Proper Design Techniques Enhance Inspection Results
Simple board-design practices that make components more visible improve the results of inspections.
Stephen F. Scheiber, ConsuLogic Consulting Services, Slingerlands, NY -- Test & Measurement World, 6/1/2000
For decades, test professionals have extolled the virtues of designing boards for testability. Unfortunately, electronic testing alone can no longer meet all the quality concerns of manufacturers. The increasing use of small components and components with hidden connections mandates the use of inspection techniques to improve quality. But before you can apply these techniques to populated circuit boards or assemblies, you must ensure that inspection will yield useful results. Just as test engineers have beaten the drums for design for test, they now beat them for design for inspection.
The guidelines that follow provide suggestions and recommendations that will enhance the success of inspection. They don’t qualify—yet—as rules, but their common-sense approach will help make inspection easier, more accurate, and more repeatable. Some guidelines apply to virtually every manufacturing situation; others may apply only to x-ray inspection.
The first guideline applies in all inspection situations:
When design engineers change a board’s “appearance,” they must follow the same engineering change order (ECO) procedures required for a change in board function.
Suppose board designers use a small-outline IC (SOIC) to replace a functionally identical through-hole DIP. Electrically, the board hasn’t changed, yet visually it has. If information about the new appearance never reaches the inspectors or the machine-vision system on the production line, an entire run of otherwise-good boards may “fail.” These false failures tax the repair facility as it tries to figure out why electrically good boards were rejected.
Both the size and appearance of components can vary considerably from lot to lot and from manufacturer to manufacturer. The two components shown in Figure 1 are supposed to be identical, but they’re not the same size. The small difference might escape a human inspector’s notice, but an automated inspection system set for the dimensions (and tolerances) of one device might reject the other.
Consider an 0603 discrete component. According to specs, it should measure 60x30 mils. Yet real parts can range in size from 50x22 to 65x34 mils, depending on the manufacturer. Device color and markings may also differ from one manufacturer, or one lot, to another, as shown in Figure 2. Thus, the next guideline:
Ensure that the look of all parts undergoing inspection remains the same from batch to batch. Parts from all vendors must be sufficiently consistent in size, color, and markings that people and machine-vision systems can recognize them as the same part.
Also, if you employ people to inspect boards, make the orientation of components as consistent as possible. You may want to find IC manufacturers that use similar markings to designate pin 1, a common “landmark” on IC packages. Pin-1 marks can vary considerably (Fig. 3) from supplier to supplier, and so they can confuse people. They make no difference to machine-vision systems, however. To aid visibility and maintain consistency for human inspectors, follow the next guideline:
Orient components on a board in the same directions, up-down or left-right. Arrange ICs so pin 1 appears in the same relative location on each board—lower left, upper right, and so on.
Figure 1. Two “identical” components should be the same size. A human inspector might accept either one, but a machine-vision system would reject the one it had not seen before. (Courtesy of GenRad.)
Figure 2. The same type of component from two different manufacturers may look different to an inspector or inspection system due to changes in color or markings. (Courtesy of GenRad.)
Figure 3. Different types of marks indicate pin 1 on ICs from several manufacturers. (Courtesy of GenRad.)
In addition to making sure that components are oriented, you also want to ensure they occupy the spaces provided for them. Anything that increases the likelihood that an inspector—human or electronic—will notice a missing component decreases false failures. A few simple steps taken during PCB layout will help inspectors distinguish between placed components and an empty component position. Therefore, the next guideline:
Provide contrasting backgrounds or fiducial marks on bare PCBs, or place marks on components to make their presence or absence more obvious.
As shown in Figure 4, a white stripe on a PCB provides a contrasting background for a dark component. In addition to indicating presence and absence, you can use the contrasting background to enhance edge detection by machine-vision systems. Good edge detection lets your inspection system make accurate dimensional measurements, a key factor in checking the accuracy of component placement. You can also look for marks on components and keep the bare PCB underneath the component free of similar markings. Keep in mind, this technique won’t ensure the placement of the proper component, just the placement of a component.
Current inspection techniques may rely also on x-rays that will reveal the structure of hidden components or hidden connections, such as those on BGAs.1 X-ray inspection technologies come in two basic forms: transmission and laminography.2 The transmission technique passes an x-ray beam through a board and provides an image of everything in the beam’s path. That image doesn’t let you easily determine whether features reside on the top or the bottom of the board. The picture becomes more confused when designers place connection pads so they appear superimposed on the top and bottom of a PCB (Fig 5a). Staggering the pads (Fig. 5b), helps separate them in x-ray images, and permits more accurate image analysis. So here’s a tip for when you plan to use transmission x-ray inspection:
Stagger contacts on the top and bottom of PCBs to increase the usefulness of x-ray inspection images.
Current PCB Suggested
Laminography scans a PCB board many times with oblique x-ray beams to build images that let you distinguish features on either side of the board. Because laminography builds its images one layer at a time, its primary enemies are lack of coplanarity and board distortions. The laminography system must locate surfaces precisely despite warpage and other shape-changing factors, such as the board’s operating temperature. Therefore, designers should consider how to reduce mechanical and thermal stresses and thus minimize board deformation. If your plans call for x-ray laminography:
Marking Marking
Figure 4. A background color that contrasts with a component’s color helps an inspector or machine-vision system determine the presence or absence of the component. A high contrast permits a measurement system to precisely determine a component’s position. (Courtesy of GenRad.)
Figure 5. a) Placing component connections in the same positions on the top and bottom of a PCB causes problems in a transmission x-ray image that yields a composite view of everything in the x-ray beam. b) Staggering the component connections decreases the interference.
Specify PCB materials and assembly procedures that minimize coplanarity problems.
No matter what x-ray technique you use, remember that metallic objects such as heat sinks, shields, transformer cores, and so on may make it difficult to image some parts of a PCB. If possible, when you use x-ray inspection, place heat sinks and other x-ray opaque materials or components on the product after inspection.
Manufacturers are adding inspection to production lines to ensure board quality, so designers must make an equal effort to create boards that encourage the best possible inspection results at the lowest cost with the lowest incidence of false failures. The above tips should help them get started. T&MW
FOOTNOTES
1. Titus, Jon, “X-Ray Systems Reveal Hidden Defects,” Test & Measurement World, February 1998. pp. 29–36.
2. Titus, Jon, “X-Rays Expose Hidden Connections,” Test & Measurement World, October 1999. pp. 28–35.
Stephen F. Scheiber has spent more than 22 years in the test industry. He consults and writes extensively about such topics as test strategies and test economics. E-mail: sscheiber@aol.com.
