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Common problems in the use of solder paste

When we use the solder paste to clean the machine, we will find some problems caused by the circuit. Let us take a look at it today.

First, the circuit board is shorted. Phenomenon profiling: Solder shorts are often present on tighter ICs or between smaller chip components.

There are four reasons for shorting the board:

1, solder paste Excess: the thickness of the steel mesh and the opening size are not appropriate, the printing support is uneven or the support points are scattered too little, the flatness of the PCB is poor and the tension of the steel mesh does not conform to the specifications and the cleaning of the steel mesh does not follow the rules, causing partial or total tin thickness.

2, printing offset is too large: PCB fixture is not safe, the machine manual may be active positioning and correction accuracy is poor, the general printing offset exceeds 1/4 welding The disc is judged to be a printing offset.

3, solder paste collapse, including the following three:

A) Print collapse Edge

The viscosity of the solder paste is low, the thixotropic is poor, and the flow collapses after printing; the wall of the steel mesh is rough and convex, and the tin is peeled off during printing. When the membrane is attacked, a similar sag phenomenon occurs, and the blade pressure is too large to form a solder paste forming damage.

Select a solder paste with moderate viscosity; use laser cutting or other better steel mesh; lower blade pressure.

B) Collapse when mounting

The placement machine is loading SOP, In QFP, QFN, and CSP type components, excessive pressure causes the shape of the solder paste to change and collapse.

C) Collapse during soldering heating

Reflow preheating is too fast, soldering The solvent component in the paste evaporates too quickly, causing the solder particles to collapse out of the weld zone.

4, fine-small components Pad orientation, length, width planning and component foot distribution, scale is not assigned.

Second, the circuit board after reflow oven exhibits tin bead phenomenon: tin beads generally occur around the Chip component, and the scale is generally 0.2---0.4 Between mm.