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:
- Precise application of the needed material (solder paste, glue)
- Creation of depots defined in shape and size
- Reproducibility of the print
- Effective cleaning of the pads
- Economy
The stencil data (necessary openings = apertures) is generated from the PCB data supplied (on request with pad reduction) or completed by the customer.
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
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 | |||
|---|---|---|---|---|---|
| Type | Aspekt ratio | leaded | >1,5 | lead-free | >1,7 |
| Type | Area ratio | leaded | >0,66 | lead-free | >0,8 |
The thickness of stencils for adhesive is usually 250µm.
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.
Possible changes of the pad form.
Text on the stencil
At Multi-CB 100 characters of text are inclusive (more text is feasible for a small fee). The text can be produced half lasered and lasered.
Half lasered text
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). Half lasered text is thus on the bottom 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
Placement of PCB data
If we get no information regarding data placement from the customer, the PCB data is placed based on the available data centrally on the stencil. Thus based on the board outline, or the pads (copper). If a different placement is wished, please let us know when ordering (text, sketch).
When ordering your stencil along with your PCB, you ideally have the stencil data already integrated in the digital production data and placed a note in your order to notify us
Examples of default layers for stencil data.
- EAGLE: tCream, bCream
- Target: PasteTop, PasteBot
- Protel: GTP, GTB
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
| Attribute | Tolerance | ||
|---|---|---|---|
| Attribute | Position accuracy | Tolerance | +/- 5µm |
| Attribute | Accuracy of pad geometry | Tolerance | +/- 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:
Finishing treatment
- Inclusive: Finish treatment bydouble-sided brushing of the surface in allfour directions, giving reductionof the surface roughnessRzto about1µm. On request we can provide your stencil with a circumferential Edge protection.
- Surcharge for: Electropolish (good for middle series) and Nano protection (big series. See SMD-Stencil finishing treatment.
