The fundamental reason such phenomena are the wetting force element on both sides of the imbalance, and both ends of the torque components is not balanced, which leads to the occurrence of such phenomena.
The following conditions lead to the uneven wetting force on both sides of the reflow solder:
1.Design and layout of pads are not reasonable.
If the design and layout of the pad has the following defects, it will cause the uneven wetting force on both sides of the element.
1)One of the welding plates on both sides of the element is connected with the ground wire or the area of one side is too large, and the thermal capacity of the two ends of the plate is not uniform.
2) the temperature difference is too large on the surface of PCB so that the heat absorption of the two sides of the plate is not uniform.
3)There will be temperature inhomogeneous at both ends of small plate element welds around large devices QFP, BGA and radiator.
Change the design and layout of the pad.
2.Problems in solder paste and solder paste printing.
The activity of the solder paste is not high or the weldability of the component is poor. After the solder paste is melted, the surface tension is not the same, which will cause the uneven wetting force of the weld plate. Two solder paste solder paste printing quantity is uneven, the multi side of the solder paste will increase the heat absorption, melting time lag, so that the wetting force is not balanced.
Choose the higher activity solder paste,improve the data of solder paste in printing,especially in the size of the window template.
3.Mounter displacement in Z axis with difference pressure.
This issue causes the element to be immersed in the solder paste and the depth is uneven, and the wetting force on both sides is unbalance due to the time difference. If the element patch shift will directly lead to a monument.
Adjust the process data of mounter.
4.The profile of reflow oven is net correct.
If the reflow furnace is too short and the temperature area is too small, the working curve of PCB heating will be incorrect, so that the moisture difference on the panel will be too large, resulting in uneven wetting force.
Adjust the right profile according to the different products.
5.Oxygen concentration in nitrogen reflow oven
Take the protection of nitrogen reflow solder wetting force will increase, but more and more examples, have erected phenomenon in the case of low oxygen content increased; usually think control the oxygen content (100~500) minus 6 square * 10 about the most suitable.