GB2380613A

GB2380613A - Package for electronic components and method for forming such a package

Info

Publication number: GB2380613A
Application number: GB0123807A
Authority: GB
Inventors: Torsten Hauck; Jorg Strogies; Anton Kolbeck; Christina Brigitte Bohm
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-10-04
Filing date: 2001-10-04
Publication date: 2003-04-09
Also published as: AU2002337129A1; WO2003032391A2; WO2003032391A3; GB0123807D0

Abstract

A package (30) for electronic components comprises substantially sheet-like elements in a stack, the elements comprising; a heat sink (32), a flexible substrate (34) carrying at least one conductor layer (36) and at least one electronic component (40) mounted on a component attach layer (38), a gap filler (42) and a lid (44); wherein the gap filler applies a pressure to the flexible substrate such that it is in mechanical contact with the heatsink when the lid and heatsink are locked together by the frame (45). The package requires no adhesive to remain together and can easily be combined with ribbon cables and is easy to disassemble.

Description

METHOD FOR FORMING A PACKAGE FOR ELECTRONIC COMPONENTS AND PACKAGE FOR ELECTRONIC COMPONENTS Field of the Invention This invention relates to a method for forming packages for electronic components and packages for electronic components. More particularly, this invention relates to packages comprising a substrate with power semiconductors.

Background of the Invention Electronic components are contained in packaging to support and protect the components, to provide heat dissipation and to provide external mechanical and electrical connectors to the components.

Typically, a package comprises a single substrate on which are mounted the electronic components and a lead frame having leads which form the external connectors to the components. Contact pads are formed on the substrate and the lead frame is attached to the contact pads using solder. Conductive lines extend over the substrate for providing electrical connections between the components and/or the lead frame. The type of connection used to connect the conductive lines to the components depends on the components. Prominent interconnect technologies are flip chip bonding and wire bonding for Direct-ChipAttach (DCA) and Surface-Mount-Technology (SMT) for discrete component attachment. For example, for surface mount and flip chip components, contact pads on the devices are soldered directly to contact areas of the conductive lines on the substrate.

Several electronic components can be arranged in a single housing to provide a system in a package (SIP). Such a SIP wherein the electronic components are mounted on a flexible multi-chip module (MCM) forms a compliant pressure package (CPP).

With the drive for electronic devices with simpler connectivity, there is a need to achieve simpler and more cost effective assembly of a package.

Especially in packages comprising power semiconductors heat must be conducted from a chip mounted on a substrate to the outside. According to the

prior art, this is achieved by laminating the flexible substrate on a heat sink that forms a part of the housing.

US patent no. 6,154, 369 discloses a semiconductor package arrangement based on an insulated metal heat sink, which consists of a heat dissipater, a dielectric material, and a conductive layer laminated thereto. A semiconductor chip is attached to the insulated metal heat sink. The generated heat is mainly transferred through the bottom surface of the semiconductor chip and the dielectric layer into the metal heat sink. The assembly concept doesn't utilise all surfaces of the semiconductor chips for the heat transfer. The actual device consists of a relative large number of subassemblies with many process steps which influences reliability and yield negatively.

There is therefore a need for a microelectronic assembly solution for power semiconductor packaging that compensates for high power loss, allows multi-chip assembly and integrates connector functions.

Summary of the Invention In accordance with a first aspect of the present invention there is provided a package for electronic components and a method for forming a package for electronic components as recited in the independent claims of the accompanying claims.

Brief Description of the Drawings A package for electronic components and a method for forming a package for electronic components in accordance with a preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: FIG. 1 is a cross-sectional schematic diagram of a stack of sheet-like elements for a package in accordance with the present invention; FIG. 2 is a schematic cross-sectional diagram of a part of a package in accordance with the present invention; FIG. 3 is a perspective schematic diagram of part of a package in accordance with the present invention;

FIG. 4 is a perspective schematic diagram of a package including the part shown in FIG. 3 in accordance with the present invention; FIG. 5 is a side cross-sectional schematic diagram of another embodiment of a package in accordance with the present invention; and FIG. 6 is a front cross-sectional schematic diagram of the package of FIG.

5.

Detailed Description of the Drawings Referring firstly to FIG. 1, a package in accordance with a preferred embodiment of the invention comprises substantially sheet-like elements in a stack 10: a heat sink 12, a flexible substrate 14 carrying conductor layer 16 and a component attach layer 18 thereupon with electronic component 20. The electronic component 20 is attached to the flexible substrate 14 by a direct chip attach technology such as semiconductor flip chip or a semiconductor wire bond.

The flexible substrate can carry surface mount components. The component 20 is followed by a gap filler 22 and a lid 24 completing the stack. The flexible substrate 14 and the heat sink 12 are pressed to each other such that both are in mechanical contact. No thermally conductive interface material, such as an adhesive or paste, are needed to transfer heat from the flexible substrate to the heat sink because of the direct mechanical contact. The direct mechanical contact between the flexible substrate 14 and the heat sink 12 is maintained by a permanent pressure on all sheets generated by the gap filler 22 being sufficiently compressed between its surrounding layers, in this stack component 20 and lid 24. The outer layers of the stack 10, i. e. heat sink 12 and lid 24 are locked to each other. Means for such locking are described later. Thus the stack 10 needs only to be compressed once and the compressed gap filler 22 maintains the pressure.

The gap filler and the flexible substrate and also the gap filler and the lid are in direct mechanical contact. The direct contact without adhesive in between enables a good heat transfer and allows a mechanical assembly of the package without adhesive. The gap filler is preferably thermally conductive.

The components may be any type of electronic component, for example, active semiconductor devices such as transistors, microcontrollers, Digital Signal

Processors (DSPs) etc. and/or passive devices such as discrete resistors, capacitors or inductors. The layers fixed to the flexible substrate 14, namely conductor layer 16 and component attach layer 18 and electronic component 20 need not be complete in the area, i. e. individual electronic components can be distant from each other, the space between them needing thus no component attach layer 18, or the conductor layer 16 needs not to cover the whole area.

Nevertheless, the gap filler 22 will fill such gaps and provide a pressure to press the flexible substrate 14 and the heat sink 12 together.

Referring now also to FIG. 2, which shows in part a CPP, package 30 comprises a stack such as stack 10 of FIG. 1 with a heat sink 32, a flexible substrate 34 carrying conductor layer 36 and a component attach layer 38 thereupon with electronic component 40. The electronic component 40 is followed by a gap filler 42 and a lid 44 that is a second heat sink here completing the stack. Component attach layer 38 can comprise underfiller and solder material. A non-conductive frame 45 locks heat sink 32 and a lid 44 to each other. The locking is not shown here but those skilled in the art know how to design one, e. g. a snap-in feature on the non-conductive frame 45. The nonconductive frame 45 further comprises connector pins 46 soldered to the conductor layer 36 upon the flexible substrate 34. The gap filler 42 fills gaps around the electronic component 40 or components and provides a pressure to press the flexible substrate 34 and the heat sink 32 together.

FIGs. 3 and 4 show the assembling advantages of forming a package 50 according to the invention in different stages. After providing the sheet-like elements a heat sink, a flexible substrate carrying at least one conductor layer and at least one electronic component, a gap filler, and a lid, the sheet-like elements are arranged in a stack and a pressure is applied to the stack such that the flexible substrate and the heat sink are in mechanical contact.

A flexible substrate 52 carries a conductor layer 54, to which semiconductor chips 56 and surface mount components 58 are soldered, forming a soft printed circuit board (PCB). The substrate 52 is positioned to a nonconductive frame 60, here a pre-molded metal leadframe having connector pins 61 that are soldered to the flexible PCB. These parts establish the complete functionality of the system including the mechanical connector function and can be manufactured as subassembly 62 in a common reflow process.

Now, the sheet-like elements are arranged to form a stack. A thermally conductive gap filler 64 (potting or foam) is applied onto the sub assembly. A metal heat sink 66 is arranged at the bottom side of the assembled flexible PCB subassembly 62, and lid 68 being here a second heat sink is arranged at top side of the gap filler 64. Now pressure is applied to the stack and a mechanical clip 70 clamped over the stack. The result is a CPP comprising parts in a fixed arrangement, which is easily producible without the need of an adhesive. A prominent application for such a package is a plug connector where a part of the frame is formed as plug.

FIGs. 5 and 6 show a package 70 according to a specialized embodiment of the invention. The package 70 comprises a heat sink 72, a flexible substrate 74 carrying conductor layer 76 and a component attach layer 78 thereupon with electronic component 80 that is shown as a single component for simplicity. The electronic component 80 is followed by a gap filler 82 and a lid 84, here a second heat sink, completing the stack. One special feature is that the heat sink 72 and the lid 84 are matching to form a complete metal housing. Another special feature is that the flexible substrate 74 is extended to form a ribbon cable extending outside of the housing through openings 86. Those skilled in the art will know how to seal the conductor layer 76 and the housing at the opening against moisture or electricity.

In summary, the present invention provides a package having a high thermal performance at a very cost effective assembly procedure, few manufacturing steps, a smaller number of assembly components are required, no adhesive is required, and the concept avoids mechanical constraints caused by thermal mismatch between heat sink metal and PCB. As a consequence the thermally induced stresses in the semiconductor chips are reduced. Therefore the soft contact between metal heat sink and PCB provides improved mechanical reliability of the device. The device is easy to disassemble as there is no process step required in order to separate the substrate from the heatsink. Therefore the device components are fully recyclable. The introduced concept is extendible to multiple substrate assembly.

Claims

Claims 1. A package for electronic components comprising substantially sheet-like elements in a stack: a heat sink; a flexible substrate carrying at least one conductor layer and at least one electronic component; a gap filler ; and a lid ; wherein the flexible substrate and the heat sink are pressed to each other such that both are in mechanical contact.
2. The package of claim 1, wherein the gap filler and the lid are in mechanical contact.
3. The package of claim 1 or 2, wherein the gap filler and the flexible substrate are in mechanical contact.
4. The package of claim 1,2, or 3, further comprising a non-conductive frame supporting the stack of the sheet-like elements and carrying connector pins coupled to the conductor layer.
5. The package according to any of claims 1-4, wherein the lid is a second heat sink.
6. The package according to any of claims 1-5, wherein the gap filler is of a potting or foam material.
7. The package according to any of claims 1-6, further comprising a mechanical clip applying a pressure to the stack.
8. The package according to any of claims 1-7, further comprising a snap-in mechanism adapted to lock the lid and the heat sink to each other.

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9. The package according to any of claims 1-8, wherein the gap filler is thermally conductive.
10. The package according to any of claims 1-9, wherein the flexible substrate is carrying the at least one electronic component in a direct chip attach technology.
11. The package according to any of claims 1-10, wherein the gap filler is compressed to maintain pressure to the flexible substrate.
12. The package according to any of claims 1-11, wherein the a flexible substrate is carrying surface mount components.
13. The package according to any of claims 1-12, wherein a mechanical assembly of the heat sink and the flexible substrate is without adhesive.
14. The package according to any of claims 1-13, wherein a mechanical assembly of the package is without adhesive.
15. The package according to any of claims 1-14, wherein the flexible substrate is extended to form a ribbon cable.
16. The package according to any of claims 1-15, wherein the substantially sheet-like lid and heat sink are extended to cooperatively form a housing.
17. A plug connector comprising the package according to any of claims 1-16.

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18. A method for forming a package for electronic components with substantially sheet-like elements, comprising the steps of: providing the sheet-like elements a heat sink, a flexible substrate carrying at least one conductor layer and at least one electronic component, a gap filler, and a lid ; arranging the sheet-like elements in a stack; applying a pressure to the stack such that the flexible substrate and the heat sink are in mechanical contact.
19. The method of claim 18, wherein the gap filler and the lid are in mechanical contact.
20. The method of claim 18 or 19, wherein the gap filler and the flexible substrate are in mechanical contact.
21. The method according to any of claims 18-20, further comprising the step mounting a non-conductive frame to the stack of the sheet-like elements, the frame carrying connector pins coupled to the conductor layer.
22. The method according to any of claims 18-21, wherein the lid is a second heat sink.
23. The method according to any of claims 18-22, wherein the gap filler is of a potting or foam material.
24. The method according to any of claims 18-23, further comprising the step applying a pressure to the stack by a mechanical clip.
25. The method according to any of claims 18-24, further comprising the step locking the lid and the heat sink to each other by a snap-in mechanism adapted therefore.

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26. The method according to any of claims 18-25, further comprising the step pre-compressing the gap filler to provide pressure to the flexible substrate.
27. The method according to any of claims 18-26, wherein the gap filler is thermally conductive.
28. The method according to any of claims 18-27, wherein the a flexible substrate is carrying the at least one electronic component in a direct chip attach technology.
29. The method according to any of claims 18-28, wherein the flexible substrate is carrying surface mount components.
30. The method according to any of claims 18-29, wherein a mechanical assembly of the heat sink and the flexible substrate is without adhesive.
31. The method according to any of claims 18-30, wherein a mechanical assembly of the package is without adhesive.
32. The method according to any of claims 18-31, wherein the flexible substrate is extended to form a ribbon cable.
33. The method according to any of claims 18-32, wherein the substantially sheet-like lid and heat sink are extended to cooperatively form a housing.
34. The method according to any of claims 18-33, wherein the package is a plug connector.