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Advances in solder ball placement for surface-mountable packages
side down. Electrical connections from
Advances in
the ball side are made to the die through a
window in the substrate. This is sometimes
known as the window BGA or WBGA
(Figure 5).
solder ball
Typical ball diameters for these devices
are 0.2 to 0.4 mm with their respective ball
pitches at 0.3 to 0.8 mm.

placement for
Wafer-level CSPs
Fig. 10b Vacuum transfer ball placement. Placement.
The usual definition for the wafer level
Fig. 1. Underfill
Overmold
chip scale package
Flip
is
Ch
an
ip Di
integrated
e
circuit
Low Positive Pressure
that has all of its manufacturing processes
surface-mountable
performed at the wafer level, with the
final stage being package singulation. The
singulated parts are then surface mountable.
Interposer
Solder Balls
packages
Wafer Wafer
Fig. 5 Cross section of WBGA
Tooling
Figure 2. Categories of SiP (Source: ITRS) Wire Bonds
Overmold
Fig 11a. Wafer tooling:
a single unit, which provides multiple elect to stack the packages themselves as
functions associated with a system or sub- a part of their surface mount process. In 0.
Pallet
Vacuum Wafer
system. A SiP may optionally contain this method, the bottom package would
2 Channels
6
passives, MEMS, optical components and be placed along with the other SMT
Fig 11b. Flux Print:
other packages and devices.”
1
components. The top package would then
Substrate
Silicon Die
Solder Balls Flux Screen
Squeegee
With its origins in the highly be dipped in flux or solder paste and placed
specialised hybrids and multi-chip modules onto the bottom package before the whole
Fig. 10a Vacuum transfer ball placement. Pick-up.Figure 5. Cross-section of WBGA.
of more than 20 years ago, the SiP has circuit board goes through solder reflow. Pallet Vacuum
Wafer
come a long way and is now a mainstream By the end of 2006, the PSvfBGA
Channels
WLCSPs entered Vacuum
Machined
mainstream SMT
option for high-density semiconductor (package stackable very fine BGA) became
assembly Toolduring the 1990s in either a 0.75
Reservoir
Fig. 11c. Ball Placement:
packaging. This is particularly true where the fastest growing new product in Amkor’s
or 0.8 mm pitch form factor (before the
space is limited and demands for increased history, and 2007 saw PoPs in mass
Direction
Ball Place
industry agreed on standard pitches). It did of Travel
Transfer Head
functionality are high, most notably in production for Apple’s new iPhone. Indeed
not take long before this was reduced to 0.5
the mobile phone, PDA and laptop most of the major mobile handset makers
mm pitch with a 0.3 mm diameter solder
markets. In fact, many of the functions have shifted to the PoP configuration
ball. These dimensions were the most
Pallet
Ball Place
currently provided by separate packages as the engine to power their flagship
common in the early 2000s. In the past few
Wafer
Stencil
are now being, or will soon be, products.
4
years, 0.4 mm pitch CSPs have become
incorporated into SiPs.
popular, the ball diameter for these being
In many areas of electronic design the MicroBGA/CSP strips
typically 0.25 mm.
lines between surface mount circuit board The backbone of the DRAM market for
WLCSPs have been growing rapidly
assembly and semiconductor packaging are several years now, the microBGA strip is
in popularity for 10 or more years, with
becoming blurred. Nowhere is this more already in very high production volumes.
the boom in portable electronics sales
evident than in the SiP. Figure 2 shows One of the most common variants is a thin
(particularly mobile telephones) being
some of the various categories of SiP, rigid substrate with the die attached active
the greatest driver for their exponential
including those with both embedded and
growth. By the end of 2007, an estimated
surface-mounted dies and passives.
10 billion WLCSPs will have been
This diagram not only shows that many
shipped, with a further 25% growth
SiPs require solder balling as one of the
predicted for 2008.
2
final stages in their assembly process but
When you look at the benefits it has to
also that some of the components within
offer, it’s not hard to see why the WLCSP
the SiP have already been solder balled
continues to be this successful, the most
prior to integration within the SiP.
obvious advantage being its minimalist
dimensions, not only in length and width,
Package on package/stacks
Figure 3. Package on package. Memory on top
of logic.
4
but also in thickness, clearly a big incentive
Strictly speaking these should be part of the
to any assembler wishing to include more
‘system in package’ group as they meet
functionality in a smaller package. But
all the requirements for inclusion; however,
that’s not where it stops. In many cases
PoPs are pretty unique in several ways and
the WLCSP also offers superior electrical
so deserve a little more attention. One
performance, largely due to the shorter
of the most interesting points is the way
(and therefore faster) signal path between
they’re used. As well as buying in pre-
the die and the circuit board to which it is
stacked packages, the SMT assembler can
Figure 4. PoP section from Samsung ‘Slim’ phone.
ultimately attached.
3
www.globalsmt.net Global SMT & Packaging - August 2008 – 13
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