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Stencil design basics

  • Solder paste stencil printing
  In this printing method, solder paste is deposited through metal stencil having openings (apertures) previously cut in the places where solder paste should fall on surface mount components pads.
  Nowadays laser cutting method of stencil manufacturing using specialized polished stainless steel became most widely used. It provides high quality of apertures formation in the stencil and accurate reproduction of their shapes and sizes set in stencil project source file. 
  
  •   Choosing thickness, material and stencil apertures sizes
  For high quality and fast solder paste deposition it is recommended to choose stencils made of polished stainless steel but not of bronze. Being more soft material, bronze causes apertures deformation in the stencil when it is stretched, which can lead to offset of solder paste prints pattern and further bridging formation between component pads or residual solder balls on the surface of assembled PCB.
  Polished surface of stainless steel facilitate better squeegee fit during solder paste deposition decreasing the scooping of already deposited solder paste and lowering the amount of solder paste residue on the stencil after squeegee pass.
  Excessive solder paste residue on the stencil will degrade its deposition at the next printing cycle, and at the same time, small paste residues can form solder balls that can then roll under low-seated components.
  Practical experience shows that for most of simple electronic products is enough to use only two stencil material thicknesses - these are 0.127 mm and 0.150 mm. 
  If PCB includes components with pitch of 0.5 mm and less, the best practice is to use a stencil made of 0.127 mm thick material. Otherwise, it is better to leave 0.150 mm. For mounting of components with more smaller pitch (including microBGA) it is recommended to use more thinner materials.
 
 
  As a reference, you can choose stencil material thickness from the table below:
 

Type of component

Pitch

(distance between middle of neighbour leads of component)

Width of component pad

Length of component pad

Width of aperture of stencil

Length of aperture of stencil

Range of stencil thicknesses

PLCC

1.25 mm

[49.2 mil]

0.65 mm

[25.6 mil]

2.00 mm

[78.7 mil]

0.60 mm

[23.6 mil]

1.95 mm

[76.8 mil]

0.15 - 0.25 mm

[5.91 - 9.84 mil]

QFP

0.65 mm

[25.6 mil]

0.35 mm

[13.8 mil]

1.50 mm

[59.1 mil]

0.30 mm

[11.8 mil]

1.45 mm

[57.1 mil]

0.15-0.175 mm

[5.91 - 6.89 mil]

QFP

0.50 mm

[19.7 mil]

0.30 mm

[11.8 mil]

1.25 mm

[49.2 mil]

0.25 mm

[9.84 mil]

1.20 mm

[47.2 mil]

0.125 -0.15 mm

[4.92 - 5.91 mil]

QFP

0.40 mm

[15.7 mil]

0.25 mm

[9.84 mil]

1.25 mm

[49.2 mil]

0.20 mm

[7.87 mil]

[1.20 mm]

47.2 mil

0.10 -0.125 mm

[3.94 - 4.92 mil]

QFP

0.30 mm

[11.8 mil]

0.20 mm

[7.87 mil]

1.00 mm

[39.4 mil]

0.15 mm

[5.91 mil]

0.95 mm

[37.4 mil]

0.075-0.125 mm

[2.95 - 3.94 mil]

0402

-

0.50 mm

[19.7 mil]

0.65 mm

[25.6 mil]

0.45 mm

[17.7 mil]

0.60 mm

[23.6 mil]

0.125 -0.15 mm

[4.92 - 5.91 mil]

0201

-

0.25 mm

[9.84 mil]

0.40 mm

[15.7 mil]

0.23 mm

[9.06 mil]

0.35 mm

[13.8 mil]

0.075-0.125 mm

[2.95 - 3.94 mil]

BGA

1.25 mm

[49.2 mil]

CIR 0.80mm

[31.5 mil]

CIR 0.80mm

[31.5 mil]

CIR 0.75mm

[29.5 mil]

CIR 0.75mm

[29.5 mil]

0.15 - 0.20 mm

[5.91 - 7.87 mil]

BGA
fine pitch

1.00 mm

[39.4 mil]

CIR 0.38mm

[15.0 mil]

CIR 0.38mm

[15.0 mil]

SQ 0.35mm

[13.8 mil]

SQ 0.35mm

[13.8 mil]

0.115-0.135 mm

[4.53 -5.31 mil]

BGA
fine pitch

0.50 mm

[19.7 mil]

CIR 0.30mm

[11.8 mil]

CIR 0.30mm

[11.8 mil]

SQ 0.28mm

[11.0 mil]

SQ 0.28mm

[11.0 mil]

0.075-0.125 mm

[2.95 - 3.94 mil]

 
 
 
  • Stencil apertures shape and size changing
  There are several major variants of stencil apertures shape and size modification that can help to avoid some defects of soldering. The following recommendations are not a “dogma” for production engineers because actual requirements to the stencil are often determined from experiment at the production facility directly. Many things depend on equipment used and the facility staff skills.
  
Proportional reduction of all apertures sizes or one of the sizes
  Usually, it is sufficient to make the stencil apertures equal by size to components pads, nevertheless for fine-pitch components (less than 0.5mm pitch) it is recommended to narrow the apertures by 0.05mm at each side to avoid bridging between neighbour component leads.
 
  Also is possible to try proportional reduction of all apertures sizes, but in this case it’s needed to keep in mind that excessive reduction can be a source of insufficient paste volume on the pad, and consequently, lead to open joint.
 
Rounding of stencil apertures corners 
Rounding of apertures corners
Fig.1 Rounding of stencil apertures corners 
 
  Rounding is made to prevent paste particles trap in stencil aperture corners, especially in small apertures. Trapped particles can drop out of the stencil onto PCB during the next printing cycle and form solder balls during soldering or inhibit solder paste entering the aperture during the next printing cycle. As a rule, it is not bad to make such rounding of apertures designed for components with 0.5mm and smaller pitch, and optionally of other apertures.
  
 
Apertures shape modification
 
  As a rule, this is used for standard chip components (resistors, capacitors, etc.) in order to move away the excess of solder paste from under component’s body to prevent bridging between its pads.
 
Modification 1 of aperture form Modification 2 of aperture form
Modification 3 of aperture form
Fig.2 Aperture shape modification variants for standard chip components
 
  Also the excessive solder paste not fixed to a component pad under the component can cause solder balls formation.
 
Cause od solde balls formation
Fig. 3 Solder balls formation caused by excessive paste under chip component
 
 
Next modification is used for MELF packages (barrel-shaped components).
 
Modification 4 od aperture shape
Fig. 4 Example of aperture shape modification for MELF package
 
  From one side, this helps to avoid bridging under the component (and also solder balls formation caused by paste residue), from another, it forms two bracket-shaped paste rollers wrapping around the barrel-shaped component and giving it no way to roll over the PCB before solder paste is reflowed.