2 Template Factors
2.1 stencil materials and engraving
Two methods, chemical etching and laser cutting, are commonly used. For high-precision screens, laser cutting methods should be used because laser-cut holes have straight walls with small roughness (less than 3 microns) and a taper.
2.2 Relationship between various parts of stencil and solder paste printing
(1) The size of the hole
The shape of the openings in the stencil and the shape of the pads on the printed board are very important for the precise printing of the solder paste. The openings in the stencil are mainly determined by the size of the corresponding pads on the printed board. In general, the size of openings in the stencil should be 10% smaller than the corresponding pads.
(2) Thickness of stencil
The thickness of the stencil and the size of the opening have a great relationship with the printing of the solder paste and the subsequent reflow. Specifically, the thinner the hole is, the more favorable the solder paste is released. It has been demonstrated that good print quality requires that the ratio of the opening size to the thickness of the stencil be greater than 1.5. Otherwise, the solder paste is not completely printed. In general, for a lead pitch of 0.5mm, use a 0.12∽0.15mm stencil thickness, a lead pitch of 0.3∽0.4mm, and a thickness of 0.1mm stencil.
(3) Opening direction and size of mesh plate
Solder paste release in the direction of the length of the pad is consistent with the direction of printing, printing effect is better than the direction perpendicular to the two.
The specific stencil design process can be implemented according to Table 3.
Table 3 Relationship between mesh opening size, mesh thickness, pad width, and lead pitch
Component type
Pin spacing
Pad width
Pad length
Opening width
Opening length
Template thickness
PLCC
1.27
0.65
2.00
0.60
1.95
0.15-0.25
QFP
0.635
0.35
1.50
0.32
1.45
0.15-0.18
QFP
0.50
0.254
1.25
0.22
1.20
0.12-0.15
QFP
0.40
0.25
1.25
0.20
1.20
0.10-0.12
QFP
0.30
0.20
1.00
0.15
0.95
0.07-0.12
0402
0.50
0.65
0.45
0.60
0.12-0.15
0201
0.25
0.40
0.23
0.35
0.07-0.12
BGA
1.27
Ø0.80
Ø0.75
0.15-0.20
BGA
1.00
Ø0.63
Ø0.56
0.10-0.12
μBGA
0.50
Ø0.30
Ø0.28
0.07-0.12
Flip chip
0.25
0.12
0.12
0.12
0.12
0.08-0.10
Flip chip
0.20
0.10
0.10
0.10
0.10
0.05-0.10
Flip chip
0.15
0.08
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