CN1219768A - Method and apparatus for heat transfer enhancing attachment - Google Patents

Abstract

Methods and apparatus for joining heat transfer enhanced connectors of ceramic chip carrier assemblies. For through-cavity ceramic substrates, the heat slug is attached to the substrate with a flexible adhesive having sufficient bond strength to secure the heat slug to the substrate and sufficient thermal conductivity to Allows for proper heat flow from the chip to the thermal slug. The adhesive is selected to provide sufficient bond strength to secure the heat slug to the substrate without unduly increasing mechanical constraints. The adhesive also provides a direct thermal path from the die to the heat slug. The heat slug is adhesively attached to the chip carrier. For planar, multilayer ceramic substrates, the heat sink is attached to the substrate with the same adhesive as described above.

Description

Method and apparatus for heat transfer enhanced joints

The present invention relates generally to electronic device packages and the construction of assemblies including heat transfer enhancing connections. More specifically, the present invention describes methods and apparatus for securely attaching metal heat slugs and heat spreaders to ceramic chip carriers using specific adhesives.

Existing assembly practices that are actually used for through-cavity ceramic lead grid array (PGA) assemblies and planar multilayer ceramic substrates include attaching metal or composite metal heat transfer enhancers to the peripheral area of one side of the ceramic substrate. The heat-transfer-enhanced connector dissipates the heat generated during the operation of the chip. The connectors are typically constructed of a copper-tungsten composite, with between 85% and 100% tungsten and the balance copper, and are typically nickel-plated to form a warmable surface for the brazing alloy. Typical brazing alloys consist of copper and silver (eg silicon bronze). The ceramic substrate must optionally have a metallized sealing band, typically consisting of a co-fired thick film of molybdenum or tungsten, followed by a layer of electroless or electrolytic nickel.

Brazing of heat transfer enhancing joints is generally accomplished by reflowing the components in a continuous belt furnace under reducing conditions such as hydrogen or forming gas to temperatures in the range of 850 to 1000 degrees Celsius, depending on the brazing alloy Composition depends. After reflow, a second layer of nickel is deposited on all metal surfaces and then covered with gold plating to prevent corrosion and enhance the aesthetics of the assembly. Once the connector is attached to the ceramic substrate, the device can be affixed to the inner surface of the connector using commercially available substrate adhesives and then wire bonded to pads on the substrate.

The assembly is encapsulated by reflowing a metal alloy cover, such as gold-plated Kovar® (registered trademark of Westinghouse Corporation), with a lower melting temperature alloy (typically an 80% gold-20% tin brazing alloy). to the appropriate metallized solder strip on the side of the substrate opposite the connection base. The assembly is then subjected to a seal leak test to ensure that there are no leaks in its seal. The required metallization of the ceramic carrier, combined with the inherently high cost of the copper-tungsten plated connectors, adds to the cost of the assembly and complicates the process.

There are two main types of heat transfer enhancing connectors. When constructing through-cavity PGA assemblies, the heat transfer enhancement connector is a heat slug. Alternatively, when constructing a planar multilayer ceramic substrate, the heat transfer enhancing connector is referred to as a heat sink. The main difference in physical construction is that in the case of through-cavity PGA modules, the chip is directly attached to the heat sink, whereas in the planar multilayer ceramic substrate structure, the chip is directly attached to the ceramic substrate.

US Patent No. 5381042 (Lerner et al.) describes a method for integrating an oxygen-free copper heat slug into a molded leadframe assembly. A wire-bond substrate is secured to the surface of the copper heat slug and wire-bonded to an aluminum conductor board before the entire assembly is epoxy-encapsulated to prevent damage to the chip. Whereas through-cavity or planar multi-layer ceramic chip carriers require the use of metal heat slugs or heat sinks to transfer heat to external heat sinks or cooling devices, respectively, the invention makes no provision for the aforementioned ceramic chip carriers to function.

The present invention is a novel method and apparatus for bonding metallic heat transfer enhancing devices to multilayer ceramic through-cavity substrates or planar multilayer ceramic substrates using commercially available adhesives. The connectors may be made of suitable alloys containing aluminum or copper. The aluminum alloy fittings can then be anodized to produce a wide variety of user-defined colors, including imitation gold to give the appearance of much more expensive brazed and gold-plated copper-tungsten fittings.

Accordingly, it is an object of the present invention to provide an apparatus and method for attaching a heat transfer enhancing device to a through-cavity or non-through-cavity, planar multilayer On the ceramic carrier, the above-mentioned adhesive will form a direct heat transfer path to the integrated circuit device and have mechanical stability.

Another object of the present invention is to form a mechanically secure connection between heat slug/heat sink materials having matched coefficients of thermal expansion (CTE). These materials include aluminum, silicon carbide, and molybdenum composites bonded with flexible epoxy resins.

Another object of the present invention is to form a cost-effective thermal conduction path between the chip and the heat slug/heat sink.

Another object of the present invention is to provide a method and apparatus for ensuring the mechanical and operational integrity of the bond between the device and the heat slug/heat sink under normal conditions of use, such as gravity, mechanical shock, Vibration, high temperature, humidity, and repeated thermal expansion/contraction cycles caused by temperature changes, etc.

Another object of the present invention is to provide a method and apparatus capable of absorbing thermally induced strain without damaging the chip carrier or related components.

Another object of the present invention is to provide a method and apparatus for masking out the unwanted epoxy fillet formed when a heat spreader is attached to the bottom of a ceramic chip carrier.

A final object of the present invention is to form a non-hermetic assembly that does not require high temperature soldering or brazing steps, and the associated substrate plating and metallization requirements.

The novel features and essential characteristics characteristic of the invention are set forth with particularity in the appended claims. The drawings are for illustration purposes only and are not drawn to scale. However, the invention itself, including its organization and method of operation, is best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

FIG. 1 illustrates a schematic diagram of a prior art method for connecting a heat sink to a through-cavity ceramic chip carrier;

Figure 2 illustrates a preferred embodiment of the invention, wherein a ceramic through-cavity chip carrier is used;

Figure 3 illustrates another preferred embodiment of the invention, wherein a planar ceramic chip carrier is used; and

Figure 4 illustrates a further preferred embodiment of the present invention.

In the field of thermal control of ceramic wire bond packages, a common method of heat removal from the substrate is by brazing a copper-tungsten composite heat slug/sink to the metallization tape of the ceramic chip carrier. The component must then be nickel or gold plated for corrosion protection and aesthetics of the component. In the case of through-cavity PGA assemblies, the substrate is back-bonded to one of the surfaces of the copper-tungsten heat slug using a suitable substrate adhesive including: solder alloy, polymer composition, or clear glass Element. Subsequent bonding and assembly operations typically begin by wire bonding the substrate to pads on the chip carrier and then encapsulating the assembly by securing a cap to the reverse side of the ceramic substrate. Although this assembly method can form a sealed environment for integrated circuit devices, it requires a series of complex process steps to complete. Additionally, the plated copper-tungsten heat slug/sink itself is expensive. In order to provide a more price-competitive packaging solution, it is important to simplify assembly and processing steps and reduce component costs.

FIG. 1 illustrates a conventional method for attaching a copper-tungsten heat slug 30 to a through-cavity ceramic chip carrier 5 by soldering the heat slug 30 to a metallization strip 35 . The brazing alloy 40 of the radiating block is usually copper-silver alloy, the weight percentage of copper is 0-20%, and the rest is silver. Typically, the same solder alloy 40 is used to connect the input/output (I/O) pins 56 to the I/O pads 55 of the carrier 5 at the same time as the connection of the heat slug 30 . However, brazing materials with a lower melting range can also be used in industrial applications to connect heat sinks.

After the brazing step, the substrate (carrier 5) is usually plated with nickel and gold for corrosion protection and to improve the appearance of the assembly. The substrate 15 is fastened to the inner bottom plate of the heat dissipation block 30 through a substrate connecting material 10, which can be selected from any solder-based alloy, such as gold-tin alloy, gold-silicon alloy, gold-germanium alloy; or Polymer-based materials such as silver-filled cyanate ester adhesives or filled epoxy formulations. Alternatively, the substrate connecting material 10 may be a transparent glass composition, and its designed reflow temperature range is 350-450 degrees Celsius.

The secured substrate 15 has substrate pads 20 which are wire bonded to corresponding carrier pads 27 of the carrier 5 . Leads 25 connect substrate pads 20 and carrier pads 27 . The assembly is then packaged by reflowing a gold plated Kovar(R) cover 50 to the substrate with solder 45. The solder 45 is usually a gold-tin eutectic mixture (80% gold, 20% tin) with a melting point of 280 degrees Celsius. This type of module structure is quite airtight due to the dedicated use of impermeable (low diffusivity) sealing materials. However, the assembly must still be leak tested to ensure acceptable assembly leakage rates.

Figure 2 illustrates a preferred embodiment of the invention. It has been determined that using less costly materials to attach the heat slug 32 to the carrier 5 results in equivalent thermal efficiency and overall assembly reliability. This can be accomplished by using well known, commercially available polymeric binders 36 . Now the heat slug 32 can be made of alloys of aluminum, copper, aluminum silicon carbide (such as AlSiC), molybdenum. In this embodiment, the heat slug 32 is directly connected to the substrate 15 and the carrier 5 . The adhesive 36 is located between the heat slug 32 and the carrier 5 , and the substrate bonding material 10 is located between the heat slug 32 and the substrate 15 .

The typical thermal conductivity of traditional copper-tungsten heat sinks is between 150 and 200 watts per meter Kelvin (W/m-k), and its coefficient of thermal expansion (CET) is well matched with the ceramic carrier, generally 6.8 parts per million -7.5 per degree Celsius (ppm/°C) to reduce stress during brazing and under actual field conditions. Molybdenum-copper alloys have high thermal conductivity, typically ranging from 180 W/m-k for aluminum to 380 W/m-k for copper, but have a much higher coefficient of thermal expansion than copper-tungsten. The thermal expansion coefficient of aluminum is generally 22-23.5ppm/℃; the thermal expansion coefficient of copper is generally 16-17ppm/℃.

It is hoped that the stress of bonding between the carrier 5 and the heat sink 32 can be reduced. One method is to choose a composite material based on a mixture of aluminum silicon carbide (AlSiC), which has a thermal expansion coefficient that can meet the needs, and its range is, for example, from 7.3 To 14ppm/℃, and also has a high thermal conductivity of about 150W/m-k. It has been determined that molybdenum is also a suitable heat slug material, its thermal expansion coefficient is close to that of the carrier 5, and its thermal conductivity is about 100 W/m-K. Surface treatments vary with each material described, but can include anodizing over aluminum and AlSiC, or nickel plating over copper and molybdenum.

The choice of heat slug adhesive 36 depends on the thermally induced stresses and strains exhibited by the material under reliability testing and actual field conditions. Flexible silicone adhesives and flexible epoxy-based adhesives have been found to meet the thermal requirements. In addition, they form a mechanically fastened assembly capable of withstanding liquid-to-liquid thermal shock testing at temperatures ranging from -65°C to 150°C without significant loss of bond strength that would fail in shear testing.

Adhesives 36 are typically supplied as premixed frozen liquids, and their viscosity at room temperature is such that they can be easily applied with automatic application tools. The thermal conductivity of these materials can vary widely, from as low as 0.18W/m-k for flexible silicones to as high as 1.0-1.5W/m-k for filled epoxies. The best thermal efficiency can be obtained by ensuring a thin bonding wire between the heat slug 32 and the carrier 5 . The means used to form a thin bonded joint thickness is typically an assembly fixture that applies a spring load to both the bottom surface of the heat slug 32 and the top of the carrier 5 while bonding the bond in a suitable oven at 150-175 degrees Celsius. Allow the agent to cure for at least one hour.

Since the use of a polymeric adhesive to attach the heat slug renders the assembly non-hermetic, a newer fast curing die attach material 10 can be used to secure the substrate 15 to the heat slug 32 . The substrate 15 has pads 20 to which leads 25 are connected. These leads 25 are also connected to pads 27 on the carrier 5 . The non-hermetic assembly also simplifies the packaging of the assembly, since the cover 42 can be bonded to the carrier 5 with epoxy adhesive 38 . Since the assembly is non-sealed, the leak test is reduced to a bubble leak test, which is simpler than the test required when an impermeable sealing material is used and constructed to seal the assembly.

FIG. 3 illustrates another embodiment of the invention in which a heat sink 34 is attached to a non-through-cavity (planar) wire bond carrier 7 by means of a well-known commercially available adhesive 36 . The adhesive bond between the carrier 7 and the heat sink 34 eliminates the need for a high temperature brazing process as discussed in detail in FIG. 1 , thus reducing process complexity. The heat sink 34 can be made of a metal with high thermal conductivity, such as aluminum and copper. Finishes for these materials include anodizing the aluminum, which mimics the typical gold-plated color, and nickel-plating copper heat sinks for corrosion protection and aesthetics.

Because the attachment of the heat sink 34 is made by the adhesive 36 which cures at relatively low temperatures (<200 degrees Celsius), all other bonding and assembly steps can be performed first. In addition, similar assembly operations can be performed simultaneously to further reduce the overall assembly cycle time. Low-cost, adhesive-bonded heat sinks can be used in both hermetic and non-hermetic assemblies without compromising functional characteristics. The heat sink 34 is an effective base for attaching an external heat sink or cooling device required for reliable system operation.

Another aspect of the invention is to make the surface of the heat slug/heat sink away from the assembly (shown as the lower side in the figure, but actually the upper side) as flat as possible. This allows for better contact with the heat sink, which results in enhanced heat transfer and better overall performance.

Figure 4 illustrates yet another embodiment of the invention. The adhesively bonded heat sink 34 can be fabricated to mask adhesive fillets created during assembly if they are deemed to detract from the overall assembly appearance. The heat sink 34 has an overhanging lip 60 that extends beyond the base in the y-z plane, blocking the adhesive fillets so that they are not apparent when viewed from the x-direction after assembly.

While the invention has been described in conjunction with certain preferred embodiments, it is evident that various alternatives, modifications and alterations will be apparent to those skilled in the art. It is therefore intended that the appended claims cover any such alternatives, modifications and changes within the true scope and spirit of the invention.

Claims (11)

1. An electronic device comprising: at least one semiconductor chip; a semiconductor substrate having at least one substrate pad and first and second surfaces capable of forming electrical connections with one or more semiconductor chips; The heat transfer enhancing connector has an upper surface and a lower surface, the lower surface of which is in contact with the first surface of the substrate through a flexible adhesive, and the adhesive is suitable for connecting the heat transfer enhancing connector to the semiconductor substrate. 2. The device of claim 1, wherein the heat transfer enhancing connector is directly attached to the semiconductor substrate. 3. The device according to claim 1, wherein the flexible adhesive is one of silicone and flexible epoxy. 4. The device of claim 1, wherein the thermal conductivity of the flexible adhesive is between 0.17 W/m-K and 1.6 W/m-K. 5. The device of claim 1, wherein the flexible adhesive has a coefficient of thermal expansion between that of the heat transfer enhancing connector and that of the semiconductor substrate. 6. The device according to claim 1, wherein the thermal expansion coefficient of the heat transfer enhancing connector is larger than that of the semiconductor substrate and the semiconductor chip. 7. The device of claim 1, wherein the semiconductor chip and the heat transfer enhancing connector are physically connected by a layer of material selected from the group consisting of solder alloys, transparent glass, and polymeric substrate attachment materials select. 8. The device according to claim 1, wherein the chip is electrically connected to the substrate by a plurality of leads. 9. The device of claim 1, wherein the heat transfer enhancing connector extends beyond the boundary of the adhesive in the y-z plane such that the protruding adhesive fillet is masked when the assembly is viewed in the x direction. 10. A method for fabricating an electronic device includes attaching a heat transfer enhancing connector to a substrate with a flexible adhesive. 11. The method according to claim 10, wherein the flexible adhesive is one of silicone and flexible epoxy.

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