Here is chapter 2 of our comprehensive guide to stencils in which we explore what to consider when deciding which stencil is right for your application.

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2.1 Stencil formats

2.1.1 Remountable stencils

Many years ago, the tension offered by meshed stencils could not be fully replicated and as such image stability was a major concern when adopting remountable stencils for fine pitch printing. Today however, there are a number of four-sided tensioning frames in the market that are capable of ensuring optimum stencil tension and
long stencil life.

Remountable stencils offer three major benefits:

  • Space saving of up to 80%
  • Cost saving of up to 65%
  • No issues regarding selection of cleaning solutions to be compatible with adhesives Genesis

Genesis mechanically tensioned stencil

Tecan’s proprietary tensioning system, Genesis, enables foils to be quickly interchanged and held absolutely flat with equal and maintainable planar tension across the whole stencil.

This simple mechanical mounting system uses no adhesives – which may degrade over time, has no need for pneumatics – which are susceptible to rupture – and does not require a loading jig. OptiGuard

OptiGuard stencils for VectorGuard frames

A patented joint development between ASM and Tecan, OptiGuard is designed for use with standard
VectorGuard™ frames.

Pin bars are used to mount the foil into an extruded aluminium profile, which is ideal when stepped or multi-level stencils are required and can be used with laser-cut, precision etched, or laser-formed stencils.

The foil is then mounted into the VectorGuard frame using air pressure. TetraBond™

TetraBond stencil and VectorGuard frame

A simple system for safe mounting and demounting, designed to optimise rigidity in the stencil. Foils are mounted into a thin aluminium extrusion using an advanced bonding system and are designed for use with Tetra™ and VectorGuard frames, whilst being backwards compatible with most other frames in the market. Other formats

In addition to these most popular formats, there are a number of alternative remountable framing systems on the market. Tecan supports stencil production/capability for most of these, including:

  • Tecfoil
  • Apex
  • FTS
  • LPKF
  • ZelFlex

2.1.2 Meshed stencils

 Stencil meshed in aluminium frame

Traditional meshed stencils, where the metal stencil element is bonded on to an aluminium frame, are still preferred by some operators.

Tecan only uses high grade epoxy adhesives and polyester fabrics in the meshed stencil manufacturing process as although meshed stencils are tensioned in four directions, the mesh material tension can degrade over time with use. Also, alternative adhesive bonds – between the frame and mesh and the mesh and stencil – may degrade with some cleaning solutions available.

Offset image positioning can often result in the start of the print stroke being close to the glue bond areas. When using semi-automatic or automatic printers with meshed stencils, don’t be tempted to make the print stroke too long or damage to the glue bonds and mesh border could result.

Typical frame sizes (mm):

  • 434 x 434 x 25.4mm
  • 508 x 508 x 38.1mm
  • 503 x 404 x 25.4mm

2.1.3 Extended stencils

  • 798 x 578 x 28/35mm
  • 736.6 x 736.6 x 38.1mm
  • 584.2 x 584.2 x 38.1mm
  • 622.3 x 392 x 19.05mm
  • 650 x 550 x 30mm

2.1.3 Extended stencils

Tecan’s extended precision screens are available in sizes of up to 1800mm x 900mm. Typically used for specialist applications such as OLED and LCD display technologies,
these bespoke meshed stencils provide the same level of printing accuracy as standard SMT sizes.

Extended stencil meshed into aluminium frame

2.2 Stencil thickness selection

Component pitch vs stencil thickness

Stencil thickness ultimately determines the volume of solder fillet that is available for the component terminations. Any aperture design modifications, used to ptimise the printing process, can be compromised by selecting an inappropriate thickness. Too thin and the fillets required may not be achieved. Too thick and paste retention may occur effectively starving the solder paste volume.

Figure 2.12 above highlights the different stencil thickness requirements for a range of surface mount components often found together on a typical assembly. In deciding which stencil thickness will give the best results it is always worth considering a multi-level stencil as this is the best way to provide components with conflicting paste volume requirements, the specific individual paste volumes they require.

The aspect ratios of materials currently available are listed below:

2.2.1. Multi-level

Multi-level stencils have become the preferred solution for many organisations due to their superior print quality and paste release properties. Other benefits include lower printed defects; less paste bridging, higher production yields and optimised quality.

Multi-level stencil features are created using photo chemical machining and subsequent laser-etching.

Multi-level stencil with raised and recessed areas

2.2.2. Stepped stencil

Stepped stencils are created by removing an isolated depth on the squeegee side of the stencil leaving the general thickness untouched.

etching from both sides

Stencils with raised areas on the squeegee surface are created by removing the majority of the top surface to leave raised islands. In these cases it is better to select nickel as the stencil material since it isn’t affected by removal of the majority of one skin.

Stencils with both raised portions and reduced thicknesses are the result of two separate processes.

2.2.3 Under-routed stencils

Localised pockets can be provided on the underside of the stencil to accommodate thicker solder pads, test points and badly registered solder resist. This enables optimum stencil gasketing.

Multi-level stencil with under-routing

2.3 Aperture design

2.3.1 Aperture reduction

So why is it necessary to reduce stencil apertures from the CAD Gerber designs? It is recommended that stencil apertures are generally reduced from the copper feature sizes to be printed. Exceptions to this rule include BGAs, µBGAs, components with a known poor co-planarity and pin-in-hole reflow applications.

Let’s consider that the Gerber is used as a datum from which both the PCB substrate and the stencil apertures are created.

PCB fabricators work to a manufacturing tolerance. Understanding the processes involved in producing the substrates can help to achieve successful printed results.

It is no good to design aperture footprints with IPC standards in mind if the PCB has been accepted with overetched features.

Although today’s stencils can be produced with tolerances of between 5-9µm, the loss of any stencil to PCB gasketing surface can lead to under-stencil contamination and ultimately bleeding, bridging shorts and rework.

Stencil gasket

Printing accuracy must always be considered, even with the best printing machine available an acceptable tolerance on its accuracy is often ± 12 – 25µm so aperture reduction must accommodate this tolerance.

Reductions also allow for any paste slump: the “collapse” of the paste bricks from their printed formation due to environmental conditions and any paste spread associated with pick and placement pressures. Aperture reduction example

2.3.2 Aperture shapes

Simply reducing all the stencil apertures by a global dimension or percentage cannot offer the advantages that specific reductions or aperture shape modifications, tailored to individual components, can achieve to reduce end of line surface mount assembly defects.

For those engineers who recognise the problems but may not be aware of the solutions, Tecan can design the stencil aperture sizes and shapes to suit.

Stencil aperture shapes

With the base pads of D-PAK components it is often necessary to reduce the outside dimensions of this pad and then window the resulting aperture to offer the solder paste volume required by the component without causing unnecessary component float or paste starvation.

Modifications for D-PAK components

2.3.3 Mid-chip solder ball elimination

Solder beading or mid-chip solder balls are the result of excess solder paste beneath the non- wetting surfaces of discrete components. Upon reflow this paste is not able to retract and when the component is drawn towards the PCB it is squashed to emerge from beneath the component.

Mid-chip solder ball