NO834067L - SOLVENT MIXTURE TO REMOVE FLUID - Google Patents

Description

The electronics industry requires circuit boards that are essentially free of residues of ionic and organic fluxes, as these contribute to making the circuit boards unusable. Stringent methods are therefore used when cleaning the circuit boards to remove both ionic and organic residues. Numerous solutions and mixtures of solvents have been tried but found unsatisfactory. The most commonly used commercial solvents are 1,1,2-trifluoro-1,2,2-trichloroethane ("Fluorocarbon" 113) in a mixture with 10.67 vol% methanol and 0.33 vol% nitromethane. This solvent effectively cleans the soldered circuit board of rosin flux used as an aid in soldering. The cleaning efficiency is measured by standardized methods in the industry, and such a method is prescribed by the military in the USA, namely a specification for "Printed Wiring Assemblies" MIL-P-28809. This test consists of spraying or immersing the cleaned card in a freshly prepared aqueous isopropyl alcohol solution for a given time, after which the resistance of the solution is measured in ohm-cm. The effectiveness of a flux removal compound is a function of the cleaning time, the composition of the flux and the type of cleaning method. If all these are equal, the more effective mixtures will give a higher value for the specific resistance when tested according to the above-mentioned test or similar standard tests.

The above fluorochlorocarbon mixture has been shown by industrial experience, and by the above test, to be an effective flux removal solvent. Below the flash point level, chlorinated hydrocarbons alone or in combination with alkanols will generally give poorer results, especially when it comes to the removal of ionic components in the flux. It is important that the mixtures used in industry have no flash point, for obvious safety reasons.

It is also known that chlorinated hydrocarbons, especially 1,1,1-trichloroethane (methylchloroform), will remove the non-ionic constituents of the rosin flux better than the above fluorochlorocarbon mixture.

Two patents that describe flux removal compositions are US Patents 3,932,297 and 4,023,984, which state 1.1.1-trichloroethane (methylchloroform) together with 1-propanol (n-propyl alcohol) and 2-propanol (isopropyl alcohol), respectively;

and an azeotropic mixture of a fluorocarbon and 1-butanol (n-butyl alcohol) is described in US Patent 3,671,446 as a circuit board cleaner.

It would therefore be advantageous to have a mixture based on chlorinated solvent which will effectively remove both ionic and non-ionic flux residues, and which has no flash point. The present invention provides such a mixture.

According to the invention, stable 1,1,1-trichloroethane (methylchloroform) solvent mixtures in combination with from 0.5 to less than 2% by volume of methanol and from approx. 3 to approx. 10% by volume of at least one alcohol with from 2 to 5 carbon atoms proved to be particularly good solvents for removing flux. These mixtures have no flash point.

A number of tests were carried out using several preparations of chlorinated hydrocarbons and a 1,1,2-trifluoro-1.2.2-trichloroethane preparation which finds extensive use in industry, for the removal of flux residues from circuit boards.

Test methods

Sample pieces with dimensions of 25.4 mm x 25.4 mm x 1.6 mm of base material for electronic circuit boards were cleaned by immersion in two cleaning baths of 75% by volume 2-propanol (isopropyl alcohol and 25% by volume water, agitated by means of a ultrasonic vibrator The clean test pieces were stored in a nitrogen-dry box until they were to be used.

Each of the clean test pieces was removed from the dry box and immediately introduced horizontally into a flux, "Alpha 711-35 MIL", and held there for 5 minutes. This flux is immediately used by circuit board manufacturers and is well known by professionals in the field. The test pieces were then suspended horizontally and dried for 5 minutes.

The sample was then held at 250°C for 15 seconds

in a horizontal position, which simulates the conditions of actual use. After the heat treatment, the test piece was again suspended in a nitrogen-dry box until use in the cleaning tests.

In performing the cleaning comparison tests, a flux coated sample was taken from the dry box and suspended by means of a clip and (1) introduced into a vapor zone of the flux removing solvent preparation for 30 seconds, (2) immersed in the boiling solvent for 30 seconds, (3) raised above the vapor zone and held in the space above this for 30 seconds, then (4) returned to the vapor zone and held therein for 30 seconds and (5) connected to a hanger for drying.

After drying, each sample was tested for purity by immersing the sample in 40 ml of a clean solvent consisting of a mixture of 2-propanol (isopropyl alcohol) and water, 75/25% by volume respectively, while the solvent was subjected to ultrasonic vibration for 5 minutes. After the sample was taken out, the specific resistance of the aqueous alcohol solution was measured using a clean 1 mm conductivity measuring bridge for each measurement. The average result for several measurements for each of the listed preparations was calculated. The higher the specific resistance, the more effective the removal of residues of the ionic flux.

Another experiment was carried out with flux-removing compositions with regard to their resistance to corrosion of aluminum. The experiment consisted of placing shavings of aluminum (Al 2 024) in a flask containing the liquid solvent mixture. A condenser was connected to the flask and the solvent heated to boiling and refluxed for 7 days, during which the chips were kept under observation. If the aluminum was not corroded at the end of the 7-day test period, the mixture was considered to have passed the test.

The flash point of each mixture was also determined. (The method used was according to ASTM-92, known as the "Cleveland Open Cup flash point method"). If the mixture had a flash point, it was considered to have failed. No observable flash point: indicates that the solvent was satisfactory or acceptable. The results from the flash point and corrosion tests are shown in table I, where failure and satisfactory results are denoted by F (failed) and P (passed) respectively.

Comparative example 1

The above test methods were carried out using a commercially available inhibited 1,1,1-trichloroethane (methyl chloroform) consisting of:

95.7% 1,1,1-trichloroethane

0.7% 1,2-butylene oxide

o.4% nitromethane

3.2% diethylene ether.

Comparative example 2

A commercially available flux-removing mixture was also tested as above. The mixture consisted of:

89.00% "Fluorocarbon 113" (described above)

10.67% methanol

0.33% nitromethane.

Comparative example 3

The above experiment was also carried out using the inhibited methyl chloroform in Comparative Example 1 (92.5%)

and 7.5% 2-butanol, which is also a commercially available product.

The percentages in examples 1-3 above as well as in

the following examples are on a volume basis unless otherwise stated.

Table I shows the results from the test for the mixtures in comparative examples 1-3 above and others known in the field. In comparative examples 7-13, 10% of different alcohols are used together with the inhibited mixture according to comparative example 1.

It should be noted that comparative examples 1-13 are of a comparative nature and do not follow within the scope of the invention.

The inhibited methyl chloroform in Comparative Example 1

is not effective in removing ionic components in the flux. Comparative examples 2 and 3 show the state of the art in the area of purification of ionic residues with commercially available mixtures which do not have a flash point. It will be seen that the fluorochlorocarbon mixture is more effective than the butanol-1,1,1-trichloroethane mixture. It will

it can also be seen from examples 4-13 that a single alcohol mixed with 1,1,1-trichloroethane will not give a preparation which will give similar results as the fluorinated mixture and still not show a flash point. Comparative examples 4 and 5 show that 1% methanol in 1,1,1-trichloroethane gives no flash point, while 2% methanol has a flash point.

A series of stabilized 1,1,1-trichloroethane-containing (Comparative Example 1) flux removing compositions containing various amounts of methanol together with other alcohols were tested on the same flux as above according to the method described above. The results are shown in Table II as Examples 14-35. These examples show that some 1,1,1-trichloroethane mixtures with methanol, 2-butanol and/or 2-methyl-3-butyn-2-ol which have no flash point, unexpected ability to remove residues of ionic fluxes than the fluorochlorocarbon mixture in comparative example 2. The preferred mixtures contain approx. 1% methanol and approx. 6% 2-butanol and/or 2-methyl-3-butyn-2-ol. The mixtures containing 0.5% methanol have a somewhat insufficient ability to remove residues of ionic fluxes, and the mixtures containing close to 2% methanol come too close to the undesirable flash point range.

The samples which show a flash point are not considered to fall within the scope of the invention. The mixture judged to be the most preferred is 1% methanol, 3% 2-butanol and 3% 2-methyl-3-butyn-2-ol.

It should be emphasized that values for specific resistance can only be compared with other values when the test conditions are practically identical. For example, data for table II are thus not comparable with those in table III, because a different/different flux was used. For the test conditions that were used for the data given in Table II, the mixtures which have a specific resistance of

>1.1 x 10<6>ohm-cm, while those having a value >15 are most preferred.

With respect to the solubility of the components in the rosin flux, when the volume of methanol is relatively small, the volume of the second alcohol component or mixture must be higher for the ionic components to be removed. When the methanol volume approaches 2%, the second component may be present in minimal amounts. 2% or more of methanol gives a product which has a flash point and thus falls outside the scope of the invention.

These mixtures should, like all 1,1,1-trichloroethane mixtures that can be used in contact with metals, especially aluminium, be stabilized to be usable in practice. Any of a number of compounds can be used as a stabilizer, including diethylene ether (1,4-dioxane), dioxolanes, nitroalkanes, 1,2-butylene oxide and the like. These are well known to those skilled in the art and have virtually no detrimental effect on the flux removal properties. As the known stabilized 1,1,1-trichloroethane mixtures do not completely remove ionic flux constituents, it is necessary to add other solvents to achieve a more complete removal of these ionic substances. The present invention provides such mixtures, which are shown in Table II and described in the specification of the invention.

The examples whose numbers are grouped by "C-" do not fall within the scope of the invention due to their flash point.

Some of the mixtures tested above, and others in which methanol and other alcohols are used, were tested on another and different flux ("Alpha" 711) which contained more ionic components than the one previously used. The results are shown in Table III.

Table III again shows the poor performance of stabilized 1,1,1-trichloroethane alone. The alcohol mixtures which do not contain methanol also show low efficiency compared to the methanol mixtures according to the present invention. It should be noted that since the flux "Alpha" 711 contains 50% solids, while "711-35 MIL" contains 35% solids, it is more difficult to clean using the same set of conditions; this is reflected in the lower specific resistance values found.

Claims (13)

1. Mixture containing 1,1,1-trichloroethane for removing rosin flux, characterized in that the (A) contains, based on the total volume of the mixture, (i) from 0.5 to less than about 2 volume% methanol, and (ii) from 3 to 10% by volume of at least one alcohol having from 2 to 5 carbon atoms, and (B) has no flash point measured by the "Cleveland Open Cup method". 2. Mixture according to claim 1, where component A(ii) is 2-butanol. 3. Mixture according to claim 1, wherein component A(ii) is 2-methyl-3-butyn-2-ol. 4. Mixture according to claim 1, where component A(ii) is a mixture of 2-butanol and 2-methyl-3-butyn-2-ol. 5. Mixture according to claim 4, where the volumes of said butanol and methylbutynol are equal. 6. Mixture according to claim 4, where the total volume of component A(ii) is from 6 to 10% by volume. 7. Mixture according to claim 1, where component A(ii) is a mixture of ethanol and 2-methyl-3-butyn-2-ol. 8. Mixture according to claim 1, where component A(ii) is a mixture of 2-butanol and 2-propanol. 9. Mixture according to claim 1, wherein component A(ii) is a mixture of 2-methyl-3-butyn-2-ol and 2-methyl-2-butanol. 10. Mixture according to claim 4, where the methanol is present in an amount of 0.5-1% by volume. 11. Mixture according to claim 1, where the mixture has a specific resistance of at least 1.1 x 10 <6> ohm*cm determined by the method according to examples 1-38.; 12. Mixture according to claim 1, where the mixture has a specific resistance of at least 1.5 x 10 <6> ohm*cm determined by the method according to examples 1-38. 13. Mixture according to claim 1, which also contains a component to stabilize the 1,1,1-trichloroethane so that corrosion of aluminum is avoided.