DocID15852 Rev 3

13/15

TN0107

Rework

15

4.3

Reassembly and reflow

After preparing the site, the new package can be placed onto the PCB and the leads are to

be inserted into the holes. Regarding placement accuracy and placement force, the process

should be comparable to the (automatic) pick-and-place process.

During the soldering process, it should be assured that the package peak temperature is not

higher and temperature ramps are not steeper than for the standard assembly process.

Soldering wire can be used to re-solder the leads. Use no-clean solder paste only, solder

wire, and flux for repair.

4.4

Coating of assembled PCBs

In some applications, coatings are used to prevent damage due to external influences such

as:

Mechanical abrasion

Vibration

Shock

Humidity

Hand perspiration

Chemicals and corrosive gases

These influences may cause:

•

Electrical leakage due to humidity.

•

Corrosion that leads to degradation of conductor paths, solder joints, and any other

metalized areas, and/or formation of electrical leakage paths. These can eventually

result in electrical shorts (electrical leakage) or open contacts.

•

Mechanical damage to conductor paths, solder joints, and components. This damage

can lead to electrical failures.

Coatings act as electrically isolating and impervious covers that adhere well to the different

PCB materials.

A wide variety of different coatings is available on the market. They differ in:

Price

Simple processability (spray, dip, casting, curing, etc.)

Reparability

Controllability

Homogeneity

In any case, please be aware of the chemical, electrical, mechanical and thermo-

mechanical interaction between the coating and the PCB and its components. Coatings can

affect component reliability.