3-D techniques have the potential to offer a quantum leap in performance levels for advanced test and measurement equipment.

Human vision and perception of the world rely on the ability to view objects as the three-dimensional (3-D) structures they are. Perceiving depth, distance, and an object’s curvature and surface relies on the human ability to translate two simultaneous images into one 3-D view. Not only does the human mind typically cope better with 3-D images, 
3-D also contains a richer source of information for computer analysis. 

In printed circuit board (PCB) surface-mount technology (SMT), a new class of advanced test and inspection equipment looks at the structures on a PCB as true 3-D objects, performing measurements and extracting images in all three dimensions. In-line automated optical inspection (AOI) for testing components and joints, solder-paste inspection (SPI) for checking the screen print process, and X-ray systems for solder-joint testing are enhanced by 3-D imaging and analysis.

AOI Systems
Current AOI machines operate by analyzing images of a PCB assembly to detect manufacturing defects. Various techniques produce multiple two-dimensional (2-D) images of the PCBs and use low-resolution analog cameras placed at angles around a device or high-resolution color digital cameras placed above the device. 

A new range of AOI combines these 2-D images to create a 3-D model of the PCB. One technique is a patented technology called solid shape modelling (SSM) that creates 3-D models for any solid structure on the PCB, such as components and joints.

For SSM to create 3-D models, the 2-D images presented to it must be very precise. Information about 3-D shapes is revealed when a device is lit from specific, known directions and angles and viewed with a single, fixed, high-resolution camera. 

When a device is lit with controllable directional light such as north, south, east, and west, studying the reflections and the shadows of the device gives information on its 3-D shape. SSM analyzes the images when the same scene is lit from eight different angles and directions and combines these into a single 3-D image model. 

2-D AOI systems produce a fixed view of the PCB from which the programmer has to determine what combination of lighting and camera is optimal for detecting a defect. With 3-D AOI, the system has the capability to rotate the scene freely in three dimensions to reveal hidden information. 

For components, 3-D AOI information reveals the height and shape of the component relative to the PCB. For post-reflow joints, it shows information on the height, volume, and shape of the solder fillet. 

Component height and shape are useful in determining the presence of dark devices often indistinguishable from the surrounding PCB. 3-D AOI systems use the relative height and shape of the component to give a robust indication of presence regardless of the size, color, or shape of the component.

For solder-joint analysis, 2-D AOI systems rely on the fillet resembling a curved mirrored surface. Various combinations of lighting and camera angles highlight the presence or absence of this mirrored surface, and from this, good or bad joints are assessed. 

A 3-D AOI system looks at the solder fillet as a true 3-D object and reveals its relative height, volume, and shape. In addition to helping 3-D repair operators view the joints, analyses of these new parameters can be used to accurately represent and classify good and bad joints. Figure 1 is an example of each type of joint when viewed with a 3-D AOI system, showing a clear difference visually for a human and mathematically for a computer.

Current 2-D AOI systems are plagued by false calls and defect escapes. SMT manufacturers reluctantly have embraced this technology as a cost-effective part of a test strategy but always with the safety net of in-circuit or functional test. 

Even the defects captured by 2-D AOI are subject to the whim, time constraints, and ability of the programmer and not tied to any industry-accepted classification method. 
3-D AOI analyzes the component and joint parameters and compares them against the mathematical specifications found in industry standards such as IPC-610A.

For the first time in SMT history, 3-D AOI offers the possibility to control the volume of post-reflow solder by using statistical-process techniques applied to quantitative measurements from solder joints. 3-D AOI is in its infancy, but the possibilities it offers already are mapping it as the future for AOI.

Solder-Paste Inspection 
SPI is a subset of AOI used directly after printing because companies claim that as many as 80% of their defects can be attributed to the paste printing process.¹ A paste inspection strategy can consist of 2-D or 3-D, in-line or off-line, and 100% coverage or sampling techniques. 

2-D SPI measurement systems provide X and Y locations and area of coverage for each paste deposit, with some bridge detection capability. 3-D SPI measurement systems have all the 2-D information but add height, volume, and additional bridge detection capability.

Typically, 2-D SPI systems use lighting rings with various colored visible light at certain angles of incidence. Usually, 3-D measurements are structured light or laser-based techniques. An active light source is presented in a way that variations in height or range affect the optical properties such as intensity, phase shift, position, and size of the scene.² Optical triangulation, a version of structured light, is used in a majority of commercially available 3-D paste-inspection systems to derive height measurements.

Companies select an SPI strategy for many reasons. The added cost benefit for catching defects early is a driver. A recent study on the 2-D vs. 3-D question shows that 3-D is preferred and even required in four out of five cases because solder-paste volume is an important predictor of long-term solder-joint reliability.¹ 

Recent technological advances allow SPI systems to inspect and detect 3-D information at line rates. Process control has become key with the dawn of lead-free solder and the popularity of 0201s, chip-scale packages (CSPs), ball grid arrays (BGAs), and ceramic column grid arrays (CCGAs), which have known requirements for solder-paste volume.³