Issue
The PAD design of the PCB does not match the component, causing component shifted after reflow.
Solution
Base on IPC industry standard of Soldering, modify the PAD design of the PCB according to the soldering pad of component, or replace the component that meets to the PAD design of PCB, as to ensure that the component will not be shift or stand reflow or after reflow process.
After the solder paste printed, we may use glue to make component adhered with PCB, to avoid shift or stand of component happens during or after reflow process.
Issue
The difference in melting time between the two sides of the components is too large, resulting non-wetting, tombstone, and shifted after reflow.
Solution
Connect temperature measuring wires on both sides of the components, measure the actual temperature of the reflow process on both sides, and adjust the furnace temperature according to the measurement results. To reduce the shift or stand of component caused by premature tin melting on either side and resulting in unbalanced torque at both ends.
Issue
Insufficient wetting during reflow, causing solder insufficient or poor PAD diffusivity.
Solution
Adjust the preheating time and liquid retention time in accordance with the reflow conditions of the solder paste and each component to ensure that the flux can maintain the best activity.
Or maybe use nitrogen to make the surface tension of the liquefied solder paste smaller than the atmospheric environment, so the wettability and expansion rate of the solder will become improved, and the oxidation of the solder surface due to high temperature during reflow will also reduced, to make secondary reflow which worked for double-sided process are also been improved.
In addition, the solder paste use a flux formula gives better oxidation, so the reflow can be better cover the soldering surface or the surface of the tin powder, in order to isolate the oxidation of high temperature reflow, and can also effectively dissolve the oxide on the metal surface, gives improvement of the welding effect.
