AN1231 Motorola / Freescale Semiconductor, AN1231 Datasheet - Page 4

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

AN1231
4
escaped as they are dropped straight down using dumbbell
or teardrop shaped pads with offset vias to the associated
power and ground planes on the circuit board. This leaves
the outer four rows containing 236 signal pins around the pe-
rimeter of the package that need to be escaped. This pack-
age itself has 23 mil diameter solder pads on it and the same
solderable surface diameter or only slightly larger is recom-
mended for the PCB. Therefore, using a 23 mil non–solder-
mask defined pad in conjunction with a 25 mil diameter via
pad a board employing eight mil lines and spaces can be
used to easily route these four outer rows using two signal
layers. The escape routing of the outer two rows can be
achieved on the topside of the PCB without using vias and
the third and fourth rows from the outside are escaped by
dropping down vias and escaping on the bottomside. This
example is illustrated in Appendix C, which shows a repre-
sentative top and bottomside signal layer routing schematic
for the 68356.
devices packaged in BGA such as Fast Static RAM devices
that utilize a 7x17 and soon a 9x17 array PBGA, the
PowerPC 603 , PowerPC 604 microprocessors as well as
the MPC105 PCI Bridge/Memory Controller for PowerPC
microprocessors that will be available in a ceramic BGA. The
fact that this 119 pin FSRAM package only contains seven
A similar package design methodology is used on other
Figure 2. Solder Pad for PBGA: a) recommended NSMD dumbbell pad shown with typical dimensions for
most 1.27 mm pitch PBGAs, b) SMD dumbbell pad for most 1.27 mm pitch PBGAs, C) SMD teardrop
pad, and d) NSMD pad as would be used for signal routing on the device layer of the motherboard.
= SOLDERMASK OVER LAMINATE
35.4 MILS
À À À À À À
À À À À À À
À À À À À À
À À À À À À
À À À À À À
a)
c)
14 MIL DRILLED VIA DIAMETER
(12 MIL FINISHED DIAMETER)
25 MIL VIA PAD DIAMETER
8 MIL TRACE WIDTH
= SOLDERMASK OVER COPPER
23 MIL SOLDER
PAD DIAMETER
4 MIL SOLDERMASK
TO PAD CLEARANCE
31 MIL SOLDERMASK
OPENING DIAMETER
rows in one direction further simplifies routing since there are
a maximum of three buried rows. As can be seen in Table 3,
no matter what geometry (diameter and SMD versus NSMD
pad configuration) is chosen for 1.5 mm pitch PBGA devices
one eight mil trace can always be routed between two solder
pads. Only if NSMD pads are used an eight mil trace can be
routed between two pads at 1.27 mm pitch. As can be seen
in Table 3, a maximum trace width of six mils would be need-
ed to use an SMD pad.
essary to make signal assignments on more than just the
four outer rows. This would require more that one buried row
on each of the outer layers to be escaped on a given PCB
layer to maintain a two signal layer board. When this be-
comes the case, trace size will have to decrease further from
the standard eight to six. Table 3 also shows how many
traces can be routed between two pads for given line and
space technologies down to three and three and all the cur-
rent standard PBGA pad diameters. For example, when and
if it becomes necessary to route two signal traces between
NSMD pads at 1.27 mm pitch, five mil trace widths will be
necessary to avoid adding extra signal layers. If SMD pads
were used those traces could only be three mils wide maxi-
mum. This example once again underscores the routing ad-
vantages of NSMD pads.
As PBGA pin count and matrix size increase it may be nec-
À À À À À
À À À À À
À À À À À
À À À À À
À À À À À
À À À À À
= SOLDERABLE SURFACE
b)
d)
MOTOROLA FAST SRAM
23 MIL SOLDERMASK
OPENING DIAMETER
À À À À
31 MIL SOLDER
PAD DIAMETER
4 MIL SOLDERMASK
OVERLAP
= BARE LAMINATE