1.5
PCB Design and Assembly Considerations
This section lists recommendations to achieve maximum board-level reliability of the 5.5 x 5.5 x 1mm VFBGA 68‑ball
package:
■
NSMD lands, i.e. lands smaller than the solder mask aperture, are preferred because of the greater
accuracy of the metal definition process compared to the solder mask process. With solder mask defined
pads, the overlap of the solder mask on the land creates a step in the solder at the land interface, which
can cause stress concentration and act as a point for crack initiation.
■
Ideally, use via-in-pad technology to achieve truly NSMD lands. Where this is not possible, a maximum of
one trace connected to each land is preferred and this trace should be as thin as possible, this needs to
take into consideration its current carrying and the RF requirements.
■
35µm thick (1oz) copper lands are recommended rather than 17µm thick (0.5oz). This results in a greater
standoff which has been proven to provide greater reliability during thermal cycling.
■
Land diameter should be the same as that on the package to achieve optimum reliability.
■
Solder paste is preferred to flux during the assembly process because this adds to the final volume of solder
in the joint, increasing its reliability.
■
When using a nickel gold plating finish, the gold thickness should be kept below 0.5µm to prevent brittle
gold/tin intermetallics forming in the solder.
1.6
Typical Solder Reflow Profile
See
Typical Solder Reflow Profile for Lead-free Devices
for information.
Advance Information
This material is subject to CSR's non-disclosure agreement
© Cambridge Silicon Radio Limited 2011
Page 22 of 110
CS-209182-DSP1
CSR8640 BGA
Data Sheet
