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Trends in the Stencil Printing Process
December 31, 1969 |Estimated reading time: 9 minutes
What trends will drive solder paste printing process technology over the next several years? This article examines two major trends to influence and drive the stencil printing process - lead-free and component miniaturization.
By Joe Belmonte
The first trend expected to drive the stencil printing process is the conversion to lead-free soldering materials. Although there are a number of electronic manufacturers that have successfully made the conversion to lead-free solder materials, most manufacturers are preparing for this transition. In 2006, many electronic manufacturers will be required to introduce a manufacturing process that will not use any materials containing lead. The RoHS regulation requires lead-free products in the member countries on July 1, 2006, so manufacturers must produce these products before that date. The RoHS regulation bans six materials, but the primary one that affects electronics manufacturing is lead. China and California are considering similar regulations; and many believe that RoHS-type regulations will become a worldwide standard over the next several years.
On the surface, this just seems like a material change. In reality, lead elimination in electronic manufacturing materials will be a significant technology change. The elimination of lead will affect virtually every area of the manufacturing process. This article focuses on the lead-free solder paste printing process. Because lead-free solder paste does not spread or “wet” as well as tin/lead solder paste, it will require adjustments in several aspects of the solder paste printing process, including stencil design and equipment accuracy and repeatability.
The second trend to affect the solder paste printing process is the introduction and growing use of miniature components such as 0.4-mm chip scale packages (CSPs) and small passive components such as the 0.40- ×0.20-mm (01005) package.
Printing solder paste onto these small PCB pads while supplying sufficient solder paste for larger components on the same assembly presents a number of challenges. The difference in solder paste volume requirements for large and small components on any particular product is difficult to satisfy. This article details these process design challenges.
Lead-free Solder Paste Printing Process
Developing and optimizing a lead-free solder paste printing process involves the same approach as developing a tin/lead solder paste printing process. Responsible process engineers first must understand performance specifications (stencil life, squeegee speed, squeegee pressure, etc.) of the selected lead-free solder paste. The engineer must then perform formal experimentation - Design of Experimentation (DoE) - to identify optimum values of all the critical operating parameters for that particular solder paste in that particular printing equipment, using that particular stencil design.
One company* has conducted several experiments over the last few years with lead-free solder paste from virtually all major solder paste suppliers, and has determined there is no measurable difference in the actual printing of lead-free solder paste compared to tin/lead solder paste. The performance of the lead-free solder paste printing process is determined entirely by the sophistication and effectiveness of the process development effort.
The significant issue with lead-free solder pastes is that they do not “wet” as well onto the PCB component pads as tin/lead solder paste. When a tin/lead solder paste was misprinted, the misprinted tin/lead solder paste would “wet” back and cover the PCB pad. However, because this wetting process does not occur as well with lead-free materials, is a more accurate solder paste printing process required?
Figure 1. PCB used in offset solder paste printing study and the particular components evaluated in this study.
A formal statistical study was conducted to answer this question. Five solder paste suppliers provided their latest tin/lead and lead-free solder pastes. A stencil was designed with the apertures offset at various distances from PCB pads (Figure 1). Each board was printed and reflowed with each of the five lead-free and tin/lead solder pastes using the same stencil and printing equipment. For example, on a 0402 chip component, eight different aperture offsets were used: 0.00", 0.02", 0.04", 0.06", 0.08", 0.010", 0.012" and 0.014" (Figure 2). The test included the following component types: BGA36, BGA256, R0402 (horizontal and vertical placement) and QFP208 (horizontal and vertical placement).
Figure 2. Example of stencil-aperture offsets from PCB pads in the horizontal direction for a 0402 chip component.
Although some lead-free pastes performed adequately and newer pastes work better than earlier formulations, this detailed experiment confirmed that tin/lead solder pastes wet back to PCB pads better than lead-free solder pastes. Therefore, a more accurate solder paste printing process is required in the use of lead-free solder pastes to compensate for this (Figure 3).
Figure 3. Results from printing the tin/lead and lead-free solder pastes on 0402 chip components in the horizontal direction.
The accuracy and repeatability of printing equipment is important to success with lead-free materials. When evaluating printing equipment, it is important to understand the suppliers’ accuracy and repeatability specifications. These specifications can be calculated using a number of methods. Other factors to consider in optimizing the accuracy and repeatability of the solder paste printing process include:
- Stencil precision - The printing process requires close alignment of stencil apertures with PCB pads. Both stencils and PCBs are rigid items that cannot be adjusted for one particular misalignment situation. It is imperative that the stencil minimizes variance to ensure a more accurate printing process. Ask your stencil manufacturer for data to verify the accuracy of the stencil process.
- PCB - It is not uncommon for PCBs to be slightly larger or smaller than the actual data used to fabricate them. Work with PCB suppliers to understand this variation.
- PCB support - It is important that the board be held firmly in place once it is in the printing position. The equipment and associated tooling must be capable of supporting the board in all three axes (X, Y and Z) during the printing process.
Miniature Components
The second factor affecting the solder paste printing process is the introduction of miniature components. These components require small PCB pads such as 0.010" and 0.009" diameter, 0.009 × 0.0125" rectangles and 0.008 × 0.010" or 0.007 × 0.011" pads for 01005 component packages.
The issue with printing solder paste through the tiny apertures required for these miniature components is not printing these components themselves. The challenge is developing a solder paste printing process that can satisfy requirements for these small apertures and the larger apertures required for various components on a product.
There are many factors that must be considered in developing and optimizing the solder paste printing process. One key factor is PCB design and fabrication. The PCB must be designed with the correct component-pad geometry, correct component-pad size and correct spacing between components. The PCB must be fabricated to ensure that the pads are the size the design data specifies. During the PCB-fabrication etching process, the board can be “over-etched,” causing some of the pads to be smaller than specified; or “under-etched,” causing the pads to be larger than specified. If you are an OEM who designs and builds products, you may have a great deal of control on PCB design and fabrication. If you are an EMS provider, you may have little control over PCB design and fabrication. One powerful tool that almost all electronic manufacturing operations have is stencil design. For relatively little cost, it may be possible to design a stencil that can compensate for most of the errors introduced during PCB design and fabrication.
There are precise calculations that must be used in the stencil design. The most important calculation when designing a stencil for miniature components is called “area ratio.” This is the ratio between the area opening of the aperture and the area of the aperture walls.
As this formula shows, stencil thickness is a variable. If a thin stencil (less than 0.005" thick) is designed, the required area ratio of the smallest component on the board may not print a sufficient volume of solder paste for the larger components on the board. As components get smaller, the challenge is to design a stencil that will satisfy solder paste volume requirements of the smallest and largest components, and be able to print without clogging the stencil aperture. Dual-thickness or “step-down” stencils are one solution, but they have several limitations.
Another viable solution for printing the correct amount of solder paste for both large and small paste-volume requirements is the use of a dual-stencil printing process. The dual-stencil printing process involves the use of a thin stencil that prints solder paste for the small-volume requirements first, while using a thicker stencil with areas of the bottom removed to “clear” or “sit over” the solder paste deposits that were printed using the thin stencil. The dual-stencil process also has been used in the Pin-in-Paste (PIP) process, where products require a large volume of solder paste for the thru-hole components being reflow-soldered, while still providing the volume required for smaller, fine-pitch components. In high-volume applications, two stencil-printing machines will be used; one system prints with the thin stencil and the second prints with the thicker stencil.
There are several equipment features that may also satisfy large and small solder paste volume on one product requirement.
- If equipped with a paste dispenser, the equipment-software control can establish a volume profile for the dispense cycle in the X dimension. This assists with managing the volume of the paste bead to increase or decrease (as needed) for component density and volume deposited - improving paste quality by minimizing exposure to the amount required.
- Enclosed print-head systems can provide for better aperture fill during the print stroke, allowing for more consistent deposit volumes.
- Wiping frequency and the type of wipe can be used to better manage these difficult applications.
- Two-dimensional post-print of the solder paste deposit can be used to look for insufficiencies on smaller components and trigger a wipe if this occurred. Stencil inspection also can be used to verify stencil cleanliness and trigger a wipe, focusing on critical components.
- Snap-off profiles assist with stencil aperture release, keeping apertures from clogging and providing for better volume deposition.
Additional research is being conducted to identify the best solution for the “wide-range” printing-process requirements caused by the introduction of miniature components. This research is focused on developing printing methods that will allow manufacturers to design a stencil that will violate aperture-ratio rules, while providing excellent solder paste printing performance.
Conclusion
The introduction of lead-free materials and miniature components to the electronic manufacturing industry will affect the solder paste printing process. With these innovations, the “process window” or process tolerance has been narrowed considerably. The process engineer must be more attentive to process development, monitoring and optimization to implement these technologies effectively.
A more accurate solder paste printing process will be required. All aspects of the printing process must be evaluated to ensure the absolute minimum variation in materials, tooling and equipment. The process engineer should work with equipment and material suppliers to understand the affects lead-free and miniature components have on the solder paste printing process. Keep updated on research in the development of the “wide-range” printing process.
* Speedline Technologies, Franklin, Mass.
Joe Belmonte, project manager, Advanced Process Group, Speedline Technologies, may be contacted via e-mail: jbelmonte@speedlinetech.com.