Special Material Selection Keeps Screw Plug Immersion Heaters Cleaning Circuit Boards
Aqueous cleaning of resins and organic fluxes from printed circuit boards is required after soldering. The aqueous cleaning solution is heated for two reasons; first it makes the solution clean at its maximum and, second, it cuts down the drying time.
There are several models of cleaning machines that use from one to three heaters per unit, depending on the volume of parts to be cleaned. The heater is rated at 7.5 kilowatts, while the maximum for the machine is 22.5 kilowatts. These machines are designed to allow the user to upgrade their basic unit as future production or process change demands.
The main constituent in the aqueous cleaning solution is city tap water, which has chlorine in it. This chlorine caused stress chloride cracking (SCC) in the 304 stainless steel heaters resulting in premature failure of the heater. Carbon and austenitic stainless steels (304, 316 and series-300) are very susceptible to SCC. In water at higher temperatures, chlorides in PPM can cause SCC. SCC occurs predominantly with stagnant solutions where chloride concentrate builds up.
In heater applications, air pockets are very conducive for chloride build up. As long as water is flowing, SCC at low temperature is not as much a problem for stainless steel. SCC is a progressive failure that produces cracking at stress levels (bends in heaters) that are well below those of a material's tensile strength. The fracture break appears brittle, with no localized yielding, plastic deformation or elongation. SCC has a characteristic appearance, which in many cases makes it easy to identify. Rather than a single crack, a whole network of very fine, feathery cracks will form. (In most cases the area that the cracks develop in will have to be magnified to see the cracks.)
The problem of stress chloride cracking can be eliminated by the following:
- Changing the sheath material to Incoloy®
- Belt polishing the elements to remove the dark oxidize
- Passivating or electro-polishing the sheath and the screw plug threads SCC can be eliminated by following the process as described above. This will increase the life of the heater which will decrease cost and increase production for the user of aqueous cleaning equipment.
Ability to provide application expertise.
Incoloy® is a registered trademark of Special Metals Corporation.