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
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.
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.
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
Positioning of PCB data on the 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:
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.
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.
