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Title:
A METHOD FOR ELECTRICAL ASSEMBLY CLEANING
Document Type and Number:
WIPO Patent Application WO/1991/011269
Kind Code:
A1
Abstract:
A mixture of a primary liquid and a secondary liquid is heated in a cleaning apparatus. The mixture is heated to at least the boiling point of the secondary liquid but less than the boiling point of the primary liquid. The secondary liquid vaporizes (102), forming a vapor layer above the primary liquid (103). Condensing elements (101) near the top of the cleaning apparatus condense the vapor (102), returning it to the heated primary liquid (103) to be vaporized again. The electronic assembly (104) to be cleaned is lowered through the vapor and then immersed in the primary liquid (103) before being positioned in the vapor (102) where it may be sprayed by the primary liquid (105) to remove any remaining contaminants. The vapor (102) mixes with the atomized primary liquid formed from the spraying, and returns it to the primary liquid (103).

Inventors:
Pfahl Jr., Robert C.
Wrezel, James A.
Hagner, Lawrence R.
Application Number:
PCT/US1991/000594
Publication Date:
August 08, 1991
Filing Date:
January 18, 1991
Export Citation:
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Assignee:
MOTOROLA, INC.
International Classes:
C23G5/024; C23G5/032; C23G5/04; H05K3/26; (IPC1-7): B08B3/00
Foreign References:
US1907875A
US4640719A
US4828751A
US4867800A
US3904430A
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Claims:
Claims;
1. A method for cleaning contaminants from an electrical assembly, the steps comprising: a) establishing a body of liquid comprising a terpene having a first boiling point and a second liquid having a second boiling point that is less than the first boiling point; b) estabHshing a body of condensible vapor above the body of liquid, substantially isolating the body of liquid from other gases, by continuously heating the body of liquid to a predetermined temperature range that is less than the first boiling point but greater than the second boiling point; c) condensing the vapor of the body of condensible vapor to form the second liquid that is returned to the body of liquid; d) placing the electrical assembly to be cleaned into the terpene: e) moving the electrical assembly to be cleaned into the body of condensible vapor; and f) spraying the electrical assembly to be cleaned with a liquid comprised of the terpene.
2. The method of claim 1 wherein the second liquid is isopropyl alcohol.
3. The method of claim 1 wherein the second liquid is a hydrochlorofluorocarbon.
4. The method of claim 1 wherein the second liquid is pentafluoropropanol .
5. The method of claim 1 wherein the second liquid is isopropyl alcohol and the predetermined temperature range is at least 82°C and less than 170°C.
6. The method of claim 1 wherein the second liquid is methanol and the predetermined temperature range is at least 65°C and less than 170°C.
7. A method for cleaning contaminants from an electrical assembly, the steps comprising: a) estabHshing a body of liquid comprising a terpene having a first boiling point and a second liquid having a second bo ling point that is less than the first boiling point; b) estabHshing a body of condensible vapor above the body of liquid by continuously heating the body of Hquid to a predetermined temperature range that is less than the first boiling point but greater than the second boiling point; c) condensing the vapor of the body of condensible vapor to form the second Hquid that is returned to the body of Hquid; and d) spraying the electrical assembly to be cleaned with the terpene.
8. The method of claim 9 and further comprising the steps of placing the electrical assembly to be cleaned into the body of condensible vapor before spraying the electrical assembly to be cleaned with a liquid comprised of the first Hquid.
9. The method of claim 9 wherein the second liquid is a hydrochlorofluorocarbon.
10. The method of claim 9 wherein the second liquid is pentafluoropropanol.
11. The method of claim 9 whererin the second liquid is isopropyl alcohol.
12. The method of claim 9 wherein the second liquid is isopropyl alcohol and the predetermined temperature range is at least 82°C and less than 170°C.
13. The method of claim 9 wherein the second liquid is methanol and the predetermined temperature range is at least 65°C and less than 170°C.
14. A method for cleaning contaminants from an electrical assembly, the steps comprising: a) establishing a first body of liquid having a first boiling point; b) estabHshing a second body of liquid having a second boiling point that is lower than the first boiling point; c) estabHshing a body of condensible vapor above the first and second bodies of Hquid by heating the second body of Hquid at a predetermined temperature that is greater than the second boiHng point; and d) condensing the vapor of the body of condensible vapor to form the second body of liquid; e) placing the electrical assembly to be cleaned into the body of condensible vapor; and f) spraying the electrical assembly to be cleaned with a Hquid comprised of the second body of Hquid.
15. The method of claim 17 wherein the second body of liquid is a hydrochlorofluorocarbon.
16. The method of claim 17 wherein the second body of liquid s pentafluoropropanol.
17. The method of claim 17 wherein the second body of liquid s isopropyl alcohol.
Description:
A METHOD FOR ELECTRICAL ASSEMBLY CLEANING

Background of the Invention

Soldering components to a circuit board assembly typically leaves contaminants such as solder flux, resins, and ionic compounds. These contaminants must be removed if the circuit board assembly is to be sealed from the elements by a protective coating. The protective coating may not properly adhere to an assembly containing contaminants. The contaminants may also be removed for aesthetic purposes, to prevent corrosion, or to increase the assembly's insulation resistance. The cleaning process to remove the contaminants is usually done with chlorofluorocarbon (CFC) based solvents in liquid or vapor form. These solvents are frequently an azeotrope of trichloro-trifluoroethane and another solvent such as an alcohol, an example being methanol. One such solvent is a fluorinated hydrocarbon manufactured by E. I.

DuPont de Nemours & Co and marketed under the trade name FREON TMS.

The cleaning process is typically accomplished by placing the solvent in a container having heating elements at the bottom and condensing elements near the top. The solvent is then heated to its boiling temperature of 43.3°C, producing a solvent vapor above the liquid solvent. The circuit board or electronic assembly to be cleaned is placed in the vapor. The electronic assembly, being at a lower temperature than the vapor, causes the vapor to condense on the assembly, thereby

eaning off the contaminants by dissolving or flushing off the contaminants.

The vapor that did not condense on the electronic assembly is retained in the cleaning apparatus by the condensing elements. These elements, typically operated at a temperature of 10°C, cause the higher temperature vaporized solvent to condense back to liquid, returning to the bottom of the container to repeat the cycle. While the condensing elements in the container keep most of the solvent in the container, some of the solvent diffuses across the air-solvent interface and therefore escapes into the atmosphere. In addition, the assemblies being cleaned may retain small amounts of solvent on their surfaces after removal from the cleaning apparatus. Also, the spilling of solvent by workers refilling the cleaning apparatus can release the solvent into the atmosphere

The solvent must be retained in the apparatus due to its expense and because of environmental concerns. In recent years, scientific studies have shown that CFCs have been depleting the protective ozone layer above the Earth, causing holes to form in this layer. This allows harmful ultraviolet rays from the sun, normally filtered out by the ozone layer, to reach the Earth. The increased ultraviolet rays will increase incidence of skin cancer and add to the warming of the atmosphere.

Alcohols such as isopropyl, methanol, or ethanol have been tried as an alternative to CFC based solvents. The assembly to be cleaned is initially immersed in and then sprayed with isopropyl alcohol, dissolving and/or washing off the contaminants. Alcohol, while it adequately removes ionic material from the assembly, may leave a white residue. Customers buying the assembly might get the impression that the assembly has not been cleaned or is defective in some way. There is therefore a need for an environmentally safe cleaning solvent that removes ionic material and does not leave a residue on the assembly being cleaned.

Summary nf the Tnvpntinti

The present invention is a method for cleaning electronic assemblies without the use of CFCs. The process is accomplished by heating a mixture of a high boiling point primary liquid, such as a terpene, and a lower boiling point secondary liquid, used as a rinsing drying agent, such as pentafluoropropaήol. The primary/secondary liquid mixture is heated in an apparatus having condensing elements near the top. The mixture is heated to a temperature that is greater than the boiling point of the secondary liquid but less than the boiling point of the primary liquid. This will cause the secondary liquid to vaporize, forming a vapor layer above the primary liquid. The vapor is retained in the cleaning apparatus condensing on the condensing elements and returned, in liquid form, to the heated mixture to be vaporized again.

The assembly to be cleaned is immersed through the vapor and then placed in the primary liquid or sprayed with the primary liquid to dissolve the flux. The assembly is then placed in the vapor and rinsed with the primary liquid either by spraying or immersion in the distilled primary liquid. The vapor layer, by mixing with the atomized primary liquid from the assembly and condensing on the condensing elements, will contain the primary liquid within the apparatus, returning it to the heated primary liquid for reuse.

Brief Description of the Drawings

Figure 1 shows the cross section of an apparatus for practicing the present method on a single electronic assembly or a batch of electronic assemblies;

Figure 2 shows the cross section of an apparatus for practicing the present method on multiple electronic assemblies continuously moving through the apparatus; and

Figure 3 shows an alternate embodiment for practicing the present method on multiple electronic assemblies continuously moving through the apparatus.

Detailed Description of the Preferred Embodiment

The method of cleaning electronic assemblies disclosed herein operates without the use of CFCs that damage the ozone layer. This is accomplished by using a primary or solvent liquid with a high boiling point and a secondary or rinsing/drying liquid with a lower boiling point. In the preferred embodiment, a terpene hydrocarbon is used as the primary or first liquid and pentafluoropropanol as the secondary or second liquid. Terpenes represent a broad family of unsaturated, isoprene-type hydrocarbons and alcohols that exist naturally in most essential oils and oleoresins of plants. For example, one simple terpene, limonene (C 10 H 16 ), is a major component of a cleaning solvent mixture marketed by Petroleum Fermentations, N.V. under the trade name BIOACT EC-7.

This solvent system possesses the following significant properties:

Boiling point at atmospheric pressure - 170°C;

Flash point at atmospheric pressure - C.O.C. 71°C and T.C.C. 47°C; and

Vapor pressure - 1.6mm Hg at 20°C. In addition, terpene, when atomized, can be explosive if exposed to air at a temperature lower than T.C.C.

Pentafluoropropanol, CF 3 CF 2 CH 2 OH, is a non- chlorofluorocarbon, alcohol-type cleaning agent marketed by

Daikin Industries LTD. under the name PEFOL. Pentafluoropropanol, which has a zero ozone depletion potential, has the following significant properties:

Boiling point at atmospheric pressure - 80.7°C; Flash point at atmospheric pressure - none; and

Vapor pressure - 33 mm Hg at 20°C.

In the embodiment shown in Figure 1, the bottom of the apparatus is filled with the primary and secondary liquid mixture. The mixture is heated to approximately 95°C, greater than the the boiling point of the secondary liquid. The secondary liquid will then vaporize into a layer (102) above the primary liquid (103). The primary liquid (103), because it has a higher boiling point than 95°C, will remain in liquid form below the vapor layer (102).

The assembly (Ϊ04) to be cleaned is first placed in the primary liquid (103) by passing it through the vapor. Next, the assembly (104) is positioned in the vapor layer (102) where it can be sprayed with the primary liquid (105) to remove the remaining contaminants. The primary liquid (105), when it strikes the assembly (104), will atomize. This atomization may present the risk of explosion if the atomized mist was exposed to the atmosphere. One purpose of the vapor (102), therefore, is to isolate the primary liquid (103) from the atmosphere.

The vapor (102) mixes with the atomized primary liquid from the assembly being cleaned. This vapor mixture is then condensed by the condensing elements (101). The now liquid primary and secondary mixture is returned to the heated primary liquid (103) below where the secondary liquid is again vaporized to repeat the cycle. The condensing elements (101), therefore, retains the vapor (102) which, in turn, retains the primary liquid (103).

Another function of the secondary liquid is to rinse off the primary liquid that is contaminated with flux. The secondary liquid then evaporates more rapidly than the primary liquid because of its higher vapor pressure. The evaporation dries the assembly.

Figure 2 illustrates another embodiment of this invention. This embodiment is suited for assembly line cleaning of electronic assemblies. The electronic assembly is first moved into a terpene wash (201) where it is emersed in heated terpene. The assembly next travels through a high

pressure terpene spray (202) to remove any remaining impurities. The assembly finally travels through an isopropyl alcohol immersion (203) to remove the terpene residue. Figure 3 illustrates yet another embodiment of this invention. In this embodiment, the electronic assembly is lowered through the vapor layer (302) for either immersion in the primary liquid (305) or spraying with primary liquid (301). The primary liquid (305) should be kept at a temperature above 83°C to prevent the vapor (302) from condensing on the surface of the primary liquid. The assembly is next immersed in the secondary liquid (306) to remove the primary liquid (305) and any contaminants dissolved by the primary liquid (305). Finally, the assembly is withdrawn through the vapor (302), sprayed with clean secondary liquid distillate (303), and allowed to drain before removal from the vapor (302). As in the first embodiment, the level of the vapor layer (302) is controlled by condensing elements (304). The distillate is produced from the distillation of the secondary liquid (306) to remove the primary liquid (305) and contaminants. While the preferred embodiment uses terpene hydrocarbons as the primary or solvent liquid and pentafluoropropanol as the secondary liquid or rinsing/drying agent, any liquids with properties similar to these can be used. Examples of a pentafluoropropanol replacement are alcohol (methanol, ethanol, n-propanol, isopropanol, etc.), an alcohol water mixture, water, or hydrochlorofluorocarbon. Examples of a terpene hydrocarbon replacement are terpene alcohol, mineral spirits, naphtha, N-methyl pyrrolidinone (NMP), or mixtures thereof. However, since these elements have varying boiling points, the primary and secondary liquid mixture will be kept at different temperatures. For example, a mixture of a terpene as the primary or first liquid and methanol as the secondary or second liquid requires a temperature of at least 65°C, the boiling point of the methanol, to 170°C, the boiling point of a terpene. A mixture of a terpene

and isolpropyl alcohol requires a temperature of at least 82°C, the boiling point of isopropyl alcohol, to 170°C.

The present invention provides cleaning of electrical assemblies without the use of ozone depleting chemicals. This method removes ionic materials without leaving a residue on the assembly while maintaining safe conditions for people near the cleaning apparatus.




 
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