DocID023948 Rev 1

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AN4208

Soldering assembly recommendations

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New device soldering

For placement of the device several solutions are possible:



Use a mini-stencil and solder paste then place the device. This is the preferred solution

to ensure homogeneity of assembly conditions if assembly of WLCSP (wafer level chip

scale package) is performed with solder paste, even if small footprints and tight

dimensions make this operation difficult.



Use no-clean flux on the site and place the device.



Dip the WLCSP in no-clean flux, and to place it on the board.

Next operation is to reflow the solder joint by applying controlled heat to the component.

This can be done in much the same way as described above for component removal, but

accurate temperature control is necessary to ensure good soldering of the joint.

Alternatively this can be done by putting the whole board in a furnace. See Figure 10 for

reflow profile recommendations.

Equipment

Systems for these operations are available at various levels of automation. Methods and

techniques used in more sophisticated automatic systems can be copied using manual

equipment. Soldering irons should be avoided for these operations. Tweezers or any picking

tools pressuring the sides or bottom (bump side) of the WLCSP must be avoided since such

tools can damage silicon and create chip outs.

Figure 11 shows an example of semi-automatic equipment for component rework. (See the

Web site of Comintec for more information.)

Figure 11. Comintec ONYX32 - Semi-automatic equipment for component rework