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Step 4 Printing
December 31, 1969 |Estimated reading time: 13 minutes
By Ray Prasad
In the reflow soldering of surface mount assemblies, solder paste connects the leads or terminations of surface mount components to the lands. There are many variables in this process paste, screen printer, paste application method and printing process. In printing solder paste, the substrate is placed on the work holder, held firmly mechanically or by vacuum, and aligned with the aid of tooling pins or vision. A screen or stencil is used to apply solder paste. This article, after briefly describing the paste printing process, will focus on some key issues in squeegee and stencil materials and then discuss printing processes for fine-pitch and through-hole components in a mixed surface mount assembly.
Solder Paste Printing Process and EquipmentThe solder paste printing process involves a series of interrelated variables, but the printer is crucial for achieving the desired quality of print. The solder paste screen printers available on the market today fall into two main categories: laboratory and production. Each category has further subdivisions because companies expect different performance levels from laboratory and production printers. For example, a laboratory application that is research and development (R&D) for one company could be prototype or production for another. Moreover, production requirements can vary widely depending on volume. Since a clear-cut classification of equipment is not possible, the best thing to do is to select a screen printer to match the desired application.
Figure 1 presents a cross-sectional view of screen and stencil. The frames of the screens and stencils are similar; the differences lie in the construction of individual openings used for depositing the solder paste. The screens or stencils have openings to match the land patterns on the substrate where solder paste must be deposited for the electrical interconnections. The screen/stencil is stretched in a metallic frame and aligned above the substrate.
Figure 1. Screen/stencil construction is illustrated.
In manual or semiautomatic printers, solder paste is placed manually on the stencil/screen with the print squeegee at one end of the stencil. In automatic printers, paste is dispensed automatically. During the printing process, the print squeegee presses down on the stencil to the extent that the bottom touches the top surface of the board. The solder paste is printed on the lands through the openings in the stencil/screen when the squeegee traverses the entire length of the image area etched in the metal mask. Figure 2 shows the location of paste, screen, squeegee, substrate and snap-off distance.
After the paste has been deposited, the screen peels away or snaps off immediately behind the squeegee and returns to its original position. This gap or snap-off distance is a function of the equipment design and is about 0.020 to 0.040". Thus, snap-off distance and squeegee pressure are two important equipment-dependent variables for good quality printing.
If there is no snap-off, the operation is called on-contact printing. This is used when an all-metal stencil or metal squeegee blade is used. If there is a snap-off, the process is called off-contact printing. Off-contact printing is used with flexible metal masks and screens.
Squeegee TypesSqueegee wear, pressure and hardness determine print quality and should be monitored carefully. For acceptable print quality, the squeegee edges should be sharp and straight. A low squeegee pressure results in skips and ragged edges. A high squeegee pressure or a soft squeegee will cause smeared prints and may even damage the squeegee, stencil or screen. Excessive pressure also tends to scoop out solder paste from wide apertures, causing insufficient solder fillets.
Two squeegee types are commonly used:
- Rubber or polyurethane
- Metal.
When using rubber, 70 to 90 durometer hardness squeegees are used. If excessive pressure is applied, the paste underneath the stencil may bleed, causing bridging and requiring frequent underside wiping. To prevent underside bleeding, it is important that the pad opening provide a gasketing effect while printing. This is dependent on the roughness of the stencil aperture walls.
Figure 2. Schematic illustration of the solder paste printing process.
Metal squeegees also are commonly used. Their popularity has grown with the use of finer pitch components. They are made from stainless steel or brass in a flat blade configuration, and are used at a print angle of 30 to 45°. Some are coated with lubricating material. Because lower pressure is used, they do not scoop paste from apertures, and because they are metallic, they do not wear easily (like rubber squeegees) and, therefore, do not need to be sharpened. They are, however, significantly more expensive than rubber squeegees. They also can cause wear on the stencil.
Different squeegee types have different ramifications in printed circuit assemblies (PCA) containing both standard and fine-pitch components. The solder paste volume requirement is very different for each component type. Fine-pitch components require much less solder volume than standard surface mount components. Pad area and thickness control the solder paste volume.
Some engineers use dual thickness stencil to apply the appropriate amount of paste at the fine-pitch and standard surface mount pads. This is the conventional approach and requires a rubber squeegee to force the paste through the stencil holes.
Other engineers take a different approach, using a metal squeegee. It is easier to prevent variation in paste volume deposition with a metal squeegee; however, this approach requires a modified stencil aperture design to prevent excess paste deposition on the fine-pitch pads. While this method has become more popular today in the industry, the rubber squeegee with dual thickness printing has not vanished by any means.
Stencil TypesAn important variable in print quality is the accuracy and smoothness of the side walls of the stencil aperture. It is important to maintain a proper aspect ratio between the stencil width and thickness. Figure 3 shows the recommended aspect ratio (aperture width divided by stencil thickness) of 1:1.5 for a stencil. This is important for preventing clogging of the stencil. Solder paste will tend to remain in the opening if the aspect ratio is less than 1:1.5.
Both the smoothness and the accuracy of aperture walls are controlled by the process in which the aperture is made. There are three common processes for making stencils:
- Chemical etching
- Laser cutting
- The additive process (electroformed).
Figure 4 summarizes the key attributes of these three stencil-making processes. Let us briefly review each process of making stencils.
Chemically Etched Stencils. The metal mask and flexible metal mask stencils are etched by chemical milling from both sides using two positive images. During this process, etching proceeds not only in the desired vertical direction but also in the lateral direction. This is called undercutting. Thus the openings are larger than desired, causing extra solder deposit. Because 50:50 etching proceeds from both sides, it results in almost a straight wall tapering to a slight hourglass shape in the center.
Because the walls of electroetched stencils may not be smooth, electropolishing, a microetching process, is one method for achieving a smooth wall. Another way to achieve smoother side walls in the aperture is nickel plating. A polished or smooth surface is good for paste release but may cause the paste to skip across the stencil surface rather than roll in front of the squeegee. This problem can be avoided by selectively polishing the aperture walls without polishing the stencil surface. Nickel plating further improves wall smoothness and provides some printing performance improvement. It does not, however, reduce aperture opening, and does require artwork adjustment.
Figure 3. Stencil aperature width vs. thickness for good printing. A mimimum aspect ratio of 1:1.5 (T:W) is required for good printing.
Laser Cut Stencils. Like the chemical etching process, laser cutting is a subtractive process, without the undercutting problem. The stencil is produced directly from Gerber data, hence improved aperature accuracy. The data can be adjusted in the Gerber file to change dimensions as necessary. Better process control also improves aperture accuracy. Another benefit of laser cut stencils is that the walls can be programmed to be tapered. Chemically etched stencils can be tapered only if they are etched from one side, but the aperture size may be too large. A tapered aperture with an opening slightly larger on the board side than on the squeegee side (0.001 to 0.002" to produce an angle of about 2°) is desired for easier paste release.
The laser cut process can produce aperture widths as small as 0.004 mil with 0.0005" accuracy, hence it is very suitable for ultra-fine-pitch component printing. Laser cut stencils also produce ragged edges because vaporized metal is transformed into metal slag during the cutting process. This can cause paste clogging. Smoother walls can be produced by microetching the walls. Laser cut stencils cannot make stepped multilevel stencils without pre-chemically etching the areas that need to be thinner. The laser cuts each aperture individually, so stencil cost depends on the number of apertures to be cut.
Electroformed Stencils. The third process for making stencils is the additive process. It also is referred to by other names such as electroformed or Galvano; however, the term electroformed is most common. In this process, nickel is deposited on a copper mandrel to build the aperture. First, a photosensitive dry film is laminated on the copper foil (about 0.25" thick). The film is polymerized by UV light through a photomask of the stencil pattern. After developing, a negative image is created on the mandrel. Only the apertures on the stencil remain covered by the photoresist. The stencil then is grown by nickel plating around the photoresist. After the desired stencil thickness is achieved, the photoresist is removed from the apertures. The electroformed nickel foil is separated from the mandrel by flexing a key process step. The mandrel can be reused. Now the foil is ready for framing as in other stencil-making processes.
Making step stencils by electroforming can be done at added cost. A significant advantage of this process is that electroformed stencils provide a good gasket effect because of the close tolerances possible. This minimizes paste seepage under the stencil, and means that the frequency of underside stencil wiping is decreased drastically, reducing potential bridges.
Chemical etching and laser cutting are subtractive processes for making stencils. The chemical etch process is the oldest and most widely used; laser cut is a relative newcomer. Electroformed stencils are the latest rage. Figure 4 summarizes the key attributes of all three processes.
Paste Printing for Fine-pitchWhen printing for fine-pitch, it becomes important to control aperature smoothness and dimensions in the stencil. This means that laser cut stencil use should be considered seriously. Fine-pitch also requires different solder paste.
Solder paste print quality is very important because it accounts for a significant number of defects on a PCA. When using fine-pitch, the problem is compounded and an unacceptable amount of bridges between adjacent leads can result.
Bridging is not the only type of defect particular to fine-pitch. If inadequate paste is deposited, either insufficient solder or complete opens result. We should keep in mind that, in most companies, fine-pitch accounts for about half of all defects combined.
Figure 4. Key features of chemically etched, laser cut and electroformed stencils.
To avoid printing problems, some people use dual-step stencils (0.005 to 0.006" for fine-pitch and 0.007 to 0.008" for other components). Figure 5 shows an illustration of a step stencil. The stencil material, generally stainless steel, is etched down to a low thickness so that the stencil aligns with the pads of fine-pitch components. Step stencils require rubber squeegees to force the paste through the holes in the stencil. This is a good method for depositing the right amount of paste at specific locations 0.005 to 0.006" paste height for fine-pitch, and 0.007 to 0.008" paste height for standard surface mount components.
A more recent method is to use metal squeegees and deposit the same paste thickness in fine-pitch and other component types. The danger with this approach is that you may either deposit too much paste on the fine-pitch lands or too little paste on the standard surface mount lands. This boils down to having good process control and stencil design. If that is obtained, a better and more consistent result than with the old method of using rubber squeegees will result.
Process complexity is increased when using fine-pitch technology. It is no longer a simple process and requires considerable process development and control to achieve consistently good paste print.
Paste-in-hole Process for Through-holeGenerally, solder paste printing is not used for through-hole, but there are circumstances when it should be considered. For example, in a mixed assembly with double-sided SMT boards (active components on both sides) and very few through-hole components on the top side, it may be cost effective to print paste for through-hole components. This process typically is referred to as a paste-in-hole process and it avoids an additional step of either hand soldering or wave soldering (using customized fixtures to hide previously reflowed soldered surface mount components).
Why should hand or wave soldering be avoided? When hand soldering is used, there always is the potential for internal damage depending on operator skill. And using special fixtures for wave soldering adds to product cost and creates the potential for solder bridging in previously reflowed soldered surface mount components.
Figure 5. Schematic of a step stencil.
Industry has been struggling with their ability to successfully solder through-hole components by using the paste-in-hole process. There are different variations of this process. However, the basic concept in all approaches revolves around printing the needed volume of solder. The volume of solder paste needed is determined by subtracting the volume occupied by lead-in-hole from the volume of the plated through-hole.
The problem is that about half the paste disappears into thin air at soldering temperatures. Additionally, there is the fine point of knowing the right ratio between the diameter of the lead and the hole so that solder can get into the hole by capillary action. The right amount of play between the hole and the lead is needed to achieve the necessary solder fill in the hole.
The paste-in-hole process is relatively new today. But some leading companies have been doing it for some time. When using the paste-in-hole process, it should be determined first if the through-hole components can withstand the reflow temperature. Also, it should be determined if the parts are moisture-sensitive. If they are, they must be baked before soldering. If they are not baked, they will "popcorn" or crack during reflow soldering.
It also should be noted that if the board has wavesolderable surface mount components on the secondary side, reflow soldering of through-hole components will not save any process steps. In such a case, wave soldering instead of reflow soldering is the most commonly used practice for through-hole components.
SummaryTo achieve good printing results, a combination of the right paste material (right rheology, i.e., viscosity, metal content, largest powder size and lowest flux activity possible for the application), the right tools (printer, stencil and squeegee blade) and the right process (good registration and clean sweep) are necessary.
Metal squeegees and stencils are the most commonly used printing tools even though they are slightly more expensive than other alternatives. As the industry moves to finer pitches, laser cut and electroformed stencil use, again more expensive options, are becoming more common.
Even the best pastes, equipment and application methods alone cannont ensure acceptable results. The user must control process and equipment variables to achieve good print quality. This is even more critical in fine-pitch printing because it requires very accurate stencil aperture alignment to the land patterns. Finally, there are times when it may be more cost effective to reflow solder through-hole components if they can withstand the reflow temperature.
REFERENCERay P. Prasad, Surface Mount Technology: Principles and Practice, Chapman and Hall, 2nd Edtion, ISBN: 0-412-12921-3.
RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the textbook Surface Mount Technology: Principles and Practice. Additionally, he is president of BeamWorks Inc. (www.beamworks.com), a supplier of Selective Automated Assembly Systems, located in Portland, OR and founder of the Ray Prasad Consultancy Group, which specializes in helping companies establish strong internal SMT infrastructure. Contact him at P.O. Box 219179, Portland, OR 97225; (503) 297-5898 or (503) 646-3224; Fax (503) 297-0330; Web site: www.rayprasad.com.