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Printing Difficult Applications

Successful ultra-fine feature solder printing has become difficult, more challenging with the continuing trend toward miniaturizing electronic assemblies, thus posing printing issues for difficult applications. At the same time, OEMs are demanding not only increasing reliability of their high-density PCB applications, but also high, near 100 percent yields, a crucial requirement in today’s highly competitive markets.

Solder Paste Printing Challenges

Greater numbers of PCBs are populated with mixed technologies, combining 0.4 millimeter (mm) and 0.5 mm pitch technology. Flip-chip, CSP, WLP, and BGA packages co-exist on the same board as these technology developments continue. Also, there are some traditional SMD connectors requiring thick solder paste. Solder-charge SMT multiple-pin connectors also require thick solder paste. But 0.4 mm pitch CSP and small bump flip-chips need a thinner stencil to achieve a higher area ratio by acquiring sufficient solder paste release.

Flip-chip and fine-pitch CSP packages normally use no-solder-mask defining (NSMD) pads because there is a large registration tolerance (+/- 3 mil) for the solder mask. If a paste printing process is used, the tendency toward short defects becomes greater. Also, there is a major tendency toward opens for small-bump CSPs and flip-chips (including WLPs) due to the thickness of the solder mask layer, plus most pick-and-placement (P&P) machines don’t have a force-control capability. This means placed chips are not evenly placed into paste during the P&P process. Chip weight alone is not sufficient to overcome the solder-paste surface tension during the reflow stage, resulting in open solder defect of one or fewer bump-joints.

Solution and Limitations

0.5 mm pitch fine-pitch CSP/flip-chip/WLP/BGA components normally have 9.8 to 11 mil pad diameter. Mask quality and registration are stable and improved due to the large gap. A 2-mil stencil aperture size can be increased (from 11 mil to 13 mil) to achieve a 13 mil stencil aperture (round). This means a 0.81 area ratio for the 4 mil thickness stencil.

0.4 mm pitch fine-pitch CSP/flip-chip/WLP/BGA components have difficulties in getting effective paste release and acceptable final assembly quality. A 9.5-mil pad size and 6.25 mil gap don’t provide enough room for a stable mask attachment, and safe clearance between the two pad paste depositions. Here, stencil aperture cannot be increased to increase area ratio. Due to the +/- 3-mil mask registration, there could be some partial exposed trace/pads. Sometimes, as a worst case, mask protection is lost. It’s best to use a
3-mil thick stencil with 10-mil round aperture and 0.83 area ratio.

The popular 100 µm (3.9 mil) diameter pad of 100 µm bump (CSP/WLP/flip-chip) poses a difficult situation for paste releasing (for 3 mil stencil, area ratio is 0.33). This very small pad is not designed for the paste process. The best approach here is to use a flux-only process with force-control P&P machines to avoid the stencil printing process. For the solder paste process, you should use 8 mil pads (200µm bump diameter) for CSP, WLP and flip-chip. Therefore, 3-mil stencils with an 8-mil circle aperture can be used (area ratio 0.67, no aperture size increase).

There are three options for the stencil materials. The chart shows the characteristics of each stencil material. Experience tells us to use new stencil material (instead of using a standard 301 stainless A) to improve paste-releasing capability. An alternative solution is to use 2-mil stencils with 8-mil circle aperture (area ratio 1.00, no aperture size increase). Also, it’s important to prepare a couple of 2-mil stencil for a large quantity production jobs because the 2-mil stencils have short working / operational life.

Stencil A B C
Material Standard 301 Stainless Fine-Grain 304 Stainless Ultra-Fine Grain Stainless
Crystal Grain Size (mircon) 25 – 30 9 1 – 2
88% Paste Releasing AR 0.75 0.62 0.55
85% Paste Releasing AR 0.66 0.57 0.51
Performance Standard Better Best
Surface Hardness Standard Very Hard Super Hard
Stencil Flatness Standard Very Flat Super Flat
Aperture Wall Surface Smoothness Standard Very Smooth Super Smooth

For prototype jobs with frequent layout changes, stencil life is not an issue. For simple boards, area ratio of more than 0.75 can be easily achieved through stencil foil thickness selection. For 0.4 and 0.5 mm pitch mixed complex jobs, 3 or 2- mil stencils cannot be used due to insufficient solder deposition on the connectors and non-fine-pitch components. You can do some aperture modification by increasing the size and changing the circle to square shape with round corners to get better area ratio paste deposition.

Newer stencil material developments provide the industry with new directions to improve the paste printing process. The traditional standard stencil foil uses mil-grade 300 series stainless steel material regarded as standard material Stencil A. It has a rough aperture wall surface featuring 25 – 30 µm grain size. The laser-cut mil-grade stainless stencil has an 88% paste releasing volume for a 0.75 area ratio. For a 0.66 area ratio, 85% paste releasing volume is recommended.

Without even a price increase, one can get a new and better stencil (Stencil B). For example, the new 304 series stainless steel is 4 specifically designed for laser-cutting and solder paste stencil printing. Called a “fine-grain material,” it has 9µm grain size. The laser-cutting fine-grain stencil has an 88% paste releasing volume for a 0.62 area ratio. For a 0.57 area ratio, 85% paste releasing volume is achievable.

With a price premium, you can get the best stencil, Stencil C with a smooth aperture wall surface. This newly developed ultra-fine-grain material features 2 µm grain size. This is 304 series stainless steel specifically designed for ultra precise laser cutting and solder paste stencil printing. This laser-cutting ultra-fine-grain stencil has an 88% paste releasing volume for a 0.55 area ratio. For a 0.51 area ratio, 85% paste releasing volume is achievable.

The new stencil material saves time and money and delivers significant benefits in comparison with 300 series stainless steel stencil including:

  1. Flatter Stencils – allows for more accurate printing with improved registration and print accuracy (including paste height control and paste releasing volume control).
  2. Harder Surface – provides a more durable stencil increasing cycle time between stencil changes, increasing stencil life for the thin stencils (2 mil and 3 mil).
  3. More Uniform Surface Aspect – allows for higher stress relaxation properties so stencils that are laser cut or etched will have more consistent results.
  4. Uniform Fine Grain Structure – yields smoother aperture walls, better paste releasing and will yield better results from zero cutting when step etching is involved.
  5. Superior Fatigue Strength – Exceptionally Smooth Surface and Elevated Wear Factor – stencils that ordinarily would require electroforming can now be laser-cut on the new stainless steel saving time and money.
  6. Super Fine Crystal Grain – it is achieved by reducing carbon which moderates re-crystallization behavior and prevents the formation of Cr23C6 (Chrome-carbide – seen as smut and rust on the surface – which prevents formation of a passivation layer); adding Nitrogen which gives solid solution strengthening; and the addition of Niobium which yields ultra-fine grain size and provides secondary protection against the formation of Cr23C6.
  7. Greater Hardness and Longer Cycle Life – super fine crystal grain, discussed above, results in a huge increase in the number of crystal grains in the stencil and thus the number of crystal grain boundaries. Since energy is absorbed at the grain boundary, the increase in grain boundaries yields higher energy absorption. This provides a stencil material with hardness greater than 430 HV and has a longer cycle life before failure in the applications.

Other Contributing Factors To Improved Printing Process

  1. Three standard global fiducial marks on the three corners of a PCB would help achieve accurate printing paste registration and accurate component placement.
  2. High performance paste printer with wet print deposit accuracy and repeatability ±25 microns (±0.001”) at six sigma, process capability index (Cpk) of greater than or equal to 2.0. The higher the Cpk, the lower the variability with respect to the process specification limits. In 6 sigma process, the Cpk is greater than or equal to 2.0.
  3. Tightly controlling the uniform thickness of the stencil foil is extremely important. The supplier provides ultra-fine-grain stencil material with ±0.2 mil across the foil surface. This thickness tolerance directly impacts paste height control tolerance and final quality of printing.
  4. Tightly controlled surface tension of framed stencil is much better than foil stencil or universal frame-less foil. It provides aperture position accuracy as well as high paste registration accuracy. For a used stencil, check the foil tension before printing to avoid solder bridging and insufficient solder defects.
  5. Fab warpage (twist and bow) will cause paste printing quality problem. It’s suggested that 0.75% (7.5 mil per inch board length dimension) be used as max warpage tolerance for standard products. For high end products, 0.5% should be used as max warpage tolerance.
  6. For all BGA, CSP, WLP, flip-chip, LGA, multiple-row QFN components, 100% post-reflow x-ray inspection should be performed and all images saved with the board serial number and reference locations. All defect information should be saved, and 100% post-placement x-ray inspection to check alignment and solder bridging before reflow. Also, new chips should be used to do rework for any soldering defects since bumped CSP/WLP/flip-chip components cannot be re-balled with the traditional BGA re-balling technology.

    Post-printing, a 100% visual inspection for paste deposition condition is very important. For high volume production, AOI is used as a tool to do this inspection before the P&P process. For any paste printing defects, touch-up can be performed with simple bamboo stick tool. This way, low area ratio and standard 4 mil stencils can be used to get good yield for 0.4 mm and 0.5 mm pitch mixed assemblies.

  7. Comparing to type 3 standard solder paste, commercially available type 4, type 4.5 and type 5 solder paste will yield better results for the ultra-fine-pitch, high mixed complex assemblies (0.4 mm and 0.5 mm pitch). With those types of pastes, area ratio requirements can be reduced and thus, allowing the use of a 4 mil stencil to provide get good paste printing results and final yields for highly complex boards.
  8. No-clean solder paste is better than water-soluble paste for the ultra-fine-pitch SMD components and complex boards (0.4 mm and 0.5 mm pitch). It also prevents the cleaning process issues and problems because of the small gap under ICs, which doesn’t allow the complete cleaning of corrosive flux residues and the chip-out problem during the washing process.
  9. Fig. 1 - Solder-charge high-density connector is the big challenge for soldering quality.

    Fig. 1 – Solder-charge high-density connector is the big challenge for soldering quality.

  10. Solder-charge high-density connector is the big challenge for soldering quality, Fig. 1. While regular BGA components have spherical solder balls attached to the leads and pads, their ball pitch is a constant and the center of the pad is the center of solder sphere; the solder-charge-style connectors employ the unique solder charge design, thereby making it a difficult application to work with.
  11. Fig. 2 - Solder-charge blocks are offset from the center making the leads appear to be in pairs.

    Fig. 2 – Solder-charge blocks are offset from the center making the leads appear to be in pairs.

  12. Unlike the uniform ball-grid arrangement of BGA components, solder-charge blocks, Fig. 2, are offset from the center making the leads appear to be in pairs. They appear as two joints with pitch minus (solder-charge width plus lead width) and the next two joints with pitch plus (solder-charge width plus lead width), as shown in Fig. 3. The solder-charge block orientation alternates from row to row; solder-charge blocks are positioned back to back.
  13. Fig. 3 – Leads appear as two joints with pitch minus (solder-charge width plus lead width) and the next two joints with pitch plus (solder-charge width plus lead width).

    Fig. 3 – Leads appear as two joints with pitch minus (solder-charge width plus lead width) and the next two joints with pitch plus (solder-charge width plus lead width).

  14. The center of the connector pads is not the center of the solder-charge block; instead it is the center of the non-solder-joint leads. They create a huge difficulty for solder-joint final quality and reliability.

Due to the above reasons, solder-charge connector manufacturers require using 35 mil (0.89 mm) diameter pads and 6 mil stencils. A majority of PCB layout designers are not familiar with this type of device. They use normal 25 – 28 mil diameter pads, usually resulting in bad quality joints and poor reliability in future. Therefore, it is best to follow the connector manufacturers’ requirements to use 35 mil pads.

Using 6 mil thickness stencils may be too thick for the small features of ultra-fine-pitch CSP/WLP/flip-chip components. Therefore, new stencil material should be used to get good yield for the small feature areas with thick stencil.

Conclusion

New stencil material provides a new method and defined relationship between the area ratio and solder paste releasing volume. Even with the most complex boards, reliable quality and good yield are achieved under the low area ratio of 0.50 – 0.55 and a 4-mil stencil limitation.

With a well-controlled printing process and other SMT processes, flip-chip/CSP/WLP paste-printing defects can be prevented, while achieving the same yield as the flux-only process yield. This helps to eliminate the high-cost and time-consuming under fill process. It also avoids large capital expenditure – using two printers and two stencils: one printer with a thin stencil to print the miniature components and a second in-line printer with a thicker stencil to cover the large solder paste volume requirement.

Through process optimization, a large process window can be found for the most complex assemblies to ensure final quality. At the same time, moderate assembly costs can be maintained to meet OEM customers’ toughest requirements in the today’s technical competitive market and cost sensitive environment.