STENCIL DESIGN FOR WAFER BUMPING/WAFER LEVEL BALL DROP
AND FLIP CHIP ASSEMBLY for Wafer Bumping
Susan Holmes and William E. Coleman VP Technology Photo Stencil
Photo Stencil LLC
Golden, Colorado, USA
sholmes@photostencil.com
ABSTRACT
Stencils have many other uses applications in addition to
printing solder paste for SMT Assembly. This paper
reviews stencil designs for Wafer Bumping Applications
where solder paste is printed on the die pad sites forming
solder paste bricks on each pad. The wafer is reflowed
during which each solder paste bricks melts and forms a
truncated sphere (solder bump) on each pad. Stencils for
Wafer Level Ball Placement are also described. There are as
number of Wafer Level Ball Drop Tools which require
stencils. The first step in this process is to print flux on the
wafer die pad sites. This is normally achieved with a thin
Electroform stencil with aperture size about 60% of the pad
size area. Aperture sizes normally vary 30um to 120um
depending on the die pitch. The Ball Drop stencil typically
has a thickness equal to the ball diameter. This includes a
spacer layer on the bottom side of the stencil to keep the
stencil from touching the flux which has already been
printed on the wafer. Aperture size is typically ball size plus
25um to 50um depending on the die pitch. Stencil
configurations and performance are described. Mixed
assembly of Flip Chip and SMT is becoming very popular.
Stencil designs to satisfy these requirements include twoprint stencil systems where a thin stencil is used to print flux
for the FC and a thicker stencil with a relief pocket
anywhere flux was printed is used to print solder paste for
the SMD. These stencil configurations and performance are
described.
Key Words: Stencils, Electroform Stencils, Wafer
Bumping, Wafer Ball Drop, Bump Height, Flip Chip.
Wafer Bumping
Normally, in a wafer bumping process, it is desirable to
maximize the bump height. The bump height is determined
by the volume of solder paste printed on the die pad as well
as the diameter of the die pad. When the solder paste melts
it forms a truncated sphere with a defined height defined by
the formula: (Volume of truncated sphere =  h/6 (3R2 +
h2), where h = height of bump and where R = radius of
attachment pad). Typically solder paste shrinks by about
50% when it becomes solid solder, so twice as much paste is
required. The amount of paste deposited can be increased
by increasing the thickness of the stencil and by increasing
the area of the aperture opening, overprinting the area of the
die pad. However there are limitations on both, mainly the
pitch of the pad sites and the Area Ratio of the stencil
aperture design. Solder paste printing can be divided into
two processes: the fill process and the transfer process. As
the squeegee blade traverses over the aperture the aperture is
filled with solder paste. With proper printing the aperture is
100% filled with solder paste. In the transfer process it is
desirable to transfer all of the paste in the aperture to the pad
below on the substrate. However paste transfer is a
competing process where the side walls of the aperture are
holding the paste inside the aperture while the pad beneath
is pulling the paste out of the aperture. How much paste is
transferred depends on the several factors including: solder
paste, aperture wall smoothness and the Area Ratio. The
generally accepted design guideline(1) for acceptable paste
release is for the Print Area Ratio to be greater than 0.66.
When the stencil separates from the substrate, paste release
encounters a competing process: will it transfer to the pad
on the substrate or will it stick to the side aperture walls?
When the area of the surface receiving the paste is greater
than 2/3 of the area of the inside aperture wall, the paste has
a good probability of achieving 80% or better paste release.
However, stencil technology has a direct influence on paste
release and thus what is the acceptable Print Area Ratio.
Stencil performance Studies(2)
have shown that
Electroformed stencil, with mirror type aperture wall finish
are capable of 80% paste transfer with Area Ratio’s less
than .5 (1/2). In addition Electroform stencils are much
better suited for the large aperture counts (typically 300,000
to 800,000 apertures) encountered in Wafer Bumping
applications.
Wafer Bumping Stencil Design Considerations
Stencil design (aperture size, stencil thickness, electroform
stencil with smooth apertures, spacing between apertures
and Area Ratio) play a major role in maximizing bump
height. Table 1 shows 6 stencil designs 400um, 300um.
And 200um pitch die pads for full array and peripheral array
die layouts. Overprinting the die pads is utilized in all 6
cases. It is desirable from a stencil fabrication viewpoint as
well as paste print viewpoint to keep the spacing between
apertures equal or slightly larger than the stencil thickness
for full array die configurations. In the first example a
400um pitch die has a 310um aperture which prints
(overprints) solder paste on a 200um die pad. Space
between apertures is 90um, the same distance as the stencil
thickness. Area Ratio is .86 very acceptable for good paste
transfer. Predicted bump height is 135um. The peripheral
arrays shown here assume a double row of I/O pads. In this
case a healthy amount of pad overprinting is available. As
seen in Table 1 this results in higher bump heights are
available compared to full array die, if higher bump heights
are desirable. Figure 1 shows solder paste bricks 280um in
diameter that were printed over a 100um pad. Figure 2
shows the solder bumps after reflow forming a truncated
sphere on the pads. Bump height was 120um with a CV of
3%.
Wafer Level Ball Placement
The first step in the Wafer Level Ball Placement process is
to print flux on the die pad sites on the wafer. This is
typically done with an Electroform stencil 20 – 50um in
thickness. The flux aperture size is typically between 60%
and 90% of the area of the die pad. Table 2 illustrates
typical Flux stencil designs for ball sizes of 30um to 250um
and pitches of 60um to 400um. Figure 3 shows an
Electroform Wafer flux stencil. This flux stencil has a foil
size of 400x400mm mounted in a 750x750mm frame.
Aperture size is 190um on a 400um pitch and the stencil foil
is 50um thick. There are slightly over 400,000 apertures in
this stencil.
After the wafer has flux printed on the die pads, the Ball
Drop Stencil is placed into the Ball
Drop Equipment over the wafer for transfer of solder balls
to the wafer. Some Ball drop Equipment requires a stand-off
layer on the bottom of the stencil to prevent the stencil from
touching the flux on the die pads. A schematic of this type
Ball Drop Stencil is shown in Figure 4. A picture of a Ball
Drop Stencil for a 300mm wafer having die pads on 400um
pitch with 250um balls is shown in Figure 5. A magnified
view shows the flux stand-off layer that runs along the die
streets of the wafer is shown in Figure 6. The flux stand-off
layer is 250um wide and 125um thick giving a total
thickness of the ball drop stencil of 258um which is 8um
taller than the solder ball. Table 3 shows Ball Drop Stencil
Designs for wafers with die pad pitch from 400um down to
60um and Ball sized from 250um down to 30um. As seen
from Table 3 ball drop aperture size is typically ball
diameter plus 50um for 150um pitch and above. There is
one exception for the 210um pitch design with a 125um ball
where the aperture size is only 35um greater than ball size.
Some of the latest imaging processes (including Laser
Direct Imaging) the G ratio can be much smaller. The B
ratio is defined as the ball diameter divided by the stencil
thickness (L1). This ratio should be .5 or greater to assure
the ball at least ½ of the ball is held in the stencil. Figure 7
shows a top view and a cross section view of an electroform
stencil 33um thick for a 100um pitch, 60um ball, and a
75um aperture. Figure 8 shows the same views for an
electroform stencil 30um thick for a 75um pitch, 40um ball
and a 50um aperture. Figure 9 shows the same view for an
electroform stencil 20um thick for a 60um pitch, 30um ball
and a 40um aperture.
Mixed assembly of Flip Chip and SMT
Mixed assembly of Flip Chip and SMT is becoming very
popular. Stencil designs to satisfy these requirements
include two-print stencil systems where a thin stencil is used
to print flux for the FC and a thicker stencil with a relief
pocket anywhere flux was printed is used to print solder
paste for the SMD. This process allows placement of Flip
Chip die and small chip devices in a pick and place single
reflow process.
The flux stencil is typically an electroform stencil between
25 and 50um thick with flux apertures typically 100um on
200um pitch. The paste stencil is typically 75 to 100um
thick with aperture sizes for 01005 and 0201 devices.
Normally this stencil is also electroformed for good paste
transfer for the chip component paste The flux relief pocket
is formed as a step in the electroform stencil process
Package densities are becoming smaller and smaller
requiring the spacing between chip component pad and die
pad to be smaller. This is a challenge for the electroform
paste stencil with relief pockets.
However recent
developments in the electroforming process utilizing LDI
have allowed significant improvement in reducing the
distance between paste apertures and the relief step pocket.
Figure 10 shows top down view of electroform stencil with
40um space between aperture and relief pocket wall.
Stencil is 85um thick with a relief pocket 60um deep.
Figure 11 shows cross section of the same stencil aperture /
relief pocket.
CONCLUSION
Electroform Wafer Bumping stencils can produce bump
heights from 71um for 200um pitch die to 135um for 400um
pitch die. Coefficient of Variations of less than 5% are
achievable.
Electroform Flux stencils are useful in printing flux on die
pads with pitches ranging from 60um to 400um.
Electroform Ball Drop stencil tools with a flux relief standoff are an effective ball drop tool for applications down to
60um die pad pitch.
Electroform stencils are used for mixed technology FC die /
SMT applications. A thin Electroform stencil 22um to
50um thick is used in the first print to deposit flux on the FC
die pad sites. An Electroform stencil with a relief pocket is
used to print solder paste for the SMT devices. Recent
advances in the electroform imaging process allow aperture
to relief step wall spacing as low as 40um.
REFERENCES
1 IPC 7525 RevB “Stencil Design Guidelines”
2 “Stencil Design and Performance for Flip Chip / Wafer
Bumping” William Coleman and R Lathrop, APEX 2004
Wafer Die
Array Type
Pitch
Pad
Size
Aperture
Size
Stencil
Thickness
Area Predicted Bump
Ratio
Height
All Dim.
microns
Full
Full
Full
Peripheral
Peripheral
Peripheral
400
300
200
400
300
200
200
150
100
200
150
100
310 cir
214 cir
136 cir
225x450
175x400
125x350
90
85
63
125
100
118
Table 1 Wafer Stencil Designs and predicted Bump Height
Figure 1 Solder Bricks: 280um diameter 90um high on 400um pitch. Pads are 100um
0.86
0.63
0.54
0.60
0.61
0.61
135
105
71
188
160
130
Figure 2 Solder bumps (truncated spheres) after reflow 120um high
Flux Stencil Design for Ball Drop
All Dim um
Ball
Size
Pitch
A1
A2
L1
250
400
190
195
50
125
210
93
98
50
80
150
60
65
40
60
100
40
45
40
40
75
30
34
30
30
60
22
25
22
Table 2 Flux Stencil design for Ball Drop Flux printing
Electroform Stencil
Figure 3 Electroform Flux Stencil 50um thick, 190um apertures , 400um pitch, >400,000 apertures
Figure 4 Schematic of Ball Drop Stencil: L1 Stencil, L2 Flux stand-off build up, A1 and A2 aperture size
Figure 5 Electroform Ball Drop Stencil for 300mm wafer
Figure 6 Electroform Ball Drop Stencil showing flux stand-off (250um wide, 125um thick)
Ball Stencil Design for stencils requiring flux relief
All Dim um
Ball Size
250
125
80
60
40
30
Pitch A1
400 300
210 160
150 100
100 75
75
50
60
40
A2
315
170
107
80
54
43
L1
133
64
50
33
30
20
L2 G ratio H ratio
258
0.75
0.53
128
0.78
0.51
90
1.00
0.63
68
0.76
0.55
45
0.83
0.75
33
1.00
0.67
G ratio = gap between apertures / L1
H ratio = Ball diameter / L1
B minimun = 40um. D minimum = 125um
Table 3 Ball Drop Stencil Design for ball sizes 30um up to 250um
Figure 7 Electroform Ball Stencil 100um pitch, 60um balls, 75um apertures, 33um thick
Figure 8 Electroform Ball Stencil 75um pitch, 40um balls, 50um apertures, 30um thick
Figure 9 Electroform Ball Stencil 60um pitch, 30um balls, 50um apertures, 20um thick
Figure 10 Electroform Relief-Step Stencil for 2nd print of paste for 0201, 01005, 85um thick, 60um deep pocket
Figure 11 Electroform Relief Step Stencil cross section 40um space between 01005 aperture and relief step pocket