Step 7: Soldering
December 31, 1969 |Estimated reading time: 6 minutes
By Doreen Tan
Solder paste is the backbone of the SMT process. SMT solder joints provide both electrical and mechanical connections. One of the best predictors of solder joint strength is its shape and one of the best predictors of solder joint shape is solder paste volume.
After SMT component placement, boards are typically wave soldered to permanently and securely affix the components to the board surface. One of the best predictors of resultant solder joint integrity is the volume of solder paste that was applied earlier in the process. Therefore, when thinking of soldering issues, the solder paste application process must be considered, and the best form of process control for paste application is solder paste inspection.
By inspecting after the print process, poorly screened solder paste can be wiped from the printed circuit board (PCB) and the board can be printed again. By using statistical process control (SPC) tools, process trends can be detected before they result in defects, and if defects are produced, they are caught before expensive rework. Automated solder paste inspection can help improve first-pass yields and reduce rework costs.
Inspection Technologies
Using multiple camera technology, as well as multiple lighting angles, 2-D systems can be fast but are sensitive to changes in color of the PCB, soldermask and paste appearance as it dries.
Figure 1. Solder paste volume is critical to final product quality.
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True 3-D systems use laser triangulation and provide accurate height and volume information in addition to area. Paste thickness can be referenced from the metal traces, board or soldermask surface. Of these three reference points, the metal traces provide a more accurate height reference because board and soldermask surfaces tend to be uneven.
Figure 2. Defective paste scanned with a 3-D system. Pad 99 (next to bottom) has no paste. Pads 100 and 101 both have low paste. Pad 100 appeared to have more paste because it was badly smeared, giving the appearance of greater pad coverage.
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Because 60 to 80 percent of solder defects stem from poor paste printing, implementing a solder paste inspection system makes the most sense in the effort to improve yield. Boards at this stage can be cleaned easily, and cost savings are greatest at the start of the SMT process.
Figure 3. The area of the BGA deposit appears normal, but contained more than twice the expected volume and height.
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The majority of gross solder paste defects will be caught by all inspection systems. The difference is when there are height and volumetric defects. Figure 1 illustrates the importance of solder paste volume. The upper diagrams show paste depositions that cover the area and are also of appropriate volume. The lower diagrams show paste depositions that can produce solder joints that pass electrical test, but which have low mechanical strengths and high failure rates. Figure 2 shows an area of defective paste that was scanned with a 3-D laser system. Note that one pad contains no paste at all, another has partial coverage, and a third has low paste volume. The low volume deposition could provide a false pass if only area information is used and height is not taken into account.
Figure 3 illustrates another example of why height information is important. In this example, the area of the ball grid array (BGA) deposit appears normal but actually contains more than twice the expected volume and height.
These examples illustrate that to perform an effective solder paste inspection, the inspection system must be able to provide not only area but also accurate volume and height information.
System Programming and Robustness
All vision systems require some time investment for program creation. Most of the systems have some sort of Gerber or CAD import tools to aid in creating the inspection programs. Most of them also are capable of storing libraries of previously created algorithms or template types.
The key is how robust the programs are once they are created. Are they robust enough to handle changes in board and mask color, paste as it dries, even lead-free paste? If the chosen inspection system requires constant algorithm or lighting adjustments, that should be taken into consideration when allocating technical resources to support the system.
In-line Inspection vs. Off-line Sampling
Once the type of inspection technology has been decided, how should the system be implemented? Some manufacturers may choose to put the system off-line, sampling a few boards every few hours. This is one way to share the inspection system between several lines. The disadvantage is the labor and handling involved, risking paste damage. Systematic defects or problems may not be detected in time. Random defects can be missed if the boards on which they occur are not selected for inspection.
In-line implementation allows every board to be inspected as it is produced. It provides immediate feedback of systematic problems and random defects if 100 percent inspection is performed. In-line process control for SMT inspection involves putting inspection tools where defects most often originate.
Figure 4. Potential locations for inspection prior to reflow in a typical SMT line. There is an increase in cost per defect as the part moves further down the line.
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Figure 4 illustrates potential locations for inspection prior to reflow on a typical SMT line. Figure 5 shows the cost of defects, and was obtained from an actual manufacturer's cost data. It shows a greater than tenfold cost increase when defects are discovered in in-circuit functional test vs. the paste print process. Because the highest number of defects is generated at the print process, the greatest cost savings can be achieved by implementing in-line solder paste inspection and eliminating paste-related defects here. This particular manufacturer chose to implement 3-D solder paste inspection with the goal of eliminating all possible defects stemming from the paste printing process.
Figure 5. Cost of defects from an actual manufacturer.
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Monitoring Tools
By default, the system should act as a go/no go station, by not allowing boards with defective paste to continue down the line. Pass/fail judgments can be made by the inspection system using specification limits set by the user.
Figure 6. Effect of operator break.
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The first perceived problem after installing an automatic inspection system is how to use the quantity of data that the system produces. An accurate and repeatable inspection system should provide defect data as well as the tools to track process variations. Some examples of such tools include real-time X-bar and R charts, X-bar and S charts, and Pareto charts.
Figure 7. Effect of change in stencil.
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Figures 6 and 7 are sample X-bar and S charts that show examples of process variation tracked by a 3-D inspection system. A process shift does not necessarily indicate a problem. Instead, it serves as an indication that something has changed. The goal is to have the process perform consistently within control. The charts show variation caused by external influences, e.g., in Figure 6, the effect of an operator break and Figure 7, a process shift due to a stencil change.
Information derived from the inspection system can be used for continuous process improvement. For example, as part of the print process operation, a manufacturer can determine that the first boards after a break should be cleaned, since the printer seems to take several print cycles to achieve consistent volume. Stencil designs can be evaluated, helping the user choose one that produces the consistent amount of paste. Optimal printer settings are determined with real-time SPC charts.
The case for in-line inspection is not to totally eliminate in-circuit test, but to use it as an important effort to improve yield and reduce rework costs.
Doreen Tan, applications manager for SVS Products, may be contacted at GSI Lumonics Inc., 22300 Haggerty Road, Northville, MI 48167; (248) 449-8989; Fax: (248) 735-2460; E-mail: tand@gsilumonics.com