It is recommended that finer pitch aperture openings be slightly smaller than the landing pad size. This is primarily for:.

• Improved gasketing between the landing pad and the underside of the stencil
• Prevent bridging on fine pitch component

Recommended Pad and Aperture Size:

Aperture width reductions must be taken equally from each side so that aperture is centered on the pad. (Fig 1.)

Aperture lengths can be reduced by similar dimensions to reduce the potential of solder balling.

Fig 1.

Apertures can be shifted to the outside edge of a pad to reduce potential for "underchip" solder balls. (Fig. 2)

Fig 2. Aperture versus Final Glue Dot Diameter.

Different aperture shapes have been found to offer the benefits of less paste utilization, consistent paste release and reduced or eliminated solder balling. Shapes to consider include:

Fig 3. Oval - Home - V-shaped.

Stencil or "foil" thickness is an important part of stencil design. Optimal paste deposition onto a PCB is impacted by the relationship that exists between the pad size, aperture opening and foil thickness. While the aperture may be appropriately sized for a pad, a stencil that is either too thin or too thick may still cause less than optimal deposition of solder paste.

This relationship is also known as "aspect." Aspect is the difference in forces that either pull paste from an aperture and on to a pad or cause paste to be held within an aperture. These forces can be quantified and represented as a measurement called the Aspect Ratio. In simple terms, for a paste to be adequately deposited on a pad, the paste surface tension must be stronger that the surface tension of the paste to the aperture wall.

A broad set of rules has been adopted that help us design stencils with appropriate Aspect Ratios depending on the type of stencil ordered. It is important that the smallest aperture on the board be used for this calculation.

For example:

a laser-cut stencil with a 16 mil leaded component (8 mil aperture width) should have a maximum foil thickness of 6 mil* [6 x 1.2 = 8]

A stencil should always have a Paste Pulling Tension of 0.2 or greater:

Pad Pulling Tension (P)		Aperture (Length (L) ´ Width (W))
	=		³ 0.6
Retaining Wall tension (R)		Stencil Thickness (T) ´ Aperture Perimeter (2 ´ (L + W))

Fig 3. Aperture Width Vs Deposit Profile.

Diameter of stencil openings for different components:

The diameter (GDD) and the height (GDH) of the glue dots depend on:

• Diameter of the stencil openings
• Stencil thickness
• Viscosity/Rheology of the glue
• Surface roughness of the stencil apertures - surface tension forces between the glue and the stencil.
• Surface topography of the PCB

To find an optimal diameter of the glue dot /stencil apertures for different components, dots with different diameters were printed onto a glass substrate then populated with various components. These combinations of components and glue dots were then inspected for the maximum dot diameter that did not contaminate the component metallizations.

In the table below are approximate GDD (and openings in the stencil) for different components when printing with a metal, laser-cut stencil of 10 mil thickness using the printing parameters described:

Metal Stencils:
Some say "Acetone is the best solvent for cleanup from both a cost and solvency standpoint"
But if a non flammable solvent with equivalent solvency is desired then Acceton is not an option!!

It is recommended to verify the compatibility of the cleanup solvent with the stencil frame adhesive.

Plastic Stencils:
Plastic stencils can be cleaned with the same recommended solvents as for metal stencils with a few additional considerations.

• A static charge may build up on the stencil during cleaning that can effect printing results.
• Plastic stencils are more susceptible to scratching during manual cleaning.
• Typically plastic stencils are much thicker than metal stencils and small apertures may require more aggressive cleaning such as high pressure spray and/or ultrasonic methods

The shape and the consistency of the glue dot depends on the rheology of the adhesive (yield point and plastic viscosity).
Pay attention that the adhesive must have been developed with a rheology designed for stencil printing, not hygroscopic and excellent adhesion with standard and difficult to glue components (Low-Stress Plastic Encapsulated Components).

Printing parameters have a crucial influence on glue dot shape and consistency. In laboratory conditions, optimal results have been obtained with the following parameters:

Advantages:
(Valid for Conventional and for the Printing Process with Thick Stencils)

• Higher throughput.
• There is no need to install dispensing machines on the line:
  - shorter lines
  - lower investment.

• Larger glue packages can be used:
  - less package waste
  - lower costs for the glue per quantity unit.
• This process is well suitable for double sided boards populated with SMD-components only.
• Adhesive deposits that are shapes are possible.

Disadvantages:
(Valid for Conventional and for the Printing Process with Thick Stencils)

• Less flexible as each PCB-Layout requires another stencil or screen.
• Thick stencils are more difficult to be cleaned than needles.
• It is not possible to print adhesive on a populated PCB.
• Not applicable for double sided reflowed boards, as it is only possible to dispense the glue after printing of solder paste (process: first side of the PCB - print solder paste, dispense the glue, place the components, reflow the paste and cure the glue ; second side of the PCB - print solder paste, place the components, reflow the paste).
• Adhesive is exposed to the environment over longer period of time - bigger temperature differences and water absorption can cause performance changes in the adhesive - it is recommended to use non-hygroscopic adhesives which have low sensitivity to temperature.
• There is a danger of dust/lint entrapment in the glue.

(Valid for Conventional Printing Process only

• It is difficult to process components with a big difference in stand-off (QFP’s and PLCC’s) as all glue dots have the same height.
  This assumes that the component stand-off height is > 10 mils.

Concerns:
(Valid for Conventional and for the Printing Process with Thick Stencils)

• There is a danger of air entrapment in the glue during the squeegee process. The danger of air entrapment is bigger with high viscosity adhesive. On the other hand, an increase of the temperature and smaller vertical squeegee movement decreases the risk of air entrapment. The use of metal squeegees, can significantly reduce the entrapment of air.
• If a printing technique is used for processing of PCBs which are populated both with SMD and also with through-hole components, then the process must be organised as follows: print the glue, place the SMD’s, cure the glue, insert the through-hole components then wave solder. During the placement of the through-hole components a strong bending of the PCB is possible which might cause a loss of SMD’s. This danger is less present when the glue is applied by dispensing because through-hole component insertion typically precedes the glue dispense and SMT placement operations.

(Valid for Conventional Printing Process only)

• If there are components on the board with a very big difference in stand-off, it may be necessary to dispense the glue manually or automatically .

(Valid for Conventional Printing Process with Thick only)

• During the cleaning process with plastic stencil a static charge can occur.
• When printing with stencils considerably thicker than 10 mils the dot consistency of small dot (< 20 mils) is compromised

The conventional printing process works fine with the limitation that all glue dots have the same height.
High dots with excellent consistency for the component range of 0402 chips to SO 28 leaded components can be obtained with a thick (10 mil) metal stencil.
A print-print print mode with a metal squeegee blade and slow board to stencil separation speed are recommended.
By adding snap-off or by leaving a controlled film of material over the apertures after the last squeegee stroke, very high large dot heights can be obtained.