Pseudo soldering inspection and control plan
Inspection items after reflow soldering:
Whether the components are missing.
Whether the components are mounted correctly.
Whether there is a short circuit.
Whether there is pseudo soldering.
The first three cases are easy to check, and the reasons are clear and easy to solve, but the reasons for the pseudo soldering are more complicated.
pseudo soldering during SMT process:
1. Judgment of pseudo soldering.
Use special equipment, e.g. online tester (commonly known as needle bed) for open/short testing.
2. Visual inspection (including magnifying glass and microscope).
When there is too little solder in the solder joints, poor solder wetting, or cracks in the middle of the solder joints, or the surface of the solder is convex spherical, or the solder is not compatible with SMD etc., you should pay attention to it.
Even a slight phenomenon will cause a hidden danger, it should be immediately judged whether there is a lot of pseudo soldering problem. The method of judgment is: to see if there are many problems with the solder joints at the same position on the PCB. If it is only a problem on an individual PCB, it may be caused by the solder paste being scratched, pin deformation etc., if it happens at the same position on many PCBs. it is likely to be caused by a bad component or a problem with the pad.
3. Causes and solutions of pseudo soldering.
(1). The pad design is defective. There should be no through holes on the pads, as the through holes will cause solder loss and insufficient solder; the pad spacing and area also need to be matched with standards, otherwise the design should be corrected as soon as possible.
(2). The PCB board is oxidized, that is, the pad is black and does not shine. If there is oxidation, an eraser can be used to remove the oxide layer to reproduce the bright light. If the PCB board is suspected to be damp, it can be dried in a drying box. If the PCB board is polluted by oil, sweat stains etc., it should be cleaned with absolute ethanol.
(3). For PCBs that have been printed with solder paste, the solder paste is scratched and rubbed, which reduces the amount of solder paste on the relevant pads, resulting in insufficient solder. It should be made up in time. The method of making up can be using a dispenser or pick up a little with small sticks.
(4). SMD (Surface Mounted Components) is of poor quality, expired, oxidized, and deformed, resulting in pseudo soldering. This is a more common reason.
- The oxidized component is black and does not shine.
The melting point of the oxidized components rises. It can be soldered with more than 300 degrees of soldering iron and rosin-type flux, because use more than 200 degrees of SMT reflow soldering and less corrosive no-clean solder paste is difficult to solder.
Therefore, the oxidized SMD is not suitable for soldering with reflow soldering furnace. Be sure to check whether there is oxidation when buying components, and use them in time after buying them. In the same way, oxidized solder paste cannot be used.
- For surface mount components with multiple leads, the leads are small and easily deformed under external force. Once deformed, the phenomenon of pseudo soldering or open soldering will definitely occur. Therefore, it is necessary to carefully check and repair in time before/after soldering.