A Basic Guide to Solder Paste Handling

Estimated reading time: 5 minutes

Solder paste is a combination of pre-alloyed spherical metal powder and flux medium. Solder paste formulations are designed with specific characteristics of tack time, stencil life and rheology (flow characteristics). As solder paste ages and is used in production, these characteristics will change. Handling solder paste properly preserves the original characteristics of the solder paste longer, resulting in less paste waste, better process yield and lower defect rates.

Environmental Effects on Solder PasteSolder paste is sensitive to heat, freezing and moisture. If exposed to these elements, the life and performance of the solder paste may be reduced dramatically.

Excessive heat. Most solder pastes are formulated to be tolerant of temperatures as high as 80°F for short periods of time. Excessive heat can cause the separation of the flux medium from the body of the paste. If this separation occurs, it will alter the rheological properties of the paste, possibly resulting in printing defects.

Freezing. In general, solder paste manufacturers do not recommend freezing solder paste. Freezing can result in the precipitation of the paste activators out of the medium, which can reduce wetting ability.

Moisture. All solder pastes are somewhat hygroscopic (having the tendency to absorb moisture), and care should be taken to avoid introducing solder paste to moist environments. The addition of moisture to solder paste can result in several defects¨ slump; solder balling; flux/solder spatter; components moving or shooting off" the board during reflow; reduced tack time; and poor wetting.

Transporting Solder PasteTo minimize its exposure to the environment, solder paste transit times should be as short as possible. Next-day delivery is common shipping practice for solder paste.

As a protector against excessive heat, solder paste may be shipped in ice packs, dry ice, gel packs or other insulating material. This is performed at the discretion of the solder paste supplier.

Solder Paste StorageSolder paste should be received in and stored as soon as it arrives. Solder paste should not remain on the receiving dock and should be stored in refrigeration at 40°F. In most cases, refrigeration will double the shelf life of the material as compared to storing it at room temperature. Refrigeration also acts as an additional protector against unforeseen environmental changes.

If solder paste is to be stored at room temperature, it is vital that the temperature and humidity be maintained at an appropriate level. Temperature should not exceed 78°F, with a relative humidity of less than 60 percent.

Solder Paste Inventory ControlWater-soluble solder pastes generally have a refrigerated shelf life of three to six months. Most no-clean solder pastes have a refrigerated shelf life of six months to one year. Using these pastes within their respective shelf lives is vital to proper paste performance. Because of the possibility of paste expiration, the use of the first in, first out" (FIFO) inventory management is recommended the oldest material in stock always should be used first.

Preparing Solder Paste¨ WarmingDo not remove any seal, open or attempt to mix solder paste until it has warmed completely to room temperature. The typical warming or stabilization time for solder paste is four to eight hours. Do not force-warm solder paste, as this may cause flux separation or other rheological problems. A simple way to accomplish proper warming is to remove the solder paste from refrigeration the shift or the night before it will be used.

If solder paste is used while it is cold, it will condense and draw moisture as it warms, possibly resulting in slump, spatter or other process defects. In addition, cold solder paste will have a dull appearance, be very difficult to stir, will not roll on the stencil or print correctly, and may stick to the squeegee blades.

Preparing Solder Paste¨ StirringAfter the solder paste has reached ambient temperature, it is necessary to ensure an even distribution of any separated material throughout the paste. Stir the paste lightly and thoroughly in one direction for one to three minutes (probably closer to one minute). Use a spatula for jars or a mixing device for cartridges to perform mixing.

Printer Area ConditionsAs stated above, heat and humidity are the nemeses of solder paste. Ideally, the printing area should be maintained at 40 to 50 percent relative humidity and 65° to 75°F. In addition, air (cool or warm) should not blow directly on the stencil top.

Solder Paste ApplicationThe initial application of solder paste onto the stencil is typically two to three times greater than applications made during the print cycle. The initial solder paste bead should be the full length of the printable pattern on the stencil, in a diameter of approximately 0.5".

During the first few print strokes, the squeegee blades will load up" with solder paste that will then flow out across the length of the squeegee blades, seeking its own level. After the first three or four boards have been printed, recheck the amount of paste on the stencil surface and add paste if necessary.

Controlling the level of solder paste on the stencil surface is vital to proper printing. It is better to add a smaller amount of paste more frequently than to add a large amount of paste less often. This method of control will ensure a constant turnover of paste, while keeping the freshest possible paste on the stencil.

Storing Opened Solder Paste ContainersDo not refrigerate opened jars or cartridges. If more solder paste is removed from refrigeration than is used, simply reseal the jar or cartridge and leave it out at room temperature until it is ready to be used again.

Storing Used Solder PasteIf desired, used" solder paste may be stored for future use. After a printing operation, simply remove any paste remaining on the stencil, place it in an empty container (do not store this paste with unused paste), and store it at room temperature.

When printing again, reapply the old paste onto the stencil with the addition of an equal amount of fresh paste. This should revitalize the used paste, and allow for acceptable printing.

Stencil CleaningWhen isopropyl alcohol (IPA) is used as a stencil cleaner and makes contact with the paste to be printed, a foreign substance is introduced into the paste. One relatively common result of this interaction is solder paste dry-out. The solder paste supplier should be contacted to determine which stencil cleaners are safe to use with their particular products.

Exposure to Solder PasteAs a final note, remember that care should be taken to avoid the ingestion of solder paste. As solder paste often contains lead or other toxic materials, gloves and gowns should be worn while using solder paste, and hands should be washed afterwards.

DAVID SURASKI is the technical marketing specialist at AIM, 25 Kenney Drive, Cranston, RI 02920; (401) 463-5605; Fax¨ (401) 463-0203; Web site¨ www.aimsolder.com.