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Pastes, Stencils, and Process Control
December 31, 1969 |Estimated reading time: 9 minutes
Concern for process defects is one reason for the growth in SMT. Paste and the printing process account for most process defects. Proper understanding of control of this part of the process will cut costs. Because high-quality stencil supply is not always guaranteed, companies should consider an in-house option. But interaction between different parameters is complex and requires further study.
By Armin Rahn, Ph.D.
With all the attention given to the elimination of lead in the soldering process, a real necessity only in the EU with RoHS - many important aspects for the electronic-joining process seem to be pushed into the background. However, production must continue and cost control is essential. Perhaps no issue has a greater impact on production costs than the defect rate.
Professionals speaking of defects may refer to several definitions. On one hand, failure of joints during usage, i.e. defects that relate to reliability questions, seem central to lead-free considerations. Then there are functional defects that occur during production, especially during soldering, such as bridges or opens. A third category of defects are often described as process indicators, which are not so serious that they require immediate intervention, but bothersome enough to reconsider certain parameters of the process. Another defect relates to components that are out-of-spec or do not work.
When analyzing the sources of process defects in surface mount production, studies identify paste, paste handling, and printing as the main causes. Depending on the study, percentages range from 40-70%. Reaction to such a Pareto diagram would indicate that this is where action should begin. Studies show that placement and reflow are of minor importance as they contribute to about 10-15% of possible defects, all of which excludes important items such as design, layout, printed wiring board (PWB) manufacturing, quality of components, and properties like solderability of boards and components.
One of the reasons SMT combined with reflow is gaining ground over flow soldering (often called wave soldering) is the possibility to control a reflow soldering operation at lower process-defect levels than with the uncertainty of the dynamics during the creation of liquid solder flow through imprecise pumping methods. Any management that recognizes defect costs as a major impact on production cost gravitates to the process instinctively or strategically that promises lower defect levels. However, this promise can only be realized with major effort and continuous control measures.
Paste
For paste treatment, we have developed rules that range from paste purchase to discarding, which a number of paste manufacturers have adopted (See Sidebar). These rules are necessary if one wants to eliminate a possible source of errors that can creep in when critical procedures are not controlled constantly. These rules are based on physical and chemical properties of different constituents of the paste, and err on the safe side. The main contributing cost is not in the price of the paste, but in loss due to problems during production or later in the field. An hour-long halt in a major production line outweighs the cost of a tin of paste by a factor of several hundred, if not several thousands of dollars. Management should realize that the occurrence of one defect can be more expensive than 500 g of paste.
Paste Printing
The printing process is a complex interaction between paste, the squeegee, and the stencil. Some of the other factors affecting successful deposition of paste on the PWB include board finish; environmental parameters (humidity, temperature, air currents); squeegee speed and pressure; and attack angle. Fishbone diagrams do not add much to the general control capability. Despite the fact that the importance of these steps in reflow soldering processes was recognized early, a systematic approach and studies have been forthcoming.
As the printing process becomes the bottleneck in production, placement equipment have developed speeds that compare favorably with cycle times of printing processes. Although a variety of hollow squeegees had been developed, their promises rarely have been fulfilled. Slow printing is still preferable to fast printing. It is the result that matters in the end.
The Stencil
The stencil often is a neglected item. However, stencils have gained their prominence because of a number of factors, most importantly, the possibility to use the thickest pastes. Manufacturers control the viscosity of their pastes largely by the amount of metal that they stir into them. Research shows that higher metal content yields better results with regard to solder balls. The addition of thyxotropic agents (mostly derivatives of castor oil) has helped as well.
There are a number of models and methods that try to combine several stencil properties to develop an optimizing tool. However, instead of throwing many aspects into a blender and pushing a button to mix them properly, a pragmatic approach states, “The target is a perfect print.” It is the result of the printing process that defines the quality of this manufacturing step and influences all other steps. When stencil printing with a metal squeegee, a good print is one in which every deposit looks like a brick entirely flat at the top with straight sides and a height that corresponds to the thickness of the stencil. Scooping is not permitted, and with the recently increased angle (approaching vertical position) of the squeegee blade, scooping should be rare or can serve as an indication that the applied pressure is too high (Figure 1).
Figure 1. Solder bumps on a wafer.
Investigations during the improvement of the printing process should concentrate on measures that can achieve a perfect print. In several defect-reduction programs that have been carried out at different manufacturing facilities, progress was quick when concentrated on printing speed, separation speed, and pressure. However, when the height of the deposits was measured over the entire surface of the PWB in one of the plants, unsatisfactory results were recorded. Although individual deposits complied with the rule of steep walls and flat surfaces, height varied greatly (Figures 2 and 3).
Figure 2. Solder paste printed on wafer before soldering.
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Figure 3. Solder paste printed on wafer after soldering.
Experimenting with squeegee speed, pressure, and angle of attack brought no improvement. Frustration grew to the point where we mistrusted the stencil and began measuring its thickness at several different locations over the surface. Surprisingly, thickness variations up to 30 μm were recorded on a stencil thickness of 160 μm. This, for a stencil with nominal thickness (no step-down), was not acceptable.
The natural reaction was to go back to the stencil manufacturer, explain the situation, and ask for another stencil. The replacement electroform stencil came with the same problems. Discussions with some technical experts from one of the major printer manufacturers confirmed our suspicion that this was not an isolated occurrence. Deviations from the nominal thickness seem to be relatively common for electroformed stencils due to the manufacturing process. Uniform deposition of metal seems to be difficult to accomplish because of field variations during the plating process.
Because of some fine-pitch patterns, the alternative was to choose a laser-cut stencil. Problems can be found with all types of stencils,1 and manufacturing problems are inherent in PWB production with tolerances of pad locations within about 75 μm.
Because there is limited influence on PWB production, one can turn to the stencil. Laser-cut stencils are only as good as their source. Statistics indicate that variation in delivered quality can occur.2 The same stencil provider may show variations either due to cost-saving measures or lack of quality control. Starting with the quality of rolled stock, the nominal thickness of the stencil may be precise (within 2 μm) or show deviations that are unacceptable. Although laser cutting can be accurate (<5 μm over the entire stencil; better by a factor of four compared to electroform), not all suppliers ensure such precision.
In-house Laser Cutting
There is the possibility to control stencil manufacturing. Affordable equipment often requires a larger number of stencils due to a high-mix or large production environment. Having the laser-cutting equipment under control not only offers the possibility to make the appropriate minor adjustments required for the perfect stencil, but also opens companies up to the possibilities of cost-saving alterations. When configuring the data for stencil cutting, a number of items should be included in the final calculation:
- The tolerance of the PWB as received from one’s supplier - different suppliers hold tolerances at different levels. The better the location of critical pads with regard to fiducials, the easier it will be to accommodate such deviations during stencil manufacturing.
- PWBs may shrink during storage rather than expand, affecting production and the stencil. Geometrically distorting the aperture image is one measure that can be taken.
- Stencils can stretch over longer periods of usage. This is particularly true in cases with fine-pitch (thin stencils) printed in a non-contact fashion. As users anticipate stencil stretching, it may be decided to have the geometry of the stencil compensate for future elongation during laser cutting. Stretching depends on the number of prints and the printing process (contact vs. non-contact).
- The chafer given to the inner wall of the aperture is important. As the chafer can be controlled, a decision depends on printer-capability material of the stencil, the stickiness of the paste, possible delays during printing, and cleaning frequency. Another consideration may be experimentation with the new approach of reverse chafer. Other advantages become apparent with time, such as quick reaction when a stencil is damaged. Avoiding long line stoppage or delay of delivery is more than a fringe benefit.
Conclusion
In-house stencil production would not be possible without proper equipment. Neither electroform nor etching methods are suited for in-house processing as they require too many steps to complete, and controlling these steps is critical for high-quality stencils.
Laser cutting, on the other hand, comprises two steps and efficient equipment is available. Modern machines can accept any format data and convert it to cutting instructions. This includes not only location, but also beam diameter and choice of chafer. Stencil material may be chosen and is not limited to nickel. Stencils have been cut from stainless-steel, nickel, and even titanium or molybdenum. Cutting speed, although usually of little concern during in-house processing, has been improved considerably. Modern lasers can cut 50,000 apertures per hour.
Special developments can be attributed to applications such as wafer bumping or BGA balling. Here, stencil thickness and aperture size is reduced. Surprisingly, the same laser that can produce thin stencil foils will also be able to produce stencils as thick as 600 μm.3
REFERENCES
- Coleman, William E.; “Stencil Printing Studies,” IPC SMEMA Council; 2003.
- Shea, Chrys, et al.; “Printing and Profiling Fine Feature Devices,” IPC SMEMA Council; 2003.“LPKF MicroCut, Face the Diamond Line,” 2005.
Armin Rahn, Ph.D., consultant, rahn-tec consultants inc., may be contacted via e-mail: rahn@vaxxine.com. For more information, contact info@lpkfusa.com, or visit www.lpkfusa.com/SMTstencil.
Tips for the Storage of Solder Pastes
- Buy pastes according to manufacturing date, not expiry date.
- Store in refrigerator at 5°-10°C. This doubles the possible storage period from the manufacturing date.
- Most pastes should not be frozen. Cold storage lengthens storage time as chemicals will react more slowly and paste is more stable against separation. Storage that is too cold (<5°C) may cause crystallization of rosins/resins. Storage that is too warm may cause a loss of solvents and possible reaction of activators.
- Use airtight containers.
- Use a first-in/first-out storage system.
- Never store beyond the usable date.
- Store cartouche in upright position to limit danger of air bubbles. Older formulations may require horizontal storage with (occasional) rotation to avoid flux separation.
- Store paste in the dark as light can polymerize rosins.
- Shelf life of paste depends on storage temperature, for example:
At 7°C, some pastes can be stored for six months. At 20°C, some pastes can be stored for only one month. At 30°C, some pastes may be stored for just two weeks.