Patent Application
20110233756
N/A
Embodiments of the invention relate generally to wafer level packaging (WLP). More specifically, invention embodiments are directed toward wafer level packages (WLPs) comprising a backside laminate or film, which includes thermally conducting fillers or fibers that, instead of insulating the top surface of a WLP, aid in the dissipation of heat via convection or the transfer of heat from the surface of a WLP to a heat sink.
Initially, the back side or the top side of the silicon die 102 was left bare with nothing placed thereon. With nothing on the back side of the silicon die 102, the flip-chip package 104 had a heat dissipation capability of up to about 20 watts. Over time, the flip-chip packaging technology evolved to provide a 50 to about 90 watt heat dissipation rating. This was done by incorporating a thermal interface material (TIM) 110 on the top surface of the silicon die 102. The thermal interface material is a grease, paste or silicon gel that has heat conductive qualities therein. The TIM 110 is not an adhesive and does not provide any structural enhancement to the prior art flip-chip package 100. On top of the TIM 110 was placed a heat spreader or lid 112 that sandwiched the TIM 110 between the top side of the die 102 and the bottom surface of the heat spreader 112. This prior art flip-chip configuration is seen more clearly in the expanded view of area 114. It is important to understand that the addition of the TIM 110 and the heat spreader 112 is manufactured on a piece-wise basis such that each individual die 102, after being cut from a wafer and flip-chip mounted on the package 104, is then processed one-by-one to include the placement of the TIM 110 on its upper surface along with the incorporation of a heat spreader 112 thereon.
The heat spreader or heat sink 112 may be made of aluminum or copper. The TIM 110 was used to minimize the thermal resistance between the top side of the silicon die 102 and the heat spreader 112. The combination of the heat spreader 112 and the TIM 110 help increase the thermal capabilities of the prior art flip-chip packages 100.
The TIM 110 is essentially thermal grease, jelly or silicon substance that is placed on the surface of the silicon die 102 and spreads out when the heat spreader or heat sink is pressed on as well as when heated. The TIM 110 increases the thermal conductivity between the silicon die 102 and the heat spreader 112. The TIM 110 is not used to hold the heat spreader 112 in place, but other means such as clips or silicon glue around the edges of the heat spreader (not specifically shown) are used to hold a heat spreader in place on prior art flip-chip packages.
One of the key drawbacks of flip-chip packaging is the manufacturing cost. Flip-chip package manufacturing is performed at the unit level thus; there are no economies of scale that are traditionally associated with a wafer level processing process. For example, each wafer may have a thousand or more dies manufactured thereon, but with flip-chip packaging each die must be cut from the wafer and individually packaged on a per-unit basis. Therefore, if there are a thousand dies incorporated in a wafer, the flip-chip packaging process would be required to be performed one thousand times; 1 time for each die. Therefore, what is needed is a device and manufacturing technique for making such a device that allows for manufacturing a complex silicon die with a multitude of electrical connections while still being able to dissipate more than 20 watts. It would be a further advantage if such a device could be manufactured at a wafer level rather than a per-unit level.
Prior art WLP packages may include a laminate on the back or inactive side of the WLP. The prior art laminate consists of a polymeric film with silica fillers and a heat curable adhesive on one side of the polymeric film. The polymeric film/adhesive combination provides a good surface for marking the part number or other information. The prior art polymeric film/adhesive combination acts as an insulative layer that limits thermal conductivity away from the prior art WLP packages. An insulative layer may be OK for low power WLP packages (less than about 20 watts), but it limits WLP devices from incorporating higher power circuitry (25 to 50 to about 90 watts) and larger WLP device sizes from 9×9 solder ball arrays to 20×20 arrays.
Thus, what is further needed is a WLP configuration that is conductive to aiding thermal heat removal such that higher power and larger WLP devices can be made available via an economic manufacturing process.
In an embodiment, a method of manufacturing a heat dissipating wafer level package (WLP) is provided. The method of manufacturing the heat dissipating WLP comprises a step of applying a thermally conductive coating on a back surface or inactive surface of a wafer, wherein the wafer comprises a plurality of wafer level package device sections. The method of manufacturing the heat dissipating WLP further comprises curing the thermally conductive coating that has been applied to the back surface or inactive surface of the wafer. In some embodiments, the step of applying the thermally conductive coating further comprises applying a laminate to the back surface of the wafer. An exemplary laminate may comprise a thermally conductive adhesive layer and a thermally conductive film layer. In other embodiments, the step of applying the thermally conductive coating may comprise spraying, sputtering or coating the thermally conductive coating or layer onto the back surface of the wafer.
In additional embodiments the method of manufacturing the heat dissipating wafer level package may further include dicing the wafer into a plurality of wafer level package device sections such that each wafer level package device section is an individual heat dissipating wafer level package.
Another embodiment of the invention comprises a heat dissipating WLP device. The heat dissipating WLP device comprises a WLP die, which has a circuit side and a back side. Covering the back side of the WLP die is a thermally conductive coating that comprises a thermally conductive filler that is dispersed in the coating. In some embodiments, the coating comprises a film, which includes the thermally conductive filler. In other embodiments, the thermally conductive coating comprises a thermally conductive filler that is in a mesh, woven, lattice, or non-woven configuration. Additionally, embodiments of the invention may comprise a thermally conductive coating that is an impressionably compliant thermally conductive coating adapted to conform to a surface of an item, such as a heat sink, pressed thereon.
In other embodiments, a heat dissipating WLP device is provided that comprises a WLP die which has a circuit side and a back side. The embodiment further comprises a coating that covers the back side of the WLP die. Dispersed within the coating are thermally conductive fillers. The heat dissipating WLP further includes a heat sink or heat spreader that has a first surface that is thermally engaged and attached to the top surface of the coating such that the coating is sandwiched between the back side of the WLP die and the heat sink or heat spreader.
For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying Drawings in which:
Referring now to the drawings, wherein like reference numbers are used herein to designate like elements throughout, the various views and embodiments of a wafer level package incorporating heat dissipation are illustrated and described, and other possible embodiments are described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated and/or simplified in places for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations based on the following examples of possible embodiments.
Embodiments of the present invention enhance the thermal performance of wafer level package (WLP) devices by incorporating into an exemplary WLP device elements and materials that provide effective heat spreading and heat dissipation about and away from an exemplary WLP device. Such heat spreading and dissipation, in accordance with embodiments of the invention, will move heat away from circuit hot-spots on exemplary WLP packages and also aid in transferring heat away from exemplary WLP packages via attached heat spreader or heat sink elements. Embodiments of the invention include a WLP with a back side laminate, layer or coating that incorporates thermally conductive fillers or fibers. The back side laminate comprising thermally conductive fillers effectively helps to spread and dissipate heat away from hot spots in an exemplary WLP and/or away from an exemplary WLP. An exemplary back side laminate with thermally conductive fillers does not insulate or provide a thermally insulative coating over the top surface (back side surface) of an exemplary WLP.
Referring now to
On the inactive or back side surface 210 of the die 202 a thermally conductive coating 212 is bonded thereon. In exemplary embodiments, the thermally conductive coating 212 completely covers the inactive or back side surface 210 of an exemplary WLP device 200. In some embodiments, the thermally conductive coating comprises a thermally conductive film. The thermally conductive film includes thermally conductive fillers or fibers. The thermal conductivity of the thermally conductive coating 212 moves heat away from hot spots, as shown by the arrows 214, in order to spread or dissipate heat about the die 200. The thermally conductive coating 212 also helps remove heat from an exemplary WLP device 200 via convection (as shown via arrows 216). Embodiments of the thermally conductive coating 212 have a substantially uniform thickness of about 20 microns (μm) to about 100 microns (μm).
Referring now to
An adhesive layer 306 is next to and against one side of the thermally conductive film layer 302. The adhesive layer 306 also comprises filler particles that are thermally conductive (not specifically shown). The filler particles within the adhesive layer may comprise one or more of the same filler particles, fibers or other thermally conductive items or elements found within the film layer 302. The adhesive layer is used initially to attach the film layer 302 to the back side surface of a wafer, which comprises a plurality die-circuit segments. Once attached, a curing process takes place that cures the adhesive 306 such that the film layer 302 is firmly bonded against the back side 210 of a die 202. When cured, some embodiments' thermally conductive particles, fibers or other elements provide a thermally conductive connection between the back side surface 210 and the top side 310 of the thermally conductive coating 212.
In some thermally conductive coating embodiments, a release layer 308 covers the adhesive layer 306. The release layer 308 is peeled away or peeled off the film layer/adhesive layer combination during the manufacturing process prior to applying the thermally conductive coating 300 (film layer 302 and adhesive layer 306) to the back side 210 of a die 202. In exemplary embodiments, the film layer 302 may have a thickness of from about 20 μm to about 100 μm. In the manufacturing process, exemplary thermally conductive coating 300 may be provided in continuous rolls.
Exemplary thermally conductive coatings will provide a thermal conductivity in the range of 5 to about 50 watts per meter Kelvin. In some embodiments, the thermally conductive coating, when cured, can have a thermally conductive rating as high as about 100 watts per meter Kelvin. The thermally conductive coating 300, which acts as a thermal interface material and has a high thermal conductivity, facilitates the heat dissipation from a WLP package by spreading heat away from local hot spots as well as transferring heat from the silicon layer 202 to a heat spreader or heat-sink that is attached to the top side 310 of the thermally conductive coating. Embodiments of exemplary thermally conductive coatings 300, being sputtered, sprayed, a thin film, a laminate, or with or without an adhesive layer, will have or add thickness to exemplary embodiments of the invention. Such exemplary thermally conductive coatings 300 may have or add a thickness of about 20 microns (μm) to about 100 microns (μm) of thickness to embodiments of the invention.
As mentioned above, in various embodiments of the invention, the thermally conductive coating may comprise a woven material having fibers that are either coated with thermally conductive material or having fibers that have thermally conductive particles colloidally suspended and/or touching each other in the fiber material. Additionally, a woven or lattice of thermally conductive strands may be embedded within the film layer 302 along with or instead of the filler particles 304. The varying embodiments of the thermally conductive coating add structural strength and integrity to the silicon wafer to which it is applied. The exemplary thermally conductive coating or layer 300 will aid in adding integrity by deterring warpage of the silicon wafer and/or the resulting individual dies during the wafer dicing process as well as during each dies operation when installed and/or operating on a PC card. During the manufacturing of the exemplary silicon wafers the back side of the wafer is polished and ground in order to decrease the overall thickness of the resulting WLP packages. Furthermore, WLP packages are increasing in dimensional size as they incorporate larger and more complex circuitry. Embodiments of the invention, which include thermally conductive coatings on the back side of a silicon wafer or die, reinforce the resulting wafer and/or die against warpage and mechanically strengthen the resulting WLP package while improving its overall thermal performance.
The film layer 302, after the adhesive and/or film layer 302 has been cured, may retain some plasticity in that it may conform to or accept indentions from an adjacent surface being pressed against it. The adhesive layer 302 of some embodiments is thus adaptable to conform to an impression of an item pressed against or embedded onto the top side of the thermal conductive coating 310. For example, a heat sink or heat spreader may be pressed against the top surface of the thermally conductive coating 310. In response to a heat sink or spreader being pressed thereon, embodiments of the film layer 302 may conform and indent to a substantially flat, slightly patterned or slightly irregular surface pressed against it so that air bubbles or gaps may be removed and a substantially continuous surface to surface contact exists between the heat sink's bottom surface and the upper surface of the thermally conductive coating 310 in order to maximize thermal conduction. By being an impressionable, compliant, plastic or conformable material, the thermal resistance between the die and a heat sink or heat spreader device may be further minimized in embodiments of the invention.
Referring now to
At step 406, an exemplary thermally conductive coating 300 may be unrolled from, for example, a roll and/or cut to the size of the wafer's back side surface. If there is a release layer 308, it is removed from the thermally conductive coating 300 and the thermally conductive coating 300 is placed on the back side of the wafer such that the adhesive layer is applied to and adheres to the back side surface of the wafer. In some embodiments, the thermally conductive coating 300 is applied to the back side of the wafer and then the excess thermally conductive coating is removed or cut away from the side edges of the wafer. The adhesive layer of the thermally conductive coating holds the thermally conductive coating in place on the back side of the wafer during this step of the manufacturing process. In some embodiments, a thermally conductive coating is sprayed, sputtered or coated onto the backside of the wafer. In such embodiments, the thermally conductive coating is a colloidal mixture of thermally conductive elements and an epoxy, polymer, silicon, ink or other fluid material that suspends the thermally conductive elements therein.
At step 408, the thermally conductive coating 300 is cured on and/or to the back side of the wafer. The curing process may be a thermal process wherein the adhesive layer 306 hardens somewhat such that removal of the thermally conductive coating 300 from the back side of the silicon wafer is difficult. In one embodiment, the curing process may take place at 150 to 200 degrees for a predetermined amount of time that ranges from a half hour to about an hour and a half. In other embodiments, the curing process may be performed using ultraviolet or other wavelengths of light. In yet other embodiments of the invention, the curing process may occur as an evaporative or chemical process wherein the thermally conductive coating goes through a chemical/molecular change during the curing process. Additionally, the curing process strengthens the integrity of the overall wafer and/or an individual dies contained therein. The thermally conductive fillers, lattice, or woven material within the thermally conductive coating provide unexpected advantages of not only establishing thermally conductivity through the coating, but also increased integrity of the resulting wafer and individual WLP devices by adding additional stiffness and mechanically-static strength.
At step 410 the top side 300 of thermally conductive coating 300 may be marked via a printing, ink jet, laser marking, etching or other marking technique in order to place a part number or other pertinent information on the top side 310 of the thermally conductive coating 300.
At step 410, an exemplary wafer may be mounted onto a frame and secured so as to be prepared for dicing. The dicing process involves cutting the silicon wafer into individual exemplary WLP parts. Each exemplary WLP part comprises a die with its active side surface. The die also has a back side or inactive surface having a thermally conductive coating substantially covering the back side. The resulting WLP part will have been manufactured via a wafer level manufacturing process rather than the individual die or unit level manufacturing process of prior art flip-chip devices. In exemplary embodiments, the singulation or dicing of the dies from the wafer effectively provide a resulting exemplary embodiment by the exemplary process.
At step 414, the individual exemplary WLP dies may be picked and placed onto an adhesive side of tape reels, which may be utilized in pick and place machines utilized at step 416 wherein exemplary WLP parts may be picked from the tape reels and placed on to a PC board 208 as shown in
A resulting exemplary WLP device provides a WLP device with a thermally conductive coating (WLP w/TCC) that is competitive with the heat dissipation abilities of prior art flip-chip devices that dissipate or thermally conduct heat away from a silicon die. An advantage of an exemplary embodiment is that additional manufacturing steps are not required to incorporate the thermally conductive coating (TCC) layer or coating onto the back side of an exemplary WLP w/TCC device. The result being a significantly less expensive manufacturing method that produces a thermally conductive silicon die at a wafer level rather than at a die or unit level. Since thousands of dies may be cut out of a single wafer, the manufacturing steps, time and manufacturing costs associated with an exemplary WLP w/TCC are significantly decreased from prior device manufacturing processes.
Referring still to
To date, heat sinks and heat spreaders are not being incorporated onto or integrated with the inactive or back side of WLP devices. Therefore, embodiments discussed herein advance the thermal dissipation capabilities of exemplary WLP packages to operate at higher continuous wattage ratings than prior WLP packages have been able to operate at. For example, exemplary WLP devices are designed to dissipate heat in a manner such that they may be rated to operate at a greater than 20 watt heat dissipation rating. Embodiments of the invention may be rated from about 20 watts to about 50 watts of continuous heat dissipation. Some embodiments may be rated at up to about 90 watts. Since WLP devices are being utilized in smaller and smaller consumer oriented devices such as mobile phones, mobile terminals, GPS devices and various other small portable hand held devices, it is advantageous to decrease the size of a device package from a flip-chip device down to an exemplary WLP device and still be able to dissipate enough heat such that higher power circuitry can be incorporated into the exemplary WLP device.
By providing an exemplary WLP w/TCC, the size of wafer level packages can grow from 7×7 ball array WLP devices to 20×20 ball array WLP devices or larger while maintaining both the structural strength of the resulting device and supporting higher power circuitry (from 20 to about 90 watts).
It will be appreciated by those skilled in the art having the benefit of this disclosure that this wafer level packaging with integrated heat dissipation provides a WLP with improved structural strength and higher power capabilities without a significant increase in manufacturing costs. It should be understood that the drawings and detailed description herein are to be regarded in an illustrative rather than a restrictive manner, and are not intended to be limiting to the particular forms and examples disclosed. On the contrary, included are any further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments apparent to those of ordinary skill in the art, without departing from the spirit and scope hereof, as defined by the following claims. Thus, it is intended that the following claims be interpreted to embrace all such further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments.
