AN1231 Motorola / Freescale Semiconductor, AN1231 Datasheet - Page 6

• Manufacturer Part Number: AN1231
• Description: Plastic Ball Grid Array (PBGA)
• Manufacturer: Motorola / Freescale Semiconductor

AN1231
6
eight mils of solder paste screenprinted onto it prior to device
placement. Besides screening solder paste and flux, pin
transfer and single point dispense of paste flux have also
been used successfully on PBGA within Motorola.
DEVICE PLACEMENT
QFPs, pick and place is much simpler and involves lower re-
quired machine accuracy. Additionally, due to tight ball pat-
tern to device edge tolerances (better than
to the pitch, placement can be performed off of the body out-
line. This is the method currently used for much smaller de-
vices in the industry using extremely fast “chip-shooters”.
Additionally, new equipment with upward looking vision or la-
sers (dual lasers or better are recommended over a single
laser system) that are specifically designed for the ball grid
array are widely available. This equipment centers off the ball
array itself and can also check for missing balls and in some
cases calculate device coplanarity real-time. Sometimes,
only balls on the outer or perimeter row are recognized and
used for placement. Finally, due to the fact that the PBGA is
self-centering in the reflow process, a device can be placed
up to 50% off pad and still be expected to align itself. The
self-centering feature, which is a result of the surface tension
Due to the large pitches involved relative to fine pitch
Screenprinted Solder Paste (Magnification of Approximately 40X, 45 )
Figure 4. Micrograph of an PBGA Test Board Pad with Eight Mils of
3 mils) relative
of the molten solder, can be easily observed by placing de-
vices deliberately off pad and reflowing.
REFLOW
of leaded devices. The process of reflowing a PBGA is
sometimes referred to as a Controlled Chip Carrier Collapse
Connection or C5 since the solder ball starts with an approxi-
mate height of 25 mils (plus paste if any) and collapses down
one to nine mils during reflow due to the weight of the pack-
age, solder paste collapse, and wetting of the motherboard
pad. PBGA devices can easily be reflowed in IR, convection
and mixed heating ovens, as well as with vapor phase. Care
must obviously be taken that each solder joint is exposed to
the solder liquidus temperature immediately following a flux-
dependent high temperature soak period in which the flux is
mobile and active. An example of an IR reflow profile (some-
what optimized) obtained by placing thermocouples under
two different devices at the middle and corner of a 4.5 x 7.5
four layer test board is presented in Figure 5. The main differ-
ence in reflowing the PBGA lies in the fact that the joints are
heated more from the package and board as opposed to di-
rect air impingement or IR exposure onto the leads. Due to
low mass of the PBGA. It is relatively easy to obtain a suit-
able profile with boards containing a variety of surface mount
and through-hole device types along with the PBGA.
Surface mount reflow of the PBGA device is similar to that
MOTOROLA FAST SRAM