembly to a video camera, where the digitized images are sent to an image processor for display, enhancement and analysis.
One strategy for overcoming production yield loss is to combine inspection and process monitoring to optimize the manufacturing process before defects are generated, while rework operations recover and validate failed assemblies. Integrating X-ray testing with rework systems can reduce cost and repair time for component defects, improve process yields, and enhance quality.
Evaluating Test AlternativesTesting is an integral part of manufacturing. Traditionally, SMT manufacturers used a combination of visual inspection, in-circuit inspection, and functional testing to find defects and improve board quality. However, these techniques may not be sufficient for identifying some defects on area array devices.
Visual InspectionVisual inspection relies on line-of-sight access to verify interconnections. The equipment used for visual inspection ranges from the naked eye to microscopes to automated optical inspection (AOI) systems. Without line-of-sight access, however, these forms of testing are ineffective.
In-circuit testing (ICT) entails a point-by-point electronic checking of solder joints (pins) with a probe or with bed-of-nails testing nodes. Unfortunately, ICT cannot identify voids or verify solder quality. In fact, pressing down on a solder joint actually can "make" a connection. Where an incomplete joint exists because of quantity or solder paste distribution, ICT actually may show that a connection exists at the time the test is performed; however, this joint eventually may come apart, causing the product to fail. With ICT, there is no way to identify a potentially poor quality solder joint. Additionally, test access to the component may be unobtainable because of the layout design.
Functional test involves applying signals (powering the board) at various points to determine if the circuit is active. As with ICT, potential reliability problems cannot be identified because functional testing only ascertains whether or not the entire unit works. More importantly, when a signal failure does occur, this test can only tell that a board is defective; it cannot identify the specific location or cause of the problem.
X-ray inspection has become a more common tool on the production floor because it detects faults in obscured solder joints that have no visual access. X-ray identifies unreliable solder joints that would pass both functional and in-circuit testing, and also can determine common failures such as misalignments, part orientation, shorts and opens the most common assembly problems.
A Closer Look at X-ray ImagingX-ray inspection systems display grayscale images, which represent shape and thickness variances of an object. High-density features produce a darker image than those with lesser density or thickness. Therefore, it is possible to quantitatively measure these features and develop a correlation between acceptable and unacceptable manufacturing process conditions.
Figure 2. X-ray inspection can reveal a number of BGA component defects, including open solder joints (a); shorted solder joints (b); lifted leads (c); and voiding (d).
During an inspection, X-rays emitted from the source pass through the circuit board to an X-ray detector (Figure 1). The images then are directed from the detector to a video camera, where the digitized images are sent to an image processor for display, enhancement and analysis. Systems are available for manual or fully automatic inspection and defect detection. X-ray inspection can reveal a number of ball grid array (BGA) component defects, whether hidden or visible, including open or shorted solder joints, lifted leads, component misregistration, chip tombstoning, voiding, and unacceptable size variations in solder bumps (Figure 2).
Reworking Defective BoardsOnce a particular component has been determined defective, it must be removed and replaced. When these operations have been executed, the reworked product should possess the same integrity as during the first inspection. During the repair process, the assembly will be subjected to at least three extra thermal cycles; therefore, it is essential that the processes be well defined and controlled so that the PCB and adjacent components are never subjected to damaging stresses.
Quality rework requires that the following operations be carried out:
- Remove the component
- Dress the site (i.e., remove residual solder and apply solder paste)
- Place and solder a new component.
Each of these functions require heating the component to temperatures above the melting point of the solder, typically 183°C, and maintaining the assembly there for some period of time. The rate at which this elevated temperature is reached and the time the assembly is held above 183°C typically is well established, but creating the necessary thermal cycle or temperature profile is not intuitive.
Fortunately, today's automated rework systems are very sophisticated and most incorporate software that facilitates rework operations by providing the operator with predefined steps. A process engineer defines the parameters for these steps, thereby guaranteeing the correct sequence of events, thermal dynamics and timing. This also allows the process to be fine-tuned and ensures repeatability the key to successful rework without damaging surrounding components.
Integrating X-ray Inspection and ReworkWhen X-ray inspection and rework are integrated, identifying the location of a defective component becomes easy. A printed circuit board assembly (PCBA) may be identified with a barcode, which may then serve as a traveler (repair ticket) to associate and communicate captured X-ray images of the board's failure region to the rework display. This paperless method of initiating rework assists the operator by pinpointing the fault, revealing the precise location of the defect and eliminating the possibility of reworking the wrong component.
Benefits of integrating X-ray inspection and rework include the following:
- Verification that rework actually is necessary (eliminating false calls).
- After rework, the assembly can return to the X-ray system for inspection to verify that it meets the inspection criteria and solder joint quality standard after rework.
- In addition to validating the rework process, the X-ray system can monitor the assembly process, detect trends and provide statistical process control data for feedback to the production floor.
- The X-ray system also can verify thermocouple location on instrumented boards and provide invaluable information to aid in process development.
- When used together with inspection and rework activities, the X-ray system provides a strategy for improving yield and validating the PCBA quality.
ConclusionWhen considering the deployment of advanced high-density component technologies such as BGAs, a new set of manufacturing considerations come into play which include X-ray inspection for failure analysis and process optimization, and advanced rework systems for control and automation of the rework process.
Newer X-ray inspection technologies are capable of 100 percent in-line inspection and in many cases they are required to test PCB assembles with BGA, microBGA and flip chip components because:
- Visual inspection has no visual access.
- ICT cannot identify voids or verify solder quality and, without test points, has limited or no physical access to the component.
- Functional test only identifies a board is defective; it cannot identify the specific location or cause of the problem.
However, X-ray inspection is just one of the new production methods for surface mount assemblies. Automating the rework process has become economically practical and even necessary. Automated rework stations provide better, faster and repeatable repairs to damaged boards. From site preparation to the thermal dynamics of reflow temperature profiles, today's automated rework stations are helping to achieve profitable yields.
The best manufacturing practices also include some means of process monitoring, process optimization, quality assessment and rework of failed assemblies. Many companies are realizing the benefits of these practices and are combining X-ray inspection with reflow and rework technology to provide manufacturers with a single source, resulting in more robust process integration and greater cost saving solutions.
FRED SCHLIEPER, worldwide independent channel sales manager, may be contacted at GenRad Inc., 8221 Arjons Dr., Suite F, San Diego, CA 92126-6319; (858) 635-8617; Fax: (858) 695-9902; E-mail: schlieperf@genrad.com.