According to authoritative statistics, the most important and critical process in the SMT process should be the solder paste printing process. Almost 70% of soldering defects are caused by poor solder paste printing.

The solder paste printing process is related to the success or failure of the SMT assembly quality, and the design and manufacture of the stencil is a key factor in the quality of the solder paste printing. Proper design can get a good solder paste printing result, otherwise it will cause the process quality to be unstable. , Defects are difficult to control. The following content will list some common SMT solder paste stencil opening optimization designs for your reference.

 

 

1.SMT steel mesh

(1) Steel mesh thickness

The thickness of the stencil should not be too small or excessive. Generally speaking, the IC situation is mainly considered. Different IC foot pitches correspond to different stencil thickness ranges; too thin a stencil will cause defects such as low tin soldering, and too thick will cause defects such as tin and tin beads; The surface of the steel mesh must be flat and smooth, and the thickness error should be within the acceptable range.

(2) Outer frame stiffness and steel mesh tension

 

The steel mesh frame is generally made of hollow reinforcement alloy. Due to the high tension of the tensioned stencil, it is generally required to be above 30N/mm2. It must withstand such high tension and the clamping pressure of the printing press. Otherwise, the position of the stencil will be shifted or the outer frame will be deformed. The stencil should not be tight, and it should not be close to the surface of the PCB during printing, causing the solder paste to leak under the stencil.

 

(3) SMT steel mesh opening design

The template opening design contains two contents: opening size and opening shape

 

The size of the opening and the shape of the opening will affect the filling and release of the solder paste (demolition), and ultimately affect the amount of solder paste leakage.

 

The template opening is designed according to the printed circuit board land pattern, and sometimes needs to be appropriately modified (enlarge, reduce or modify the shape), because the structure, shape, and size of the pins of different components require different amounts of solder paste.

The greater the disparity in the size of the components on the same PCB and the higher the assembly density, the greater the difficulty of template design.

The most basic requirements for the design of steel mesh formwork openings:

Aspect ratio = opening width (W) / template thickness (T)

Volume ratio = opening area / hole wall area

The width to thickness ratio/area ratio of the rectangular opening:

Aspect ratio: W/T＞1.5

Area ratio: L×W/2(L+W)×T＞0.66