PCB recommendations ESDA14V2-1BF3
6/9 Doc ID 15894 Rev 1
4 PCB recommendations
4.1 Design
For optimum electrical performance and highly reliable solder joints, STMicroelectronics
recommends the PCB design recommendations listed in Ta bl e 3 .
Note: A gold layer finishing on the PCB pad that is too thick (0.2 µm maximum) is not
recommended (low joint reliability).
To optimize the natural self centering effect of CSP on the PCB, PCB pad positioning and
size have to be properly designed (see Figure 14)
Micro vias
An alternative to routing on the top surface is to route out on buried layers. To achieve this,
the pads are connected to the lower layers using micro vias. Only SSBU via technology is
approved.
Figure 14. Solder mask opening
Table 3. PCB design recommendations for solder bar pitch 400 µm
For NSMD PCB
non solder mask defined
Oblong pad: 370 x 180 µm
– Micro via SSBU allowed
– Micro via SBU to be avoided
– Micro via SBU filled (under qualification)
Tra ck:
– Only one track per pad
– Maximum track width = 100 µm
Track layout must be symmetrical to the die axis (to homogenize stress
and welding attraction during reflow assembly)
For SMD PCB
solder mask defined
Oblong pad:
– Micro via SSBU allowed
– Micro via SBU to be avoided
– Micro via SBU filled (under qualification)
PCB Pad Finishing Cu – Ni (2-6 µm) - Au (0.2 µm max)
370 µm
370 µm
100 µm
450 µm
180 µm
180 µm
255 µm
Non solder mask defined
Solder mask defined
