The soldering parameters such as the thickness of the stencil, the final soldering paste thickness and planarity and the velocity of the temperature change during the reflow must be rigorously controlled since they can produce dangerous side effects on the microphone structure. A different soldering paste thickness can generate different forces on microphone pads and on the package as well as an excessively fast temperature gradient can mechanically stress the device package.
• Stainless steel stencils are recommended
• Stencil thickness of 75 - 100 μm (2.95 - 3.94 mils) is recommended for screen printing
• Stencil aperture should have a rectangular shape
• The openings of the stencil for the signal pads should be between 70 - 90% of the PCB pad (The recommended stencil openings for each ST microphone are shown in the following figures)
• Optionally, for better solder paste release, the aperture walls should be trapezoidal and the corners rounded
• The fine IC lead 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
• The final soldering paste thickness should allow for proper cleaning of flux residuals and clearance between sensor package and PCB
• The soldering paste thickness, filling the stencil openings, must be as homogeneous as possible
• The reflow profile depicted in Figure 21 must be followed
• The vapor phase reflow soldering process must be avoided since during the vapor stage some particles can enter in the sound inlet and contaminate the MEMS sensor
• If possible, sealing the microphone port hole with foam or with tape during the soldering process is recommended
• Do not clean the PCB with any ultrasonic method
• Manual soldering should be avoided since it is difficult to ensure the proper temperature gradient and temperature variation over time
For further details (i.e. land pattern, stencil openings) please download the application note AN4211 from the ST website
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