Contents:
Additional applications information, such as reflow profiling, eliminating voiding, tombstoning & solder beading, bar solder drossing and flux conformal coating compatibility can be found in AIM's numerous Technical Articles: Read Technical Articles
_______________________________________
Screen Printing Process
Snap Off
- Select a good flat substrate.
- Position the substrate to be printed under the stencil.
- Lower the stencil, or raise the board to the print height.
- Remove all vacuum holding the board by turning it off, closing of vacuum gates is not sufficient.
- Adjust stencil snap-off so that there is a definite gap between the bottom of the stencil and the substrate surface across the entire substrate surface.
- Using your finger, deflect the stencil surface down toward substrate, lowering the snap-off distance slowly until the stencil just makes contact with the substrate surface across the entire printable area.
- For manual printers, lock the stencil height into place and reset dial indicators to 0.00", for automated printers, record the new snap-off distance, save changes and repeat steps 2, 3, & 6 to verify adjustment.
Separation Distance
- Select a good flat substrate.
- Position the substrate to be printed under the stencil.
- Lower the stencil, or raise the board to the print height.
- Remove all vacuum holding the board by turning it off, closing of vacuum gates is not sufficient.
- Adjust stencil snap-off so that there is a definite gap between the bottom of the stencil and the substrate surface across the entire substrate surface.
- Using your finger, deflect the stencil surface down toward substrate, lowering the snap-off distance slowly until the stencil just makes contact with the substrate surface across the entire printable area.
- For manual printers, lock the stencil height into place and reset dial indicators to 0.00", for automated printers, record the new snap-off distance, save changes and repeat steps 2, 3, & 6 to verify adjustment.
Print Speed
Squeegee Pressure
- In the case of rubber or poly squeegee materials, the lower durometer (hardness), blades generally require more downward pressure in order to provide the desired 'clean wipe' of the stencil surface. Herein lies the problem, the more pressure exerted by the squeegee blades, the more gouging or scooping of solder paste out of the stencil apertures occurs. There are two basic fixes to this, you can try printing at a slower print speed, which will enable you to reduce overall squeegee pressure, or upgrade to a harder durometer blade material. For standard PCB assembly, somewhere around 80 to 100 durometer is typical.
- A less visible problem with some types of poly blades is as more downward pressure is applied to the squeegee blade, the poly material will deflect more, much more than a metal blade assembly, which changes the angle of attack between the squeegee blade material and the stencil surface. This can cause problems with paste roll, or the filling of apertures, and solder paste bleed out.
- Another typical problem with using poly blades, is that they will wear very quickly, increasing defect levels as they deteriorate.
- A harder durometer or metal blade squeegee will require less overall pressure than a poly blade, and it is much easier to control solder paste deposition across the length of the blade. A metal blade squeegee will not gouge or scoop the solder paste from stencil apertures, allowing better volume control as well as a better defined solder deposit.
- Select a good flat substrate.
- Position the substrate to be printed under the stencil.
- Adjust the squeegee pressure so that excess solder will remain across the stencil surface after a print cycle.
- With solder paste applied to the stencil, perform a print cycle.
- Observe the amount of solder paste remaining on the stencil surface
- Add a small amount of squeegee pressure and perform another print cycle.
- Again observe the amount of solder paste remaining on the stencil surface. It should be somewhat less that the first pass.
- Continue to repeat steps 6 & 7 until all that remains visible on the stencil surface is a very thin skim coat of solder paste.
Transporting Solder Paste
-
Solder paste should be shipped on an overnight basis in order to reduce its exposure to environmental conditions. This may be accomplished by land (if possible) or by air. This is especially important during hot/humid weather.
-
International shipments of solder paste should be made as expediently as possible. A transit time of two, three days maximum, is preferable. Solder paste should never be shipped by sea.
-
The inclusion of ice and/or other special packaging materials may be warranted in certain situations, i.e., shipping solder paste during very hot weather; shipping paste internationally; shipping paste of an extremely sensitive composition.
Storing Solder Paste
-
Solder paste should not be allowed to remain on the dock after receipt. Solder paste should be transferred immediately to the proper storage area.
-
Most solder pastes normally have a shelf life of three to six months when stored at room temperature (22°C). Refrigerated: 0°C-12°C (32°F-55°F) / Unrefrigerated < 25°C (< 77°F). Refrigeration will normally double the shelf life of solder paste, while protecting the paste from the varying levels of heat and humidity often found in warehouses and offices.
-
Do not combine new and used solder paste in the same container. This may degrade the new paste.
-
Paste should not be re-refrigerated after the seal has been broken. Any opened material should be resealed and stored at room temperature when not in use. If possible, solder paste should be packaged in quantities no greater than the total daily usage of paste.
Preparing Paste For Use
-
Solder paste should not be used cold.
-
Solder paste should be allowed to reach ambient (room) temperature naturally and completely before breaking the seal of the paste package. DO NOT FORCE WARM THE PASTE. The package of solder paste should be exposed to room temperature for six to eight hours in order to reach ambient temperature. Eight hours (overnight) is recommended for optimal results.
-
Once the solder paste has reached ambient temperature, the seal of the package may be broken. Before applying, paste should be stirred lightly and thoroughly for one to four minutes. Solder paste should be stirred in one direction, either with a spatula (for jars) or with a mixing rod (for cartridges), as per instructions. Stirring the paste ensures an even distribution of any separated material (flux and metal) resulting from storage.
Stencil Application
-
A sufficient amount of solder paste should be applied (by hand or automatic dispenser) to allow for a smooth and even roll during the print cycle. A roll diameter of 3/8 to 1/2 inch (the size of a dime) is normally sufficient for stenciling. The amount of paste applied for the initial application typically should be two or three times the size of the desired roll. This will allow the squeegee blade to "load up" and allow the paste to flow out across the length of the squeegee blade.
-
The length of the roll should correspond to the length of the squeegee blade, which should be slightly larger (1/2 inch on each side) than the board width.
-
Small amounts of fresh solder paste should be applied to the stencil at frequent, controlled intervals. This will help to maintain the paste’s chemistry and workable properties. To best maintain the chemistry of the solder paste, smaller amounts of paste should be added at frequent intervals, rather than large amounts of paste less often.
-
After printing, the SMT process should continue in a timely manner to prevent the solder paste from drying out on the PCB and other time-related problems.
Environmental Conditions
-
For optimal results, the temperature and humidity of the areas in which printing, populating, and reflow take place should be maintained at a stable level. A temperature of 22° - 26°C (72° - 80°F) at 45% ± 5% relative humidity is optimal. Open/Tack times of 24 hours and greater have been achieved at these conditions. In addition, these times have been seen to increase to 48 hours when printed paste has been stored under Nitrogen.
-
Conversely, if temperature and humidity conditions are not maintained at optimal levels, the time that the paste may be exposed to the environment and paste functionality may decrease significantly. Solder paste is also more prone to defects such as slumping, solder balling, and viscosity changes when applied in environments of excessive temperature/humidity.
-
Generally, solder paste will remain functional for the longest periods of time between printing to populating and populating to reflow when temperature and humidity are controlled. When temperature and/or humidity are not controlled, there is a trade-off in the time that the paste remains functional, as well as the functionality of the solder paste.