PCB design rules

AN4159

10/27

Doc ID 023639 Rev 3

Stencil design and solder paste application

The soldering paste thickness and pattern are important for the proper pressure sensor

mounting process.

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Stainless steel stencils are recommended

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Stencil thickness of 90 - 150 µm (3.5 - 6 mils) is recommended for screen printing

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The final soldering paste thickness should allow proper cleaning of flux residuals and

clearance between sensor package and PCB

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Stencil aperture should have a rectangular shape with dimensions up to 25 µm (1 mil)

smaller than PCB land

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The openings of the stencil for the signal pads should be between 70 - 80% of the PCB

pad area

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Optionally, for better solder paste release, the aperture walls should be trapezoidal and

the corners rounded

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The fine IC leads pitch requires accurate alignment of the stencil to the PCB. The

stencil and printed circuit assembly should be aligned to within 25 µm (1 mil) prior to

application of the solder paste

Process consideration

In case of use of no self-cleaning solder paste, proper board washing after soldering must

be done to remove any possible source of leakage between pads due to flux residues.

The PCB soldering profile depends on the number, size and placement of components on

the board. The soldering profile should be defined by experience more than the pressure

sensor soldering profile only.

No solder material reflow on the side of the package is allowed since LGA packages show

metal trace out of package side.

Solder heat resistance and environmental specification:

The second level interconnect category on ST ECOPACK® lead-free package is marked on

the inner box label, in compliance with JEDEC Standard JESD97. Soldering conditions

maximum ratings are also marked on the same label.

LGA packages for pressure sensor are qualified for soldering heat resistance according to

JEDEC J-STD-020, in MSL3 condition.

4.1

Power supply and sequencing, fail-safe I/Os

There are 2 voltage supplies, VDD_IO and VDD in LPS331AP.

VDD is the core voltage supply to the internal circuits, power on reset, and sensor.

VDD_IO is the supply for the I2C blocks and interface signals.

The operating voltage for both VDD_IO and VDD is from 1.71 V to 3.6 V.

In order to prevent possible leakage in the operational condition, it is necessary to ensure

VDD_IO <= VDD.