Browse Prior Art Database

DOUBLE STENCIL WAFER BUMPING

IP.com Disclosure Number: IPCOM000009277D
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-14
Document File: 6 page(s) / 330K

Publishing Venue

Motorola

Related People

Aubrey Sparkman: AUTHOR [+3]

Abstract

As I/O count increases and individual die designs are made smaller, it becomes increasingly important to reduce the pitch of solder bump con- nections. Motorola transfered C4 (evaporated) bump technology from IBM. This is a very mature, robust, and reliable but expensive wafer bumping process. Motorola has therefore invested several million dolllars to develop a lower cost wafer bump- ing process using electroless Ni and Au for the Under Bump Metallurgy (UBM) and a stencil print- er to print solder paste which is then processed through a reflow oven to form the solder bump.

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Page 1 of 6

Developments Technical 0 M MO-LA

DOUBLE STENCIL WAFER BUMPING

by Aubrey Sparkman, Harold Downey and Martie Knauss

INTRODUCTION

  As I/O count increases and individual die designs are made smaller, it becomes increasingly important to reduce the pitch of solder bump con- nections. Motorola transfered C4 (evaporated) bump technology from IBM. This is a very mature, robust, and reliable but expensive wafer bumping process. Motorola has therefore invested several million dolllars to develop a lower cost wafer bump- ing process using electroless Ni and Au for the Under Bump Metallurgy (UBM) and a stencil print- er to print solder paste which is then processed through a reflow oven to form the solder bump.

  The purpose of this project is to develop full array wafer bumping at pitches comparable to and

smaller than C4 evaporated bumps at the lower cost of stencil printed bumps.

CURRENT BUMP CAPABILITIES FOR 5 MIL BUMPS WITHOUT DOUBLE STENCIL

In the table in Figure 1, a comparison is made

between the three different wafer bumping technolo- gies that can be obtained today in Motorola. The quotes for getting 8 inch wafers bumped were obtained as well. One can see from the table that while the Stencil printed bump will cost the cus- tomer the least, a 5 mil tall bump can not be obtained at any array with pitch less than 13 mils. At 10 mils array pitch, a bump 3.5 mil tall bump is the largest that can be printed and at 8 mils array pitch, a 2.5 mil tall bump may not be possible.

1 E-PlatelStencil

                  C4lE3. Peripheral Pitch (pm) 225 200 275

Array Pitch (q~) 225 200 337

Bump Density, /mm 20 25 9

Bump Height (pm) 125 125 125

Quote for 8" Wafer $275 %I55 *,,I3

Fig. 1

0 Motomla. Inc. ,999 161 June 1999

[This page contains 14 pictures or other non-text objects]

Page 2 of 6

Developments Technical 0 M MOTOROLA

SINGLE STENCIL WAFER BUMPING PROCESS

  A summary of the current Stencil process as shown in Figure 2. After the wafers have electroless NiIAu plated on them, they are ready for the solder paste printing step. A stencil printing machine loads the wafers and aligns them to a stencil fixed in the stencil printing machine. The printing step shows a

  2 3 PRINT REFLOW CLZAN

Fig. 2 Stencil Print Bump Process Flow

  The solder paste paste deposits must be separat- ed by a sufficiently wide gap to prevent them from flowing together which causes the bumps to short together. This separation gap requirement is one of the pitch limiting factors of the stencil printed wafer bumping process.

  Since the solder paste has twice the volume of the solder in the reflowed bump, aperture volume is a critical factor. But the stencil must be thin enough

to get good paste release and this reduction in sten- cil thickness causes a reduction in aperture volume.

Reduction of aperture volume causes a reduction in solder bump height which is another critical limiting factor for the stencil printed wafer bumping process.

  Motorola has a patent on a process which uses photoresist to act as...