AN1850 Freescale Semiconductor / Motorola, AN1850 Datasheet - Page 5

• Manufacturer Part Number: AN1850
• Description: Flip-Chip PBGA Package Construction Assembly and Board-Level Reliability
• Manufacturer: Freescale Semiconductor / Motorola

1.2 Motherboard Solder Pad Geometries
Figure 5 and Figure 6 contain the recommended solder pad geometries for the FC PBGA and the CBGA,
respectively. A pad with a 0.58 mm diameter solderable surface that is 0.05 mm smaller than the 0.635 mm
package pad is recommended for the FC PBGA. The preferred CBGA pad is larger with a diameter of 0.72
mm. For maximum solder joint reliability, it is recommended to maintain the solder pad diameter within
the range shown (0.72±0.037). However, board-mounted solder joint reliability data in 0 to 100
cycling on a CBGA using a PBGA-sized motherboard pad shows that reliability is not impacted greatly; see
Section 1.3, ÒSurface Mount Assembly.Ó Even when the smaller motherboard pad is used for the CBGA, a
minimum solder paste volume of 0.079 mm3 (see Table 1) must be applied to each pad during board
assembly.
Note that the pads shown in both Figure 5 and Figure 6 are non-soldermask (NSMD) or etch deÞned. They
have an annular soldermask clearance of 0.075 mm around the copper pad. This conÞguration is
recommended for the best solderability, especially when a hot air solder leveled (HASL) surface Þnish is
used. However, there are other factors that affect what motherboard pad type is chosen. The NSMD pad is
best for solderability, but because it has a smaller copper diameter than an SMD pad and does not have the
added strength provided by the soldermask overlap, it may be weaker. It may be more likely to fail during
bending, severe ramp rate thermal cycling, component rework or site dress after rework. Failure can occur
by the pad lifting out of the board or by the trace cracking where it enters the pad, especially if the trace has
a width of less than 0.30 mm.
The via and via-pad diameters shown in Figure 5 and Figure 6, as well as soldermask clearances, can be
adjusted as necessary to meet individual design rules. The interstitial plated through-hole diameter is shown
as 0.30 mm for CBGA and greater than or equal to that for the FC PBGA. A via size increase may be
possible due to the decreased solder pad diameter associated with the FC PBGA. Soldermask clearance
around the solder pad should be speciÞed such that soldermask does not encroach on the pad at the worst-
case soldermask to artwork registrational tolerance of the printed circuit board fabricator. The surface Þnish
for CBGA or FC PBGA may be any consistently solderable surface such as organic solderability protectant
(OSP), HASL, electroless or electrolytic nickel/gold or immersion silver.
Via
Figure 5. Recommended FC PBGA Motherboard Solder Pad Geometry
Flip-Chip PBGA Package ConstructionÑ Assembly and Board-Level Reliability
Vi
a
Solder Pad
Cross Section
Solder Pad
³
³
0.30+mm (12+ mil) wide line between pads
0.58 ± 0.037 mm (3 ± 1-1/2 mil)
0.076 ± .025 mm (3 ± 1 mil)
Clearance between copper pad and solder mask
Solder mask away from copper pad
PCB Laminate
Copper diameter
3.30 mm (12 mil) finished plated through-hole
0.65 mm (25 mil) annular pad
Liquid photoimageable solder mask
°
C thermal
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