The laser scanner is equipped with two detectors that are designed to eliminate “shadowed” areas where the projected spot cannot be detected. The high-speed data acquisition rate of the 3-D laser scanner allows the system to collect full 3-D image data rather than sample cross sections. This results in an accurate representation of the size and shape of each feature on the board. Figure 2 shows a diagram of the 3-D laser scanner.

2-D Versus 3-D Images

Figure 3: 2-D Image Figure 4: 3-D Image Figure 3 is a 2-D image of a section of a CSP site on a printed circuit board. The solder paste appears dark grey in this image and can be difficult to separate from the background. There is no height information available. The only information useful in this 2-D image is area and registration.

Figure 4 is an image of the same section of the board, but this time in 3-D. Note how the solder paste can be clearly differentiated from the background. In addition to area and registration, volume and height data is available.

The next image shows a profile through the paste. The differences in heights of the solder paste can be seen here. Figure 6 shows a perspective view of a corner of the CSP site. The data is the same, but presented differently. The topology of the paste can be seen in the perspective view. Accurate measurements can thus be made due to the density of data acquired by the 3-D scanner.

How the Process Can Benefit from Solder Paste Inspection
Solder paste printing (even though the printing equipment and process has become much more reliable over the past 10 years) is still a complex process where many process variables, materials, environmental influences (temperature and humidity), and human factors, meet. Most boards today are printed “On Contact” meaning that the stencil is intended to be placed flat against the circuit board being printed and that the pressure of the squeegee moves the paste over the stencil, forcing it forward and down into the stencil apertures. It sounds simple, but it is consistently reported that this process step continues to be responsible for the greatest number of end of line defects. Changing the process to accommodate smaller packages, new stencil designs and paste formulations will make it more difficult to maintain high yields. Use of solder paste inspection equipment should help ease this transition.

First, by finding defects where they occur, in-line inspection can reduce rework costs by spotting problems before the go through the reflow oven. The cost to repair a defect increases substantially as it moves undetected through the assembly process. Poorly screened solder paste can be washed from the PCB and the board can be printed again before a costly set of components are placed on it. Common defects that an in-line solder paste inspection system should be able to detect include bridging, insufficient paste, excessive paste, and paste mis-registration. Misplaced or missing components can be easily corrected before reflow. After solder reflow, these same types of defects can be much more costly to repair, and there is risk of damaging the board or the components during the
rework process. Also it is difficult to pinpoint the exact cause of a defect after reflow. The most commonly reported end of line assembly defect is solder bridging. Although bridging is frequently attributed to an excess volume of solder paste, it is difficult to make this judgement with certainty after reflow because other factors such as component leads or placement may be at fault. Using in-line solder paste inspection can eliminate solder defects
as a source of problems and help clarify the remaining causes of problems in the process. By eliminating solder paste defects from the process the remaining causes of problems can be uncovered and addressed.

Second, a good in-line inspection tool does more than sort good product from bad. By providing accurate, repeatable measurements of important process parameters, it’s easy to get valuable process control data from an automated system. Process control data is a valuable window into the SMT assembly process. Because the data includes not only defects, but also information from every board built, it’s easy to see the normal process variations and identify the causes of defects. X-bar R charts are frequently recommended as a good method of detecting abnormal variation in the paste printing process.

Third, SMT inspection tools that provide accurate, repeatable solder volume and height measurements can also be used to accelerate process refinement and help reduce product introduction cycles. Both the CSP and 0201 packages are still at an early stage of their introduction cycle and much remains to be learned before their usage will become commonplace. For 0201 passives in particular additional study is required before the relationship between solder paste volume and beading defects is well understood. The leading edge of the SMT process is continually changing, with new materials, new components, and new assembly methods being introduced to reduce costs and improve product performance.When the assembly process changes, it should be re-qualified and
characterized to ensure high yields. In many cases, process studies can also be done on existing production methods to further improve yields. This type of engineering work is much easier with reliable data from in-line inspection tools that are monitoring critical steps of the process.

Conclusion
The solder paste deposition process continues to be the leading source of end of line defects in SMT assembly process. Process changes driven by the usage of emerging CSP and 0201 packaging technology will further complicate the paste deposition process. Solder paste volume will continue to be the best predictor of a good solder joint at the end of the line. In-line, 3-D solder paste inspection can be used to detect solder paste problems
before they result in expensive re-work or scrap. End of line inspection may prevent defective parts from leaving the factory, but does little to improve the quality of the product in the first place. A more effective strategy would be to improve first pass yields and prevent these defects from occurring in the first place.