Apparatus and method to substantially minimize low-cycle fatigue of electrical connections

[0002] The present invention relates generally to electronic devices, and more particularly to an apparatus and method to substantially minimize breakage or low-cycle fatigue of electrical connections between components.

[0003] Electronic devices and integrated circuits are being required to perform more functions at ever increasing speeds. Component densities are also increasing while packaging size requirements are decreasing. The higher component densities, higher operating frequencies and tighter packaging requirements are resulting in the generation of more heat that must be managed for proper operation and longevity of current and future high performance electronic devices and circuits.

[0004] Components in an electronic device can be electrically connected to one another by solder connections. For example, a microelectronic die, chip or other component may be electrically connected to a printed wiring board (PWB) or the like by a plurality of solder ball connections or a ball-grid array (BGA). Because of possible size variations and differences in composition between the electrically connected components, they can expand and contract at different rates of thermal expansion during thermal cycling as the components build up heat under operating conditions and cool down during low duty cycles or non-operating conditions. Additionally, variations in ambient temperature under both operating and non-operating conditions can also cause the components to expand and contract at different rates. The repeated thermal expansion and contraction of the components can result in low-cycle fatigue and breakage of the solder connections or BGA connecting the components. Low-cycle fatigue can be an especially significant problem with ceramic components that have low expansion relative to typical PWB materials.

[0005] Thermal expansion mismatches can be managed to some extent by mechanical arrangements, such as aramid-reinforced printed wiring boards, heat sinks and by attachment of leads or columns to the components to increase mechanical compliance during thermal cycling. Solder fatigue can still occur using these mechanical arrangements because a contributing factor is the need to thermally control the actual solder connections or BGAs by transferring heat away from the connections. Additionally, these mechanical arrangements are passive and cannot react or adapt to changing conditions during the operational cycles of an electronic device.

[0006] Accordingly, for the reason stated above, and for other reasons that will become apparent upon reading and understanding the present specification, there is a need for an apparatus and method to substantially minimize the low-cycle fatigue or breakage of electrical connections between components caused by the thermal expansion mismatch between the components. There is also a need for an apparatus or method to substantially control low-cycle fatigue that is adaptive to changing conditions during the different operational cycles of the components and that can thermally control the actual electrical connections by actively transferring heat away from the connections.

[0007] In accordance with the present invention, an apparatus to prevent low-cycle fatigue or breakage of at least one electrical connection between a first component and a second component includes at least one thermoelectric device to thermally contact at least the first component and includes at least one sensor to couple to at least one of the first component and the second component. A controller is coupled to the at least one thermoelectric device and to the at least one sensor. The controller controls operation of the thermoelectric device to prevent low-cycle fatigue of the at least one electrical connection by transferring heat from at least the first component in response to signals from the at least one sensor.

[0008] In accordance with an embodiment of the present invention, an electronic device with thermal expansion control includes a first component and a second component electrically coupled to the first component. A thermoelectric device thermally contacts at least the first component and at least one sensor is coupled to one of the first component or the second component. A controller is coupled to the at least one sensor and to the thermoelectric device to transfer heat from at least the first component in response to signals from the at least one sensor.

[0009] In accordance with another embodiment of the present invention, a method to prevent low-cycle fatigue of at least one electrical connection between a first component and a second component includes sensing at least one of temperature, strain or deflection of at least one of the first component or the second component and transferring heat from at least one of the first component and the second component in response to the sensing of at least one of temperature, strain or deflection.

[0010] In the drawings, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes or primed (X′) represent different occurrences of substantially similar components.

[0011] FIG. 1 is a block schematic diagram of an electronic device and an apparatus to prevent low-cycle fatigue of electrical connections between two components of the device in accordance with an embodiment of the present invention.

[0012] FIGS. 2A and 2B are block schematic diagrams of an electrical device and an apparatus to prevent low-cycle fatigue of electrical connections between two components of the device in accordance with another embodiment of the present invention.

[0013] FIG. 3 is a block schematic diagram of a controller for use in an apparatus to prevent low-cycle fatigue of electrical connections between components in accordance with an embodiment of the present invention.

[0014] FIG. 4 is a block schematic diagram of a controller for use in an apparatus to prevent low-cycle fatigue of electrical connections between components in accordance with another embodiment of the present invention.

[0015] In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments can be utilized and that process or mechanical changes may be made without departing from the scope of the present invention.

[0016] FIG. 1 is a block schematic diagram of an electronic device 100 and an apparatus 102 to prevent low-cycle fatigue of electrical connections 104 between a first component 106 and a second component 108 of the electronic device 100 in accordance with an embodiment of the present invention. The apparatus 102 includes a thermoelectric device 110 to thermally contact at least one of the first component 106 and the second component 108. In the embodiment shown in FIG. 1, the thermoelectric device 110 is disposed between the first component 106 and the second component 108 and can thermally contact both components 106 and 108 to transfer heat between the components 106 and 108. Alternatively, the thermoelectric device 110 can thermally contact only one of the components 106 or 108. The thermoelectric device 110 can be a Tellurex Zmax® thermoelectric module or the like as manufactured by the Tellurex Corporation of Traverse City, Mich.

[0017] At least one sensor 112acan be coupled to at least one of the first component 106 and the second component 108. In the embodiment shown in FIG. 1, a sensor 112bcan be attached between the components 106 and 108. The sensor 112acan be a temperature sensor. Each of the sensors 112bcan be a strain/deflection sensor, an optical sensor, a hall effect device or sensor, a capacitive sensor, a differential variable reluctance transducer or the like. The sensors 112 can be all one type or multiple different types of sensors can be used together at different locations on the components 106 and 108, depending upon the application, design constraints and environmental conditions. Each of the sensors 112 are electrically connected to a controller 114 and the controller 114 is electrically connected to the thermoelectric device 110. The controller 114 controls the operation of the thermoelectric device 110 to transfer thermal energy or heat between the components 106 and 108 in response to signals from the one or more sensors 112.

[0018] FIG. 2A is a block schematic diagram of an electronic device or system 200 incorporating an apparatus 202 (shown in more detail in FIG. 2B) to prevent or substantially minimize low-cycle fatigue or breakage of electrical connections 204 between a first component 206 and a second component 208 in accordance with another embodiment of the present invention. The first component 206 can be a microelectronic die disposed in a package, a semiconductor chip or other electronic device and the second component 208 can be a printed wiring board (PWB) or the like. The electrical connections 204 can be a ball grid array (BGA) of solder balls or similar electrical connections. The apparatus 202 includes a thermoelectric device 210 that is thermally coupled to the first component 206. While the thermoelectric device 210 is shown as being coupled to the first component 206, it should be noted that depending upon design constraints, the thermoelectric device 210 could also be thermally coupled only to the second component 208 or the thermoelectric device 210 could be thermally coupled to both the first component 206 and the second component 208 similar to the embodiment shown in FIG. 1. In a further embodiment (not shown), a plurality of different thermoelectric devices 210 may be present and the different thermoelectric devices 210 could be coupled to each of the first and second components 206 and 208.

[0019] At least one sensor 212 is coupled to at least one of the first component 206 and the second component 208. In the embodiment shown in FIG. 2A, at least one temperature sensor 212acan be attached to each of the first and second components 206 and 208 proximal to a center of the BGA connections 204 and at least two strain/deflection type sensors 212bcan each be disposed proximate to side edges 214 of the first component 206. Other types of sensors 212, such as those discussed with reference to FIG. 1, could be used as well depending upon design and environmental requirements.

[0020] A heat sink 216 can be thermally coupled to at least the thermoelectric device 210. In the embodiment shown in FIG. 2A, the heat sink 216 has a first portion 216athat thermally contacts the thermoelectric device 210 and a second portion 216bthat is bonded to the second component 208 by a bonding material layer 218. The bonding material layer 218 can thermally insulate the second component 208 from the heat sink 216 and form an air gap 220 between the heat sink 216 and the second component 208. Alternatively, rather than integrally forming the first and second portions 216aand 216bof the heat sink 216, separate heat sinks could thermally contact both the first component 206 and the second component 208.

[0021] In FIG. 2B, a controller 222 (not shown in FIG. 2A) is electrically coupled to the temperature sensors 212aand to the strain/deflection sensors 212band to the thermoelectric device 210. The controller 222 controls the operation of the thermoelectric device 210 in response to signals from the sensors 212aand 212bto transfer heat from the first component 206 to the heat sink 216.

[0022] FIG. 3 is a block schematic diagram of a controller 300 in accordance with an embodiment of the present invention that can be used for the controller 114 of FIG. 1 or the controller 222 of FIG. 2B. Inputs to the controller 300 can include a first component temperature signal T_1Cand a differential strain control signal Δε. The first component temperature signal T_1Ccorresponds to a temperature sensed by the temperature sensor 212athat is coupled to the first component 206. The differential strain control signal Δε can be generated by the at least one strain/deflection sensor 212bcoupled between the first component 206 and the second component 208. The at least one strain/deflection sensor 212bsenses the difference in strain or deflection between the first component 206 and the second component 208. The differential strain signal Δε is then a measure of the difference in strain or deflection between the first component 206 and the second component 208. The strain/deflection sensor 212bcan generate the differential strain signal as a function of a first strain signal corresponding to the strain on the first component 206 and a second strain signal corresponding to the strain on the second component 208. The differential strain control signal Δε is applied to a first proportional-integral-differential (PID) control circuit or algorithm 310 to generate a strain-control drive current signal i_∈ as a function of the differential strain control signal Δε. Adequate control of the thermoelectric device 210 to control the temperature and strain/deflection of the first component 206 can be obtained in most circumstances by using only the proportional control segment or portion of the first PID 310 to generate the strain-control drive current i_∈ as a function of the differential strain control signal Δε; although, the integral and differential control portions of the first PID 310 can also be used to improve stability and control accuracy. The strain-control drive current signal i_∈ can be applied to a first limiter circuit or algorithm 312 to limit the strain-control drive current signal i_∈ to a predetermined range. Accordingly, if the absolute value of |i_∈|≦i_max, then the first limiter 312 sets i_∈=i_∈. If the absolute value of the |i_∈|>i_max, then the first limiter 312 sets i_∈=i_max(i_∈/|i_∈|) The strain control drive current i_∈ is limited to not overdrive the thermoelectric device 210.

[0023] The first component temperature signal T_1Csensed from the first component 206 by the sensor 212acan be applied to a second limiter circuit or algorithm 314 to limit the temperature signal to a predetermined range or quantity T_exceed. If T_1C>T_max, then the second limiter 314 sets T_exceed=T_1C−T_max. If T_min≦T_1C≦T_max, then the second limiter sets T_exceed=0 and if T_1C<T_min, then the second limiter 314 sets T_exceed=T_min−T_1C. T_maxis a maximum design temperature that the first component 206 can reach to effectively control the low-cycle fatigue of the electrical connections 204 connecting the first and second components 206 and 208 and T_minis a minimum operating temperature.

[0024] The T_exceedsignal is applied to a second proportional-integral-differential (PID) control circuit or algorithm 316 to generate a temperature-limiting drive current signal i_Tas a function of the T_exceedsignal. Adequate control of the thermoelectric device 210 to control the temperature and strain/deflection of the first component can be obtained by using only the proportional control segment or portion of the second PID 316 in most applications to generate the temperature-limiting drive current signal i_Tfrom the T_exceedsignal. Although, the integral and differential control segments of the second PID 316 can be used to improve stability and control accuracy of the temperature and strain/deflection of the first component 206. The first and second PIDs 310 and 316 can be digitally implemented as well as in analog form.

[0025] The strain-control drive current signal i_∈ is combined with the temperature-limiting drive current signal i_Tin a fourth summing junction, summer or combiner 318 to generate a combined current signal i_∈/T. In the embodiment shown in FIG. 3, the combined current signal i_∈/Tcan be the difference between the strain-control drive current signal i_∈ and the temperature-limiting drive current signal i_T. The combined current signal i_∈/Tcan be applied to a third limiter circuit or algorithm 320 to limit the combined current i_∈/Tto a predetermined level or range so as to not overdrive the thermoelectric device 210. If |i_∈/T|≦i_max, then the third limiter 320 sets i_∈/T=i_∈/Tand if |i_∈/T|>i_max, then the third limiter 320 sets i_∈/T=i_max(i_∈/T/|i_∈/T|). The limited, combined current i_∈/Tis then applied to the thermoelectric device 210 to control the operation of the thermoelectric device 210 to transfer thermal energy Q to cool the first component 206 or to transfer heat from the first component 206. It should be noted that the controller 300 can be implemented digitally as well as in hardware or an analog implementation.

[0026] FIG. 4 is a schematic diagram of a controller 400 that can be used for the controller 114 of FIG. 1 or the controller 222 of FIG. 2B in accordance with another embodiment of the present invention. The controller 400 includes a first component expansion circuit or algorithm 402. The first component temperature signal T_1Ccorresponding to a temperature sensed from the first component 206 is applied to the first component expansion circuit or algorithm 402 to generate a first component expansion or strain signal ε_1Cas a function of the first component temperature signal T_1C. The first component expansion or strain signal ε_1Ccan be generated according to the relationship ε_1C=α_1C(ΔT_1C), where α_1Cis the thermal expansion coefficient for the first component 206.

[0027] The second component temperature signal T_2Csensed from the second component 208 is applied to a second component expansion or strain signal circuit or algorithm 404 to generate a second component expansion or strain signal ε_2Cas a function of the second component temperature signal T_2C. The second component expansion or strain signal can be generated according to the relationship ε_2C=α_2C(T_2C) where α_2Cis the thermal expansion coefficient for the second component 208. A reference expansion ε_refcan be generated by a reference expansion circuit or algorithm 406 according to the relationship ε_ref=(α_2C−α_1C)T_ref. The reference temperature signal T_refcan be a temperature at which both components 206 and 208 are neutral or have zero strain or deflection, such as about room temperature.

[0028] A differential strain control signal Δε can be generated by combining ε_1C, ε_2Cand ε_refin a first summing junction, summing algorithm, summer or combiner 408. In the embodiment shown in FIG. 4, the differential strain control signal Δε can be generated by determining the difference between ε_1C, ε_2Cand ε_ref. The differential strain control signal Δε is applied to a first proportional-integral-differential (PID) control circuit or algorithm 410 to generate a strain-control drive current signal i₅₃₁ as a function of the differential strain control signal Δε. As previously discussed, adequate control of the thermoelectric device 210 to control the temperature and strain/deflection of the first component 206 can be realized by using only the proportional control segment of the first PID 410 to generate the strain-control drive current i_∈. Although, the integral and differential control segments of the first PID 410 can be used to improve stability and control accuracy of the temperature and strain/deflection of the first component 206. The strain-control drive current signal i_∈ can be applied to a first limiter circuit or algorithm 412 to limit the strain-control drive current signal i_∈ to a predetermined range similar the first limiter 312 in FIG. 3 as previously discussed.

[0029] The first component temperature signal T_1Ccan also be applied to a second limiter circuit or algorithm 414 similar to the limiter circuit or algorithm 314 of controller 300 to limit the temperature signal sensed from the first component 206 to the predetermined range or quantity T_exceed. The T_exceedsignal is applied to a second proportional-integral-differential (PID) control circuit or algorithm 416 to generate a temperature-limiting drive current signal i_Tas a function of the T_exceedsignal. As previously discussed, adequate control of the thermoelectric device 210 is possible by using only the proportional segment of the second PID 416; although the integral and differential segments of the second PID 416 can be used to improve stability and control accuracy of the temperature and strain/deflection of the first component 206.

[0030] The strain-control drive current signal i_∈ is combined with the temperature-limiting drive current signal i_Tin a second summing junction, summer or combiner 418 to generate a combined current signal i_∈/T. In the embodiment shown in FIG. 4, the combined current signal i_∈/Tcan be the difference between the strain-control drive current signal i_∈ and the temperature-limiting drive current signal i_T. The combined current signal i_∈/Tcan be applied to a third limiter circuit or algorithm 420 to limit the combined current i_∈/Tand not overdrive the thermoelectric device 210. The limited, combined current i_∈/Tis then applied to the thermoelectric device 210 to cause the thermoelectric device 210 to transfer thermal energy Q to cool the first component 206 or to transfer heat from the first component 206.

[0031] Accordingly, as the temperature and strain on the first component 206 and the second component 208 change during the operational cycles of the electronic device 200 (FIG. 2), the strain-control drive current signal i_∈ and the temperature-limiting drive current signal i_Twill change in correspondence therewith as will the combined current signal i_∈/T. Thus, the apparatus 202 (FIG. 2) continually adapts to changing thermal conditions to control the operation of the thermoelectric device 210 to transfer heat from the first component 206, or if the thermoelectric device 210 is coupled to the second component 208, to transfer heat form the second component 208.

[0032] The first and second PIDs 410 and 416 can have digital as well as analog implementations and the controller 400 may also be implemented in software or digitally or in an analog implementation.

[0033] The present invention thus provides an apparatus and method to substantially minimize the low-cycle fatigue or breakage of electrical connections between components caused by the thermal expansion mismatch between the components. The present invention also provides an apparatus and method that is adaptive to changing conditions during the different operational cycles of the components to substantially control low-cycle fatigue.

[0034] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.