Stencil Design Guidelines

Our ALPHA DIMENSIONS Stencil Engineering System allows off-line programming of any custom design rule(s) that will be applied automatically upon your request to your

1. Area Ratio

• The relation between the surface of the aperture and the inside surface of the aperture walls in the stencil. The major difference with Aspect Ratio is that Area Ratio is more suitable for shapes such as circles. Since solder paste has a certain adhesion force, it will stick to the walls of the aperture and to the pad.
• A ratio of ≥ 0.66 pad to wall is considered acceptable in our industry (for example a 13.5 mil circle in a 5 mil thick stencil). See formula.
• This factor has the single biggest impact on Transfer Efficiency and Repeatability of the solder paste deposits.

• Transfer Efficiency is the percentage of solder paste of the Theoretical Volume of the aperture that is transferred onto the pad.

	Transfer Efficiency (%) = TE = VD/VA
	Volume APERTURE = VA = Theoretical Volume Stencil Aperture
	Volume DEPOSIT = VA = Actual or Predicted Volume of Deposit

• The biggest impact on Area Ratio is stencil thickness:
• Area Ratio = A_P/A_W         ≥ 0.66
  
  {PAD AREA / WALL AREA}

2. Aperture Size

• It is recommended that aperture openings are made smaller than the landing pad size. This is called reduction or cropping. The main reasons for this are:
  1. Improved alignment accuracy & repeatability between the PCB     pad and the stencil aperture.
  2. Control solder paste volume (prevent bridging & mid ship solder     balling).
• Aperture width reductions must be taken equally from each side so that aperture is centered on the pad.
• Aperture lengths can be reduced by similar dimensions to reduce the potential for solder balling.
• For certain components, such as BGA’s, CCGA’s or other component types that require large volumes of solder, it is recommended to make the stencil apertures larger than the landing pad size. This is called overprinting.

3. Aperture Shape

• Different aperture shapes have been found to offer the benefits of better volume control or to prevent defects such as solder balling, bridging, voiding etc. Shapes to consider include:

• Remember a square gives the biggest aperture volume for any given size. The difference in aperture volume is 27% (4/π).

4. Stencil Thickness

• 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.

• Stepped Area(‘s) are recommended when the component mix featured on any given board does not allow the stencil to be manufactured with one overall thickness. Guidelines can be found in our GLB-AMG-0302 Procedure and Reference Documents.
• Based on the relationship of Area Ratio on Transfer Efficiency, influencing paste release and volumetric repeatability, changing the stencil thickness locally might have a favorable impact on the absolute volume and standard deviation of the deposit.
• A local thinner stencil (STEP DOWN) will be used when a more favorable Area Ratio (when < 0.7) can be achieved for a fine feature device.
• A local thicker stencil (STEP UP) will be used when the Area Ratio is already > 0.7 and an absolute larger volume is required than the Theoretical Volume of the aperture provides.

Please contact Cookson Electronics for a stencil manufactured using the Cookson Standards for Stencil Design. These standards are available worldwide through our ALPHA DIMENSIONS network and through our global procedure GLB-AMG-0302.

Once you start using these design rules, we can convert them into your personal design rules which allow you to have them customized to meet all your printing application requirements.

Stencil Manufacturing Method

• Laser cut stainless steel stencils are recommended for printing adhesives.
• For apertures whose minimum span is less than 15 mils (0.375mm), laser cut electro-polished stencils should be used.
• For applications with wider tolerances, chemically etched stencils can be used successfully.

Stencil Thickness

• 6 mil (150μm) is the typical stainless steel stencil thickness used. For most applications using the typical range of passive components, a stencil thickness of between 6 – 12 mils (150 – 300μm) is appropriate.

Aperture Design

• The following table provides recommended aperture shapes and dimensions for the most common components attached using SMD adhesives.
• Cookson recommends the double dot or slot configuration because they give the greatest process window for adequate adhesion and proper placement of the component.
• The pinched slot or oval are modifications of the typical slot and are designed to improve the aperture’s air release characteristics.
• Recommended Aperture Sizes, based on an 8 mil (200μm) thick laser cut stencil.
  

 TB ALPHA CUT Laser Cut Stencils