SMD-Stencils

SMD-Stencils are precision tools which have to comply with high requirements, as they take a key role in the SMT (surface mount technology) process. We offer following SMD-Stencils:

Solder paste stencils are used to apply solder depots on printed circuit boards with a screen printing technique. SMD components (SMD = surface mounted device) are placed within these depots subsequently and soldered with a following reflow- or vapor phase process.

Stencils for adhesives are primarily used for mixed assembly, as substitution for a dispenser, and are necessary before component mounting and wave soldering.

Our lasered SMD-Stencils (made of stainless steel) allow a precise placement and strenght-of-shape of the solder- / adhesive- depots on the PCB and therefore accomodate the small track-widths and spacings as well as the hightech multipin components. Our SMD-Stencils are perfectly suited for finepitch and BGA assembly.

Terms in this Design-Aid:
top side = squeegee side, top view
bot side = opposite side of squeegee, bottom view
aperture = hole in the stencil, stencil pad

Stencil requirements

The quality of the finished assembly is influenced by the SMD-Stencil used. The size of the apertures (openings), and thickness of the stencil determine the amount and shape of the applied paste.

The purpose of the SMD-Stencil is:

  1. Precise application of the needed material (solder paste, glue)
  2. Creation of depots defined in shape and size
  3. Reproducibility of the print
  4. Effective cleaning of the pads
  5. Economy

To ensure an optimum result for the production of your SMD-Stencils, please note the following design hints.

Stencil thickness

For the choice of the stencil thickness (for solder paste), you can adhere to the following guidelines. Decisive is always the smallest component used. The final say should always have your assembler!

Type Pitch typical stencil thickness
Type BGA Pitch 1,25mm typical stencil thickness 150µm – 200µm
Type Finepitch BGA Pitch 1,00mm typical stencil thickness 115µm – 135µm
Type Finepitch BGA Pitch 0,50mm typical stencil thickness 75µm – 125µm
Type 0402 Pitch   typical stencil thickness 125µm – 150µm
Type 0201 Pitch   typical stencil thickness 75µm – 125µm
Type 01005 Pitch   typical stencil thickness 65µm – 90µm
Type PLCC Pitch 1,25mm typical stencil thickness 150µm – 250µm
Type QFP Pitch 0,65mm typical stencil thickness 150µm – 175µm
Type QFP Pitch 0,50mm typical stencil thickness 125µm – 150µm
Type QFP Pitch 0,40mm typical stencil thickness 100µm – 125µm
Type QFP Pitch 0,30mm typical stencil thickness 75µm – 125µm

Rule of thumb for detection of stencil thickness

SMD-Stencil thickness

The stencil thickness shown is rounded down and adjusted to suit the available material thicknesses! The standard thickness is 150μm.

Detection by aspect ratio and area ratio

The calculation of necessary stencil thickness can also be made via aspect ratio and area. Recommended values:

Type leaded lead-free
TypeAspekt ratio leaded >1,5 lead-free >1,7
TypeArea ratio leaded >0,66 lead-free >0,8
SMD stencil thickness calculation by aspect ratio and area ratio

The thickness of stencils for adhesive is usually 250µm.

Stencil pads

Stencils are generally lasered from the bottom-side. The production-related conical cross-section is then wider on the side opposite to the squeegee and allows a better peeling after the aplication of solder paste / adhesive (glue).

Processing of the stencil pads

Generally we recommend a reduction of the stencil pads (apertures) by 10%. This could be the done by the customer (with export of the paste data) or is done by us on request.

The following pad operations are carried out by Multi-CB on request:

  • Reduction of the pads by up to 10%
  • Absolute circular reduction of the pads

Modification of the pad form

Generally, you should (along with your assembly) make a decision in respect of angular or rounded pads.

If you have relatively large pad areas in your design, adding outbreak grids to apertures can help to reduce the adhesive forces.

SMD pad possible changes

Possible changes of the pad form.

Text on the stencil

At Multi-CB the text on the stencil is inclusive. The text can be produced half lasered and lasered through.

Half lasered text

The SMD-Stencil will be turned for half lasered text, half lasered text is thus on the top side (squeegee side) of the stencil.

Lasered-through text

As the half lasered text, also the lasered-through text is cut from the bottom side. You can decide whether the text is readable from the bottom or top side. To ensure best stability, lasered text is executed in a special font which prevents breaking out of the letters.

Font for lasered text of a SMD stencil

Font for lasered text

Positioning of PCB data on the SMD-Stencil

Positioning of PCB data on SMD-Stencil

The positioning of the PCB data on the SMD-Stencil follows this standard:

A) Customer sends stencil data with included contour / data positioning:

The positioning of the customer is used.

B) Only the PCB-(panel)contour is available:

The stencil is centered on the PCB contour.
Advantage: The stencil covers the entire PCB

C) PCB-(panel)contour is not available:

The stencil is centered on the pads (paste data).

If your SMD-Stencil is to be manufactured deviating from the standard (e.g. centering the stencil on paste data with given board contour), please indicate this in the notes.

Standard layers for paste data / also recommended for stencil contour:

  • EAGLE: tCream, bCream
  • Target: PasteTop, PasteBot
  • Protel: GTP, GTB
  • KiCad: f.paste, b.paste

Stencil holder

On request, we can add the required perforation to your stencil for all established, licence-free quick clamping frames / holders.

Possible systems are amanongs others:

  • QuattroFlex
  • ZelFlex
  • ESSEMTEC
  • PAGGEN
  • ...

In this case, please send us the data sheet of the quick clamp together with your order or contact us.

Tolerances

AttributeTolerance
AttributePosition accuracyTolerance+/- 5µm
AttributeAccuracy of pad geometryTolerance+/- 8µm

IPC handbook

In the IPC-design directive for SMD stencils (IPC-7525A) is mentioned, as other applicable policy for the cleaning of stencils and misprinted printed circuit boards, IPC-7526 (Stencil and Misprinted Cleaning Handbook).

This Directive is not commercially available and can be downloaded for free.

IPC-7526 – free download

Our service

In the production of our SMD laser stencils, we place great value on 100% quality. That's why we measure all the pads for dimensional accuracy and the real presence of the pads after production of your SMD stencil.

Registration marks

The application ofregistration / fiducial marks ensures the correct positioning of the stencil to the PCB, by the vision system of the assembly company.

For vision systems operating from below, the registration marks are usually half-lasered. This is possible because SMD stencils are generally lasered from below.

Does the vision system of the assembly company work from above, the registration marks are lasered through, just like a normal pad.

The number of registration marks is specified by the customer, per single PCB usually 3-4 pieces.

Typical types of registration marks:

SMD stencil registration mark examples.

Finishing treatment

Additional information: