The problem of less tin in printing solder paste is mainly due to the following reasons:
1) The thickness of the stencil is not suitable.
The square and rectangular openings of the steel mesh structure can be easily calculated by length x width (l x w).
Pore wall area = 2t(l+w).
Area ratio = lw (2t(l+w)).
Figure 1Stencil cut-outs.
Circular perforated structure of steel mesh: Porous area = d 2 4;Bore wall area = dt;Area ratio = d (4t).
In the process of printing solder paste, four main forces are considered: the downward pull force of the PCB pad on the solder paste, the gravity of the solder paste, the adhesion of the hole wall to the solder paste, and the adhesion of the residual solder paste from the stencil to the solder paste in the hole. The forces that play a dominant role in the print quality are the downward pull of the PCB pad on the solder paste and the gravity of the solder paste. The sum of these two forces needs to be greater than the adhesion of the hole wall to the solder paste to ensure proper mold release. The greater the thickness of the stencil, the greater the adhesion of the hole wall to the solder paste, which increases the difficulty of demoulding.
A mismatch between the minimum opening and stencil thickness can lead to a problem with less tin. If the length of the minimum opening of the stencil is more than 5 times the width of the opening, the width of the opening needs to be 1 of the thickness5 times more (l>5w, w t>15)。When the length of the hole is not more than 5 times the width, the area of the hole should be 066 times more (l<5w, opening area hole wall area > 066)。
2) The solder paste alloy solder powder is not the right size.
The solder paste printing process needs to follow the "five-ball rule". Square The shortest side of a rectangular opening needs to hold at least five solder particles (the upper limit of the particle size is the benchmark). If the width of the rectangle opening of the stencil is 02 mm, you need to use No. 4 tin powder (20-38 m). If it is a circular opening, the diameter of the opening should be 8 times the size of the tin powder particles. The tin powder particles are too large, and the powder is easy to get stuck in the open pores, resulting in the problem of less tin for molding.
Calculation process: 38 m x 5 = 190 m = 019mm<0.2mm
3) The roughness of the steel mesh hole wall is too high.
Stencil manufacturing processes include etching, laser cutting and electroforming. There is a protrusion in the middle of the steel mesh made by etching, which is not conducive to demoulding. The roughness of the hole wall of the laser cutting steel mesh is large, and there will be burrs on the hole wall, which is easy to lead to the problem of less tin printing. The roughness of the hole wall can be observed by a stereo microscope. The electroformed steel mesh hole wall is the smoothest, and the opening is in an inverted trapezoidal shape, which greatly improves the demoulding effect.
4) The tension of the steel mesh is not up to standard.
Insufficient stencil tension can easily cause the stencil to fall during demoulding, which affects the release of solder paste and leads to insufficient amount of solder paste on the PCB.
5) The scraper pressure is too large or too small.
The effect of excessive scraper pressure is reflected in the insufficient thickness of the solder paste. During printing, the squeegee blade, which is not rigid enough to be subjected to high pressure per unit length, is deformed to a certain extent, and the solder paste inside the large hole is scooped out, causing the middle of the solder paste spot to collapse, creating a saddle-like printing effect (Fig. 2). Step blades can be used for this purpose, or the printing pressure can be slightly reduced. Conversely, too little scraper pressure will also cause the solder to be scraped cleanly, resulting in less tin.
6) Deterioration of printing solder paste.
Solder paste can become viscous and caking after being left for a long time. Solder paste with high viscosity will stick to the squeegee, resulting in insufficient amount of solder paste and less tin on the stencil during printing. The viscosity variation of the paste should be as small as possible to meet the longer print times and board times. In addition, when the yield is low, the amount of solder paste used should be reduced to avoid excessive solder. Too many times of solder paste is also not good for maintaining print quality. Generally, it is not recommended to have more than two times, as too much reheating will lead to degradation of solder performance. In addition, it should be noted that the solder paste on both sides of the scraper should be transferred to the solder paste tank in time and fully stirred for 2-5 minutes before reuse.
7) Printing The demoulding speed is too fast.
The shear-thinning nature of the solder paste allows it to be closer to the liquid state at a fast shear rate. If the printing speed is too fast, the solder paste fills the holes poorly, forming roof-like printing solder paste spots. The fast demoulding speed will also make the solder stick to the hole wall and not be able to release smoothly, resulting in less tin.
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