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Solder Thermal Switch

IP.com Disclosure Number: IPCOM000015768D
Original Publication Date: 2002-Apr-01
Included in the Prior Art Database: 2003-Jun-21
Document File: 5 page(s) / 57K

Publishing Venue

IBM

Abstract

Problem: Although circuit boards have long been protected by various fail-safe devices (ex: fuses, relays, circuit-breakers, etc), giving individual chips that same level of protection is typically not done. This can have catastrophic results not only for the chips that fail, but also for additional components, either active or passive, that are on the circuit board or substrate. This is especially true as flip-chip, DCA (Direct Chip Attach) or CSP (Chip Scale Packaging) become the package of choice for many applications.

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Solder Thermal Switch

    Problem: Although circuit boards have long been protected by various fail-safe devices (ex: fuses, relays, circuit-breakers, etc), giving individual chips that same level of protection is typically not done. This can have catastrophic results not only for the chips that fail, but also for additional components, either active or passive, that are on the circuit board or substrate. This is especially true as flip-chip, DCA (Direct Chip Attach) or CSP (Chip Scale Packaging) become the package of choice for many applications.

These newer packages allow for an ever decreasing circuit footprint while increasing functionality. However, as the functionality of the individual chips increases, so does their value. It is thus highly desirable to save valuable chips on a circuit board or substrate in the event that one of them should fail. It is also desirable to prevent damage to neighboring components or the board or substrate in the event of a chip that fails or over-heats. Incorporating a fail-safe device on individual chips is thus an ideal solution.

Solution: A Solder Thermal Switch is a customizable fail-safe device that may be incorporated into individual chips as part of the interconnect assembly. Even if other circuit elements on the chip designed to prevent catastrophic failures should fail, solder thermal switches can still protect the board and neighboring circuits. This is so because these do not depend on silicon circuit elements but rather the temperature of the chip to become active.

As is well known, typical chips mounted on a board have a Max. operating temperature beyond which damage can occur not only to the over-heating chip, but to the board and surrounding chips and components as well. Some chips may contain circuit elements to detect over-currents, etc. However, any on-chip circuit elements may fail catastrophically, including those designed to indicate abnormal conditions. In such an event, there is nothing to stop a thermal runaway from damaging other components on the circuit board or substrate as well as the substrate itself.

A solder thermal switch may be incorporated into the interconnection packaging of the chip. As is shown in Fig. 5, a solder thermal switch is mounted in thermal contact with the chip and in electrical contact with the chip and board. As is shown in Figs 1 - 4, the key of a solder thermal switch is a solder of a certain melting temperature that is deposited in one form factor, but upon reaching the melting temperature, changes into another form factor, thereby either making or breaking an electrical connection. Several versions of such solder thermal switches may be readily fabricated to accomplish various off-chip circuit functions.

For example, as is seen in Fig. 1, a over-temp condition on a chip will cause the solder as deposited by IMS (Injection Molded Solder) to go from a flat circular form factor seen in "A" to a spherical form factor seen in "B". In effect this c...