AN2348

Soldering assembly recommendations

Doc ID 12282 Rev 5

11/17

5.2

PCB design recommendations for 2-bar Flip Chips

For optimum electrical performance and highly reliable solder joints, STMicroelectronics

recommends the PCB design recommendations listed in Table 4.

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 11).

Figure 11.

Solder mask opening

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.

Table 4.

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)

Track:

– 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