Method and apparatus to clean and cleanliness test printed circuit boards

• US 5,246,023 A
• Filed: 06/14/1991
• Issued: 09/21/1993
• Est. Priority Date: 04/24/1990
• Status: Expired due to Fees

• Current Assignee: Austin American Technology Corporation
• Original Assignee: Electronic Controls Design, Inc.
• Inventors: Austen, Paul M., Breunsbach, Rex
• Primary Examiner(s): Stinson, Frankie L.
• Application Number:
  US07/715,441
• Time in Patent Office: 830 Days
• Field of Search: 134/902, 134/57 R, 134/109, 134/110, 134/111
• US Class Current: 134/57.00R
• CPC Class Codes: H05K 2203/075 Global treatment of printed circuits by fluid spraying, e.g. cleaning a conductive pattern using nozzles
• CPC Class Codes: H05K 2203/0783 Using solvent, e.g. for cleaning; Regulating solvent content of pastes or coatings for adjusting the viscosity
• CPC Class Codes: H05K 2203/086 Using an inert gas
• CPC Class Codes: H05K 2203/1509 Horizontally held PCB
• CPC Class Codes: H05K 2203/163 Monitoring a manufacturing process
• CPC Class Codes: H05K 3/26 Cleaning or polishing of the conductive pattern
• CPC Class Codes: Y10S 134/902 Semiconductor wafer

Abstract

The present invention is an apparatus to clean and cleanliness test printed circuit wiring boards and includes an inner chamber and an outer enclosure encasing the inner chamber. A rack within the chamber is adapted to hold a plurality of PCBs having electronic components assembled thereon. An operator programmable microprocessor controller causes a saponifier solution to be introduced into the interior chamber. The saponifier solution is pumped through a diffusion assembly in the chamber to spray the solution onto the PCBs. Upon completion of this cleaning cycle, the saponifier solution is drained by gravity out of the chamber. This is followed by a water rinsing cycle utilizing the same hardware and procedure. Once the rosin saponification cleaning cycle and water rinsing cycle are complete, the system controller empties the wash chamber and pumps in a standard resistivity test mixture of 75% isopropyl alcohol (IPA) and 25% water. A MIL-P-28809 cleanliness test is then performed on the PCBs. The PCBs are rinsed in the test IPA mixture by the diffusion assembly at a set temperature for a set time. If the test IPA mixture has a resistivity of ≦2 MΩ/cm³, the PCBs are determined to be contaminated and are rejected for re-cleaning.

Claims

1. An apparatus for cleaning flux and other contaminants from printed circuit boards and other electronic assemblies comprising:

a cleaning chamber;

means for holding electronic assemblies in the cleaning chamber;

means providing at least one wash cycle including a cleaning liquid spray means wherein a cleaning liquid is sprayed onto the electronic assemblies within the chamber and drain means operable to drain the cleaning liquid from the chamber when the wash cycle is completed;

means providing at least one alcohol-water rinse cycle operable after completion of the wash cycle including alcohol-water spray means for spraying an alcohol and water mixture onto the electronic assemblies within the chamber to rinse the electronic assemblies, said alcohol-water spray means including said cleaning liquid spray means;

and testing means for testing the resistivity level of said mixture as an indication of the cleanliness of the assemblies, including means for recirculating and filtering the mixture as it is sprayed over the assemblies and means for controlling the temperature of the mixture while it is sprayed,

wherein said testing means tests the level of ionic contamination in the alcohol-water mixture and includes (a) temperature monitoring means for monitoring the temperature of the mixture in the chamber, (b) resistivity measuring means for monitoring the resistivity of the mixture in the chamber, (c) volume measuring means for monitoring the quantity of mixture in the chamber, (d) specific gravity measuring means for monitoring the specific gravity of the mixture, and (e) control means operable to (1) operate said mixture spray means continuously during a test cycle to spray mixture in said chamber onto said assemblies;

(2) operate the temperature monitoring means and resistivity monitoring means during operation of said mixture spray means; and

(3) terminate the test cycle when either the resistivity of the mixture drops below a predetermined level, the temperature of the mixture is at a predetermined minimum level, or a predetermined period of time elapses from the start of the test cycle, whichever first occurs.

2. The apparatus of claim 1 including means providing an air purge of the cleaning chamber operable after the wash cycle including inert gas means for introducing an inert gas under pressure into the chamber to purge air from the chamber.

3. The apparatus of claim 2 including mixture drain means for draining mixture from the chamber, mixture fill means for adding mixture to the chamber and mixture circulating means for circulating mixture between the chamber and a mixture holding tank.

4. The apparatus of claim 1 including means for terminating the rinse cycle when the resistivity of the alcohol and water mixture exceeds a predetermined level or when a predetermined period of time elapses, whichever occurs first.

5. The apparatus of claim 1 including a means for heating and monitoring the temperature of the alcohol and water mixture in the chamber and control means operable to terminate the rinse cycle when both the temperature and the resistivity of the mixture achieve predetermined levels.

6. The apparatus of claim 5 including means for maintaining the alcohol-water mixture at a predetermined ratio.

7. The apparatus of claim 6 wherein said mixture is an isopropyl alcohol and water mixture.

8. The apparatus of claim 7 wherein said mixture is 75% isopropyl alcohol and 25% water.

9. The apparatus of claim 1 wherein the total volume of said mixture in the apparatus is less than the volume of said chamber.

10. The apparatus of claim 1 wherein said filter means includes means for cleansing the mixture of ionic contaminants within each rinse cycle and between successive rinse cycles.

11. The apparatus of claim 10 including a mixture holding tank wherein the cleansing means includes circulating means for circulating the mixture between the chamber and the mixture holding tank and said filter means includes means for filtering contaminants from the mixture positioned in the path of travel of the mixture between the chamber and tank.

12. The apparatus of claim 11 including control means operable to operate said circulating means after termination of one rinse cycle and before commencing the next successive wash cycle.

13. The apparatus of claim 1 wherein said control means includes means for adjusting the specific gravity of the mixture when the specific gravity measuring means senses a change in specific gravity of the mixture from a predetermined value.

14. The apparatus of claim 1 wherein said control means includes means for adjusting the volume of mixture in the chamber when said volume measuring means detects a volume of mixture in said chamber above or below a predetermined volume.

15. An apparatus for cleaning flux or other contaminants from printed circuit boards or other electronic assemblies comprising:

a chamber;

a door to allow access to the chamber which when closed forms a seal for the chamber;

a rack for holding printed circuit boards or other electronic assemblies stationary within the chamber, the rack being slidable out of the chamber when the door is open;

a plurality of rotatable arms located within the chamber with a plurality of spray nozzles positioned on the rotatable arms;

a means for introducing deionized water into the chamber;

a primary pump to continuously recirculate the water and other liquids in the chamber through the spray nozzles onto the printed circuit boards nested on the rack;

a drain to evacuate water and other liquids from the chamber;

a means to introduce heated water into the chamber;

a holding tank to contain a saponifying agent with a second dedicated pump to introduce the saponifying agent into the chamber for mixture with heated water thereby forming a saponifying solution;

a detection means to determine when the saponifying solution in the chamber has reached a predetermined level, the saponifying solution being recirculated by the primary pump for a predetermined time period and thereafter evacuated out of the chamber through the drain;

a means to monitor air flow between the chamber and the surrounding atmosphere to prevent the build up of a combustible gas mixture;

a holding tank to contain an alcohol and water mixture;

a sensor means in the holding tank to determine the temperature and percentage by volume of the alcohol mixture before introduction into the chamber;

a heating means to increase the temperature of the alcohol mixture to a predetermined level before its introduction into the chamber;

a third dedicated pump to introduce the alcohol mixture into the chamber;

an alcohol replenishing tank to contain absolute alcohol;

a fourth dedicated pump and corresponding feed connection to introduce the absolute alcohol from the alcohol replenishing tank into the alcohol holding tank when it is determined that the alcohol mixture in the holding tank drops below a predetermined mixture percentage by volume; and

a detection means to determine when the alcohol mixture in the chamber has reached a predetermined level,

said primary pump being operable after the saponifying solution is drained from the chamber and the mixture is pumped by the third pump into the chamber, to recirculate the mixture through the spray nozzles for a predetermined time period while being exposed to a conductivity testing means to determine the resistivity of the alcohol mixture thereby determining the relative cleanliness of the electronic assemblies.