When performing manual repair or reworking, it is necessary to pay attention to the following two issues, particularly because of temperature change and residual stress.
1. In order to prevent damage (cracks) to the component caused by localized rapid heating and heat shock, preheat the chip or take other measures to reduce heat shock to the chip.
2. The board temperature is lower than that used for reflow soldering, so a difference in residual stress occurs during cooling, and the mechanical strength (resistance to board bending) tends to decrease. In order to increase the strength, it is necessary to maintain a high board temperature during soldering.
Repair/rework task flow
Repair/rework consists of the following two tasks.
Repair task flow
- Removal task: Removing a capacitor mounted on a board
- Mounting task: Mounting a new capacitor
1. Coat with flux.
2. Preheat the board.
3. Heat the mounting area to melt the solder.
4. Remove the capacitor.
5. Cool the board.
6. Add solder.
7. Position the capacitor on the board.
8. Preheat the board.
9. Heat the mounting area to melt the solder.
10. Cool the board.
* When only mounting capacitors such as when creating product prototypes, perform only the mounting task. In these cases, the solder-adding task consists of coating with solder.
The figures below show standard temperature profiles (Figure 1, Figure 2). The solder composition is assumed to be Sn-3.0Ag-0.5Cu. Thermocouples are used to measure the temperature on the front and back of the board.
1. Preheat the entire board from the back side for 90 s.
2. Next, heat the area around the mounted component on the front of the board, and also heat from the top and bottom of the board to melt the solder.
The ultimate temperature during heating must be increased in locations where it is particularly difficult for solder to melt, even in a multilayer board (Figure 2).
Temperature measurement method
- Temperature measurement: K thermocouple
- Thermosetting resin: Epoxy resin
The temperature is measured in two locations on the front (the surface the capacitor is mounted on) and back of the board. Position a thermocouple in contact with the board in the center between the lands and fix it in place using thermosetting resin.
The preheater should have a wide heating area and be capable of preheating the entire subject board.
It is recommended to use a spot heater (hot-air heating).
* This is because spot heaters are easily obtained local heating devices.
Points when selecting tools for small components and high-density mounting boards
When handling 0603-size or smaller components and high-density mounting boards, care must also be taken to select the proper tools in addition to the heaters. The following points should be considered when selecting tools for the above tasks.
1. Magnifying device
It is recommended to use a microscope as the magnifying device for the tasks. An area heater is used to preheat the board, so an arm-type stand should be used.
When removing a capacitor from a high-density mounted board, it is recommended to use precision tweezers with a tip thickness of 0.1 mm or less to avoid contact with surrounding components.
Repair task procedure
The procedure for repair or reworking of a 0603-size capacitor on a high-density mounted board is described below.
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