Patent Application
20110260314
1. Technical Field
The present disclosure relates to a die package.
More specifically, the disclosure relates to a die package of the type comprising a die being placed on and in direct contact with a low heat sink element and having a package case as well as leads being made of conductive material, and protruding from said package case.
The disclosure also relates to a method for realizing a double side cooling of a die package.
The disclosure particularly, but not exclusively, relates to a die package of the SMD (acronym of “Surface Mount Device”) type and the following description is made with reference to this field of application for convenience of explanation only.
2. Description of the Related Art
As it is well known, in order to improve the electrical performances of an integrated device being housed into a package, in particular of a power device housed into a power package, heat removal is a key point.
In this aim, the design of the current standard power packages is made by taking into account that a low side flange (also indicated as the heat sink) whereto a power die is bonded, and in particular normally soldered, as well as Input-Output or I/O pins are connected by wire bonding. In particular, wires connect the top surface of the power die to leads, which are used to connect it with the external world.
More in particular, when considering a PowerMOSFET device, wherein a current is vertically flowing through the silicon bulk of the die comprising the device, the heat sink actually works as a heat spreader. In this case, the drain contact of the PowerMOSFET device is to be considered as a heat spreader too.
A known packaged PowerMOSFET device is schematically shown in
The package 1 in particular comprises a die 2, in turn including the PowerMOSFET device, being housed in a package case 3 and being placed in direct contact with a heat sink 4. The die 2 is connected to the external word, i.e., to the outside of the package 1, by wires, 5a and 5b, being in turn connected to leads or pins, 6a and 6b, protruding from the package case 3.
It is also known that it is possible to improve the electrical performance of a packaged die by substituting the wires by means of a metallic strap called “clip” that is soldered over the surface of the die, as schematically shown in
In this case, the package 7 comprises the die 2 which is also placed in direct contact with the heat sink 4, being for instance a flat portion of at least one lead, in particular the lead 6a. The die 2 is connected to the other lead 6b by a clip 8, which has a flat portion in direct contact with the top surface of the die 2.
In particular, the clip 8 is made of metal, normally copper (Cu), and thus substantially improves the electrical parameters of the die 2, also helping to remove the heat from the die surface whereat such heat is generated.
It can be verified that the use of a clip is normally able to decrease the thermal resistance of the package, comprising the die which is connected to this clip, by a 5-10% with respect to a similar package comprising a wire bonded die.
It is also known that the assembly process of this kind of clips needs a special coating of the contact side i.e., the front side of the silicon die, that is normally finished by a metallic layer, in particular aluminum (A1). In order to make such a surface, i.e., the front side of the silicon die, able to be soldered, it should be finished by means for instance of a so called wet metal, for example titanium-nickel-gold (Ti-Ni-Au) or chrome-nickel-gold (Cr-Ni-Au).
It is thus evident that such a special coating step would increase the cost of the silicon die.
Starting from this concept, many ideas upon the need to remove the heat from a packaged die came up in the field of semiconductor die manufacturing.
One of the most efficient method to remove heat from a packaged die is the so called “Dual side cooling”. According to this known method, the heat is dissipated from the bottom of the package housing the die, in particular through the heat sink thereof as well as from the top of the die, by adding a strap or clip soldered to the silicon top of the die, connecting it to the outside world. The exposed area of such an added strap is then attached to an external heat sink or external clip and heat is simultaneously removed from top and from bottom of the packaged die.
In particular, this external clip is biased as it is directly into contact with the silicon top surface of the die (also indicated as source). In particular, the clip is connected to an I/O terminal per die (a source terminal in this case), thus ensuring the correct biasing. It can be verified that the thermal resistance of the resulting packaged die or package is improved by 40-45% if compared to a same package with a die of identical size being cooled with a single side cooling solution of the known type, such as a die being connected to an internal clip only.
Today's power packages with double side cooling are made by soldering a metal clip, that is bonded from a top silicon surface to the package case where an external heat sink will be connected, as schematically shown in
In particular, the package 9 comprised a chip 2 being connected to an internal heat sink 4, the chip 2 being placed in direct contact with the internal heat sink 4. Moreover, the package 9 comprises a metal clip 8 being placed in direct contact, and in particular soldered, with a top silicon surface 2a of the die 2 and being in contact with the leads 6a and 6b. More in particular, the metal clip 8a has a top surface 9a that emerges from the package case 3 and can be connected to an external heat sink, not shown, in order to realize the required double side cooling.
Current methods for making a double side cooling of power packages are quite rigid and the corresponding assembly process window is very tight, thus impacting with yield and field return. As an example, the solder quantity involved in soldering the clip needs to be very much accurately dispensed so as to avoid a damage of the silicon making the dice, such a die damaging occurring if the quantity of the solder is above a maximum limit and molding compound thereof flashes. Moreover, die damaging occurs if the quantity of the solder is below a designed quantity, causing problems tied to the flatness of the final clip.
Furthermore, it is well known that the die thickness tolerance, obtained by back grinding, needs also to be accurately considered. In particular, it needs to be within +/−5 micrometers with respect to standard +/−20 micrometers of the die thickness.
In particular, the assembly process which is making dual side cooling technology possible is the molding, indeed. However, a molding process for this kind of technology is very complex and needs, in addition to basic material such as a molding compound, a foil, to be more precise, a top foil and a bottom foil, to be used. Foil material is normally a PET carrier with a glue which activates under the effect of the molding temperature.
Furthermore, the price of the machine involved in such an assembly process is very high and material involved very costly. Foil is interposed between the mold cavity and the package to mold for the following reasons:
1- Molding flashes propagation. If lower heat sink and/or upper exposed clip are covered by a layer of compound, package solderability and ability to dissipate heat would severely affected. In order to get rid of this issue, a strong flashes removal process needs to be provided.
2- Silicon chip damage. Foil is needed to absorb the difference in thickness of all the used layers. If the value of the sum of all the layers composing the package (lower heat sink, solder between heat sink and chip, chip, solder between chip and clip) is above the actual mold cavity, molding clamp pressure would break the weakest element that is the chip. Foil (normally 50-70 micrometers thick) squeezes under the effect of the clamp pressure and then works as sealer for flashes and as pressure/thickness mismatch absorber. The outcome of this process is it is very sensitive to all elements thickness and to mold design. In addition to that, foil equipped molded systems are very expensive (25% more than standard ones), with a low MTBA (acronym of “Mean Time Between Assistance”), i.e., the mean time which separates two assistance operations performed by an user which works on the system, and not very easy to maintain. Foil is also very expensive and the final package cost is strongly affected by the molding process.
Moreover, known dual side cooling packages are quite rigid in design and need to be specifically designed for this purpose.
Some embodiments include a die package having a dual side cooling and the corresponding method having structural and functional characteristics which allow to overcome the limits which still affect the method and packages realized according to the prior art.
One embodiment includes a die package with a spring element which connects leads and a front side of a die, thus allowing a final user of the package to place an external heat sink and thus provide a dual side cooling of the die package.
One embodiment includes a die package of the type comprising a die placed on and in direct contact with a low heat sink element and having a package case as well as leads, being made of conductive material, and protruding from said package case, characterized in that it further comprises an upper heat sink element being shaped as a spring element and being in contact between said die and said leads and emerging from said package case in an opposite end with respect to said low heat sink element.
More in particular, various embodiments comprise the following supplemental and optional features, taken alone or in combination.
According to one embodiment, said upper heat sink element can be a spring shaped strap.
According to another embodiment, said upper heat sink element can comprise at least an upper portion emerging from said package case so as to act as an upper heat sink.
Moreover, according to this embodiment, said upper heat sink element can further comprise at least a low portion in contact with said die and at least a lateral portion in contact with said leads.
According to a further embodiment, said upper heat sink element can be soldered to said die.
According to yet another embodiment, said die package can further comprise a solder layer being provided at least between said die and said low heat sink element.
Moreover, according to a further embodiment, said upper heat sink element can comprise a first lateral portion connected to a first lead and a second lateral portion connected to a second lead, said leads protruding from opposite ends of said package case.
According to this embodiment, said upper heat sink element can further comprise a first low portion being in contact with said die and being also connected to said first lateral portion as well as a second low portion being also in contact with said die (and connected to said second lateral portion.
Moreover, yet according to this embodiment, said upper heat sink element can further comprise an upper portion being substantially C shaped and in contact with said first low portion and said second low portion.
Furthermore, according to this embodiment, said die package can further comprise a solder layer, in turn having a first portion being interposed between said first lateral portion of said upper heat sink element and said first lead, a second portion being interposed between said second lateral portion of said upper heat sink element and said second lead, a third portion being interposed between said first low portion of said upper heat sink element and said die, a fourth portion being interposed between said second low portion and said die as well as a fifth portion being interposed between said die and said low heat sink element.
Furthermore, according to another embodiment, said upper portion of said upper heat sink element emerging from said package case is shaped in order to leave an area of said package case free.
According to yet another embodiment, said die package can further comprise a solder layer only between said die and said low heat sink element, said first and second lateral portions of said upper heat sink element being into direct contact with said first and second leads respectively, and said first and second low portions of said upper heat sink element being into direct contact with said die.
Moreover, according to an embodiment, said upper heat sink element can comprise a single low portion being in contact with said leads and said die.
According to this embodiment, said die package can further comprise a solder layer between said upper portion and said single low portion of said upper heat sink element.
One embodiment is a method for realizing a double side cooling of a die package comprising the steps of:
providing a die package comprising a low heat sink element;
providing conductive leads; and
placing a die on and in direct contact with said low heat sink element,
providing an upper heat sink element in contact with said die and said leads and having at least an upper portion, said upper heat sink element being shaped as a spring element; and
molding a package case made of insulating material and including said die, said upper heat sink element and said low heat sink element in such a way that said conductive leads protrude from said package case and said upper portion of said upper heat sink element emerges from said package case of a so obtained die package in an opposite end with respect to said low heat sink element.
According to another embodiment, said method can further comprise a step of placing an external heat sink in contact with said upper heat sink element.
Moreover, according to another embodiment, said method can further comprise a step of providing a solder layer, being interposed at least between said die and said low heat sink element.
Finally, according to this embodiment, said step of providing said solder layer can realize portions of said solder layer being interposed between said upper heat sink element, said leads and said die, respectively.
The characteristics and advantages of the die package and the method according to various embodiments will be apparent from the following description of an embodiment thereof given by way of indicative and non limiting example with reference to the annexed drawings.
In such drawings:
With reference to such figures, and in particular to
It should be noted that the figures show schematic views of the die package and are not drawn in scale, being on the contrary drafted so as to emphasize the important features of the illustrated embodiment.
The die package 10 comprises a die 12 being placed on and in direct contact with a low heat sink element 14 and has a package case 13 made of insulating material, in particular of plastic material. Leads 16a, 16b, being made of conductive material, in particular of Copper (Cu), protrude from the package case 13.
Advantageously according to an embodiment, the die package 10 also comprises an upper heat sink element 11, in particular shaped as a spring element.
More in particular, the upper heat sink element 11 comprises at least a low portion in contact with the die 12, at least a lateral portion in contact with at least one lead and an upper portion emerging from the package case 13 so as to act as an upper heat sink.
In essence, the upper heat sink element 11, being a strap which is “spring” shaped, is soldered into the die package 10 and connects a front side or surface of the die 12, leads 16a and 16b and the external world, i.e., the outside of the die package 10.
Also advantageously according to an embodiment, the die package 10 can also comprise a solder layer 15 being provided at least between the die 12 and the low heat sink element 14 and in case between the die 12 and the low portion of the upper heat sink element 11 as well as between lateral portions of the upper heat sink element 11 and the leads 16a and 16b.
According to the embodiment shown in
Moreover, the upper heat sink element 11 comprises a first low portion 11c being in contact with the die 12 and being also connected to the first lateral portion 11 a as well as a second low portion 11d being also in contact with the die 12 and connected to the second lateral portion 11b. Finally, the upper heat sink element 11 comprises an upper portion 11e being substantially C shaped, in contact with the first low portion 11c and the second low portion 11d and emerging from the package case 13 in an opposite end with respect to the low heat sink element 14.
The upper heat sink element 11 is thus a profiled strap, in particular being made of metallic material and free to be compressed under the effect of mold clamps which are to be used to realize the package case 13.
In other words, advantageously according to an embodiment, the upper heat sink element 11 being spring shaped adapts itself to the thicknesses of the several components or layers comprised into the package case 13, as the low heat sink element 14 and the die 12, and in case the solder layer 15, as well as to the relevant tolerances.
According to the embodiment shown in
The die package 10 according to an embodiment is also shown in
According to an alternative embodiment, the die package 10 only comprises the solder layer 15 between the die 12 and the low heat sink element 14, the first lateral portion 11a of the upper heat sink element 11 being into direct contact with the corresponding first lead 16a, the second lateral portion 11b of the upper heat sink element 11 being into direct contact with the corresponding second lead 16b, the first low portion 11c of the upper heat sink element 11 being into direct contact with the die 12 and the second low portion 11d being into direct contact with the die 12, as shown in
Moreover, according to a further alternative embodiment, the upper heat sink element 11 comprises a single low portion 11f, being in contact with the leads 16a and 16b and the die 12, and being in case separated to the upper portion 11e by a further portion 15f of the solder layer 15, as shown in
An embodiment also relates to method for realizing a double side cooling of a die package 10 comprising the steps of:
providing a die package 10 comprising a low heat sink element 14;
providing conductive leads 16a, 16b; and
placing a die 12 on and in direct contact with the low heat sink element 14.
Advantageously according to an embodiment, the method also comprises the steps of:
providing an upper heat sink element 11 in contact with the die 12 and the leads 16a, 16b and having at least an upper portion 11e, such a upper heat sink element 11 being shaped as a spring element; and
molding a package case 13 made of insulating material and including the die 12, the upper heat sink element 11 and the low heat sink element 14 in such a way that the conductive leads 16a, 16b protrude from the package case 13 and the upper portion 11e emerges from the package case 13 of the die package 10.
In this way, the upper heat sink element 11, being “spring” shaped, is free to be compressed under the effect of the molding step in order to compensate the clamping effect and obtain a flat upper side of the die package 10. Moreover, the free compression of the “spring” shaped upper heat sink element 11 ensures no damage on the die 12.
The method further comprises a step of placing an external heat sink in contact with the upper heat sink element 11, thus improving the dual side cooling of the die package 10.
Finally, the method can further comprise an optional step of providing a solder layer 15, being interposed at least between the die 12 and the low heat sink element 14 and preferably having portions 15a, 15b, 15c, 15d being interposed between the upper heat sink element 11 and the leads 16a, 16b and the die 12, respectively.
In summary, advantageously according to an embodiment, even mismatch among the layers, leading to planes tilting, will be “flattened” and exposed package surface, in particular the upper portion 11e of the upper heat sink element 11, which is the second side the heat is dissipated, will always be flat.
This is achieved by designing the upper heat sink element 11 as a profiled clip being always in interference with a mold cavity of the die package 10 as it closes.
In particular, it should be remarked that interference will ensure very low compound flash propagation and the spring effect of the so shaped upper heat sink element 11 will ensure no damage on the die 12.
In addition to that it can be verified, for instance by a FEM analysis, that the die package 10 according to an embodiment shows an excellent behavior, when compared to a standard package, under power/thermal cycles (measured in cycles) where the stress on the die 12 and solder layer 15 is induced by thermal expansion/contraction of all the elements comprised into the die package 10.
In particular, such an analysis, being realized by the applicant, showed an improvement equal to 15-20% with respect to a standard package.
From an assembly process point of view, advantageously according to an embodiment all assembly process steps are simplified and standard existing equipment can be used as well. As an example, solder paste dispensed quantity, as well as the die thickness tolerance are no longer an issue as the final compensation is given by the spring effect of the upper heat sink element 11.
In particular, the main assembly process of the die package 10 according to an embodiment is always mold. However, advantageously according to an embodiment, top and bottom foils are no longer needed and standard mold can be used. The above described interference would control flash propagation and will not cause stress/damage on the die 12.
That turns into low investment for equipment, for material and less rigidity in package design.
In addition to that, many current packages can be easily changed from single to dual side cooling with very limited investment on material/equipment.
Advantageously according to one embodiment, the only requirement is to provide the package with an upper heat sink element 11 being a dedicated spring clip and silicon chip design. In particular, it is well known that the geometry of the die surface (the metalized one) and the chip surface being soldered thereon should be designed in accordance one another.
Moreover, according to the first alternative embodiment shown in
This will further improve thermal resistance of the die package 10 of
Finally, according to the second alternative embodiment of
It should be noted that, event packages of the SMD (acronym of “Surface Mount Device”) have been considered, also through hole and other types of packages may profit as well of the proposed upper heat sink element 11 realizing a dual side cooling mechanism for the die package 10.
In essence, the die package 10 comprising the upper heat sink element 11 shaped as a spring element would resolve all process and assembly issues by considering a spring elements which connect leads and a surface of the die 12 as well the package case 13, allowing a final user to further place an external heat sink and realize the double side cooling of the die package 10.
The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2010A 000718 | Apr 2010 | IT | national |
