The screen printing process is critical to the electronics manufacturing industry and has undergone significant developments since its inception. Significant effort has gone into understanding all the critical parameters since very few processes can cause as many production problems as out-of-control screen printing. In this issue of EMPFasis, we will review the screen printing process with a focus on the fundamentals necessary in the development and maintenance of an effective solder paste printing process.

The solder paste printer was designed to accurately apply solder paste to printed wiring boards so the electronic components can be held securely and conduct electric current. Additionally, the printing machine also allows for the solder paste to be applied in a controlled manner to prevent inconsistencies and defects.

There are 5 major sub-assemblies that make up most solder paste printing machines. They are:

• Print Head: The print head is used to support the squeegee blades and their associated driving mecha-nisms and travels forwards and backwards above the stencil along precision guide rails.
• Squeegee System: The squeegee is driven across the top of the stencil in a controlled manner to flood solder paste through stencil apertures onto printed wiring boards. Squeegee speed, balance, and pressure greatly affect the quality of the print.
• Stencil/Screen Frame: The metal foil stencil is mounted to an aluminum frame under tension with a nylon mesh and a bonding adhesive. A universal frame allows for small or odd-size frames to be used on the existing prin-ter, thus expanding the capabilities of the screen printer.
• X, Y Table: The X, Y table supports the printed wiring board during the screen printing cycle. More advanced tables may include a “work nest” and a vacuum system, to secure the board in place.
• Vision System: Vision systems help ensure accurate registration of the stencil to the printed wiring board. Automated vision systems allow for high speed and accurate alignment without operator interference.

Squeegee selection
Understanding squeegees and the selection criteria used to determine the appropriate types for specific production applications is necessary. The available squeegee materials for printing solder paste are metal and rubber.

Metal squeegees work well with stencils rather than screens because of the rough screen surface. One advantage to the metal squeegees is that there is very little “scooping” of paste from the apertures. The metal squeegees are also popular in the printing process because of reductions in set-up time, print variability, and sensitivity. Some disadvantages of the metal squeegees include a higher cost and their vulnerability to damage if bumped or dropped.

Rubber squeegees work well with traditional screen printing due to their ability to conform to the mesh screen. Also, they can be sensitive to other printing variables such as squeegee pressure and aperture size. The rubber can be specified at different hardness levels in order to accommodate different process requirements. Higher durometer readings, which indicate a harder rubber, are recommended for stencil printing in order to avoid “scoopage”.

Stencils/screens
The stencil/screen determines the pattern in which the solder paste is printed onto the printed wiring board. Solder paste is placed directly on top of the stencil, and then the squeegee forces the paste through the apertures onto the printed wiring board. The stencil/screen also serves as a gasket to prevent paste from being printed onto unwanted areas and allows for accurate transfer of the solder paste onto the designated patterns.

Apertures are typically created by chemical etching, laser cutting, or electroforming. Each has its advantages and disadvantages. The best method will depend on the application. With the trapezoidal-shaped aperture, the tapered walls of the aperture allow the volume of solder paste at the bottom of the aperture to greatly exceed the solder paste at the top. This allows for easy release of the paste from the aperture (similar to sand from a tapered pail). This design also helps to prevent clogging of the aperture during the printing process.

During the separation of the stencil from the board, insufficient gasketing may occur. This is where the aspect and area ratios are implemented. The aspect ratio is the width of the aperture divided by the thickness of the stencil (W/T). The area ratio is the area of the pad divided by the area of the aperture walls (LxW/2(L+W)T). Generally, the aspect ratio should be greater than 1.5 mils and the area ratio should be greater than .66 mils in order to ensure adequate material release. When the pad area is greater than .66 mils of the aperture wall area, a complete paste transfer should occur.

Machine operation parameters
The print speed determines how fast the squeegee moves across the stencil and is a critical variable due to the time that solder paste needs to roll into the apertures and onto the printed wiring board. A good starting point is 1 inch/second and then optimize the process from there. Additionally, slower speeds are usually required for finer pitch technologies (around 20 mil pitch).

The print stroke is the distance the squeegee travels when printing onto a printed wiring board. Usually, 0.5" past the furthest aperture on the stencil is appropriate. This distance is usually enough to allow for the paste to begin rolling for the next print. Too long of a stroke could possibly cause damage to the squeegees and stencil bonding.

“Snap off” is the distance at which the table moves down after separating the stencil from the printed wiring board after printing. The separation takes place at a slower speed to allow for the solder paste to release from the apertures. For off-contact printing, the snap off speed and the off-contact gap are critical.

The print mode is probably the most commonly used printing method in production. In this mode, only one printing stroke is used for each printed wiring board, printing forward for the first board, then reverse for the next.

Squeegee pressure is the pressure applied to the squeegee during the print stroke. Too little pressure can cause smearing of the paste on the stencil, poor paste deposition, and the incomplete transfer of the paste to the printed wiring board. Too much pressure can cause “scooping” of the paste from larger apertures (especially with rubber squeegees). Also, too much pressure can cause excess wear on the stencil and squeegees, and may cause “bleeding” of the paste between the stencil and the printed wiring board. Typically, just enough pressure to wipe the paste from the stencil surface is adequate. Ideally, a barely noticeable film of flux is left on the stencil after the print stroke.

Solder paste failure modes
The following are some of the most common modes of failure for solder paste:

Powder failure - The flux has both a reactive and protective character that attempts to minimize oxidation of the metallized powder. As the paste ages, the reactive nature of the activator decreases and can cause powder oxides to grow. As the powder oxidation process proceeds, solder balling and printing defects become evident in manufacturing.

Rheologic failure - Due to high stresses imparted on a paste in the stencil printing process, both the viscosity and thixotropic nature of the paste can change over time due to degradation of the flux paste medium. This change in material will typically cause problems in printability of the paste or, potentially, in its slump or tack behavior after printing.

Solvent separation - With the high stresses imparted from the stencil printing process and an increase in ambient temperature, it is possible that the solvent(s) within the solder paste can separate from the flux paste system. In manufacturing, this is seen as solvent pooling around the printed paste, which can cause significant solder balling due to the volatile nature of solvents.

Moisture adsorption - Moisture adsorption is generally due to the hygroscopic nature of the solvents used (e.g., glycols) or due to other additives that have some affinity for water. Solder paste moisture will primarily cause an increase in solder balling due to increased volatility of the water in the flux system.

Loss of powder suspension - Uniform powder suspension provides a homogeneous characteristic along with consistent rheology under stress. In addition, suspension does increase the protection of the powder from accelerated oxidation due to direct environmental exposure. Powder suspension failures can be temporary or permanent. A temporary failure can easily be repaired by mixing the paste back into a homogeneous mixture. A permanent failure will have no suspension character regardless of the amount of mixing.

Conclusion
Controlling your solder paste printing process need not be a major undertaking. Once you have established the optimum process for your product complexity, monitoring and controlling the key parameters mentioned in this article should adequately satisfy your needs. If you would like additional information on screen printing or are interested in screen printing training, please call the EMPF Helpline at 610-362-1320 or logon to the EMPF website at http://www.empf.org.