Fig. 10b Vacuum transfer ball placement. Placement.
Fig. 1. Underfill
Overmold
Flip Chip Die
Low Positive Pressure
Interposer
Solder Balls
Wafer Wafer
Fig. 5 Cross section of WBGA
Tooling
Wire Bonds
Overmold
Fig 11a. Wafer tooling:
0.
Pallet
Vacuum Wafer
2 Channels
6
Fig 11b. Flux Print:
Substrate
Silicon Die
Solder Balls Flux Screen
Squeegee
Advances in solder ball placement for surface-mountable packages
Fig. 10a Vacuum transfer ball placement. Pick-up.
Stencil printing ball placement
Pallet
Vacuum Wafer
Fig. 10b Vacuum transfer ball placement. Placement. Channels
process overview Vacuum
Machined
Whilst there are some advantages to Fig. 1. Underfill
Overmold
Flip Chip Die
Tool
Low Positive Pressure Fig. 11c. Ball Placement:
the vacuum transfer method, the stencil
Fig. 10b Vacuum transfer ball placement. Placement.
Reservoir
printing method has also proven to be Direction
Ball Place
Fig. 1. Underfill
Overmold
highly effective. The system works by
Flip Chip Die
Low Positive Pressure of Travel
Transfer Head
firstly transporting the wafers/panels/
strips/components into the flux printing
machine where a vision alignment
InterposerFig. 10b Vacuum transfer ball placement. Placement.
Solder Balls Pallet
Ball Place
process occurs. Having accurately
Wafer Wafer
Wafer
Stencil
Fig. 1. aligned Underfillthe wafer to the emulsion
Overmold
mesh
Figure 9. Solid state drive - another ball array package. Figure 10a. Vacuum transfer ball placement - pick-up.
Flip Chip Die Fig. 5 Cross section of WBGA
Low Positive Pressure
Tooling
flux screen, the wafer is then screen
Interposer
Solder Balls
Fig. 10b Vacuum transfer ball placement. Placement.
printed with flux deposits on all ball
Wire Bonds
Overmold Wafer Wafer
pad
Fig. 1.
s. After
Underfill
Overmold
flux prFinlipt Cinhipg ,D itehe parts are
Fig. 5 Cross section of WBGA
Tooling
Low Positive Pressure
Fig 11a. Wafer tooling:
transported to the ball place machine
Wire Bonds
Overmold
where a similar vision alignment
0.
Fig 11a. Wafer tooling:
Pallet
Vacuum Wafer
process occurs. The wafer/panel/
Interposer
2
strip/component is th
Solder Balls
Channels
en brought into
6
Wafer Wafer
intimate contact with the underside of
0.
Interposer
Solder Balls
Fig 11b. Flux Print:Pallet Vacuum
Wafer
Fig. 5 Cross section of WBGA Substrate
Tooling
the ball place stencil. The ball transfer
Si
2 Channels
Flux
Wafer
licon Die
Wafer
Solder Balls Screen
6
Squeegee
heaFig. 5 Cross section of WBGAd then au
Wire Bonds
tomatically tr
Overmold
averses the
Tooling
Fig 11b. Flux Print:
topside of the stenciWire Bondsl, depositinOvermoldg a
Fig. 10a Vacuum transfer ball placement. Pick-up.
SubstrateFig 11a. Wafer tooling:
Silicon Die
Solder Balls Flux Screen
solder ball into each of the stencil’s
Squeegee
Figure
Fig 11a. Wafer tooling:
10b. Vacuum transfer ball placement - placement. Figure 11a. Wafer tooling.
Pallet
Vacuum Wafer
apertures as it meets them, these
0.
Channels
apertures being previously well aligned
Fig. 10a Vacuum transfer ball placement. Pick-up.Pallet
Vacuum
Vacuum Wafer
2 0.
Machined
Pallet Channels
Vacuum Wafer
to the printed flux. See secti
6
onal
2
views
Tool Channels
Fig. 11c. Ball Placement:
Pallet
Vacuum Wafer
6 Reservoir
in Figures 11a to 11c, which show the
Channels
Fig 11b. Flux Print:
Substrate
Fig 11b. Flux Print:
Vacuum
process
Substrate
Silic
for
on D
wafer
ie
Siliconlevel
Solder Balls
Machined
Die ball
Solder Ballsplacement. Flux
Flux
Screen
Direction
Ball Place
Tool Squeegee
Screen
Squeegee of Travel
Transfer Head
Reservoir
Fig. 11c. Ball Placement:
Fig. 10a Vacuum transfer ball placement. Pick-up.
ScrFig. 10a Vacuum transfer ball placement. Pick-up.een & stencil Direction
Ball Place
Typically the flux screen will be a
of Travel
Transfer Head
Pallet
Vacuum Wafer
Pallet
Vacuum Wafer
stainless steel, 45˚ mesh with a few µm of
Channels
Vacuum Channels Pallet
Ball Place
Machined
photoimageable
Vacuum
emulsion on the wafer
Wafer
Machined
Stencil
Tool
Tool
side. The screen’s mesh wire diameter
Reservoir
Fig. 11c. Ball Placement:
,
Reservoir
Fig. 11c. Ball Placement:
mesh pitch, emulsion thickness and
Direction
Ball Place
Pallet
Ball Place
Figure
of Travel
11b. Flux print.
Transfer Head
Figure 11c. Ball placement.
Wafer
Stencil
aperture size are calculated according to
Direction
Ball Place
of Travel
Transfer Head
the ball diameter, such that the printed
flux volume to solder ball volume ratio
is constant for all ball diameters. Once
Pallet
Ball Place
Wafer
Stencil
fabricated, the screen is checked against
Pallet
Ball Place
the specification and then measured for,
Wafer
Stencil
amongst other things, aperture size, mesh
tension, emulsion integrity and image
stretch/shrinkage.
The ball place stencil is best comprised
of two layers, the top layer being either
stainless steel or electro-formed nickel
Figure 12. Printed flux dots on a wafer. Figure 13. Placed solder balls on a test wafer.
and the bottom (or stand-off) layer being
a photoimageable dry-film resist. The
purpose of the stand-off layer is to hold the
Miniaturisation solder ball diameters have been reduced
top metal layer away from the pre-printed
Driven by the constant demand for greater proportionally too.
flux. Another benefit of this stand-off
functionality from ever-smaller portable Another interesting development
material is that it provides a relatively
electronic devices, it’s no surprise that is a rapidly growing requirement for
soft surface in contact with the active
the ICs used to power these devices are 50 to 100 µm ball placement for flip
side of the parts to be ball placed, thereby
also shrinking. In turn this means that chip wafer bumping. This is normally
minimising the possibility of damage.
ball diameters and pitches are reducing performed by electro-plating or by
incrementally every few years. Some stencil printing solder paste. Many
Key challenges for ball placement
examples of this are the standard CSP predicted the cost of solder balls (priced
Given the explosive growth in ball array
pitch has gone from 0.8 mm to 0.5 mm, by the 1000) would be prohibitive for
packages and the growing diversity of
then around 2005 to 0.4 mm, and now this application with its very high I/O
package types, ball placement equipment
0.3 mm. Also, the standard BGA pitch counts per wafer. However it now seems
manufacturers are now faced with some
has dropped from 1.27 mm to around 0.65 that, at least for some applications, the
unique challenges.
mm over the same period. Clearly the benefits of throughput and bump height
www.globalsmt.net Global SMT & Packaging - August 2008 – 15
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