Method of forming an electronic package with a solder seal

• US 6,119,920 A
• Filed: 12/20/1996
• Issued: 09/19/2000
• Est. Priority Date: 12/20/1996
• Status: Expired due to Fees

• Current Assignee: RF Monolithics, Inc. (Murata Manufacturing Co Limited)
• Original Assignee: RF Monolithics, Inc. (Murata Manufacturing Co Limited)
• Inventors: Johnson, Paul O., Guthrie, Frank E.
• Primary Examiner(s): Ryan, Patrick
• Assistant Examiner(s): ELVE, MARIA ALEXANDRA
• Application Number:
  US08/770,268
• Time in Patent Office: 1,369 Days
• Field of Search: 228/124.6, 228/123.1, 228/56.3, 228/245, 228/214, 228/219, 228/220, 228/234.2
• US Class Current: 228/123.1
• CPC Class Codes: B23K 1/008 Soldering within a furnace B23K1/012 takes precedence
• CPC Class Codes: H01L 23/10 characterised by the material or arrangement of seals between parts, e.g. between cap and base of the container or between leads and walls of the container
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
• CPC Class Codes: H01L 2924/01079 Gold [Au]
• CPC Class Codes: H01L 2924/09701 Low temperature co-fired ceramic [LTCC]

Abstract

A method of attaching a lid with at least a metallic surface thereon to a substrate to enclose a surface acoustic wave electronic circuit using a fluxless solder preform having an undesired oxide surface and associated with the metallic surface on the lid and in superimposed and abutting relationship with a gold seal ring surrounding the electronic circuit to form a unit that is heated in a furnace having a hydrogen atmosphere such that the hydrogen removes any undesired oxide surfaces from the solder preform and the metallic surface on the lid by combining with the oxygen therein to form moisture so as to remove any oxide surfaces from the solder preform and the lid metallic surface and enable a complete bonding of the fluxless solder preform with the gold seal ring and the lid metallic surface thereby forming a hermetically sealed package.

Claims

1. A method of simultaneously attaching a plurality of solderable lids to a like plurality of surface acoustic wave circuits on a common substrate to form a plurality of SAW circuit packages that can be separated into individual packages, the method comprising the steps of:

forming a plurality of SAW circuits on a substrate in a predetermined pattern array, each of said SAW circuits having a solderable seal ring surrounding said circuit;

arranging a like plurality of said solderable lids in a lid fixture in a corresponding predetermined pattern array, each of said solderable lids having at least a solderable peripheral flange;

providing a like plurality of solder preforms each having an undesired oxide layer thereon and that has a shape for mating with corresponding ones of said solderable seal rings and said solderable peripheral lid flange;

placing said substrate in superimposed relationship with said lid fixture so as to arrange each of said fluxless solder preforms in superimposed abutting relationship with a corresponding one of said solderable seal rings and solderable peripheral lid flanges; and

heating said superimposed substrate and lid fixture in a furnace having a reducing atmosphere until any undesired oxide layer in said preform, seal ring, or solderable lid flange combines with said reducing atmosphere to create substantially oxide-free bonding surfaces and said solder melts and bonds to said solderable seal ring and solderable lid to sealingly attach said solderable lids over and around corresponding ones of said SAW circuits on said substrate.

2. A method as in claim 1 further comprising of the step of attaching each of said fluxless solder preforms to said periphery of a corresponding one of said lids in a fixed relationship.

3. A method as in claim 1 further comprising of the step of heating said superimposed substrate and lid fixture in said furnace in a hydrogen reducing atmosphere to form said oxide-free bonding surfaces.

4. A method as in claim 1 further comprising of the steps of:

forming a ground terminal on each of said surface acoustic wave circuits; and

electrically connecting said ground terminal on each of said SAW circuits to a corresponding one of said solderable seal rings such that when said lid is soldered to said solderable seal ring, a Faraday shielded package is formed.

5. A method as in claim 1 wherein said step of heating comprises the step of heating said superimposed substrate and said lid fixture to a temperature greater than approximately 350°

C.

6. A method as in claim 5 further including a step of heating said superimposed substrate and lid fixture in said furnace at 385°

±

20°

C. for 10±

6 minutes to react said hydrogen with said oxygen in said undesired oxide surfaces according to the equation

space="preserve" listing-type="equation">MxOy+yH.sub.2 →

xM+yH.sub.2 O

thereby removing the oxide from said fluxless solder preform and solderable lid and enabling the fluxless solder preform to adhere to said solderable seal ring and solderable lid.

7. A method as in claim 6 further including the step of adhering said melted solder to said solderable seal ring and solderable lid to form a hermetically sealed package.

8. A method as in claim 1 further including the step of forming said solderable seal ring from gold alloy.

9. A method as in claim 1 further comprising the step of forming said hydrogen atmosphere of a minimum of 10% hydrogen and 90% nitrogen.

18. A SAW circuit package formed in accordance with the method set forth in claim 1.

10. A method of hermetically enclosing a SAW circuit comprising the steps of:

forming a SAW circuit on a ceramic substrate;

surrounding the SAW circuit with a gold seal ring on said ceramic substrate;

associating a fluxless solder preform with a solderable lid used to cover said SAW circuit;

at least said fluxless solder preform having an undesired oxide coating thereon and a shape for mating with said gold seal ring;

placing said solderable lid over said SAW circuit with said solder preform in superimposed abutting relationship with said gold seal ring to form an electronic circuit package; and

heating said electronic circuit package in a furnace having a reducing atmosphere until any undesired oxide coatings combine with said reducing atmosphere to create substantially oxide-free bonding surfaces, said solder melting and attaching to said gold seal ring to sealingly attach said solderable lid to said substrate and form a hermetically sealed electronic circuit package.

11. A method as in claim 10 further comprising the step of attaching said fluxless solder preform to said solderable lid in a fixed relationship prior to placing said solderable lid on said substrate.

12. A method as in claim 10 further comprising the step of heating said electronic circuit package in said furnace in a hydrogen atmosphere to form said oxide-free bonding surfaces.

13. A method as in claim 10 further comprising the step of forming said solderable lid entirely of metal.

14. A method as in claim 13 further comprising the steps of:

forming a ground terminal on said SAW circuit; and

electrically connecting said ground terminal on said SAW circuit to said gold seal ring such that when said metal lid is soldered to said gold seal ring a Faraday shield is formed for said circuit package.

17. A method as in claim 10 further comprising the steps of:

forming said solderable lid of a non-metallic material;

forming a solderable ring on said peripheral flange to make said lid a solderable lid; and

attaching each of said fluxless solder preforms to said peripheral flange of said lid.

19. A SAW circuit hermetically sealed in accordance with the method set forth in claim 10.

15. A method of hermetically enclosing a SAW circuit comprising the steps of:

forming a SAW circuit on a ceramic substrate;

surrounding the SAW circuit with a gold seal ring on said ceramic substrate;

associating a fluxless solder preform with a solderable lid used to cover said SAW circuit;

at least said fluxless solder preform having an undesired oxide coating thereon and a shape for mating with said gold seal ring;

placing said solderable lid over said SAW circuit with said solder preform in superimposed abutting relationship with said gold seal ring to form an electronic circuit package; and

heating said electronic circuit package in a furnace having a reducing atmosphere to a temperature greater than approximately 350°

C. until any undesired oxide coatings combine with said reducing atmosphere to create substantially oxide-free bonding surfaces, said solder melting and attaching to said gold seal ring to sealingly attach said solderable lid to said substrate and form a hermetically sealed electronic circuit package.

16. A method as in claim 15 further including the step of heating said circuit package in said furnace at 385°

±

20°

C. for 10±

6 minutes to react said hydrogen with oxygen in said undesirable oxide according to the equation

space="preserve" listing-type="equation">MxOy+yH.sub.2 →

xM+yH.sub.2 O

thereby removing the undesirable oxide and enabling the fluxless solder to adhere to said solderable lid and gold seal ring.

20. A SAW circuit hermetically sealed in accordance with the method set forth in claim 15.