0201s: an inspection nightmare? Both optical and x-ray inspection should be used to reduce scrap and rework in assembling 0201s.

In a world of cell phones, pagers and laptops, much discussion has centered on the challenges of assembly, as board densities increase and components become ever smaller. The growing popularity of area array devices such as ball grid arrays (BGAs), microBGAs and flip chips in electronic products attests to the trend toward miniaturization.

Not to be outdone are the surface-mount passives, with the emergence of 0201 passive devices (Figure 1). Technically, 0201s are like 0603s and 0402s, only much smaller. Specifically, an 0201 capacitor measures 0.024 in. X 0.012 in. (0.6 mm x 0.3 mm). As with microchips, ensuring board quality in the assembly of 0201 components is a matter of effective, timely inspection and the right equipment.

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Clearly, 0201s are here to stay, for two reasons. First, they use 66 percent less board real estate than 0402s, which is a significant advantage considering the increased circuit densities in handheld and portable products. Second, their performance is comparable to that of 0402s; for example, 0201 capacitors can actually outperform 0402s at high frequencies. For these reasons, 0201s can already be found in many communication devices, sensors, medical electronics and even global positioning systems developed for the automotive industry.

Possible Assembly Problems

Without question, 0201 passives complicate the processes of stencil printing, component placement and reflow. While the following conditions are not unique to 0201s, they are more likely to occur due to the extreme small size of the 0201s and the close tolerances of the corresponding interconnects:

* Tombstoning. In this condition, the component stands on its end, like a tombstone (Figure 2). Tombstoning is primarily caused when the solder at one end of the component achieves liquidous state before the solder at the other end. The surface tension created by the liquidous solder "pulls" the component upright, separating the terminal from the pad at the end where the paste has not yet reflowed. Uneven solder wetting between the two ends of the component due to poor solderability can also cause tombstoning. High flux wetting speeds or thermal profiles that heat too fast can also result in uneven heating of the solder.

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* Pillowing. A relatively new phenomenon, pillowing occurs when the component is soldered correctly at one end but is only resting on the solder joint at the other end, as if it is lying on a pillow of solder. Pillowing can be considered a type of tombstone, but it is more difficult to detect and it may pass through both in-circuit test (ICT) and x-ray inspection. However, new tools have been developed to detect pillowing during automated optical inspection (AOI).

* Solder bridging. This condition is an undesirable conductive path between components or between the two ends of a given component. Solder bridging is caused by the inherent surface tension within molten solder during reflow, due to poor solder paste deposition, incorrect thermal profiles or inaccurate placement of components. Applying the flux binder improperly can also result in bridging.

* Solder beading. Small beads (balls) of solder can adhere to laminate, masks or components (Figure 2). They result from the displacement of solder paste from solderable surfaces and can be caused by the wrong stencil thickness or by improper design of a stencil opening or an attachment pad.

* Inconsistent solder volume. Too much or too little solder across a given board or varying solder volumes between components on the same board can cause defects. Errors in stencil thickness, stencil aperture design, print process parameters, solder paste powder size or solder paste viscosity are the main causes of inconsistent stencil deposition volumes.

Optical Inspection

Given the difficulties of assembling 0201 passive components, board quality can only be ensured through timely and adequate inspection, without impacting throughput. Ideally, such inspection should occur with an inline system located immediately after the critical process stages of stencil printing, placement of chips and passives, and reflow.

Fortunately, for many applications, a state-of-the-art AOI system with one or more CCD cameras can detect placement and reflow defects for 0201s. Some AOI systems are able to keep pace with high-speed placement machines, so they can be integrated in the assembly line.

One example is an advanced type of AOI called vectoral imaging, which also uses a CCD system. But vectoral imaging differs from standard AOI in several ways. The most common AOI method relies on analyzing pixel-grid values to verify the location of components. This method is based on comparing a reference image of a component with an actual component on a board. Once the search model is trained, the AOI system can then look for an exact match, and, where the match occurs, the location of the component is known.

The problem with this method primarily concerns components that are rotated or are larger or smaller in scale than the reference model. In fact, the orientation of the component relative to the camera is a factor, as shading varies with positioning. Other optical and electronic characteristics, including illumination intensity, focus, distortion, noise and gain, also have an effect, as do backgrounds with confusing clutter and partially hidden components. As a result, match-ups can sometimes be difficult to achieve with standard AOI systems.

In contrast, vectoral imaging first identifies and isolates the key individual features within a component image and then measures salient characteristics such as shape, dimensions, angles, arcs and shading. The process then checks the spatial relationships between the key features of the reference image and the features of the component being inspected. By analyzing the geometric information from the features and spatial relationships, vectoral imaging can precisely determine the position of the component on the board, without regard for the component's rotation, size or appearance.

To make this analysis, vectoral imaging uses a CCD system in which a single pixel encompasses an image size of 32 microns. For a typical screen area of 1280 X 1024 pixels, the field of view is 1.3 in. x 1.6 in. Within this field of view, the system can detect variations as slight as 1/10 of a pixel (3.2 microns), so it can find such conditions as skewed components (Figure 3) and tombstones. Because vectoral imaging can determine the orientation of a component relative to the location specified on the computer-aided design (CAD) drawing, it can be used to gauge the performance of a pick-and place machine, noting drifting problems and other deviations when they occur. Once vectoral imaging locates the component, it can check for pillowing, joints or bridges.

Overall, vectoral imaging is more accurate than standard AOI for determining the presence, absence and location of components, including 0201s. However, all types of AOI are inadequate for joint quality inspection, so x-ray inspection also must be used.

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X-Ray Inspection

X-ray stations are employed primarily for the inspection of solder bridging and solder balling. Both conditions can occur due to poor solder paste deposition, improper thermal profiles or improper component placement. Improper placement of flux binder can also cause solder bridging. Improper stencil thickness, stencil opening design or attachment pad design can all cause solder balling.

Both conditions cannot be seen optically because the CCD camera's line of sight is blocked by the component. Usually, random checks are made during assembly using an off-line x-ray system. Where bridging or balling is detected, more frequent inspection may be warranted until the errant process has been corrected. A standard x-ray inspection system can be used to periodically inspect for these features.

An inline inspection system with standard AOI and x-ray combined may be able to conduct 100 percent inspection. However, this approach will slow down the line due to the large amount of time needed by the x-ray inspection process. An alternative is to conduct 100 percent inline AOI and periodic off-line x-ray inspection, usually after reflow to verify process control. This approach is just as reliable, and it is also quicker and less expensive.

Conclusion

As with other area array components, 0201 passive devices are challenging the abilities of assembly equipment to deposit solder paste, mount components and reflow finished assemblies. To locate defects when they occur and to reduce both scrap and rework at the end of the line, inline optical inspection is required, backed by x-ray systems to detect possible solder bridging and balling.

Acknowledgments

The authors would like to thank Pete Iasso and Tom Zanatta of Symbol Technologies Inc.

Jean-Marc Peallat is a technical sales manager at Vision Inspection Technology LLC, Haverhill, MA; e-mail: [e-mail]. Vikram Butani is general manager at VJ Electronix Inc., Bohemia, NY; e-mail: [e-mail].