Patent Grant
7205672
The present invention relates generally to integrated circuits. More specifically, the invention relates to thermal connections for integrated circuits to provide improved heat sensing and voltage control.
For various reasons it may be desirable to conduct heat away from a flip chip. In the prior art, solder bumps, which are used for electrical connections, were also used to conduct heat from a flip chip. Such solder bumps, may not provide sufficient heat conduction.
It is desirable to provide an improved method and apparatus for conducting heat away from a flip chip.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, an apparatus is provided. Generally, a zener diode flip chip with an active side and a back side opposite the active side and a positive thermal coefficient resistor are provided. A thermally conductive connection is between the positive thermal coefficient resistor and the back side of the zener diode flip chip.
A method for providing cooled flip chip is also provided. Solder paste is placed on a back side of a flip chip. A heat sink is placed against the solder paste. The solder paste is reflowed.
These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well-known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention.
To facilitate discussion,
In other embodiments, two or more solder reflows may be used. For example between the step of placing the heat sink against the solder paste (step 108) and the step of placing the solder bumps against the mounting structure (step 112) the solder paste may be reflowed to connect the heat sink to the flip chip, so that the heat sink and flip chip may be marketed or provided as a single unit.
The large area of the solder paste connected to the back side of the flip chip and the heat sink allows for a larger amount of heat transfer from the flip chip compared to smaller area connections between the solder bumps and the mounting structure. The heat sink may have various forms. Heat sinks with fins are known to provide improved cooling.
The improved heat transfer provides a preferred embodiment of the invention where the flip chip may be a zener diode and the heat sink may form part of a Positive Temperature Coefficient (PTC) resistor, which is a resistor that increases resistance as temperature increases, and where generally at a certain temperature the resistance greatly increases.
The solder bumps 606 are placed against a mounting structure, such as a printed circuit board or a lead frame (step 516). The solder bumps 606 and the solder paste 608 are reflowed (step 520).
In another embodiment, a first reflow of the solder paste may be performed before placement on the mounting surface. Such a reflow allows the PTC resistor and the zener flip chip to be joined and marketed as a single unit.
In various electronic systems, such as in a battery recharger it is desirable to provide a circuit which causes a circuit to open if too high of a voltage is applied.
In operation, the voltage source 1004 applies a charging voltage across the terminal 1008, 1012. The charging voltage C causes the cell 1020 to charge and produces a current, which passes through the PTC resistor 612 at a low resistance. During normal operation the charging voltage C is less than the reverse bias of the zener diode 604, so that little or no current passes through the zener diode 604. If the voltage source 1004 malfunctions and provides a voltage greater than C and greater than the reverse bias of the zener diode 604, current will begin to flow through the zener diode 604, causing the zener diode 604 to heat up. Heat from the zener diode 604 is transferred through the solder 608 to the PTC resister 612. The heat from the zener diode 604 causes the resistance in the middle layer 624 of the PTC resistor 612 to increase to the point where the circuit is virtually an open circuit. PTC resistors are also known as resettable fuses or self-resetting fuses. When overheated resettable fuses create a high resistance creating a virtual open circuit. When cooled resettable fuses automatically close the circuit. Prior art circuits that do not use a zener diode use the self-heating of the PTC resistor to open the circuit. Such self-heating may be too slow, which could cause damage to the cell and/or circuitry before the circuit is opened. The zener diode 604 causes the PTC resistor 612 to heat up quicker so that damage to the cell and or circuit may be prevented. Since the layer of solder paste between the zener diode 604 and PTC resistor 612 may be relatively thin, heat is able to pass from the zener diode 604 to the PTC resistor 612 quickly, providing a quicker response time. In addition, the first and second flip chip terminals 808, 812, each formed by a plurality of solder bumps, provide a large conductive cross section to allow a large current to pass through the zener diode 604, which allows faster heating of the zener diode 604.
Such battery rechargers are used on portable electronic device, where compactness is desirable. The inventive flip-chip and PTC resistor combination provides the desired compactness. The terminals for the zener diode and PTC resister may be placed so that they may be mounted on the same mounting structure, which may provide an easier mounting.
In other embodiments of the invention other thermally conductive mechanically connective layers may be used in the place of the solder paste layer to provide a thermally conductive mechanical connection between the flip chip, such as the zener diode, and the heat sink, such as the PTC resistor. Such thermally conductive mechanically connective layers may be a thermally conductive epoxy. Thermally conductive is at least as half as thermally conductive as solder and more preferably is as thermally conductive as solder. Providing a preferred thermal connection with a layer of thermally conductive material, provides a large cross-sectional area that conducts the heat and a thickness, which is much less in size than the cross sectional area, so that the distance that the heat travels may be minimized. The conductive mechanical connection, such as solder or epoxy, provide a mechanical connection that is able to support the weight of the flip chip since the conductive mechanical connection is stronger than support provided to a flip chip by a thermal compound of thermal greases, which may be used for a conductive connection.
While this invention has been described in terms of several preferred embodiments, there are alterations, modifications, permutations, and substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and substitute equivalents as fall within the true spirit and scope of the present invention.
This application is a continuation of application Ser. No. 10/011,158 entitled “FLIP CHIP MOUNTED TO THERMAL SENSING ELEMENT THROUGH THE BACK SIDE OF THE CHIP,” by Gregory Smith and John W. Oglesbee (now abandoned), filed on Dec. 5, 2001, from Which priority under 35 U.S.C. §120 is claimed, and incorporated herein by reference. This application is also related to the commonly assigned application Ser. No. 10/010,029 entitled “INTEGRATED CIRCUIT PACKAGE WITH THERMAL SENSING ELEMENT”, by Gregory Smith and John W. Oglesbee (now abandoned), filed on Dec. 5, 2001 and incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4459632 | Nijman et al. | Jul 1984 | A |
| 4649333 | Moore | Mar 1987 | A |
| 4780598 | Fahey et al. | Oct 1988 | A |
| 5488254 | Nishimura et al. | Jan 1996 | A |
| 5533256 | Call et al. | Jul 1996 | A |
| 5587882 | Patel | Dec 1996 | A |
| 5644461 | Miller et al. | Jul 1997 | A |
| 5796160 | Kozono | Aug 1998 | A |
| 5811876 | Haga et al. | Sep 1998 | A |
| 6002239 | Maloizel | Dec 1999 | A |
| 6255141 | Singh et al. | Jul 2001 | B1 |
| 6331764 | Oglesbee et al. | Dec 2001 | B1 |
| 6489879 | Singh et al. | Dec 2002 | B1 |
| 6519154 | Chiu | Feb 2003 | B1 |
| 6770513 | Vikram et al. | Aug 2004 | B1 |
| 20010015477 | Singh et al. | Aug 2001 | A1 |
| 20020130398 | Huang | Sep 2002 | A1 |
| 20020145194 | O'Connor et al. | Oct 2002 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10011158 | Dec 2001 | US |
| Child | 10740239 | US |
