Fine pitch solder deposits on printed circuit board process and product

• US 5,492,266 A
• Filed: 08/31/1994
• Issued: 02/20/1996
• Est. Priority Date: 08/31/1994
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation, Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Original Assignee: International Business Machines Corporation
• Inventors: Hoebener, Karl G., Hubacher, Eric M., Partridge, Julian P.
• Primary Examiner(s): Heinrich, Samuel M.
• Application Number:
  US08/298,983
• Time in Patent Office: 538 Days
• Field of Search: 228/248.1, 228/180.22, 228/118, 228/215, 228/224, 228/225, 228/56.3, 427/123, 427/282
• US Class Current: 228/248.1
• CPC Class Codes: B23K 2101/36 Electric or electronic devices
• CPC Class Codes: B23K 3/06 Solder feeding devices; Solder melting pans
• CPC Class Codes: H01L 21/4853 Connection or disconnection of other leads to or from a metallisation, e.g. pins, wires, bumps
• CPC Class Codes: H01L 2224/11 Manufacturing methods
• CPC Class Codes: H01L 2224/1131 in liquid form
• CPC Class Codes: H01L 2224/11334 using preformed bumps
• CPC Class Codes: H01L 2224/1147 using a lift-off mask
• CPC Class Codes: H01L 2224/13 of an individual bump connector
• CPC Class Codes: H01L 2224/13099 Material
• CPC Class Codes: H01L 2224/13111 Tin [Sn] as principal constituent
• CPC Class Codes: H01L 2224/81024 Applying flux to the bonding area
• CPC Class Codes: H01L 2224/81801 Soldering or alloying
• CPC Class Codes: H01L 24/11 Manufacturing methods for bumps on insulating substrates H01L21/4853
• CPC Class Codes: H01L 24/13 of an individual bump connector
• CPC Class Codes: H01L 24/81 using a bump connector
• 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 2924/01004 Beryllium [Be]
• CPC Class Codes: H01L 2924/01006 Carbon [C]
• CPC Class Codes: H01L 2924/01029 Copper [Cu]
• CPC Class Codes: H01L 2924/01033 Arsenic [As]
• CPC Class Codes: H01L 2924/01042 : Molybdenum [Mo]
• CPC Class Codes: H01L 2924/0105 : Tin [Sn]
• CPC Class Codes: H01L 2924/01078 : Platinum [Pt]
• CPC Class Codes: H01L 2924/01082 : Lead [Pb]
• CPC Class Codes: H01L 2924/01322 : Eutectic Alloys, i.e. obtained by a liquid transforming into two solid phases
• CPC Class Codes: H01L 2924/014 : Solder alloys
• CPC Class Codes: H01L 2924/10253 : Silicon [Si]
• CPC Class Codes: H01L 2924/14 : Integrated circuits
• CPC Class Codes: H01L 2924/15787 : Ceramics, e.g. crystalline carbides, nitrides or oxides
• CPC Class Codes: H05K 2201/09727 : Varying width along a single conductor; Conductors or pads having different widths
• CPC Class Codes: H05K 2201/10674 : Flip chip
• CPC Class Codes: H05K 2203/043 : Reflowing of solder coated conductors, not during connection of components, e.g. reflowing solder paste
• CPC Class Codes: H05K 2203/0485 : Tacky flux, e.g. for adhering components during mounting
• CPC Class Codes: H05K 2203/0557 : Non-printed masks
• CPC Class Codes: H05K 2203/1476 : Same or similar kind of process performed in phases, e.g. coarse patterning followed by fine patterning
• CPC Class Codes: H05K 3/1216 : by screen printing or stencil printing
• CPC Class Codes: H05K 3/1225 : Screens or stencils; Holders therefor
• CPC Class Codes: H05K 3/3485 : Applying solder paste, slurry or powder thick film methods for applying conductive paste or ink patterns H05K3/12

Abstract

A method, and product created thereby, by which low melting point solder suitable for reflow connection of components is formed on select contacts of a printed circuit board. The method is particularly suited to the fabrication of populated printed circuit board having fine pitch devices including flip-chip devices, connected on a board including conventional coarse pitch surface mounted components. The fine pitch contacts of the board are exposed through holes in a stencil characterized in its ability to withstand solder reflow temperatures, not be wettable by solder, and have a coefficient of thermal expansion relatively matching the printed circuit board. Low temperature solder paste is screen deposited into the stencil openings. With the stencil fixedly positioned on the board, the solder paste retained by the stencil pattern is reflowed to selectively form on the underlying contacts of the printed circuit board. Thereafter, according to a preferred practice of the invention, the stencil is removed from the board and the board is subject to previously practiced depositions of flux and paste in preparation for fine and coarse pitch component placement and ensuing solder reflow. Alternate practices of the invention involve concurrent deposition of fine and coarse pitch solder and retention of the stencil following fine pitch solder fellow.

Claims

1. A method of forming fine pitch solder deposits on a circuit board, comprising the steps of:

positioning a metallic fine pitch stencil over the circuit board with openings aligned to select circuit board fine pitch contacts;

screen depositing solder paste into openings of the positioned stencil;

reflowing the solder paste while holding the positioned stencil against the circuit board to form solder on the select circuit board contacts; and

releasing the stencil without removing the solder on the select circuit board contacts.

2. The method recited in claim 1, comprising the further step of:

reflowing the solder on the select circuit board contacts to connect placed components.

3. The method recited in claim 1, wherein the stencil is formed from a material not wettable by solder.

4. The method recited in claim 2, wherein the stencil is formed from a material not wettable by solder.

5. The method recited in claim 1, wherein the stencil has tapered openings and is characterized by a coefficient of thermal expansion similar to that of the circuit board.

6. The method recited in claim 2, wherein the stencil has tapered openings and is characterized by a coefficient of thermal expansion similar to that of the circuit board.

7. The method recited in claim 2, comprising the further step of:

screen depositing solder paste through a coarse pitched stencil for select components, before performing the step of reflowing the solder.

8. The method recited in claim 2, comprising the further step of:

selectively depositing flux to hold components, before performing the step of reflowing the solder.

9. The method recited in claim 2, wherein the placed components are flip-chip die.

10. A method of attaching fine pitch terminal components to a printed circuit board, comprising the steps of:

placing a metallic non-wetting fine pitch stencil over the circuit board with openings aligned to select circuit board contacts;

screen depositing a solder paste into the openings of the placed stencil;

reflowing the solder paste while holding the placed stencil against the circuit board to form solder on the select board contacts;

releasing the stencil without removing the solder on the select circuit board contacts;

depositing flux on the solder over the select circuit board contacts;

placing components; and

reflowing the solder on the select circuit board contacts to connect placed components.

11. The method recited in claim 10, comprising the further step of:

screen depositing solder paste through a coarse pitch stencil for select components, before performing the step of reflowing the solder.

12. The method recited in claim 10, wherein step of placing components comprises the placing of component terminals into the deposited flux before performing the step of reflowing the solder.

13. The method recited claim 10, wherein the placed components are flip-chip die.