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TN0991

Soldering assembly recommendations

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Soldering assembly recommendations

3.1

PCB design recommendations

For optimum electrical performance and highly reliable solder joints, ST follows the PCB

design guidelines listed in Table 4 and Table 5.

PCB pad positioning and size must properly designed to optimize the natural self-centering

effect of the WLCSP on the PCB.

Note:

A too thick gold layer finishing on the PCB pad is not recommended since it results in low

joint reliability.

Micro vias

An alternative to routing on the top surface is to route out on buried layers. To achieve this,

the pads must be connected to the lower layers using micro vias.

3.2

PCB assembly guidelines

To mount a WLCSP on a PCB, ST recommends the use of a solder stencil with an aperture

of 330 x 330 μm2 maximum and a stencil typical thickness of 125 μm. WLCSP are fully

compatible with the use of near-eutectic SAC solder paste with no-clean flux.

ST has qualified WLCSP packages using JEDEC STD-020D soldering reflow profile shown

in Figure 8:

•

Dwell time in the soldering zone (with temperature higher than 220 °C) has been kept

as short as possible to prevent component and substrate damages.

•

Peak temperature must not exceed 260 °C.

•

especially when the temperature is above 150 °C.

Table 4. PCB design recommendation for 0.5 mm ball pitch packages

PCB design list

Recommendation

PCB pad design

Non-solder mask defined micro via underbump allowed

PCB pad size

Ø = 300 μm max (circular) - 250 μm recommended

Solder mask opening

Ø = 340 μm min (for 300 μm diameter pad)

PCB pad finishing

Cu - Ni (2-6 μm) - Au (0.2 μm max)

Table 5. PCB design recommendation for 0.4 mm ball pitch packages

PCB design list

Recommendation

PCB pad design

Non-solder mask defined micro via underbump allowed

PCB pad size

Ø = 260 μm max (circular) - 220 μm recommended

Solder mask opening

Ø = 300 μm min (for 260 μm diameter pad)

PCB pad finishing

Cu - Ni (2-6 μm) - Au (0.2 μm max)