Browse Prior Art Database

Control of a Dual Vapor, Vapor Phase Solder System

IP.com Disclosure Number: IPCOM000039936D
Original Publication Date: 1987-Aug-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 82K

Publishing Venue

IBM

Related People

Belardinelli, RD: AUTHOR [+4]

Abstract

Vapor phase soldering systems typically utilize two vapors for operation. The primary vapor, usually produced by a dense, inert fluid which boils above the melting point of the solder to be reflowed, and the secondary vapor, typically trichlorotrifluoroethane, which is less dense and has a lower boiling point than the primary vapor, yet is more dense than air. Banks of cooling coils provide a means of containing these vapors in a vapor sump. Two sets of coils are used, one for each vapor. In addition to the primary cooling coils, the secondary vapor serves as a "blanket" to help contain the primary vapor in the system, thus preventing the primary vapor from escaping to the atmosphere.

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Control of a Dual Vapor, Vapor Phase Solder System

Vapor phase soldering systems typically utilize two vapors for operation. The primary vapor, usually produced by a dense, inert fluid which boils above the melting point of the solder to be reflowed, and the secondary vapor, typically trichlorotrifluoroethane, which is less dense and has a lower boiling point than the primary vapor, yet is more dense than air. Banks of cooling coils provide a means of containing these vapors in a vapor sump. Two sets of coils are used, one for each vapor. In addition to the primary cooling coils, the secondary vapor serves as a "blanket" to help contain the primary vapor in the system, thus preventing the primary vapor from escaping to the atmosphere. This leaves the secondary vapor exposed to the atmosphere, and therefore any secondary vapor which does not condense on the cooling coils is lost to the environment. In order for the systems' vapors to be stable, the secondary vapor in the machine must be precisely monitored and controlled. If this is not done, the equipment may lose control and release both secondary and primary vapor to the atmosphere. A method of automatically monitoring and controlling the addition of secondary fluid by using information from thermosensors which are precisely located in the system is described in this article.

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An electronic fluid which boils at 419oF and trichlorotrifluoroethane which boils at 118oF will be used as typical fluids in the following discussion. The principle point for control is at the top of the primary blanket or at the bottom of the intermix zone which is sensed with thermosensor T1 (Fig. 1). By keeping that point at a temperature just below the boiling point of the primary fluid, 415oF +/- 2, it is possible to know exactly where the top of the primary vapor zone is at all times. This is accomplished by adding trichlorotrifluoroethane to the system until the secondary blanket is heavy enough to suppress the primary vapor slightly. The thermosensor actually reads the top fringe of the primary vapor zone just where the intermix zone begins. A second thermosensor T2 is placed in the top of the secondary vapor zone. After the system has achieved the correct temperature for the primary vapor, it then uses thermosensor T2 to maintain the correct secondary temperature. The correct temperature must be achieved in the primary zone first, then in the secondary. This ensures that the correct amount of secondary vapor is always present in the system. If the secondary vapor is allowed to decrease, the primary vapor will climb above the primary coils and condense on the secondary coils. This primary/secondary mix is then collected in an acid stripper tank and recycled back through the system. The trichlorotrifluoroethane supply now becomes enriched with primary fluid in the feedback loop, causing a runaway condition which will eventually lead to the loss of the blanket. Therefor...