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Understanding the Requirements of a Mass-Imaging Platform: The Impact of Interconnect Miniaturization
June 25, 2008 |Estimated reading time: 12 minutes
As the frontiers of feature size and interspace between devices become increasingly challenging, the industry must reevaluate the performance of the toolsets used in the SMT arena. The days when a tolerance of tens of microns was acceptable are long gone--acceptance has now moved into the sub 10-micron domain. This issue is especially critical within the print process; it is this pre-placement toolset that is most sensitive to the miniaturization program running through the industry.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
When we take a step back and reflect on what is required from a printing process at this new level of miniaturization, we can quickly understand that the solder paste volumes required are crossing into semicon territory. Indeed, it is clearly possible to count the solder particles that make up a 0.3mm CSP deposit, the scale is so small. It is, therefore, paramount the composition of this pre-placement toolset is fully realized.
The intention of this paper is to break down the elements of a print platform into major mechanical and process subsections. Within the mechanical section, the elements investigated will be the co-planarity of rail systems and the tooling nest; whereas, with the process section, the elements investigated will be the composition of the squeegee assembly. Each element is fully explored using analytical methods to comprehend causes and effects. The separate modules will then be combined to enable an aggregate picture of the process and, thus, allow a conclusion of which component parts are the most critical and the level of accuracy required for the SMT challenges ahead.
Experimental Process
For each experiment, the following set-up was conducted--the solder paste used was a type 4 lead-free commercially available material, the solder paste was refrigerated between each run and allowed to stabilize to room temperature before use and the material was replaced after five refrigeration cycles.
The stencil used throughout the investigation was a 100-micron stainless steel, laser-cut, mesh-mounted foil; the aperture dimensions and metal thickness were measured and these values were used to calculate the process capability.
The 10 substrates used were pre-numbered and run in order; the board design followed the Dek08 pattern and was fabricated from 1mm FR4. The 0.4mm QFPs and CSP were the focus of this investigation (Figure 1). The 0.4mm QFP appears in opposite corners of the substrate and this feature is used to understand if the deposition process is symmetrical. The 0.4mm CSP represents the smallest and most challenging device; this will be used to understand the deposition capability.
The solder paste was measured using a Cyber Optics SE300 inspection machine; the machine was calibrated before the investigation and was subject to a satisfactory gage R&R. The process set-up was reset to the same level before each experiment, the stencil was cleaned using an automatic ultrasonic Sabermax stencil cleaner, the solder paste was loaded on the front of the image and the squeegee pressure mechanism was calibrated.
This investigation was split into two areas of research--transformed process elements and simulated interspace of the print process. The print platform was checked against the manufacturers calibration and build specification after each experiment and, thus, only the forced change was affecting the response.
Figure 1: Image of test substrate.
Transformed Process Elements Module
The process elements that were adjusted for this investigation were squeegee angle and blade material. The motive for exploring the angle was attributable to the fact that the manufactured attack angle is instantaneously changed with the inclusion of pressure; this phenomenon is illustrated in Figure 2.
Figure 2: Illustration of the effect of attack angle with respect to pressure.
Since the attack angle is a well documented variable within the print process, the approach of this investigation was to fabricate a variable angle squeegee assembly such that the attack angle could be independently set. Figure 3 shows an illustration of this assembly. The outcome of this work was to understand if a specific angle gave an increase in deposition capability.
Figure 3: Drawing of the adjustable squeegee assembly.
The average deflection of the squeegee blade was measured to be 8 degrees under the following conditions: 200-micron thick stainless steel squeegee blade, 15mm overhang, length of 200 mm and 5 Kg of pressure. It was decided that a range between 35 and 60 degrees would give ample resolution for this investigation.
The finish of the squeegee blade was also investigated and it was concluded that the interface between the solder paste and the metal face of the blade would contribute to the print process. It was decided that the investigated finishes would include the standard stainless steel blade, a stainless steel tetra carbon coated blade and a chromium-coated blade. The purpose of the coating was to introduce a reduction of surface tension and therefore reduce the "stick-scion" between solder paste and squeegee blade.
Simulated Interspace Module
The simulated interspace elements that were adjusted within this investigation covered the co-planarity of the following interfaces--stencil, substrate and tooling.
The motive for including these elements in the investigation were attributed to the fact that within a print process three individual components are interfaced during the print stroke; these been the tooling, board and stencil. If a process is set-up incorrectly, it is possible to create interspaces at these interfaces (Figure 4).
Figure 4: Diagram showing the possible interspace conditions within a print process: a) correct set-up, b) interspace created between substrate and stencil and c) interspace between tooling and substrate.
To fully understand the importance of these individual elements, and their impact on the print process, a set of experiments were set-up to identify the impact.
The first experiment conducted isolated the impact that the co-planarity of the substrate to stencil had on the print process. To achieve this objective, the following techniques were used: the rail system was systematically deformed using a shimming material, this gave the ability to "dial-in" a predetermined amount of deformation and therefore adjust the substrate to stencil co-planarity, the overall effect of this would be to produce a varying amount of interspace between the top of the substrate and bottom of the stencil.
The second experiment within this interspace module was to investigate the effect of creating an interspace between the tooling and substrate. Figure 5 outlines the test strategy. Figure 5: Break down of experiments.
Results
The method of contrasting and comparing the results from the separate experiments was carried out by statistical analysis (Minitab) and surface profiles of individual solder paste deposits. This approach allows for both quantitative and qualitative aspects to be used in the analysis, an important factor as the market both demands a perfect looking print (sharp definition, flat top, etc.) and the statistics to prove that the process is stable and capable.
The limit sets used to calculate the Cp and Cpk values are shown in Table1, the stencil measurements were used to calculate the theoretical volume; the transfer efficiency and tolerance were derived from previous assembly investigations.
Table1: Limit sets.
The results of the investigation are shown below--each experiment has the statistics and associated charts followed by the surface profiles of the solder paste deposits (for reporting purposes, only board 5 is shown).
Transformed Process Elements Module
Experiment 1 and 2: Table 2: Statistics data from experiment #1.Table 3: Process capability charts from experiment #1.
Table 4: Surface profiles from experiment #1. Table 5: Statistics data from experiment #2.
Table 6: Process capability charts from experiment #2.Table 7: Surface profiles from experiment #2. Figure 6: Chart showing transformed process elements (experiment #1 & #2). Simulated Interspace Module
Experiment 3:
Table 8: Statistics data from experiment #3.Table 9: Process capability charts from experiment #3.Table 10: Surface profile from experiment #3. Experiment 4:Table 11: Statistics data from experiment #4.
Table 12: Process capability charts from experiment #4. Table 13: Surface profile from experiment #4. Figure 7: Chart showing simulated interspace (experiment #3 & #4). Conclusion
Before comparing and contrasting these results two general observations should be noted: 1) for most process engineers, the print process is the first process to be set-up and the last process to be questioned when things go wrong and 2) within the industry, it is a well-known fact that over 60% of all defects are attributed to the print process.
So, why do engineers tend to over look the print process? Why does the print process provide the majority of defects?
The first module focused on the transformed process elements; within this section, the squeegee attack angle and squeegee blade material were chosen as the transformed factors.
Data from Table 1 and 2 suggests that the outcome of changing the squeegee angle affects the process capability--increasing the squeegee angle from 35 to 40 degrees to 55 to 60 degrees causes deposited volumes closer to the nominal volume and increases the process repeatability. Decreasing the squeegee angle increases the volume deposit and reduces the process repeatability.
To better understand why this occurs, we need to consider what is happening at the tip of the squeegee. As the blade traverses over an open aperture the solder paste material, under hydrodynamic pressure, is forced into the opening. If the transfer force is too high, the solder paste material will start to compress and force itself under the blade and create a "wake." It is this compression and wake effect that creates the high volume and reduced process capability.
This "wake" effect is most notable when observing the QFP results in Table 2, with the blade set at a low angle the charts display a duel peaked curve, after further analysis this bi-modal effect represented the split between North/South and East/West deposits.
Analyzing the difference between these two aperture orientations makes it clear that the amount of time that the squeegee blade has to fill the North/South apertures (300 ms) is considerably longer than the East/West (46 ms). It is therefore possible that the high transfer force, associated with the longer fill time of the North/South apertures, creates a greater opportunity for the "wake" effect to take place and, therefore, higher volumes and reduced process capability proceeds. The East/West apertures have less fill time and, therefore, are not subject to "wake" issues.
The surface profiles shown in Table 4 also coincide with the observations discussed above--the deposit-to-deposit consistency increases and the QFP deposits become more "brick" shaped when the squeegee angle is increased from 35 to 40 degrees to 55 to 60 degrees.
The second experiment in this transformed process section was to understand the influence the squeegee blade material has upon the print process. The two materials chosen were Tetra carbon and Chromium--both surface finishes are recognized for their friction reducing properties.
From the results shown in Table 5 it is clear to see that the Chromium finished blade is the clear winner from this "bake off," the chromium blade performs well when imaging the 0.4mm CSP device. This would indicate that the property of the chromium coating positively influences the interface of solder paste to squeegee blade and consequently improves the filling of small apertures.
The results from the surface profile (Table 7) show a slightly different picture. The profiles from both tests show very little difference in print quality--both look acceptable. It is by analyzing this scenario that we can start to understand why a print process could be overlooked when fault finding an end of line yield issue. Taking this example into a real life situation, the engineer would look at the profiles from the Tetra carbon results and almost certainly sign off the process as acceptable, but the data is otherwise indicating a different story, it is understandable how a print process could be wrongly diagnosed without a full examination.
The results illustrated in Figure 6 overlay all the results from the transformed process module; it is clear to see that a 55 to 60 degree squeegee coated in chromium gives the best overall results.
The second module focused on the influence of simulated interspaces within the print process; within this section the effect of interspace relating to the stencil to substrate and tooling to substrate were investigated.
The results from Table 8 and 9 illustrate the relationship of increasing the interspace between the stencil and substrate and print quality. The statistics indicate that as the interspace is reduced the process indices increase; thus, signifying greater process control. The surface profiles shown in Table 10 also confirm this theory.
Investigating the information shown in Table 9 provides a clear understanding as to why this phenomenon occurs. It can be seen from the 0.4mm CSP (300 microns and 200 microns) distribution chart two distinctive peaks are visible. Unlike the QFP device, the CSP is not asymmetric, therefore, this twin peaks effect is not related to the fill process, but more likely associated with the release process.
To better understand how this effect could cause poor print quality, we need to consider that, during the moment of aperture fill, the squeegee is pushing (gasketing) the stencil onto the substrate. As the squeegee moves away from the aperture, the stencil will start to peal away from the substrate causing a "false separation" step. It is during this "false separation" step that the material inside the filled apertures will be subjected to interference and, depending upon the amount of stencil peel and friction of solder paste to its interfaces, will depend how "fractured" the solder paste will become inside each independent aperture. This effect is most striking when evaluating the surface profiles from Table 10 and the print deposits of the 0.4mm CSP exhibit a "fractured" structure in which the deposits are varying in shape and volumetric quantities.
The reason why the <100 micron interspace results showed no detrimental effect can be explained by the principle that if the "stencil peel" does not exceed the solder paste deposit height, then the solder paste within the aperture will not have been completely fractured and will still follow the standard release method.
The results from this experiment conclude that if the interspace is greater than the stencil mask thickness, the influence of the "false separation" will cause a detrimental print quality.
The second experiment in this module investigated the influence of including an interspace between the substrate and tooling assembly. The results from this experiment are shown in Tables 12 and 13. As can be seen from the statistical data, the addition of the interspace decreased the print quality in both accuracy (Cp) and repeatability (Cpk). Analyzing surface profiles reveals a similar image as those of the stencil to board interspace results; thus, it can concluded that this tooling to substrate interspace has also caused a "false separation" step within the print process.
The results illustrated in Table 7 overlay all the results from the simulated interspace module. Any interspace reduces the process capability--this is especially apparent on the fine pitch CSP devices.
Throughout this investigation, the following discoveries have been made:
Squeegee angle significantly influences the print quality.
The squeegee material significantly influences the print quality.
Interspaces between the substrate and stencil, greater than the print thickness, significantly influences print quality.
Interspaces between the tooling assembly and substrate, greater then the print thickness, significantly influences print quality.
- Quantitative and qualitative processes are not always in agreement.
Merely observing print quality does not tell the whole story. The print could look acceptable, but the capability of that process is questionable. This situation would lead an engineer to wrongly accept the process capability and overlook the print process.
We have also seen that very small changes can have large impact; the inclusion of an interspace within the print set-up has been highlighted as a major cause of variation. In the real world of solder paste printing, these interspaces can be introduced through incorrect substrate solder mask thickness and incorrectly manufactured tooling assemblies, etc. These influences can cause the reported headline rate of 60% defect associated with the print process. The engineer must fully appreciate the sensitivity that these small "environmental" influences can have on the solder paste print process.