NXP微控制器的BGA封裝的PCB布線指南

　　When building a BGA footprint the number one consideration is ensuring the ball pattern

　　and outline matches the device package. This includes correct orientation of ball A1，

　　matching all ball column x row locations， and the ball-to-ball pitch. Solder joint reliability is

　　also of primary concern. For cost sensitive applications， minimizing the number of PCB

　　layers required to route the BGA is a consideration. The BGA land pattern footprint plays

　　a key role in solder joint reliability， and the number of PCB layers required to route the

　　balls.

　　Land pad design

　　The PCB BGA land pads have to be designed to ensure solder joint reliability and

　　provide optimum manufacturability. The two basic types of BGA land pad design are：

　　? The Solder Mask Defined （SMD） land pad

　　? The Non-Solder Mask Defined （NSMD） land pad; recommended type for PC

　　Solder Mask Defined （SMD） land pad

　　The SMD type of BGA land pad design is characterized by the copper pad being larger

　　than the solder mask opening above this pad. Thus the solder joint area of the land pad

　　is defined by the opening in the solder mask.