KR0168841B1 - Metal electronic package and the manufacture method - Google Patents

Abstract

The present invention relates to a metal package surrounding an electronic device to which the electronic device is attached to a cuttable die attach pad. The die attach pad is bonded to the base of the package with a thermally conductive medium. The base is bonded to the die attach pad and leadframe at the same time to reduce thermal degradation of the seal.

Description

Metal electronic package and manufacturing method thereof

1 is a cross-sectional view of a metal package known in the prior art.

2 is a schematic representation of a lead frame with an electronic device fixed in accordance with the present invention.

3 is a cross-sectional view of a metal package according to an embodiment of the present invention.

4 is a cross-sectional view of a metal package according to a second embodiment of the present invention.

* Explanation of symbols for main parts of the drawings

10: package 14: cover parts

16: lead frame 20: lead wire

22 electronic device 36 die attach pad

The present invention relates to a method of manufacturing a package surrounding an electronic device and to a package produced thereby. This package is characterized by improved resistance to thermal degradation and improved heat dissipation from the enclosed electronic device.

Several basic package designs are used in the electronics industry to accommodate integrated circuits. Integrated circuits, usually made of semiconductor material and silicon, must be protected from the external environment and electrically connected to external components. While meeting these demands, there is a need to minimize package manufacturing and assembly costs.

The electronics industry generally follows one of two policies: either the maximum protection of electronic devices using, for example, ceramic dual-in-line packages, or the minimum cost, for example, by encapsulated plastic packages. There are obvious disadvantages to both concepts.

The ceramic dual-in-line package (CERDIP) consists of two pieces of alumina or other ceramic glass bonded to the lead frame with suitable brazed glass to form a hermetic enclosure. The problem with CERDIP is the poor heat dissipation. To maximize the heat dissipation capacity of CERDIP, beryllium oxide is sometimes replaced by aluminum oxide. The thermal capability of the package is improved but the cost is also significantly increased. In general, CERDIP is usually used for high electronic requirements, for example computer or military purposes using state-of-the-art technology.

Plastic packages generally include an electronic device mounted on a lead frame that is typically encapsulated with a plastic resin such as epoxy. This plastic package is less expensive to manufacture than CERDIP and is more suitable for automated methods. Plastic packages are usually not hermetic and are not suitable for high electronic requirements. In addition, plastic packages are poor in heat dissipation.

Throughout this specification the term hermetic is used in accordance with document Military Specification 883B and is defined as a package cavity leak rate of less than 5 × 10 ⁻⁸ cm 3 / sec as measured using helium tracer gas.

To overcome the shortcomings of CERDIP and plastic packages, there is a metal package. The metal package consists of a lead frame disposed between the metal or metal alloy cover and the base part. The lead frame is bonded to the base and cover using an electrically insulating adhesive, in particular a brazing glass or a polymer adhesive. In general, solder glass is used when a hermetic package is required. Examples of metal packages using brazed glass can be found in US Pat. Nos. 4,524,238, 4,532,222, 4,542,259, 4,607,276 and 4,656,499 to Butt.

A problem associated with metal packages is the mismatch of thermal expansion coefficients between metal parts and conventional CERDIP brazed glass. Metal parts are copper or copper-based alloys and have a coefficient of thermal expansion (CTE) of 160 to 170 x 10 ^-7 ° C, while low temperature brazed glass is designed to match the CTE of the original aluminum oxide CERDIP package and is about 49 x 10 ^-7 ° C. Has a CTE of If the CTE mismatch is greater than 10%, the glass breaks due to the stress received during the thermal cycle, resulting in loss of airtightness.

The method of matching the coefficient of thermal expansion is based on the use of an improved seal, such as described in US Pat. No. 4,704,626 to Mahlikar et al., And also as described in US Pat. Nos. 4,752,521 and 4,801,488. Involves the use of appropriately selected fillers such as calcium fluoride to change the coefficients.

The hermetic metal package also has the advantage of good dissipation of heat generated by the electronic device during operation. The metal package includes a cavity and the surface of the electronic device is not subjected to stresses during thermal cycles such as those occurring in molded plastic packages.

Metal packages are also used to replace parts in plastic packages. As in the glass sealed package, a metal lead frame is disposed between the metal or metal alloy base and the cover part. The package is sealed with a polymer adhesive, usually epoxy. Examples of metal packages with polymeric adhesive seals are US Pat. Nos. 4,461,924, 4,480,262 and 4,594,770, both issued to Bert and US Pat. No. 4,105,861 to Hascoe.

Polymer adhesives are more flexible than sealed glass. That is, it is not necessary to match the thermal expansion coefficient of a package component with an adhesive agent. Thermally induced stress is absorbed and attenuated by the polymer adhesive. The difficulty with metal packages using polymer adhesives is described in Mil. Spec. No confidentiality in accordance with 883D.

In addition, the polymer is not thermally stable and tends to deteriorate during the post-curing heating cycle. One method of evaluating package seals is commonly referred to as the pressure cooker test. The test involves placing the sealed package in a pressure cooker at 121 ° C., 100% relative humidity, and 2100 gm / cm 2 (30 psi) pressure. The metal package sealed with the polymer adhesive was found to peel off in a relatively short time during this test. This test is an accelerated environmental test and indicates how well the package protects the electronic device from air and moisture.

According to the present invention, there is provided a lead frame comprising a die attach pad located at the center. Electronic devices, in particular silicon semiconductor chips, are bonded to die attach pads. The die attach pad is adhered to the package base and the base and cover parts are sealed to the lead frame. The package undergoes at least one less sealing cycle than prior art packages, which significantly improves the useful life of the seal.

In another embodiment of the present invention, holes are provided in the base part. The die attach pad is sealed over the hole. This embodiment enhances the removal of heat from the electronic device.

It is therefore an object of the present invention to provide a metal package sealed with a polymer that is more resistant to thermal degradation.

Another object of the present invention is to provide a metal package with a polymer seal which requires fewer assembly steps than conventional metal packages.

Another object of the present invention is to provide a metal package with improved heat dissipation characteristics.

The above and other objects and advantages of the present invention will become more apparent from the following description and the same reference numerals refer to the drawings showing like elements.

1 is a cross-sectional view of a conventional housing or packaging 10 that encapsulates an electronic device 22. A typical manufacturing method is to attach the lead frame 16 to the base part 12 using the first sealing material 28 which is the first bonding means. The first seal is usually a low melting braze glass or a polymer adhesive. Typical solder glasses used to seal electronic packages are lead boron glass matrices comprising one or more glass components. One example of brazed glass is a mixture of 75-85% PbO, 0.5-16% ZnO and 8-15% B ₂ O ₃ . Brazed glass with an appropriate coefficient of thermal expansion can be used. Typical polymer seals are thermoset epoxy such as novalac.

Typical sealing profiles for brazed glass are 10 minutes at about 410 ° to 430 ° C., and typical curing cycles for epoxy are about 30 to 120 minutes at about 150 ° to 170 ° C.

After the first seal is cured and the lead frame is secured in place, the electronic device 22, often a silicon-based semiconductor chip, is attached. The electronic device is directly attached to the base component 12 with die attach material 32 or to a buffer (not shown) that is fastened to the base component. The buffer system is disclosed in European Patent Application No. 86 102 059.2 issued September 17, 1986.

The coefficient of thermal expansion (CTE) of the base part or buffer determines the type of die attach material 32 used. If the CTE of the base component or buffer is within about 10% of the CTE of the electronic device, that is, within 49 x 10 ^-7 ° C, a hard solder such as 98% Au / 2% Si can be used. If the CTE of the base part or buffer is not close to the CTE of the electronic device, a more flexible die attach material is used. For example, a soft solder such as 95% Pb / 5% Sn or an electrically conductive polymer adhesive such as silver mixed epoxy may be used.

Heat treatment is required to activate the die attach material. If Au / Si solder is used, this heat treatment is generally about 5 seconds at about 420 ° C. A temperature of about 200 ° C. is used for about 20 seconds for Pb / Sn solder and a temperature of about 150 ° C. for about 30 minutes for mixed polymers. Regardless of the die attach material selected, the first seal 28 is subjected to a second heating and cooling cycle.

The lead wire 20 is then bonded to the electronic device 22 and the internal lead portion 18 to electrically connect the electronic device to the outside via the lead frame 16. The lead wire is usually a thin strip of gold or aluminum wire or copper foil, if a method known as tape automated bonding (TAB) is used. Bonding may be done by ultrasonic welding, thermal bonding or thermal pressing. The heat generated during some adhesion may be transferred through the lid by performing another heat cycle on the first seal 28. Typical adhesion temperature is 240 ° C. for about 5 seconds.

Next, a second sealing material 26, which is a second bonding means, is applied to the cover part 14. The first seal is not required as described in US Pat. No. 4,704,626, described above, but is typically selected to have the same composition as the first seal 28 to ensure chemical compatibility. The cover part 14 is then positioned adjacent to the lead frame 16 and glued to the side of the lead frame on the opposite side of the base part. A suitable sealing profile is needed to cure the second seal 26. The first seal member 28 is thermally cycled once more.

The prior art package is thus completed. If the first and second seals 28, 26 are brazed glass, the package is airtight and if it is epoxy the package is not airtight. It has been found that repeated thermal cycles of the first seal 28 adversely affect the integrity of the package. One embodiment of the present invention is to provide a housing product for an electronic device in which the repeating thermal cycle of the prior art is not performed on the first seal.

2 and 3 show a housing or package 10 assembly for an electronic device 22 according to the invention.

Referring to FIG. 2, a schematic of a lead frame 16 as shown in the art is shown. The lead frame 16 is typically made of an electrically conductive material. The lead frame consists of alloy 42 (iron-nickel alloy containing 58% iron and 42% nickel), KOVAR (iron-nickel-cobalt alloy containing 54% iron, 29% Ni and 17% Co), copper or copper-based alloy Is made. The lead frame may be plated or coated with a second material to improve adhesion to the seal and to improve corrosion resistance and appearance. Usually, lead frames for metal packages are copper-based, such as C724 as described in US Pat. Nos. 3,341,369 and 3,475,227 to Caule et al. Or C724, US Pat. No. 4,594,221 to Caron et al. Alloy. Copper-based alloys similar to alloys C638, C724, C7025 form a refractory oxide layer that facilitates bonding of the alloy to the braze glass. Alternatively, copper alloys, essentially pure copper with a certain amount of additives, are also used as lead frames because they have higher electrical conductivity than other copper alloys and higher strength than pure copper. Typical copper alloys used for lead frames are C194 (2.35% Fe, 0.03% P, 0.12% Zn and the remaining Cu).

The lead frame is made of an internal lead portion 18 for adhering to the electronic device 22 and an external lead portion 19 for electrically connecting the electronic device to the external device by insertion into a printed circuit board, for example. It consists of a finger 17. The lead frame also includes tie bars 34 for supporting the lead fingers 17 during the assembly operation. The tie bar 34 is cut so that the leads are electrically isolated from each other when the lead frame is in place. Some of the lead frames also include a centered die attach pad 36 that is connected to the tie bar by a die attach pad support 38. Lead frames with die attach pads are commonly used in plastic encapsulated packages to provide a place to place the chip and provide electrical contacts to the back of the chip. The use of die attach pads in plastic capsule sealed packages is described in US Pat. No. 4,697,203 to Sakai et al. The use of die attach pads with metal packages is described in US Pat. No. 4,656,499.

According to the present invention, electronic device 22, which is a semiconductor chip usually made of silicon, is attached to die attach pad 36 by die attach material 32, although other semiconductor materials such as germanium or gallium arsenide may also be used. . Depending on the composition of the lead frame 16 and the corresponding coefficient of thermal expansion of the lead frame, the die attach material may be lead-based eutectic solder such as gold-based Au / Si or 95% Pb / 5% Sn, or lead-based 60% Pb / 40. It may be a soft solder such as% Sn or 92.5% Pb / 5% Sn / 2.5% Ag, or a polymer adhesive such as a thermoset epoxy. The list of die attach materials is described by way of example and not by way of limitation, any suitable die attach material may be used within the scope of the present invention. When epoxy die attach material is used, it may be conductive or insulating depending on whether a backside electrical contact is required. Epoxy can be made conductive by mixing conductive metal (eg silver) powder with epoxy.

The electronic device 22 is then connected to the internal lead 18 through the lead wire 20. These wires are usually made of thin, about 0.025 mm (0.001 inch) diameter gold or aluminum strands. Alternatively, a thin strip of copper thin film is used for TAB adhesion as described in US Pat. No. 4,330,790 to Burns. The wire 20 is bonded by conventional wire bonding techniques to the place of adhesion on the electrical working surface of the electronic device 22. Both ends of the wire are bonded to the inner lead portion 18 of the lead frame 16 to form an electrical connection between the electronic device 22 and the outer lead portion 19.

A lead frame 16 comprising a die attached and wire bonded electronic device 22 is ready for assembly according to the present invention. Next, a description will be given with reference to FIG. 3 showing a package assembled according to the present invention.

In a first method embodiment, a base part 12 is provided which comprises a first seal 28 and a third seal 40 which is a third bonding means. The first seal 28 is selected from the group consisting of non-conductive braze glass, ceramics, thermosetting polymers and thermoplastic polymers. The third sealant 40 is selected from the group consisting of brazed glass, ceramics, metal alloy solders, thermosetting polymers and thermoplastic polymers. Depending on whether the back side of the chip is electrically connected to or insulated from the package base, the third sealing material 40 becomes an insulator or a conductor. The third seal is usually made electrically conductive by mixing the conductive metal powder. Carbon fillers may be used. The third sealant 40 is further characterized by high thermal conductivity. The third sealant 40 suitable for the present invention is a soft solder such as epoxy or Pb / Sn mixed with silver. The lead frame 16 is disposed adjacent to the sealing material containing surface of the base part. Frequently the thickness of the first seal 28 is thicker than the thickness of the third seal 40, the difference being compensated for by downsetting the die attach pad support 38. The down setting also helps maintain electrical insulation between the inner lead and the die attach pad support. Alternatively, after curing the first and third sealants, the die attach pad support 38 may be cut.

The lead frame is bonded to the base part by appropriate curing cycles for the first and third seals. An advantage of this embodiment over the embodiments described below is that the package during curing is opened so that chlorine by-products such as chloride ions from the epoxy escape into the atmosphere.

The cover part 14, which does not need to be the same as the first seal but comprises a non-conductive second seal 26 selected from the same group as the first seal 28, is disposed adjacent to the non-adhesive side of the lead frame. The cover part is sealed to the lead frame by a suitable heat cycle to form a housing for the electronic device.

An improvement of the present invention over the prior art is that the first seal is subjected to only one additional heat cycle, reducing the thermal degradation.

A second embodiment of the invention is that the first and second seals 28, 26 are sealed at the same time. The lead frame 16 with the electronic device 22 already die attached and wire bonded is disposed between the base part 12 and the cover part 14. The first seal 28 is disposed between the first side of the lead frame and the base part. The third seal 40 is disposed between the die attach pad and the base part. The second seal 26 is disposed between the cover part and the second side of the lead frame. Appropriate heat cycles are selected to cure the three seals simultaneously.

An advantage of the second embodiment of the present invention over the prior art is that only one curing cycle is performed on the first seal and no excessive heat cycle is performed. The structure of the electronic package made in accordance with the present invention is similar to the metal package hermetically sealed with solder glass described in US Pat. No. 4,656,499 to Bert under the enclosed semiconductor casing. However, the assembling method of the butts is completely different, and no improvement is required to reduce the thermal degradation of the sealing material.

Another embodiment of the invention is shown in FIG. The housing for the electronic device is assembled using any of the embodiments described above. Holes 42 are provided in the base part 12. This hole allows the reaction by-products generated during the closure cycle to escape. The first seal 28, the second seal 26 and the third seal 40 can be cured at the same time and the accumulation remaining in the enclosure 30 of the housing is less than in the embodiment described above. The third sealing material 40 is a ring seal surrounding the hole 42. The seal 40 adheres the die attach pad to the base 12.

A second improvement realized by the holes 42 in the base part 12 is the proximity to the external influence of the chip. When the electronic device 22 is activated, electricity passes through the device. Some of the current is converted to heat due to the internal resistance of the semiconductor circuit. This heat reduces the operating life of the semiconductor chip and therefore is preferably removed. Conventional means of removing the generated heat is a metal radiator disposed below the chip to supply forced air or forced cooling fluid. These radiator means can be found in an article entitled, `` Ernel R. Winkler's future package heat transfer from the April 1985 edition of Electronics Packaging and Production will adversely affect PCB design.

The aperture 42 allows forced air and forced fluid cooling means to be located in proximity to the electronic device 22. The use of cooling materials with large heat capacities, for example helium gas, results in improved cooling. Improving this cooling is becoming increasingly important because the density of electronic circuits on semiconductor devices increases, resulting in more heat generated during chip operation.

Solids having a larger heat capacity than liquid, such as copper or silver, may be inserted into the holes 42 or may be used as a conduit (heat radiator) for transferring heat from the electronic device.

Another embodiment of the invention comprises a second hole 44 in the cover part 14. This hole 44 is the outlet of the reaction byproduct generated during the curing cycle. After the curing cycle is finished, an inert thermally conductive gas, such as helium, or a thermally conductive or electrically nonconductive liquid or powder is optionally added to the package cavity 30 to provide additional heat dissipation means for the electronic device 22. It may be injected selectively. The hole is then sealed by applying a suitable patch, for example solder drops, to the cover part outer surface 46. Alternatively, the aperture 44 may be sealed by suitable means such as plastic. The holes 42 and 44 may be used alone or in combination as necessary.

Although the present invention has been described with respect to a metal package, the above technique is suitable for a ceramic package or a plastic package such as CERDIP.

It can be seen that the electronic package assembling means and the products of these assembling means according to the present invention satisfy all the above-mentioned objects, means and advantages. While the present invention has been described with respect to specific embodiments thereof, it is apparent from the above description that there are many alternatives, modifications and variations to those skilled in the art. Accordingly, all such alternatives, modifications and variations are intended to fall within the spirit and scope of the following claims.

Claims (37)

A method of manufacturing a package (10) comprising an enclosure (30) surrounding an electronic device (22), the method comprising: a plurality of circumferential die attach pads (36) having first and second faces Providing an electrically conductive lead frame 16 consisting of two lead fingers 17; Providing the first, second and third bonding means 28, 26, 40, providing the base part 12, providing the cover part 14, and providing the base part 12. ) Attaching the base part 12 to the die attach pad 36 with the third bonding means 40 at the same time as the first bonding means 28. Wow; Bonding the cover part (14) to the second side of the lead frame (16) with the second bonding means (26); And said electronic device (22) is bonded to said die attach pad (36) and electrically connected to said lead finger (17). The method according to claim 1, wherein the die attach pad (36) is cut from the lead frame (16) prior to the step of adhering the cover part (14) to the lead frame (16). 3. A method according to claim 2, wherein the third bonding means (40) is thermally conductive. 4. A method according to claim 3, wherein the third bonding means (40) is an epoxy mixed with silver. Method according to claim 1, characterized in that the first and second bonding means (28, 26) are sealing glass or polymer adhesive. Method according to claim 5, characterized in that the first and second gluing means (28, 26) are identical. 7. A method according to claim 6, wherein the first and second bonding means (28, 26) are thermoset epoxy. 8. The cover part (14) according to claim 7, wherein the base part (12) is bonded to the first surface of the lead frame (16) and the die attach pad (36) while the second bonding means (26) Bonded to the second surface of the lead frame (16). 9. The method of claim 8, further comprising providing a first hole 42 in the base part 12 prior to adhering the die attach pad 36, wherein the first hole 42 is provided to the die. Smaller than the attachment pad (36), the die attach pad (36) seals the first hole (42). 10. The method according to claim 9, wherein the third bonding means (40) is ring-shaped and is disposed around the edge of the first hole (42) and between the base part (12) and the die attach pad (36). Package manufacturing method to use. A second hole (44) is provided in said cover part (14) and said second hole (44) after said cover part (14) is sealed with respect to the second face of said lead frame (16). ) Is sealed. 12. A method according to claim 11, wherein a thermally conductive medium is introduced into the enclosure (30) through the second hole (44) before sealing the second hole (44). 13. The method of claim 12, wherein the thermally conductive medium is selected from an electrically nonconductive gas, liquid or powder. 12. The method of claim 11, wherein the second hole (44) is sealed with an epoxy or solder plug. 15. The method of claim 14, wherein the second hole (44) is sealed with a solder plug made of an alloy of lead and tin. A package comprising an enclosure (30) surrounding an electronic device (22), comprising: a base component (12) comprising a first hole (42); A cover part 14; A first and second opposing side surfaces disposed between the base part 12 and the cover part 14, and positioned to seal the first hole 42 and electrically connected to the lead finger 17. A lead frame (16) comprising a plurality of lead fingers (17) arranged around a die attach pad (36) disposed centrally to support the electronic device (22); First bonding means (28) for sealing the base part (12) to the first side of the lead frame (16); Second bonding means 26 for sealing the cover part 14 to the second side of the lead frame 16 and a third for sealing the die attach pad 36 to the base part 12. Package comprising adhesive means (40). 17. The package according to claim 16, wherein the third bonding means (40) surrounds the edge of the first hole (42) and is disposed between the base component (12) and the die attach pad (36). . 18. The package according to claim 17, wherein the third bonding means (40) is thermally conductive and is selected from the group consisting of brazed glass, thermosetting polymer adhesives and thermoplastic polymer adhesives. 19. The package according to claim 18, wherein the third bonding means (40) is an epoxy mixed with silver. 20. The package according to claim 19, wherein the first and second adhesive means (28, 26) are electrically nonconductive and are selected from the group consisting of brazed glass, thermoset polymer adhesives and thermoplastic polymer adhesives. 21. A package according to claim 20, wherein the first and second bonding means (28, 26) are identical. 22. A package according to claim 21, wherein the first and second bonding means (28, 26) are thermoset epoxy. A package comprising an enclosure (30) surrounding an electronic device (22), comprising: a base component (12); A cover part (14) comprising a second hole (44) sealed with a plug; A first and second opposing side surfaces disposed between the base component 12 and the cover component 14 and disposed about a die attach pad 36 disposed centrally and electrically connected to the lead finger 17. A lead frame (16) comprising a plurality of lead fingers (17) adapted to support said electronic device (22); First bonding means (28) for sealing the base part (12) to the first side of the lead frame (16); Second bonding means (26) for sealing the cover part (14) to the second side of the lead frame (16); And third adhering means (40) for sealing said die attach pad (36) to said base part (12). 24. The package according to claim 23, wherein said third bonding means (40) is thermally conductive and is selected from the group consisting of brazed glass, thermosetting polymer adhesives and thermoplastic polymer adhesives. 25. The package according to claim 24, wherein the first and second adhesive means (28, 26) are electrically nonconductive and are selected from the group consisting of brazed glass, thermosetting polymer adhesives and thermoplastic polymer adhesives. 26. A package according to claim 25, wherein the first and second bonding means (28, 26) are identical. 27. The package according to claim 26, wherein the enclosure (30) is filled with a thermally conductive medium. 28. The package of Claim 27, wherein the thermally conductive medium is electrically nonconductive and is selected from the group consisting of gas, liquid and powder. 29. The package of claim 28, wherein said thermally conductive medium is helium. A package comprising an enclosure (30) surrounding an electronic device (22), comprising: a base part (12) having a first hole (42); A cover part 14 having a second hole 44 sealed with a plug; The first and second opposing side surfaces disposed between the base component 12 and the cover component 14 and arranged to seal the first hole 42 and electrically connected to the lead finger 17. A lead frame (16) comprising a plurality of lead fingers (17) disposed around a centrally disposed die attach pad (36) adapted to support the electronic device (22); First bonding means 28 for sealing the base part 12 to the first side of the lead frame 16 and for sealing the cover part 14 to the second side of the lead frame 16. A second bonding means 26 for the purpose and a third bonding means 40 disposed around the edge of the first hole 42 and disposed between the base part 12 and the die attach pad 36. Package characterized in that. 31. The package according to claim 30, wherein the third bonding means (40) is thermally conductive and is selected from the group consisting of brazed glass, thermosetting polymer adhesives and thermoplastic polymer adhesives. 32. A package according to claim 31, wherein the first and second adhesive means (28, 26) are electrically nonconductive and are selected from the group consisting of brazed glass, thermoset polymer adhesives and thermoplastic polymer adhesives. 33. A package according to claim 32, wherein the first and second bonding means (28, 26) are identical. 31. A package according to claim 30, wherein the plug that seals the second hole (44) is solder or polymer adhesive. 35. The package of Claim 34, wherein the plug is solder consisting of lead and tin. 31. A package according to claim 30, wherein the enclosure (30) is filled with a thermally conductive electrical insulating medium. 37. The package of Claim 36, wherein the enclosure is filled with helium.

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