When printing solder paste onto a PCB there is a lot of factors to consider.
In the listing below the most essential factors are mentioned.

All the factors in the five groups are differently important but all plays a role in the final result and it is important to consider all aspects to reach the quality needed in the products produced.

Printing equipment can be divided into two main groups; In-line and off-line.
For "small" production sites an off-line screen printer should be adequate but for high out-put placement lines, where the product cycle-time is short, an in-line system can be necessary.

Fig 1. Squeegee blade.

Before investing in an in-line machine there are several things to consider:

1. the out-coming print quality.
   Since approximately 70% of all faults found on SMD PCBs come from the solder printing process, it is important that all process parameters are known and under control.
2. Inspection of the solder paste print is necessary. Especially if printing fine pitch. This could be done either by operator, vision or laser inspection systems.

Since the introduction of laser-cut stainless steel stencils (in the beginning of the 1990's), metal stencil printing has been the dominant method.

The printing squeegees can have different designs and made of different materials. E.g. square rubber rods, thick rubber plates, flat metal plates or other combinations. Today the commonly used squeegees, for metal stencil printing, are thin metal squeegees (See Fig. 1. above).

The squeegees must have a very smooth and none-sticking surface and at all times a sharp printing edge. This will ensure that the solder paste will roll more easily on top of the stencil and help prevent clogging of the stencil apertures.
The newest development in solder paste printing is the direct printing. This system replaces the squeegees with a printing head that press the solder paste directly through the stencil apertures using a piston. This type of printing system is not widely spread and is mostly used in high run production because of the large amount of solder paste in use.

To make a perfect solder paste print, the PCB support must hold the PCB in a locked position and absolutely parallel to the stencil.
If the PCB is hold by vacuum cups, be aware that the solder paste easily can be sucked into the small via holes leaving the solder lands with too little solder paste. It will also result in solder balling.
The PCB support must be designed for both flexibility and fast change over. In general, to secure a good print-ability the printer construction must be rigid and the squeegee axis, stencil and PCB support must be precisely parallel. 

To secure an accurate print onto the solder lands a vision system is necessary. For off-line printers a so-called manual system could be adequate. The operator adjusts the PCB position to fit an overlay picture of the solder paste pattern or cross-hair lines.
Automatic vision alignment of PCBs is on the other hand necessary for in-line screen printers. This type of vision system usually uses fiducial marks on both stencil and PCB as adjustment reference. The PCB or stencil is then moved to fit the other.
The modern screen printers can be provided with a lot of options such as computer control, vision or laser print control systems, environment control, automatic PCB support set-up and underside stencil cleaning.

Metal stencils can be made of different metals. Besides stainless steel, they can be made of copper, bronze or nickel.
There are 3 different metal stencil manufacturing methods:

• Etching,
• electroforming
• and laser cutting.

The apertures in both laser-cut and electroformed stencils have very sharp edges and are slightly conic.
This makes the solder paste easily slip of the aperture edges and thereby secures a uniform print. See photographs for stencil types below.

For more info about stainless steel stencils (laser-cut, Chem-etch, Electropolishingtake andElectroforming) look at Metal Stencils Overview.

Fig 2a. Lasercut.	Fig 2b. Electroformed.	Fig 2c. Etched.

The metal stencil is attached to the printing frame using tensioned mesh or directly using a special frame with a gripping system.
Mesh attachment is a little more expensive but handling the loose stencils, for the direct attachment systems, easily damage the stencils and thereby results in poor printing quality.

If properly handled a stainless steel stencil will last more than 10.000 prints.The thickness of the metal stencil is typically 150 microns but 100, 125 and 200 microns is also available. The thickness should be chosen depending on the job in hand.

For very fine pitch such as 0.3 mm lead pitch 100 or 125 micron stencil could be used and for lead pitch down to 0.5 mm 150 micron stencils can be used. The stencil thickness together with the aperture sizes also determines the amount of solder paste present to form each solder joint during reflow soldering.

As a guideline the minimum stencil aperture width must be at least 3 times (preferable 5 times) the diameter of the largest solder particle and the stencil aperture width should also be larger than the stencil thickness.
Rounded aperture corners will reduce clogging of fine pitch apertures and smearing. The top surface of the metal stencil should be slightly roughened to make the solder paste roll perfectly during printing.

The main reason for printing solder paste onto the PCB is to supply solder alloy for the solder joints. To reach this objective, the solder paste print must be aligned correctly, the correct amount of solder paste for each joint must be present and the print should form an even layer of paste for perfect component placement.

The solder paste on top of the stencil is partly rolled and pressed into the stencil apertures and onto the PCB solder lands by a moving and angled squeegee.
The squeegee angle must be between 45 to 60 degree (usually not adjustable) and the rolling solder paste should have a diameter of 15 to 20 millimetre for optimum conditions.

Fig 3. Stencil printing.

As a main rule, thin steel squeegees should be used for metal stencils and thick rubber squeegees, as hard as possible, for mesh stencils.
Rubber squeegees used on stainless steel stencils will wear out quickly and cause severe scooping in large apertures.
Steelsqueegee used on mesh stencils will damage the mesh after only a few prints.

The squeegee printing edge must be sharp to secure a well-defined print. If using an old worn out squeegee with rounded printing edge the squeegee angle is reduced and the solder paste will not roll as desired.

Several items are important to reach a good result when printing solder paste onto PCBs.
The parameters: Squeegee down stop, Squeegee pressure, Printing speed, Snap off, Separation speed,Printing area and stencil cleaning are explained below.

The squeegee down stop is a mechanical stop that prevents the squeegees to move further down. It must be adjusted only to just touch the stencil surface.
However, if the squeegee axis and the stencil are not perfect parallel it can be necessary to over-adjust the down stop to compensate. But, if the down stop is adjusted too far down, both stencils and squeegees will wear out rapidly.
Some machines do not have a mechanical down stop but a squeegee origin point. Usually were the squeegee just touches the stencil. The squeegee pressure is the added using a spring-loaded system.

The squeegee pressure should be as little as necessary to scrape the stencil clean of solder paste particles when printing.
If adjusted correctly, a thin layer of flux will remain on top of the stencil.
The amount of pressure is determined by printing speed and stencil type.

Usually the solder paste manufacturer gives a hint towards the printing speed window; Typically between 20 and 80 mm per second.
The possible printing speed is determined by the solder pastes thixotropic behaviour. The solder paste must be soft and fluid when printed but jelly-like and stable when printed onto the PCB solder lands.
The more fluid the paste is when moved and rolled the higher print speed can be used.

The printing speed must be set so the solder paste rolls perfectly on top of the stencil. The printing speed is a major factor in the printing cycle time and one is therefore interested in the highest speed possible without compromising the print quality.

Snap off is the distance between the stencil underside and the PCB placed in print position but without the squeegee touching the stencil.
For metal stencil printing the snap off should be zero. This is also called contact printing.

(For mesh screen printing the snap off should be set to between 0.5 and 3.0 mm. Here the snap of plays a role in the amount of solder paste printed onto the solder lands. A high snap off will result in a thicker layer of solder paste.)

The speed of separation between stencil and PCB after printing is important. A too rapid separation speed when printing fine pitch will result in clogging of the stencil apertures. A too fast separation will also result in tailing and form high edges around the solder paste deposits. The ideal separation speed depends on the solder paste and the stencil aperture wall smoothness. On the other hand, a slow separation speeds will slows down the printing cycle time.

To ensure that the solder paste is rolling correctly before the aperture pattern is reached, the squeegee movement should start 80 - 100 mm or 2 times the solder paste circumference outside the pattern area.
To the sides the squeegee overlap should be minimum 20 mm.

In general, if all printing parameters are in control, stencil underside cleaning should not be necessary.
Stencil underside cleaning can be done by hand or automatic.

My experience with automatic stencil wipers without vacuum has been badly. The wiper does not clean the stencil underside but simply moves the solder paste particles from around the apertures to the complete stencil underside. And when performing the next print the solder particles are transferred to the PCB where they are found all over the surface.

Stencil cleaning prior to use is important to prevent dust and dirt to enter the solder joints. The stencil should of cause also be cleaned for solder paste after use.

Solder paste:
The solder pastes function is basically to supply solder material to the soldering spot, hold the components in place prior to soldering, clean the solder lands and component leads and finally to prevent further oxidation of the solder lands.

PCBs:
The flatness of the PCBs and the solder land flatness are both essential to the printing quality.
If the PCBs are bend or twisted, the result can be large variation in the solder paste layer. And especially for fine pitch printing the solder land flatness is important.
For PCBs with lead pitch at 0.65 mm and less it is recommended to use NiAu solder land surface. HAL (hot air levelling) is simply not flat enough.

The PCBs must also be clean and without fingerprints that will cause poor wetability in the reflow process.
Fingerprints on the solder lands can also result in too little solder paste or none solder paste, because the solder paste will not stick to the surface and are left inside the stencil apertures. Especially PCBs with NiAu surface are sensitive to these problems.

Dust and dirt from the air that ends up on the PCBs and stencils can cause defects such as bridging and poor wet-ability in the reflow soldering process.
A small piece of fibre or hair between two fine pitch solder pads can easily cause bridging. It is therefore very important that the PCBs are stored in sealed packages and if necessary cleaned before use.
Air draught in the production area, can speed up evaporation of the solvents in the solder paste and thereby make the solder paste dry out.
Also a high temperature will make the solder paste dry out quickly. If the room temperature in the production area varies a lot, it will be very difficult to control the printing process.
The viscosity of the solder paste changes with the temperature and the solder paste print will sometimes be perfect and other times the paste will slump and result in bridging. Check the solder paste supplier’s data for the temperature window.

The solder paste printing is a very sensitive and delicate process. Therefore the operators has to be trained and experienced.
The operators should be able to foresee problems and adjust the process to secure a good printing quality.
Things like; position of deposit, shape of deposit and volume should be controlled frequently.