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STEP 9: Test & Inspection
December 31, 1969 |Estimated reading time: 7 minutes
From an inspection point of view, there are two types of solder joints in PCB assembly: visible and hidden. AOI is used to inspect visible joints, while AXI inspects hidden ones. Combining these technologies into a single system (AOXI) can reduce overall costs. Some sophisticated systems can even inspect double-sided PCBs with AOI and X-ray simultaneously, significantly increasing throughput rates.
By Udo E. Frank, Ph.D.
Both automated optical inspection (AOI) and automated X-ray inspection (AXI) are based on the analysis of digital images. Because it does not matter how the images are created, the image-processing technology used in AOI can also be easily used in AXI. With AOI, the illuminated surface of the inspection object creates a visible image (grayscale pattern) through the absorption or reflection of visible light in a detector (camera). With AXI, the irradiated volume of the object, through the absorption and scattering of X-ray radiation, also creates a visible grayscale image in a detector (usually an image intensifier and camera). The difference between AOI and AXI is therefore only a question of how the image is produced; there is no fundamental difference in the way resulting images are analyzed.
By using standardized inspection-sample libraries, inspection routines that include both AOI and AXI can be created, allowing a unified AOXI methodology. Defect post-classification software does not differentiate as to how the images have been generated. Only the defect criteria count.
AOI Sees What Is Visible
What is the best way to use each of these methods? This question can be answered with a series of examples in which the visibility of the inspection object diminishes with each case. AOI is a method of inspecting surfaces. A 2-D surface is illuminated orthogonally (straight on) or obliquely, and the resulting image is captured by a camera as a 2-D image. A bare circuit board shows up realistically along the X and Y axes, because only the two dimensions of its flat surface appear in the image. Defects, which manifest themselves as variations in the surface structure of the X and Y axes, are easily identified and analyzed using an orthogonal view, in which the optical axis of the camera is at right angles to the surface of the circuit board.
Figure 1. Example of defect detection only possible with AOI (SOIC wrong polarity).
Once a circuit board is populated with components, its surface becomes uneven and 3-D, while the image remains 2-D (Figure 1). From an orthogonal viewpoint, an X/Y displacement is generally detectable. Identification of all solder joint defects, however, necessitates the use of the third dimension along the Z-axis, with an oblique camera angle. A combination of both views permits detection of all defects.
Because SMT is, by definition, surface-oriented, it favors AOI. Components are placed on the paste prior to soldering and the solder joint usually remains visible. Even with increasing component density, an oblique view still permits defect detection. The limitations of optical inspection becomes noticeable only when component height increases at higher packing densities. Shadowing makes it increasingly difficult to discern solder joints on the surface of the circuit board.
Gull-wing solder joints such as QFPs and SOICs, where leads are bent outward, are easily visible because they are outside the component. With J-lead components such as PLCCs, whose leads are bent inward, solder joints are still outside the component, but have moved underneath it. The externally tinned area, however, remains visible.
AXI Sees What Is Hidden
For heavy components or components subjected to great mechanical strain that must be mounted more solidly, thru-hole technology (THT) is used. Here, the pins or wires to be soldered are inserted into holes in the board and soldered in place. The tinned front side of the solder joint is usually hidden by the component itself, and the degree to which the hole has been filled with solder (barrel fill) is unknown. Only the tinning on the backside of the solder joint is visible.
Solder joints situated entirely beneath the component, as with BGAs, microBGAs, CGAs, or MLFs, are hidden completely and no longer viewable using AOI. These solder joints can only be assessed with X-ray inspection. With BGAs, there is no longer a 1-D row of pins along the edge of the component. Instead, BGAs have an array of solder balls hidden beneath them that connect the component to the circuit board. Here, AXI a necessity; there is no other technique by which the quality of BGA connections can be assessed at production rates.
Viewing in the Third Dimension
AXI is a volumetric inspection method. The 3-D object undergoing inspection is penetrated by X-rays, and a 2-D shadow image is projected onto the detector surface. The fundamental difference between AOI and AXI is that AOI displays a surface, whereas AXI displays volume. AOI shows the surface structure, and AXI the internal structure, for example, solder joint voids. AXI reveals concealed solder joints and their shape. This characteristic of X-ray inspection can be used to make the third dimension of a solder joint visible. In an orthogonal view, a BGA appears as a regular pattern of dark dots. Bridges, inadequate or excessive solder, solder splatter, misalignment, and voids can all be readily detected. In an oblique view, it is possible to look at the third dimension of the solder joint and determine if there is any deviation from the barrel shape of a good BGA solder joint, if the tinning of the solder ball is adequate, and whether or not the solder joint is open (Figure 2).
Figure 2. Oblique X-ray view of BGA solder joints. The middle connection in the right-hand outside column is open.
An oblique view also permits AXI to inspect THT solder joints, since the bone-like structure of these shows up well. The degree of filling, tinning of the upper and lower surfaces, and even the extent of voids can be analyzed (Figure 3). In the case of gull-wing solder joints, AOI can deliver the best results. When the landing pads are hardly larger than the pins, however, it becomes advantageous to evaluate the heel tinning and extent of voids. This also can be accomplished with an oblique view in AXI.
Figure 3. Oblique X-ray view of three THT solder joints. Two of them show insufficient solder in the insertion hole (barrel fill).
There are no clear-cut dividing lines between AOI and AXI. For certain inspection tasks, both deliver good results. In general, the more visible an inspection structure is, the more effective AOI is; the more hidden a structure is, AXI is more effective.
Figure 4. Example of defect detection only possible with AOI (label verification).
In determining the best combination of AOI and AXI, it pays to examine the factors of complexity and noise. AOI depicts the surface of the inspection object. All internal structures are hidden from view, which has the advantage of not confusing the issue. The degree of complexity is comparatively low. As a result, features in the image can be more easily identified with their real counterparts. Optical images also are virtually free of noise, as the photon flux and light intensity are high. This means that optical images can be acquired in a few milliseconds. With large-format megapixel cameras and simultaneous inspection of both sides of double-sided PCBs, AOI permits rapid inspection at high throughput rates. Typical structural defects that can be detected by using AOI include incorrect polarity or labeling (Figure 4).
Inspection Depth and Throughput
AXI shows the internal structures of an inspection object. Surface structures become visible only when they cause variations in absorption. The assignment of image features to their real counterparts must take into consideration their depth within the volume. The degree of complexity is comparatively high. Because components within the volume may overlap, it is sometimes necessary to switch among various positional combinations of source, inspection object, and detector.
In addition, X-ray images display a noticeable noise, because although the dose rate (intensity) of X-ray radiation in the highly magnifying microfocus range is sufficient, it is not comparable with the intensities of visible-light wavelengths. Noise can be considerably reduced by averaging out the grayscales over a period (integration), although this takes a few milliseconds. AXI permits the inspection of concealed solder joints with high throughput. Structural features that can be seen only with AXI include the shape of BGA solder joints or barrel fill in THT solder joints.
The general rule of thumb for automatic high-speed, high-depth inspection is: Inspect as much as possible using AOI to maintain a high throughput rate, and check everything that is hidden with AXI to achieve complete inspection coverage. An AOXI system, which contains both inspection methods, is an ideal combination for this purpose because visible and hidden solder joints can be inspected and analyzed in a single pass. AOXI systems can meet high throughput demands using parallel inspection - one PCB is inspected optically, while another is X-rayed simultaneously. The investment and floor-space requirements can be half those of two separate systems, but with the same benefits.
Udo E. Frank, Ph.D., senior application manager, Viscom AG, may be contacted at 49 51 94996 0; e-mail: uef@viscom.de.