Material Compatibility

Compatibility of Electronics Cooling Fluids with Equipment Construction Materials:

Beyond being an effective cooling medium, an electronics cooling fluid must not affect the physical characteristics of the other components of the cooling system. The proper dielectric cooling fluid for the application will be compatible with all circuit board and other materials that the fluid will contact. As few fluids were specifically developed for use with computer components, long term compatibility has to be carefully considered. Fluids can affect components in a variety of ways; by causing rubber or plastic components to swell and soften, by causing some components to harden and crack, or by delamination of circuit boards.

 

One of the ways that fluids are tested for potential incompatibilities is to determine the fluid’s solubility toward rubbers and similar compounds. A common test measures the lowest temperature at which aniline will dissolve in the oil. Aniline is a chemical that has a molecular structure that is similar to a variety of rubber compounds. A low value of aniline point indicates that the oil is a more aggressive solvent, which is closely linked to rubber and plastic swelling and softening, circuit board delamination and other long term compatibility problems.

 

Aniline Points of Heat Transfer Fluids

Mineral Oil

White Oil USP

Vegetable Oil

Iso-Paraffin

Aniline Point,
ASTM D611, C.

74

94

60

92

 

Being paraffinic in nature, the white oil and isoparaffin have the highest aniline points, and therefore the least solvency toward materials. The vegetable oil is an ester, which exhibits high solvency.

There are more direct ways to measure compatibility of materials and propensity to delaminate or destabilize circuit board material, increasing entropy of the system.

Accelerated aging studies can indicate potential problems in material compatibility. Standard Test Method ASTM D3455 (“Standard Test Methods for Compatibility of Construction Material with Electrical Insulating Oil of Petroleum Origin”) can be modified to apply to a wider range of materials, beyond petroleum oils. The modified method ages materials in an oven at 150 C., for 14 days, then evaluates the samples for changes appropriate to that material. Elastomers are tested for weight change, composite materials are tested for tensile strength and evidence of delamination, etc. Further information on material compatibility testing in dielectric fluids can be found in the references (9).


The photo below shows a circuit board aged in a vegetable-based dielectric fluid. Solvent attack and the beginning of delamination can be seen (marked by arrows in this photograph).


Circuit Board Aged in Natural Ester (Vegetable Oil)

 

Circuit Board Aged in Vegetable Oil
Circuit Board Aged in Vegetable Oil

 

 

The next photo shows the same circuit board in a less aggressive fluid, synthetic hydrocarbon. Note the absence of pits and areas of dissolved circuit substrate.

 

Circuit Board Aged in Isoparaffin
Circuit Board Aged in Isoparaffin

 

Of the oils tested, the white oil and isoparaffin oils have the least solvency power and will be less aggressive to paints, varnishes, rubbers, and other materials than fluids that contain vegetable oils or conventional mineral oil.

Fluids chosen should be compatible with gasket and sealing materials that are commonly used in electronics applications. Some of these materials are:

 

Nitrile Rubber

Silicone Rubber

Buna-n Rubber

Viton

Cork

Fluorocarbon Rubber

 

For more on the subject of fluid molecular structure and its effects on material compatibility, please contact the author at davidsundin@outlook.com