Stencil and screen printing Q&A: a list of some of the most frequently asked questions concerning screen printing in electronics manufacturing.

This article is a review of frequently asked questions and answers concerning stencil and screen printing issues in the electronics manufacturing industry. Specifically, it addresses issues such as how to determine appropriate speed for various solder paste applications; using solder paste to fill via hole devices; and various rubber squeegee blade durometers and when to use them. The article also discusses terms such as downstop, contact printing and snap-off printing and processes such as stencil design and 2-D inspection. Some of these processes are carefully illustrated with graphics.

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Q: How fast can I print solder paste?

A: The relationship among force, speed and paste formulation must be maintained to get acceptable solder paste prints. Some formulations have to be printed quickly, while other formulations must be printed slowly to provide the best results. If the squeegees hydroplane and leave a thin coat of paste or flux on the top of the stencil, the force should be increased to wipe the stencil top clean. The force should not be increased to a point where the paste will no longer roll. The rolling of the paste during the print stroke is an indicator that you will get an acceptable print. Printing too fast can result in incomplete fill of apertures, especially on the leading edge of a pad. Hydroplaning will result in extremely peaked prints and incomplete coverage due to paste not properly releasing from the apertures.

Q: Why is paste left on my stencil after the squeegee prints the board?

A: A few things could cause this problem, and we will discuss two of the most common causes. First, although you may have the correct force applied to the squeegees, the downstop--or the distance the squeegees travel down into the stencil--may be too small. The second reason for this problem could be a lack of appropriate support under the board. With inadequate board support, the board can deflect downward under the pressure of the squeegees, and the resulting blade angle will not allow the squeegees to clean the stencil. Inadequate support under the board could also cause differences in squeegee pressure applied to the board as it deflects downward.

Q: Can I use solder paste for printing through-hole devices?

A: Yes, printing has become an accepted process for this type of board assembly. The process of using solder paste to fill via holes in a printed circuit board (PCB) for through-hole assembly is most commonly called intrusive soldering or pin-in-paste printing. The stencil aperture must be designed properly to get the correct amount of solder to fill the via holes to ensure reliable joints.

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Understanding that solder paste generally has a shrink factor of 50%, the first step is to calculate the volume of paste required. To do this step, the volume of paste required to fill the hole minus the lead volume should be calculated. The solder paste volume is twice the solid solder requirement, and the aperture can be calculated using the stencil thickness and available area around the through-hole pad. The paste can be overprinted, and it will pull back to the solderable surface during reflow.

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The volume of solder paste required to fill the hole equals the hole volume minus the volume of the lead multiplied by 2 (Figure 1):

V = ([3.14 * D * [D/4]] - [3.14 * d * [d/4]]) * 2 or V = 3.14(D * [D-d] * d)/2

To calculate the required stencil aperture for a square aperture, the size equals the square root (volume of paste required/stencil thickness) (Figure 2):

A = (V/T[).sup.1/2]

Squeegee Blades

Q: Why are there different rubber squeegee durometers, and when should I use them?

A: Squeegees can be obtained in several different durometers for stencil and screen printing. Typically, when printing with screens, one would use polyurethane blades with a range from 60 to 80 shore A. The screen's mesh should prevent the blade from scooping material off the substrate when the softer materials are used. Blades ranging between 90 to 110 shore A are normally used for stencil printing. However, when using polyurethane blades with stencils, scooping on larger apertures can become a problem. Metal blades are preferred for stencil printing. However, for stencils with only fine apertures or step stencils, a polyurethane blade will produce more consistent prints and may reduce wear on the stencil. Step stencils are stencils that contain sections that are relieved, or stepped to a smaller thickness than the rest of the stencil. These stencils are typically used for boards with mostly large apertures and one or two fine-pitch devices.

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Q: What is the contact angle for squeegee blades?

A: The contact angle for trailing edge blades with no pressure applied is 60[degrees] for metal blades and 50[degrees] for rubber blades. The force on the blade should be sufficient to give a clean wipe of the top surface of the stencil but not so high that it will coin the stencil and cause premature failure. Coining is the term given to the creation of a permanent impression of the board edges left in the bottom of the stencil when excessive pressure is applied to the stencil outside of the board edges.

Downstop and Snapoff

Q: What is downstop?

A: The term downstop is used as the distance the squeegee blade is programmed to travel below the top of the substrate during a print stroke. The substrate will support the stencil during the print stroke; however, if the blade travels beyond the edge of the substrate and supporting rail, the stencil can be deflected and cause premature fatigue of the stencil. Properly setting the downstop will prevent this effect since it will not allow the blade to provide enough squeegee travel to apply this excessive force on the unsupported stencil. However, if the downstop is set too low, the desired print pressure will not be achieved due to its inability to properly flex the squeegee blade. Typical downstop settings are 0.065 in. (1.6 mm) to 0.075 in. (1.9 mm).

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Q: What are the differences among contact printing, snap-off printing and slow snap-off?

A: Contact printing is achieved when the entire stencil contacts the substrate during the print process. After the material is squeegeed through the apertures, the stencil and substrate are separated in a vertical and uniform rate (Figure 3).

Snap-off printing is achieved when the stencil or screen has a desired gap between them when at rest (Figure 4). During the print stroke, the squeegee deflects the stencil to bring it into contact with the substrate. Only at the point where the squeegees apply force on the stencil is the stencil and board in contact. The stencil or screen will peel away from the substrate as the squeegee blades move forward. Snap-off printing is used in stencil printing for high-density substrates where uniform release rates do not occur repeatably, when faster cycle times are desired and when screen printing is performed.

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Slow snap-off printing is the process of the stencil and substrate slowly separating after the print stoke is complete. Since different pastes have varying aperture release characteristics, this adjustable setting is used to allow the paste to settle down after the print stroke and more cleanly release from the apertures.

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Q: When would snap-off printing be appropriate to use?

A: Snap-off printing is used when high-density stencil printing or screen printing is performed. The snap-off setting is the distance between the stencil/screen when at rest before the squeegee blades deflect the stencil/screen to the surface of the substrate being printed. This process allows the stencil or screen to roll-on and peel-off the substrate and produces a uniform release rate of the material being printed from the apertures. For high-density printing, the separation of the stencil or screen will be different from the edges and center due to the cohesive forces if contact printing is used.

Stencil Design

Q: I would like to put two images on a stencil; how far apart should the images be?

A: Normally, the images should be at least 3 in. apart for trailing edge blade printing. If allowance is made for a little print over-travel, the distance should be increased to 4 in. (100 mm). The maximum board size for a 29 in. x 29 in. stencil would be limited to no larger than 6 in. (150mm).

Q: What is the minimum size stencil foil I can use for effective print results?

A: When using trailing edge blades, the minimum size stencil foil in the y direction is calculated by taking the substrate size and adding 7 in. (178 mm). When using diamond-shaped blades, the minimum size stencil foil is the substrate size plus 1 in. (25 mm). The x direction should be a minimum of 1 in. (25 mm) greater than the squeegee blade length.

Q: Why should board images be centered on the stencil?

A: To get a true vertical separation of the stencil from the substrate, the image should be centered on the stencil. If the image is offset, uneven peel as the separation occurs may happen due to the cohesive forces of the flux that seeps between the stencil bottom and substrate.

Q: Do I always need fiducials for proper board-to-stencil alignment?

A: Fiducials are always preferred for best alignment results. However, if traditional fiducials are not available, using board pads and stencil apertures as fiducials can provide acceptable results. A unique area on the board should be used for alignment, such as an end pad of a quad flat pack (QFP). Note that care must be used to keep the stencil apertures clean, since clogged apertures can cause alignment problems. The size and shape of the aperture will be different from the clean aperture used during programming.

As shown in Figure 5, this setup creates a unique image because the system also learned the empty space to the left of the target pad. No other acceptable match to this image exists anywhere else in the camera's field of view.

The problem with the setup in Figure 6 is that the correlation box has not been created in a way where it is unique. Therefore, the possibility exists that the system would find an incorrect image match, which would cause the system to align to the pad at the bottom of the field of view rather than the intended top pad.

2-D Inspection

Q: Why should I use 2-D verification?

A: If your process never changed, you would not need an inspection or verification system. However, most PCBs have challenging components or applications, specifically in fine-pitch devices, which require some form of verification. A 2-D system is typically designed to be a process verification tool; it should not necessarily replace your inspection system. A 2-D system helps you develop your process and verifies that your process stays in control. These systems are designed to help you verify that you are getting consistent stencil release and paste coverage according to your pre-established benchmarks. It will help alert you to any deviation from the established process, enabling you to make necessary adjustments before costly additional processes such as washing or rework take place.

Q: How does 2-D work?

A: Up to 90% or more of all print defects and trends can be identified by verifying the amount of paste covering the target pad. This method identifies the lack of paste being deposited on the pad resulting in unacceptable solder joints.

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The most effective type of 2-D inspection incorporates a gray-scale comparison technique to determine the percentage of paste that is covering the target PCB pad. This technique compares the uncovered area of a printed pad against the saved area of the bare pad, determined at the time the device is first programmed (Figure 7). With this data, the calculation in Figure 7 is used to determine the percentage of paste coverage.

Q: What are some strategies for optimizing the process?

A: Finding and correcting paste application problems are best done as they occur. Given that with some exceptions inline inspection is not designed to inspect every single pad on every single board, ways do exist to determine how to program the inspection system to take advantage of the key devices and areas to use as data models. Inspection of these will transfer over to all devices on all boards being processed in any given run. To learn how to develop the best data models, you must understand how to check an assembly for the type of components that fit into these categories. Also, fully understanding the flexibility of the tools available in the inspection software is important to best determine how to apply these data models in the most effective manner for any individual stencil printing application.

Conclusion

With the right equipment, extensive knowledge and experience, stencil and screen printing challenges can be effectively addressed. The reduction of paste waste associated with squeegee printing and operator intervention during changeovers and new product setup is a requirement necessary in achieving highly reliable, fast and accurate printing results. Careful consideration of all printing issues helps manufacturers to produce high quality boards and improve the overall PCB assembly process.

Alden Johnson is a senior applications engineer with Speedline Technologies, Franklin, MA; email: [e-mail].