 |
|
|
 |
||
|
||||||||
| Contents | Previous page | Next page |
Cleaning of printed boards
is required to remove flux residues and other contaminants which are left behind
after soldering . Cleaning or washing of the boards prevent potential electrical
failures due to electromigration. Cleaning operations does the removal of the
following contaminants:
I.) Ionic contaminants
ii) Non-ionic contaminants
iii) Particulate contaminants
Fluxes that are water soluble generally produce ionic(also called polar)
contaminants that require aqueous cleaning. Non-ionic(also called non-polar)
contaminants produced by rosin fluxes require non-ionic solvents such as
trichloroethane.
 |
Water soluble fluxes need to be cleaned thoroughly due to corrosive
elements that are present in the flux residues, aqueous cleaning is ideal.
Because rosin is not soluble in water, when aqueous cleaning is used for rosin
fluxes, alkaline chemicals called saponifiers are added to the water. The
efficiency of cleaning treatment is significantly increased if the cleaning is
assisted by ultrasonic vibration. However, the ultrasonic vibrations are not
confined to the cleaning fluid and the surfaces to be cleaned, but are also
transferred into the electronic components, which may be damaged . Bonding
wires, inside active components between the die and the bonding pads may break
if the bonding wires are free and have not been tightly encapsulated in plastic
or any other packaging material. High-power, high frequency vibration induce
fracture in external leads also. The following set of parameters is generally
accepted:
§
maximum frequency of 40 kHz
§
maximum time of ultrasonic load 1 to 5 minutes
§
maximum power of 10 W/ litre
§
boards in racks, so that they cannot touch each other.
In all cases, the time
interval between soldering and cleaning be reduced to a minimum (less than an
hour) to obtain good cleaning results.
|
Web design by AnandSoft |