Stencil design and solder paste application

TN0018

6/9

DocID12707 Rev 5

3

Stencil design and solder paste application

The thickness and the pattern of the soldering paste are important for the proper MEMS

sensor mounting process.



Stainless steel stencils are recommended for solder paste application;



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



The openings of the stencil for the signal pads should be between 70% and 90% of the

PCB pad area;



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

the corners rounded;



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

circuit board. The stencil and printed circuit assembly should be aligned to within 25 μm

(1 mil) prior to application of the solder paste.

4

Process considerations



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

application board. For this reason it is not possible to define a unique soldering profile

for the sensor only. The customer should use a time and temperature reflow profile

based on PCB design and manufacturing expertise.



In order to reduce residual stress on the components, the recommended ramp-down

temperature slope should not exceed -3 °C/s.



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

show metal traces on the side of the package.



If “self-cleaning” solder paste is not used, the board must be properly cleaned after

soldering to eliminate any possible source of leakage between adjacent pads due to

flux residues.



The final volume of soldering paste applied to each PCB land is recommended to be

within 20% among (all) the PCB land pads.



Based on the Jedec 9702 standard, a component shows negligible output variation up

to stress intensity of 500 me (microstrain).