Stencil printing is a process that uses a stencil, solder paste, and printer to deposit solder paste on printed circuit boards in order to establish electrical connections between components. The solder paste acts as both an adhesive during component placement and helps form solder joints during reflow. The printing process involves carefully aligning and securing the board before the stencil prints the paste using a squeegee. Inspection after printing is important to check for defects before the next stage of component placement.

1. STENCIL PRINTING

2. INTRODUCTION
 Stencil printing is the process of depositing solder paste on the printed wiring
boards (PWBs) to establish electrical connections.
 It is immediately followed by the component placement stage.
 The equipment and materials used in this stage are a stencil, solder paste, and
a printer.
 The stencil printing function is achieved through a single material namely solder
paste which consists of solder metal and flux.
 Paste also acts as an adhesive during component placement and solder reflow.
The tackiness of the paste enables the components to stay in place.
 A good solder joint is one where the solder paste has melted well and flowed
and wetted the lead or termination on the component and the pad on the board.
 In order to achieve this kind of a solder joint, the component needs to be in the
right place, the right volume of solder paste needs to be applied, the paste
needs to wet well on the board and component, and there needs to be a residue
that is either safe to leave on the board or one that can easily be cleaned.

3. PRINTING PROCESS
 The process begins with loading the board into the printer. The internal
vision system aligns the stencil to the board, after which
the squeegee prints the solder paste. The stencil and board are then
separated and unloaded. The bottom of the stencil is wiped about every
ten prints to remove excess solder paste remaining on the stencil.
 A typical printing operation has a speed of around 15 to 45 seconds per
board. Print head speed is typically 1 to 8 inches per second. The
printing process must be carefully controlled. Misalignment of motion
from the reference results in several defects, hence the board must be
secured correctly before the process begins. A snugger and vacuum
holders are used to secure the X and Y axes of the board. Vacuum
holders must be carefully used, as they may affect the pin-in-paste
printing process if not secured properly.
 The longest process is the printing operation, followed by the separation
process. Post print inspection is crucial and is usually performed with
special 2D vision systems on the printer or separate 3D systems.

4. DESIGN
 Vision systems in the stencil printing
machines use global fiducial marks for
aligning the PWB. Without these fiducially
the printer would not print the solder paste in
exact alignment with the pads. The PWB
should have close dimensional tolerances so
that it mates to the stencil. This is necessary
to achieve the required alignment of solder
blocks on the pads.

5. MASKING
 The required accuracy in alignment can also be achieved by controlling the flow
of solder on the PWB during reflow soldering.
 For this purpose, the space between the pads is often coated with a solder
mask. The solder mask materials have no affinity to the molten solder and
hence, no positive bonding is formed between them as the solder solidifies. This
process is often referred to as Solder masking.
 The mask must be centered correctly. The mask protects the PWB against
oxidation, and prevents unintended solder bridges from forming between closely
spaced solder pads.
 Also the height of the solder mask should be lower than the pad height to avoid
casketing problems.
 If the height of the solder mask is greater than that of the pad, then some of the
solder paste would settle in the empty space between the mask and the pad.
This is what is referred to as casketing. It is a seal that fills the space between
two surfaces to prevent leakages.
 Casketing is a problem as the excess solder paste around the pad may be more
than a nuisance factor for circuits having very small line spacing.

6. FINISHING
 The pads on the PWB are made of copper
and are susceptible to oxidization.
 Surface oxidization on the copper will inhibit
the ability of the solder to form a reliable
joint.
 To avoid this unwanted effect, all exposed
copper is protected with a surface finish.

7. APERTURE FILL AND RELEASE
 The core of a well printed PWB lies in the fill and release of solder paste
into the aperture. When the stencil is in contact with the PWB, solder
paste is applied over the top surface of the stencil using a squeegee.
This causes the aperture to fill with solder paste.
 The PWB is then lowered from the stencil. The amount of solder paste
which is released from the stencil apertures and transferred to the PWB
pads, determines whether or not the print is good.
 Ideally, all volumes of solder paste should be equal to the volume of the
corresponding stencil aperture. In reality however, this is never the case.
 Hence, a print is considered to be good if a certain fraction of the paste
is released. One way of quantifying print performance is to calculate
the transfer efficiency.
 This is mathematically stated as:
 Transfer efficiency = (Volume of printed deposit) / (Theoretical maximum
volume)In the above expression, the theoretical maximum volume is
simply the open volume of the stencil aperture.

8. HISTORY
 The use of stencils dates back over 37 thousand
years, as evident in Neanderthal cave art found in
Spain.
 These paintings are outlines of hand prints; it is
theorized that Prehistoric man or woman would place
their hand against the wall, and then blow finely
crushed pigment around it.
 These stencils were accompanied by shapes from
the natural world and daily life: animals, hunting
scenes, and ritual all figure prominently.

9.  Over time, the use of stencils spread throughout the ancient world.
 Many of the detailed drawings and intricate art found in Egyptian tombs
or along the walls of the city of Pompeii were results of stencils.
 Stenciling was a simple and well-thought-out way to reproduce the same
pattern over and over again.
 The Chinese were the first to develop a paper-based stencil, around 105
AD, and used the invention to advance their printing techniques. Soon,
stenciling made the transition to cloth and colorful patterns were
transferred onto garments.
 The Japanese improved upon the technique by bonding delicate stencils
together with human hair or silk.
 Their method of dyeing fabrics with stencils is known as Katazome, and
used the paste resist method.
 With this technique, a paste (usually composed of rice flour) is pushed
through a stencil, defining a pattern on cloth.
 The places where the sticky paste adheres to the cloth resist color in the
dyeing process, therefore creating a negative stenciled design on the
fabric.
 Stenciling eventually spread to Europe, thanks to the trade routes from
the East. The technique of stenciling became quite popular, and was
used to add color to master prints. Stencils became handy in mass
productions of items, including manuscripts, playing cards, book
illustrations, fabrics, and wallpaper.