Achieving consistency in the process of visually inspecting SMT board assemblies can be problematic. Many of the manual inspection tools in use (magnifiers, eye loupes, etc.) do not facilitate consistency as an individual or group of inspectors moves from board to board.

To address this challenge, the EMPF uses the I Systems SMD-3D semi-automatic inspection system. It is primarily used in the Demonstration Factory for solder paste inspection and verification. It is also utilized in Boot Camp training, BGA Rework and Repair, and Advanced Packaging classes to illustrate the necessity of 3D inspection systems.

Solder paste printing is still a complex process where many process variables, materials, and environmental influences (temperature and humidity) meet with the human factor. Most boards are printed “on contact.” This means the stencil is intended to be placed flat against the top surface of the circuit board, and 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 the line” defects. Changing the process to accommodate smaller packages, new stencil designs, and paste formulas will make it more difficult to maintain high yields. The use of 3D solder paste inspection equipment should help ease this transition.

3D inspection systems are capable of providing both height and area measurements. True solder volume information can be acquired if a sufficient number of height data points can be measured. 3D has the added benefit of being insensitive to color and contrast changes.

Benefits of a 3D solder paste inspection system
1.) By finding defects where they occur, 3D inspection can reduce rework costs by spotting problems before they go through the reflow process. 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 board and the board can be printed again, before a costly set of components are assembled.

Common defects that a 3D solder paste inspection system should be able to detect are: bridging, insufficient paste, excessive paste, and paste misalignment. After solder reflow, these same types of defects can be much more costly to repair, and there is a risk of board or component damage during repair. It will also be difficult to pinpoint the cause of a defect after reflow. Probably the most common end of the assembly line defect reported is solder bridging. Although bridging is frequently attributed to an excess of solder paste, it is difficult to make this judgement with certainty after reflow, because other factors such as component leads, placement, or a dirty stencil may be at fault. Using a 3D solder paste inspection system can eliminate solder defects as a source of problems and help clarify the remaining causes of problems in the process.

2.) A good 3D inspection system does more than assist the operator in sorting good product from bad. By providing accurate and repeatable measurements of important process parameters, it is easy to get valuable process control data. This data reveals not only defect information, but also normal process variations, which helps to identify the causes of defects and process trends. Process chart generation is frequently part of the systems software package and can be a good method of detecting abnormal variations in the paste printing process.

3.) SMT inspection tools that provide accurate and repeatable solder volume and height measurements can also be used to accelerate process refinement and help reduce product introduction cycles. CSP packages are still at an early stage of their introduction cycle, and much remains to be learned before their usage becomes commonplace.

3D inspection can provide the user with the following data:

• Measurement of stencil apertures
• Part skew
• Dimensional measurement of mechanical parts
• Comparison of parts to a known good image
• Image documentation, annotation, part number, lot number, or image capture archiving
• X, Y, Z measurement capability
• Volume calculation
• 3D image profiles

The solder paste deposition process continues to be the leading source of end of the line defects in SMT Assembly. Process changes driven by the usage of emerging CSP 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. 3D solder paste inspection can be used to detect solder paste problems before they result in expensive rework or scrap. End of the line inspection may prevent defective parts from leaving the factory, but this does not address original product quality issues. A more effective strategy would be to improve first-pass yields and prevent these defects from occurring in the first place.

The leading edge of the SMT process is continually changing with new materials, new components, and new assembly methods being introduced to reduce the cost and improve product performance. When the assembly process changes, it should be requalified to ensure high product yields. In many cases, process studies can also be done on existing production methods to further improve yields. This type of engineering work is simpler to perform with reliable data from 3D inspection machines that are monitoring critical process steps.

For further information or a demonstration of the I Systems SMD-3D profile system, please contact Jeff Stong at (610) 362-1200, extension 224, or jstong@aciusa.org.