Solder Paste Printing

In surface mount assembly reflow soldering, solder paste is used for the connection between surface mount component leads or terminations and the lands. There are many variables, such as paste, screen printer, paste application method and printing process. In printing solder paste, the substrate is placed on the work holder mechanically or by vacuum, and aligned with tooling pins or vision. Either a screen or stencil is used to apply solder paste. In this column, I will focus on some key paste printing issues, such as stencil design and printing processes; in next month's column, I will discuss printing processes for fine-pitch and through-hole components in a mixed surface mount assembly.

Printing Process and Equipment

In the solder paste printing process, the printer is crucial for achieving desired print quality. Screen printers available 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 R&D for one company could be prototype or production for another. Moreover, production requirements can vary widely depending on volume. Because a clear-cut equipment classification is not possible, the best thing to do is to select a screen printer to match the desired application.

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 stencil bottom touches the top board surface. Solder paste is printed on the lands through the openings in the stencil/screen when the squeegee traverses the entire image area length etched in the metal mask.

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". Snap-off distance and squeegee pressures 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 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 Types

Squeegee wear, pressure and hardness determine print quality and should be monitored carefully. For acceptable print quality, squeegee edges should be sharp and straight. A low squeegee pressure results in skips and ragged edges, while a high squeegee pressure or a soft squeegee will cause smeared prints and may even damage the squeegee and stencil or screen. Excessive pressure also tends to scoop solder paste from wide apertures, causing insufficient solder fillets.

Two squeegee types are common: rubber or polyurethane squeegees and metal squeegees. When using rubber squeegees, 70 to 90 durometer hardness squeegees are used. When applying excessive pressure, paste bleeding underneath the stencil may cause bridging and will require frequent underside wiping. To prevent underside bleeding, the pad opening must provide a gasketing effect while printing. This is dependent on the roughness of the stencil aperture walls.

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 30° to 45° print angle. Some squeegees 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 hence do not need to be sharpened. They cost significantly more than rubber squeegees, and can cause stencil wear.

Using different squeegee types has ramifications in printed circuit assemblies (PCA) with 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 solder paste volume.

Some engineers use dual-thickness stencil to apply the appropriate paste amount at fine-pitch and standard surface mount pads. Other engineers take a different approach — they use a more expensive metal squeegee that does not require frequent sharpening. It is easier to prevent variation in paste volume deposition with a metal squeegee, but this approach requires a modified stencil aperture design to prevent excess paste deposition on fine-pitch pads. The approach has become more popular in the industry, but rubber squeegees with dual-thickness printing have not vanished.

To achieve good printing results, a combination of the right paste material (viscosity, metal content, largest powder size and lowest flux activity possible), the right tools (printer, stencil and squeegee blade) and the right process (good registration, clean sweep) are necessary.