SMD components are the key process to ensure the assembly and assembly efficiency of PCB. The mounted components should be accurately picked and placed one by one to the specified target position of the printed circuit board according to the assembly drawing and the schedule requirements of the assembly board. This target location generally refers to the center position of each component during PCB design.
The following are the process requirements for smt soldering:
1. The type, mode, nominal value and polarity of each number component shall be in accordance with the assembly drawing and schedule requirements.
2. Mounted components should be intact.
3. Mounting components solder terminals or pins not less than 1/2 thickness to be immersed in solder paste. For general components, the amount of solder paste to be extruded (length) should be less than 0.2mm; for narrow-pitch components, the amount of solder paste extrusion (length) should be less than 0.1mm.
The termination or pin of the component should be aligned and centered with the land pattern. Due to the self-positioning effect during reflow soldering, the component placement position is allowed to have a certain deviation, and the allowable deviation range is as follows.
Rectangular component: Under the condition that PCB circuit board pad is designed correctly, the width of the component is 1/2 or more on the pad in the width direction; the length direction of the component, after overlaps of the soldering end and the pad, and the pad protruding portion is larger than 1/3 of the height of the welding end; when there is a rotation deviation, more than 1/2 of the width of the welding end of the component must be on the pad. In particular, the solder end of the component must be in contact with the solder paste pattern.
Small outline transistor (SOT): SOT allows X, Y, θ (rotation angle) to be biased, but the pins (including the toe and heel) must all be on the pad.
Small outline Integrated Circuit (SOIC): SOIC allows placement bias for X, r, θ (rotation angle), but must ensure that 3/4 of the device’s lead width (including toe and heel) is on the pad.
Four-sided flat package device and ultra-small package device (QFP): QFP should ensure that 3/4 of the pin width is on the pad, allowing X, r, θ (rotation angle) to have small placement deviation; allow pin toe is a small amount protruding from the pad, but must have a 3/4 pin length on the pad and the root of the pin must also be on the pad.