NO146165B - APPLICATION OF A SOLVENT MIXTURE FOR CLEANING OF SOLARED ELECTRONIC COMPONENTS - Google Patents

Description

The present invention relates to the use of a solvent mixture when cleaning electronic components, e.g. boards with printed circuits. A printed circuit board consists of a board or cardboard on which an electrical circuit is applied using printing and etching techniques. Various electronic components are then mounted on the plate and connected to the circuit by soldering. Such electronic components can be, for example, transistors and capacitors. Soldering the circuit boards

often res in a soldering iron. The plates are first brought to pass a device for applying solder paste to the plates. The task of the solder paste is to remove oxides from the surface to be soldered. The solder paste often consists of wood resin (rosin).

It is preferably applied in liquid form by, for example,

be sprayed on the plate.

When the solder paste is applied, the printed circuit board is brought

in contact with a bath of molten solder. After the soldering operation, the board must usually be cleaned to remove the solder paste and other contaminants. This is necessary for several reasons. Electronic components produce heat during use which can increase their temperature to more than 60°C. The solder paste can become electrically conductive at this elevated temperature, which can result in unwanted creepage currents. If the solder paste contains halogens, creep currents can occur at room temperature.

Another reason why cleaning is necessary is that it is usually desirable to apply a plastic film to the printed circuit board. Solder paste, even in small amounts, makes it difficult or impossible to apply said plastic to the printed circuit board because the solder paste prevents adhesion of the plastic to the surface of the board.

Another reason is that insufficiently cleaned electronics

ke components become sticky and therefore difficult to handle at the same time as their appearance deteriorates.

It is known that printed circuit boards can be cleaned with different solvent mixtures, such as a mixture of ethanol and methyl chloroform or a mixture of isopropanol and trichlorotrifluoroethane. The use of such mixtures has caused certain disadvantages, such as poor cleaning effect, harmful effect on the electronic components or fire hazard due to the fact that the mixtures are flammable.

According to the present invention, for cleaning soldered electronic components, a solvent mixture is used which consists of at least one alcohol containing 4-6 carbon atoms in the molecule, in an amount of 3-50% by weight, and a chlorinated hydrocarbon selected from perchlorethylene, trichlorethylene and methyl chloroform, in an amount of 50-97% by weight. Solder paste and other contaminants are thus removed with the aforementioned solvent mixture.

The alcohol included in the solvent mixture can be aliphatic or cyclic in nature. Pentanol has been found to be a particularly useful alcohol for this purpose, especially when used in combination with perchlorethylene. It is preferred to use a mixture containing 10-20% by weight methanol and 80-90% by weight perchlorethylene. This mixture is not flammable, it has a low vapor pressure which results in little loss of solvent on evaporation, and it can be easily recovered by azeotropic distillation. Among the pentanol isomers, it is preferred to use 1-pentanol.

It is preferred to use mixtures containing perchlorethylene because such mixtures seem to have the least harmful effect on the electronic components. Such components are usually coated with a plastic film and are usually marked with paint of different colours. Mixtures containing perchlorethylene combine a low attack on such plastic films and marks with a high cleaning effect against solder pastes and other contaminants. In mixtures based on perchlorethylene, it is preferred to use alcohols that have a boiling point of 115-150°C. A preferred content of such alcohols is 5-32% by weight. Alcohols which are preferably used in combination with perchlorethylene are 3-pentanol (boiling point 116°C), 1-butanol (b.p. 118°C), 2-pentanol (b.p. 119°C), 3-methyl-l-pentine-3- ol (b.p. 121°C), 3-methyl-3-pentanol (b.p. 122°C), 2-methyl-l-butanol (b.p. 128°C), 3-methyl-l-butanol (b.p. 130°C), 4-methyl-2-pentanol (b.p. 133°C), l-hexen-3-ol (b.p. 134°C), mixed isomers of primary amyl alcohol (b.p. 127-139°C), 1-pentanol (b.p. 138°C ), 3-ethyl-l-pentin-3-ol (b.p. 138°C), cyclopentanol (b.p. 141°C), 2-ethyl-1-butanol (b.p. 146°C) and 2-methyl-1-pentanol ( b.p. 148°C). Other useful alcohols for use in combination with perchlorethylene are 2-methyl-3-buten-2-ol (b.p. 99°C), 2-butanol (b.p. 99°C), 2-methyl-2-butanol (b.p. 102°C ), 2-methyl-3-butyn-2-ol (b.p. 104°C), 2-methyl-l-propanol (b.p. 108°C), 2,2-dimethyl-l-propanol (b.p. 114°C), 3-hexen-l-ol (b.p. 156°C), 1-hexanol (b.p. 158°C) and cyclohexanol (b.p. 161°C).

In mixtures based on trichloroethylene, it is preferred to use alcohols that have a boiling point of 90-115°C. A preferred content of such alcohols is 5-20% by weight. Pre-

dry alcohols for use in combination with trichloroethylene are 2-methyl-3-buten-2-ol (b.p. 99°C), 2-butanol (b.p. 99°C), 2-methyl-2-butanol (b.p. 102°C) , 2-methyl-3-butyn-2-ol (b.p. 104°C), 2-methyl-l-propanol (b.p. 108°C), 2,2-dimethyl-l-propanol (b.p. 114°C) and 3 -pentanol (b.p. 116°C). Other useful alcohols for use in combination with trichlorethylene are 1-butanol (b.p. 118°C), 2-pentanol (b.p. 119°C), 3-methyl-l-pentin-3-ol (b.p. 121°C) and 3- methyl-3-pentanol (b.p. 122°C).

In mixtures based on methyl chloroform, it is preferred to use alcohols with a boiling point of 82-107°C. A preferred content of such alcohols is 5-15% by weight. Preferred alcohols for use in combination with methyl chloroform are 2-methyl-2-propanol (b.p. 83°C), 2-methyl-3-buten-2-ol (b.p. 99°C), 2-butanol (b.p. 99°C) , 2-methyl-2-butanol (b.p. 102°C) and 2-methyl-3-butyn-2-ol (b.p. 104°C). Other useful alcohols for use in combination with methyl chloroform are 2-methyl-l-propanol (b.p. 108°C), 2,2-dimethyl-l-propanol (b.p. 114°C), 3-pentanol (b.p. 116°C) and 1-butanol (b.p. 118°C).

The examples below indicate a few particularly useful mixtures. All percentages are % by weight.

The solution lb landing used according to the invention can in certain cases produce a perfect cleaning effect on a printed circuit board without any mechanical treatment of the board. If, for example, the printed circuit board has been soldered using the solder pastes "Zeva", type ^ 2p' ^ra the German company Zevatron GmbH, the solder paste can be removed by simply dipping the board at normal temperature in a mixture consisting of 13.5 wt. % 1-pentanol and 86.5% by weight perchlorethylene.

In other cases, it may be desirable to improve the cleaning effect by treating the plate at a higher temperature, or by simultaneously subjecting the plate to a mechanical treatment such as brushing. A brushing operation also removes insoluble contaminants. It is therefore preferred to process the printed circuit boards in an apparatus in which the boards are exposed to the simultaneous action of solvent and brushes. The temperature of the solvent is preferably 20-50°C. The solvent is preferably passed through the apparatus in countercurrent to the plates. The used solvent that leaves the apparatus is preferably recovered by distillation, and the thus purified solvent is re-fed into the apparatus. The purification of the solvent by distillation is facilitated if it has a composition as close as possible to the azeotropic mixture. A mixture of 1-pentanol and perchlorethylene has an azeotropic composition at about 15% by weight 1-pentanol and 85% by weight perchlorethylene, which boils at 119°C. However, it should be noted that there is a loss of solvent in the cleaning apparatus due to evaporation. The component with the lowest boiling point evaporates most easily. In a mixture of 15% by weight 1-peritannol and 85% by weight perchlorethylene, the evaporation ratio between the two components at a temperature of 20-50°C will be different from the azeotropic ratio, which will result in the solvent mixture in the apparatus being diluted with regard to perchlorethylene. Therefore, new solvent added to the apparatus to replace the loss by evaporation should have an elevated percentage of perchlorethylene. If, for example, a mixture containing 85% perchlorethylene is desired in the apparatus, the new solvent should for example contain 90 or 95% perchlorethylene, depending on the design of the apparatus.

Taking into account the loss by evaporation and the desired azeotropic distillation, it is preferred to give the solvent a percentage content of alcohol which depends on the boiling point of the alcohol as illustrated in the accompanying drawing, where figure 1 relates to a mixture of an alcohol and perchlorethylene, fi -

Figure 2 relates to a mixture of an alcohol and methyl chloroform and Figure 3 relates to a mixture of an alcohol and trichloroethylene. IN

in each of the figures, the abscissa indicates the boiling point of the alcohol in degrees Celsius, and the ordinate indicates the % alcohol by weight in the mixture. The preferred mixture is within the range indicated

of the charts. For example, it can be deduced from figure 1 that the percentage content of 1-pentanol (b.p. 138°C) in a mixture with perchlorethylene is 5-20% by weight. The preferred percentage content of cyclopentanol (b.p. 141°C) is 5-18% by weight. The preferred percentage content of 4-methyl-2-pentanol (b.p. 133°C)

is 7-24% by weight. The preferred percentage content of 2-ethyl-1-butanol (b.p. 146°C) is 5-13% by weight. It is a particular advantage of the invention that most solvent mixtures used have a high flash point, which reduces the risk of fire. The alcohols that have a high boiling point produce azeotropic mixtures that have no flash point. For example, the azeotropic mixture of 1-pentanol and perchlorethylene has no flash point at any temperature up to the mixture's boiling point, which is 119°C.

Claims (2)

1. Use of a solvent mixture consisting of 3-50% by weight of at least one alcohol containing 4-6 carbon atoms and 50-97% by weight of at least one chlorinated hydrocarbon selected from perchlorethylene, trichloroethylene and methylchloroform, for cleaning soldered electronic components for solder paste and other pollutants. 2. Use as stated in claim 1, where the solvent mixture consists of perchlorethylene and an alcohol which has a boiling point of 115-150°C, whereby the percentage content of the alcohol falls within the range indicated in the diagram in fig. 1.