Reasons for the shift of Components in the SMT Process

The main purpose of SMT processing is to accurately and precisely mount the surface-assembled components to the fixed position of the PCB. But in the process of mounting processing, there will be some problems, which will affect the quality of the SMT, and one common problem is the component shift. So what are the reasons for the shift of the components？Let China PCB assembly factory SYS Tech introduce to you today.

The reasons for the shift of different packages are different, and the common reasons are:

(1) The wind speed of the reflow soldering furnace is too large (mainly occurs in the BTU furnace, small and high components are prone to shift).

(2) The vibration of the conveying rail and the conveying action of the mounter (heavier components).

(3) The pad design is asymmetrical.

(4) Large-size pad lifting (SOT143).

(5) Components with fewer pins and larger spans are easy to be pulled diagonally by the surface tension of the solder. For such components, such as SIM cards, the width of which pads or stencil openings must be less than the component pin width plus 0.3mm.

(6) The dimensions of the two ends of the components are different.

(7) The components are not uniformly stressed, such as the anti-wetting thrust of the package, and stuck situation in positioning holes or mounting slots.

(8) There are components that are prone to exhaust, such as tantalum capacitors.

(9) Generally, the solder paste with strong activity is not easy to shift.

(10) All the factors that can cause the tombstone will cause shift.

Deal with on specific reasons

Due to when reflow soldering, the components are in a floating state. If accurate positioning is required, the following work should be done well:

① The solder paste printing must be accurate and the stencil opening size cannot be more than 0.1mm wider than the component pins.

② Reasonably design the pad and the installation position so that the components can be automatically calibrated.

③ When designing, the matching clearance between the structural parts should be appropriately enlarged.