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Stencil Printer Optimization Study
December 31, 1969 |Estimated reading time: 9 minutes
By John V. Stevenson and Derek Drabenstadt
Reducing the variation in the pad areas, the Six Sigma philosophies can be brought to bear on the stencil printing process.
The solder paste printing process is perhaps the most critical operation for assembling a printed circuit board (PCB) populated with surface mounted components. Industry experts agree that solder defects are caused, in part, by poorly printed solder paste.1 As many as 60 percent of soldering defects can be traced to improper deposition.2 Therefore, accuracy and repeatability of the stencil printer is highly desirable throughout the entire printing process. There are many variables that can effect the formulation of a solder paste pad. Some industry experts suggest that there are a minimum of 39 process variables that must be controlled.3 By reducing the variation in the printed paste pad area, the opportunity exists for achieving a Six Sigma process.
To optimize the stencil printing process, the Six Sigma philosophies of reducing the variation in the process and targeting an average response were used to complete this task. The Six Sigma tool, a mathematically sound statistical technique, was used to accomplish this undertaking. After mapping out the entire printing operation, a list of factors were generated for the semiautomatic printer and solder paste used for this experiment. The semiautomatic printer used on the line had a manual setting with a gauge for the print pressure setting. The solder was a high-metal-content, fine-pitch mesh paste.
Experimental DesignFirst experiment. After careful consideration of all of the possible parameters associated with the stencil printer, factors were selected for experimentation. Table 1 shows the level settings associated with each factor in the experiment. The levels were set at bold limits, but not outside the inference space of possible processing conditions.
A two-level fractional factorial design was used with a resolution value of four. This design is known as a screening experiment where factors with the largest effects are identified. Resolution refers to the amount of information that may be obtained from an experiment. In a resolution IV design, main effects are separated from other main effects and any two-way interactions that may exist. This specific design allows for main effects to be "screened out" of the experiment, but an inability to separate those interactions from themselves remains. The full array of test combinations used in this experiment is listed in Table 2. All test combinations were assembled in random order to minimize the effect of experimental error of any factors not controlled in this experiment.
An assembly composed of numerous fine-pitch components with a high solder defect rate was selected. The stainless-steel stencil had a thickness of 0.007". The solder paste supplier remained the same; the paste was comprised of a 90 percent metal content with a -325/+500 fine-pitch mesh. Normal storage procedures and packaging of the solder paste were performed. Environmental conditions, such as temperature and humidity, were monitored and treated as "noise."
A solder paste inspection machine* was used in this experiment to measure the printed paste pads. This off-line piece of equipment had been previously verified to have accuracy and precision. The laser system uses the board fiducials to give it excellent repeatability. A 2-D inspection of solder paste length and height was performed on each assembly. Five different component locations were measured on the PCB. Three of the measured component locations were 0.020, 0.025 and 0.030" pitch pad. The length and height of the printed paste pad were measured and recorded at each test combination. These measurements were then compared against the aperture size and thickness of the stencil and used as the experimental response variable. The measured output was the summation of all five measured paste pad areas as shown in the following equation:
Area = Absolute value of (length x height measured - length x height of stencil aperture)
Results and DiscussionSignificant factors. The main effects were printing speed, stencil type, board support, vacuum and thaw time of the solder paste. The factors that were found to be significant were not unexpected. Printing boards at the slower speed usually gives a better "brick" definition. Literature suggests that laser stencils provide exceptional dimensional accuracy over metal-deposition and chemically etched stencils when printing through fine-pitch stencil apertures. Also, installation of a board support jig underneath the PCB during the printing operation allows for a more rigid surface to be maintained when the squeegee blade travels across its surface. One of the fundamentals of solder paste printing is holding the PCB firmly in place by either vacuum or mechanical means.4 Following the manufacturer's recommendations for thaw time of the solder paste was also significant in the experiment for printing fine-pitch component paste pads. The objective was to determine if stricter controls needed to be defined for thawing the solder paste.
Two-way interactions were also found to exist in this experiment but were confounded with other interactions. Confounding is when the mathematical pattern of a certain factor or interaction is equal to the same mathematical pattern of other factors or interactions. Detection of which interactions were the most significant was difficult, as these two-way interactions were confounded with numerous other two-way interactions. The objective of this experiment was to screen out the factors with the largest effects and then move to another experiment to find interactions and any other factors.
Next experiment. In the following experiment, many of the factors from the initial screening experiment were held constant. A laser stencil was used to apply solder paste onto all PCBs. All were processed with the vacuum option turned on and with the board support jig installed. Processing conditions remained the same as the initial screening experiment, and environmental factors were again recorded and treated as noise variables.
Another fractional factorial design was selected for this experiment. Five factors (Table 3) were manipulated in this resolution V design that allowed for all main effects and two-way interactions to be analyzed without any confounding patterns. Print pressure and snap-off distance were again chosen because it was believed that these factors had an effect in the first experiment, but were not seen because of the confounding patterns. The level settings were changed for the solder paste thaw time because they may not have been bold enough. One of the level settings was changed for the printing speed to further optimize this factor. Level settings were also changed for the print pressure and snap-off distance based on the visual observations at settings from the first experiment. Print direction was also added to the experiment based on previous observations that differences between the front and back wipes of the squeegee blade might be occurring.
All test combinations were assembled in random order to minimize the effect of experimental error of any factors not controlled in this experiment. As with the initial screening experiment, the solder paste inspection machine* was used to inspect the PCBs. The same PCBs used in the first experiment were used for the treatment combinations. The treatment combinations and results of this experiment are shown in Table 4.
The experiment yielded one factor and two interactions as being significant. The main factor was print pressure. Print speed/print pressure and print pressure/snap-off distance interactions also existed. The findings of these results were to be expected, as literature suggests that these are critical variables in the stencil printing process. Because a fractional factorial array was used in this study, not all combinations were tested. However, a least-squares method was derived using the empirical data from the main factors and interactions. The equation in Figure 1 was used to predict the outcome of the printed paste pad area, which allowed for optimization within the inference space of this experiment. The predictive equation allows values to be selected inside the experimental inference space to estimate a printed paste pad outcome at a 95 percent confidence level.
Figure 1.
Test subject. Using statistical software, a predictive profile was generated for optimization of those significant effects. The desirability functions of the predictive model (Figure 2) showed that the print pressure should be set at contact+100 and the snap-off distance at 0.010". The print speed was selected to be 0.750" per second based on an analysis of its interaction with print pressure. Initial implementation of these experimental findings required a pilot study. A decision was made to try the parameters suggested by the predictive profile on another assembly with fine-pitch components in its design that was also notorious for its defect rate.
Figure 2. Desirability function for significant effects.
For the pilot study, any factor that was held constant in the previous two experiments again remained constant. The solder paste thaw time from the refrigerator was held at 24 hours. This matched the supplier's recommendations, as the results of the second experiment did not show a 48-hour thaw time to be any better. Print direction was not held constant, as the results of the second experiment did not show significant variation between the front and back wipes of the squeegee blade. Print pressure was set at contact+100, snap-off distance was set at 0.010" and printing speed was set at 0.750" per second. A statistical analysis of the measurements of 20 PCBs (Table 5) shows that the values fit a normal distribution. These data points fitting a normal distribution shows that the most commonly occurring data points fell in the middle of the normal distribution curve and that the least common values tail off symmetrically in both directions. All assemblies were processed through its usual production sequence. Upon reflow soldering of the assemblies, visual inspection to IPC-A-610 Class III requirements showed 0 solder defects with these test samples.
Limitations of the StudyThis study provided an optimization of the stencil printing process by defining those factors that have a significant effect on manufacturing defects. Some limitations were evident:
- Conclusions could only be drawn from the specific equipment used in this experiment.
- Solder paste suppliers and type were not analyzed, which could be performed in follow-up experiments.
- Other factors exist, such as aperture design and squeegee blade type, that could also improve processing. Budget and time constraints limited these opportunities.
ConclusionThe philosophy of Six Sigma can lend itself to reducing defects and variation in all processes. Many of the mathematical and statistical techniques found in the Six Sigma toolbox can be applied throughout the manufacturing process to minimize costs and increase productivity. Specifically, the power of the tool was demonstrated by the specific optimization of the stencil printing process on this SMT line. A progression of two experiments found many effects and interactions that needed to be controlled to reduce defects on the line. Since the completion of these experiments, procedures have been put in place to define the correct material, handling and equipment needed for operation of this process. At the same time, control charts have been installed on the line to verify and track any variation in the solder paste length and height on all fine-pitch PCBs. More importantly, there has been a dramatic decrease in defects and rework on the SMT line.
- Malcolm TD-3.
REFERENCES1 Phil Zarrow, "Vive l'Evolution," Circuits Assembly, December 1997, p. 24.
2 Bob Ries, "3-D Post-Printing Inspection," Circuits Assembly, June 1998, p. 40.
3 Richard Clouthier, "The Complete Solder Paste Printing Process: Stencil Aperture Area Aspect Ratio," SMT's Guide to Soldering & Printing, January 1999, p. 6.
4 Phil Zarrow, "Step-By-Step SMT, Step 4: Printing," Available: http://www.smtmag.com/stepbystep/ step4.html.
JOHN V. STEVENSON is senior process engineer at Fusion Lighting Inc., 7524 Standish Place, Rockville, MD 20855; (301) 284-7240; Fax: (301) 926-7258; E-mail: JohnS@fusionlighting.com. DEREK DRABENSTADT is senior process engineer at Veeder-root Co., 6th Ave. at Burns Crossing, P.O. Box 1673, Ahoona, PA 16603; (814) 696-8105; Fax: (814) 695-7605.