Fig. 16a PumpPrinting Flux – The Print Stroke
Print Direction
Fig. 16a PumpPrinting Flux – The Print Stroke
Squeegee
Flux
PumpPrint Stencil
Print Direction
Squeegee
Flux
PumpPrint Stencil
Advances in solder ball placement for surface-mountable packages
SMT
Flip Chip Die
Ball Pad Substrate
Capacitor
Fig. 16a PumpPrinting Flux – The Print Stroke
2. Top referencing or mechanical clamping.
Fig. 16b PumpPrinting Flux - During Stencil Separation.
By applying pressure to the ball side of
Print Direction
the parts to be ball placed, either before
SMT
Flip Chip Die PumpPrint Stencil
Ball Pad Substrate
or during the placement operation, the
Squeegee
Capacitor
warpage can be temporarily relieved. Of
Fig. 16b PumpPrinting Flux - During Stencil Separation.Flux
PumpPrint Stencil
course this pressure must be applied to
PumpPrint Stencil
Flux
areas other than the ball pads as these
are already printed with flux, and this
should only be done if the parts can
tolerate being straightened without
Flux SMT
Flip Chip Die
SMT
Flip Chip Die
Ball Pad Substrate
Capacitor
suffering internal damage.
Ball Pad Substrate
Capacitor
3. Accommodation. Some systems are able
Fig. 16b PumpPrinting Flux - During Stencil Separation.
to accommodate a certain amount of
SMT
Flip Chip Die
warpage. An example of this is the Figure
Ball
PumpPrint Stencil
Pad
16a. PumpPrinting
Substrate
flux—the
Capacitor
print stroke Figure 16b. PumpPrinting flux—during stencil separation.
Fig 16c Recessed, Two Layer Ball Place Stencil
pin-transfer of flux using pins or pads
mounted in a compliant or conforming
Ball Place Stencil Metal Layer
bed. Ball Place Stencil Stand-off Layer
Fig 16c Recessed, Two Layer Ball Place Stencil
Flux
Warpage will also have some effect on
Ball Place Stencil Metal Layer
alignment accuracy and for this reason a
Ball Place Stencil Stand-off Layer
‘best fit’ alignment algorithm that can also
SMT
Flip Chip Die
Ball Pad Substrate
be weighted or biased towards one fiducial
Capacitor SMT
Flip Chip Die
Ball Pad Substrate
Capacitor
or alignment mark is preferred.
Ball-side SMT SMT
Fig 16c Recessed, Two Layer Ball Place Stencil
Flip Chip Die
Ball Pad
In order to firstly print flux and later place
Substrate Capacitor
solder balls onto what is effectively a 3D
Ball Place Stencil Metal Layer
surface, both the flux and ball place tools
Ball Place Stencil Stand-off Layer
or stencils must have relieved or recessed
Figure 16c. Recessed, two-layer ball-place stencil Figure 17a. Virtual panel tooling for 40 single substrates.
areas that can accommodate the surface
features of the part to be placed.
For flux printing, one solution to this
problem is the PumpPrint® stencil (Figure
SMT
Flip Chip Die
Ball Pad Substrate
Capacitor
16). These stencils are machined from a
relatively thick plastic and have a series
of circular apertures or slot configurations,
which act like an array of dispensing
nozzles that are already perfectly positioned
over the required location of each dot. The
increased thickness of the PumpPrint®
stencil facilitates machined recesses
on the underside to fit over pre-placed
components.
Ball placement by stencil printing also
Figure 17b. Virtual panel tooling for 40 single substrates. Figure 18. Automated robotic wafer handling
lends itself to placement over existing
components, provided a full thickness
two-layer stencil is used (by full thickness
Once in the machine, the parts are lifted substrates before individual vacuum towers
I mean at least one ball diameter total
out of the boat one by one, and each part are raised to collect the now mechanically
stencil thickness).
is flux printed then ball placed as a single aligned parts and present them to the flux
Clearly systems which ‘pick & place’ a
component with its own vision alignment. screen or ball place stencil.
single solder ball at a time using a nozzle
This is a fully automated solution, with
will have no problems in clearing pre-
the only disadvantage being that, unless Silicon wafers
placed SMT components; however, the
multiples of machines are used, the To reduce product changeovers and
speed of these machines is usually not
throughput is not as high as panel-based tooling costs, any ball placement machine
cost effective for medium to high volume
production. components that are specific to a particular
production.
For high volume ball placement of wafer diameter or thickness must be
singulated parts, Virtual Panel Tooling, or minimised. One helpful solution is the
Singulated substrates
VPT, can process a whole JEDEC carrier of Universal Wafer Pallet, a two-part wafer
For lower volumes or Auer boat processes,
singulated parts at once, with roughly the tooling solution that can accommodate
SinguLign™ or its equivalent has many
same cycle time as is required for a panel. any wafer diameter from 100 to 300 mm,
benefits to offer. In these systems,
This is achieved by lifting each part out of or any wafer thickness from 200 µm to
singulated parts are loaded onto Auer
the carrier using two conical pins on over 1 mm. This is achieved by using a
boats and processed ‘a boat at a time.’
each of two corners. These pins centre the universal base plate, and a relatively low
18 – Global SMT & Packaging - August 2008 www.globalsmt.net
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