PCB Assembly Rework & Repair Guidelines
PCBA Boards can be reworked and repaired. However, only reasonable operations based on scientific guidelines can guarantee the quality of PCBA boards;
PCBA rework and repair basis
Rework and repair of PCBA must be performed in accordance with PCB design documents and repair regulations, and there also must be a unified rework and repair process.
The rework times allowed for each solder joint
Rework is allowed for defective solder joints, and rework for each solder joint shall not exceed three times, otherwise it will cause damage to the solder joints.
Use of removed components
In principle, the disassembled components should not be used anymore. If they need to be used, they must be screened and tested according to the original electrical performance and process performance of the components, and the installation is allowed only when they meet the requirements.
Times of desoldering on each pad
Each printed pad should be desoldered only once (that is, only one component is allowed to be replaced). The thickness of the intermetallic compound (IMC) of a qualified solder joint is 1.5～3.5μm, and the thickness will increase after remelting, even when it reaches 50µm, the solder joint becomes brittle, the solder strength decreases… As a result, there are serious reliability risks under vibration conditions;
And remelting IMC requires a higher temperature, otherwise the IMC cannot be removed. The copper-plated layer at the exit of the through hole is the thinnest, and the pad is easy to break from here after remelting; with the thermal expansion of the Z axis, the copper layer deforms, and the pad is separated due to the obstruction of the lead-tin solder joint.
In the case of lead-free, the entire pad will be pulled up. The PCB is delaminated after heating due to moisture in the glass fiber and epoxy resin, and the pad is easy to warp and separate from the substrate.
Bow and twist requirements for surface mounting and mixed mounting PCBA assembly and soldering
The bow and twist of surface mounting and mixed mounting PCBA assembly and welding are less than 0.75%.
Total times of PCB assembly repairs
The total times of repairs for PCB assembly is limited to six; In other words, excessive repairs and modifications affect PCBA reliability.