Thermal measuring and testing – Temperature measurement – Composite temperature-related paramenter
Reexamination Certificate
1998-10-30
2001-12-18
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
Composite temperature-related paramenter
C374S162000, C116S207000, C116S216000, C283S081000, C283S114000, C053S507000
Reexamination Certificate
active
06331076
ABSTRACT:
BACKGROUND OF THE INVENTION
Solder paste is typically maintained in a refrigerated state to preserve the integrity of its composition and to thereby prolong its useful shelf life. Refrigeration is needed because when solder paste is subjected to prolonged heat exposure, it typically suffers from chemical deterioration and physical suspension problems. Moreover, when solder paste deteriorates due to heat exposure, it often fails to meet expected performance standards as a bonding agent.
Consequently, the storage and handling of solder paste is an area which has come under increasing scrutiny in the drive to reduce variation in surface mount technology (SMT) manufacturing processes. Typically, a container of solder paste is stamped with a six-month date signaling the expiration of the solder paste's useful shelf life (i.e., the duration of time during which the freshness or integrity of the paste can be ensured, given proper handling of the paste). The duration of six months is based on a preestablished thermal forecast for the solder paste over the course of its life with a margin of safety provided.
The solder paste's actual shelf life, however, is dependent on the nature of temperature excursions that the paste actually experiences from the time it is manufactured to the time it is reflowed on a board. To preserve the quality of the paste, these temperature variations need to be monitored and controlled. Temperature variations are typically monitored by logging, over time, the temperature of the refrigerated environment where the solder paste is maintained.
SUMMARY OF THE INVENTION
By its very nature, the traditional process for monitoring the thermal history of solder paste is unreliable because it fails to consider that the paste may see excess temperatures during transport and during unexpected contingencies, such as a temporary loss of power to a refrigerator. It also lacks precision because it monitors the temperature of the environment where the solder paste containers are stored rather than the actual temperature of each solder paste container. The faithful execution of the traditional monitoring process is further complicated in that the production, transport and storage stages in the life of solder paste are filled with discontinuities, such as when a container is transferred from a shipping container to a refrigerator.
Moreover, the objective of the traditional monitoring process is modest. It seeks only to ensure that the quality of the solder paste can be preserved for the duration of the baseline six-month shelf life in accordance with the expiration date stamped on the container.
A solder paste product of this invention overcomes these limitations and offers greatly enhanced capabilities regarding the monitoring of the solder paste's useful shelf life. The solder paste product includes a container filled with solder paste and a time-temperature indicator positioned to measure the solder paste's cumulative exposure to heat.
In accordance with a method of this invention, the time-temperature indicator is attached to the container.
The time-temperature indicator is preferably a label including an indicator composition, e.g., a diacetylene monomer, that changes color as a function of heat exposure. At any given time, the color of the indicator composition is a function of its cumulative exposure to heat. The higher the temperature, the faster the indicator composition will change colors. If the temperature is dropped, the rate at which the indicator changes color will slow. The label may also include a colored reference marker, designed, in conjunction with the indicator composition, to provide a color match between the reference marker and the indicator composition as the solder paste approaches the exhaustion of its useful shelf life. In accordance with one aspect of the invention, the indicator composition is such that if the label is maintained at a fixed temperature, the logarithm of the time that will elapse before the color of the indicator composition is darker than that of the reference marker is an approximately linear function of the temperature.
The time-temperature indicator of this invention offers the advantages of providing the user with a reliable and easy-to-read indication of the condition of solder paste within a container. If a time-temperature label is attached to a jar of solder paste before it leaves the manufacturer, the label will show the temperature excursions to which the jar has been subjected through the shipping, storage and handling process. In contrast to conventional practice, the method of this invention establishes an expiration date based not on off-site speculation as to the thermal conditions that the solder paste will experience over the course of its life, but rather, on those temperatures to which the solder paste is actually subjected. As a result, a consumer will know that a batch of solder paste, which has been well refrigerated throughout its life, can be safely used beyond the traditional six-month expiration date. Conversely, the label on another batch, which was subject to a failure in the refrigeration chain, may indicate to the user that the useful shelf life of the paste has been reduced and, accordingly, that the paste should soon be used. This knowledge is likely both to enhance the proper utilization of “fresh” solder paste and to increase consumer confidence levels in product quality.
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Goodwin Jeanne-Marguerite
Gutierrez Diego
Hamilton Brook Smith & Reynolds P.C.
Manufacturers' Services Ltd.
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