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Understencil Wiping: An SMT Blind Spot
December 31, 1969 |Estimated reading time: 8 minutes
The complex stencil-printing process is at the heart of SMT production. With more than 39 variables to optimize in the printing process, it’s no surprise that 50-90% of all defects originate on the stencil printer. Changing stencil roll paper, however, may improve yields. This paper compares three types of stencil rolls and suggested improvements.
By Kimberly F. Abbett and Michael D. Jones
It is hard to survive without making assumptions. Just as computerized “data compression” programs such as “ZIP,” “STUFFIT” and “JPEG” make large files more manageable, humans use brand names, political parties, logos, “trusted sources” and assumptions to minimize the brain-strain about new people, products and events.
In computers and people, an unwanted side effect of data compression is data loss. As multiple JPEG compressions significantly can damage digital images, human “compression schemes” add fuzziness to our perceptions. Taken to extremes, our assumptions become “blind spots,” which can quietly undermine logical thinking and complex analyses. These blind spots are frequent enough that they become a perceived truth.
The worst aspect of blinds spots, of course, is their invisibility. We all have them, but cannot see them. We forget the information that has been lost; and the risk of error increases as our assumptions solidify. It was precisely this scenario that nearly derailed a recent R&D project. Despite having developed a better mousetrap, one researcher nearly was stymied because SMT process engineers had developed a blind spot about the process of understencil wiping.
Into the Lab
One company* that makes the paper used in stencil rolls saw signs that stencil wipes were not keeping up with advances in SMT assembly. Anecdotal evidence accumulated about falling yields, problematic solder joints and problems with lead-free pastes. A researcher reviewed the situation, aiming to determine if there was a problem; and, if so, what fabric would solve it.
Stencil printing is an intricate balance of technologies, therefore, this is a difficult process. Process engineers must optimize as many as 39 processes to obtain profitable yields.1 Several industry experts have estimated as many as 50 to 60% of all SMT defects stem from the stencil printing process,2 and some believe that the real number may be closer to 90%. Therefore, it would seem that fixing the stencil-printing process could fix the entire SMT line.
The researcher assembled a wide array of synthetic and natural fibers for testing. In general, there are two types of papers used in stencil wiping: papers made with synthetic fibers held together with glues (called binders), and papers made from a mix of natural and synthetic fibers that do not use glue.
Synthetic fibers found in paper usually are polyester or rayon (Figure 1a). In general, about 30% of the paper’s weight is found in binders. These papers have a harder surface, lower tensile strength, absorb less contamination and are less expensive than glue-less versions.
Figure 1a) Cross-section microphotograph of polyester paper made with binders, enlarged 200 times. The fibers are coated with glue, making surfaces hard and unable to absorb contamination. Most empty space in the wipe has been filled with binders, reducing the ability to pick up solder paste.
Other wipes** are made using a process called “hydro-entangling.” This process involves laying raw fibers of polyester and cellulose onto a conveyor belt and intertwining the fibers with high-pressure jets of water. Heat and pressure then are used to dry the slurry into the proper configuration. If lacking glue, hydro-entangled papers have a softer feel. They are more absorbent, thicker and stronger than ordinary paper (Figure 1b).
Figure 1b) Cross-section of microphotograph of a hydro-entangled polyester cellulose paper enlarged 200 times. The flat, rough cellulose fibers (thin, top layer) are highly absorbent, but weak and prone to lifting. The rounder polyester fibers (middle and bottom) provide structural strength and open space to pick up solder paste.
A new material consisting of a hydro-entangled, single-fiber polymer based on regenerated cellulose was developed (Figure 1c). This fabric, which is completely synthetic, contains no natural fibers or binders. Under a microscope, the long, thin, hard fibers are identical and homogenous, giving the fabric an open structure. The material is a pale, off-white color and highly absorbent. The fabric scores well on the solder paste pick-up tests for non-liquid contamination.
Figure 1c) Cross-section of microphotograph of hydro-entangled paper enlarged 200 times. The fibers are smooth, consistent and hard. The paper includes extensive voids and cavities to pick up solder paste. There is no cellulose to rip or shred, or binders to dissolve and contaminate the stencil.
All potential fabrics were tested; and typical results are presented in Table 1. This research answered two questions. First, it proved that there was substantial variation in the ability of different fabrics to remove solder paste. Second, one fabric was able to consistently out-clean and out-perform other tested fabrics (at least, in the lab). How it would work in the field, however, remained unknown.
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Field Trial Headaches
The first problem encountered was that few engineers were interested in testing new stencil wipes. They were eager to test new printers, squeegees, solder pastes and stencil designs. But there was great resistance to testing new stencil rolls. They felt the stencil wiping paper made little difference to the printing process. This was a huge blind spot.
After nearly two years, a few engineers agreed to test the fabric. More than 1,000 stencil rolls were consumed in field trials throughout the U.S., Mexico and Europe. During these trials, it was discovered that fabric out-performed both traditional, hydro-entangled fabrics and polyester-binder fabrics in several areas. The most significant improvements were found in the areas of improved yield, reduced rework and cost reduction of consumable supplies.
Yield Improvements
A large U.S. subcontractor administered one of the first field trials. The test was run on five SMT lines using a solder paste*** and a stencil roll with binders. Normal yield of 97.2% was lower significantly than the yield using the developmental paper, which jumped to 99.5% on the 392 boards tested. This was a promising development.
A separate test in the U.S. studied the quality of the solder joints. Using 2-D and 3-D inspection systems, engineers noticed an 8% increase in the average height of solder paste deposited on each inspected pad. The result was a statistically significant increase in yield due to fewer solder paste problems, such as solder balls, voids, insufficient solder paste and solder bridges, and a concomitant reduction in rework.
One of the most intriguing test results came from a facility in Mexico. It has been suggested that solvents have a deleterious effect of the rheology of solder paste. When engineers discovered that the developmental paper out-cleaned the current stencil roll, he reprogrammed the printer to clean without using any solvent (wiping dry). The result was a reduction in defects from a rate of 8,104 parts-per-million to zero parts-per-million. The combined test run was 4,000 boards. Further research is planned to examine if this result can be duplicated in other facilities.
The most clear-cut measurements of improvement came from a large subcontractor in northern Europe. Using paste on printers with electroform stencils, they were having trouble sustaining yields. Part of the problem was their current stencil roll, a polyester-and-glue product that provided a capability index (“CPk”) of 0.88.3 Because the company’s standard was CPk 1.30, engineers knew they had a problem. However, due to the stencil blind spot, they were unable to resolve it until samples of the developmental paper were available.
The paper delivered a CPk of 1.70, substantially better than a 5-sigma quality level. Standard deviation of the old paper was twice the size of the new paper. This means that the new paper enabled a printing process that was tighter around the mean; so fewer boards will be produced outside printing parameters. Using a few rough estimates about board size and density, this printing performance suggests that old paper would produce 800 bad solder joints every 2000 boards, while the new paper would have none. In short, changing the stencil roll added more than 2 sigma to the production quality.
Rework and Consumables
Everyone in the industry acknowledges the expense of rework. If an enterprise could focus on Philip Crosby’s “do it right the first time” program, savings would fall to the bottom line.
Rework was the focus of a prolonged test in Hungary. When the developmental paper was used compared to the previous version, first pass yields jumped from 95% with the old paper to 99%, leading to a 77% drop in rework. Rework savings were estimated at 168,000 euros annually.
Another company in Hungary found an unexpected savings from the developmental paper. This company was able to reduce cleaning cycles per shift from 50 to 25. This doubled the life of the stencil roll, resulting in an overall reduction in roll costs of 12.4%. Because large facilities often have six-figure budgets for stencil rolls, these savings can be substantial. Related savings were found in misprints and solder-paste waste.
Conclusion
This research challenged the conventional industry wisdom. Statistics show significant differences in the performance of different stencil wiping papers. One stencil wiping fabric also offered improved performance in laboratory tests. In the field, this material significantly improved yields, rework and the cost of consumables. This research helped lift the blinders from the eyes of the industry, showing that a small issue in the printing process - paper - can have large consequences on yield and cost.
* DuPont ** DuPont’s Sontara stencil wipes *** Indium 92J solder paste
References
- Richard Clouthier, “The Complete Solder Paste Printing Process: Stencil Aperture Area Aspect Ratio,” SMT Magazine, January 1999.
- “Paste Printing,” Assembly Magazine, April 1, 2003.
- “Capability Indices” are beyond the scope of this article, but generally measure the production performance of a system in relation to a specified standard deviation and mean. A CPk of 2.0 equals Six-Sigma quality. For more details, visit www.qualityadvisor.com; search for “CPk” references.
For a complete list of references, please contact the authors.
Kimberly F. Abbett, Ph.D., senior engineer, DuPont Corp., may be contacted at (219) 508-2886; e-mail: kimberly.f.abbett@usa.dupont.com. Michael D. Jones, vice president, Micro Care Corp., may be contacted at (860) 827-0626; e-mail: mikej@microcare.com.