(1) Correct components
It is required that the type, model, nominal value and polarity of each pcb assembly number component should conform to the assembly drawing and schedule requirements of the product, and the wrong position cannot be mounted.
(2) The position of the mounting is accurate
The terminals or pins of the components are aligned and centered as much as possible on the land pattern, and the solder end of the components are in contact with the solder paste pattern. The components mounting must meet the process requirements. The mounting positions of the two-end Chip components, wing-shape pins and J-pin devices, and ball-pin devices are as follows.
1 Two-end leadless Chip component. The two-terminal leadless Chip components have a relatively large self-positioning effect. When mounting, more than 1/2 of the component width direction is overlapped on the pad of the pcb, and the two ends of the length direction are simply soldered to the corresponding solder. The solder paste pattern on the disk, as shown in Figure 1, can be self-aligned when reflow soldering; but if one of the terminals is not attached to the pad or does not touch the solder paste pattern, it will occur shift and tombstone, as shown in Figure 2.
2 Wing-shaped pins and J shape pin devices. For SOP, SOJ, QFP, PLCC and other devices, the self-positioning effect is relatively small, and the mounting offset cannot be corrected by reflow soldering. If the mounting position is over the allowable deviation range, it must be manually adjusted before entering the reflow oven. At the same time, the mounting coordinates should be corrected in time. Mounting position requirements for wing and J shaped pin devices are shown in Figures 3 and 4.
● The pin width direction to the pad: P > 3/4 of the pin width
● Pin length direction: The heel and toe of the pin are on the pad.
3 spherical pin device. Since the pad area of the ball-shaped lead devices such as BGA and CSP is relatively large with respect to the area of the element body, the positioning effect is very good, so as long as the following two points are satisfied (see Fig. 5)
● The solder balls of the BGA are aligned with the corresponding pads.
● The maximum offset between the center of the solder ball and the center of the pad is less than 1/2 of the solder ball diameter.
(3) The pressure (SMD height) is suitable
The mounting pressure is equivalent to the Z-axis height of the nozzle, and the shaft height should be exactly and appropriate.
The Z-axis height is too large, which is equivalent to the mounting pressure being too small. The soldering end or the lead of the component is not pressed into the solder paste, floating on the surface of the solder paste, the solder paste cannot stick to the component, and during transferation, mounting and reflow soldering ,It is easy to produce positional movement; in addition, because the Z-axis height is too large, the components are dropped from the height when mounting, which is equivalent to free fall, which will cause the mounting position to shift, as shown in Fig. 6. The height of the Z-axis is too small, which is equivalent to the excessive mounting pressure, and the amount of solder paste is too much, which is easy to cause adhesion of the solder paste. It is easy to bridge when soldering, and the position of the alloy particles in the solder paste is also slipped and move, more serious, it will damage the component, as shown in Figure 7.
The correct Z-axis height requires that the distance between the bottom side of the component and the upper surface of the PCB pad (H) is approximately equal to the diameter of the largest alloy particle in the solder paste. For example, the maximum particle size of the No. 3 powder is ∅45μm, and the most suitable nozzle height of H≈5oμm, as shown in Figure 8.