Rework

TN0107

12/15

DocID15852 Rev 3

4

Rework

If a defective component is observed after the board assembly, the device can be removed

and replaced by a new one. Repair of single solder joints is generally possible, but requires

proper tools. For example, repairing the solder joint of an exposed die pad cannot be done

by a soldering iron.

Whatever rework process is applied, it is important to recognize that heating a board and

components above 200 °C may result in damage. As a precaution, every board with its

components has to be baked prior to rework. For details, please refer to the international

standard J-STD-033.

In any case, mechanical, thermal or thermo-mechanical overstress has to be avoided, and

rework has to be done according to JEDEC J-STD-033A, IPC-7711 and IPC-7721.

4.1

Device removal

If a defective component is sent back to the supplier, no further defects must be caused

during the removal of this component, because this may hinder the failure analysis by the

supplier. The following recommendations should be considered:

•

Moisture: depending on the MSL, the package may have to be dried before removal. If

the maximum storage time out of the dry pack (see label on packing material) is

exceeded after board assembly, the PCB has to be baked according to the

recommendations of the PCB manufacturer. Otherwise, too much moisture may have

been accumulated and damage may occur (“popcorn” effect).

•

Temperature profile: during the desoldering process it should be assured that the

package peak temperature is not higher and temperature ramps are not steeper than

for the standard assembly wave process.

•

Mechanics: be careful not to apply high mechanical forces for removal. Otherwise

failure analysis of the package can be impossible, or PCB can be damaged. For large

packages, pipettes can be used (implemented on most rework systems); for small

packages, tweezers may be more practical.

4.2

Site redressing

After removing the defective component, pads on the PCB have to be cleaned to remove

solder residues. This may be done by vacuum desoldering or wick.

Don’t use steel brushes because steel residues can lead to bad solder joints. Before placing

a new component, it may be necessary to apply solder paste on PCB pads by printing

(special micro-stencil) or dispensing.