MMA8491Q
Sensors
Freescale Semiconductor, Inc. 19
6 Mounting Guidelines
Surface mount printed circuit board (PCB) layout is a critical portion of the total design. The footprint for the surface mount
packages must be the correct size to ensure proper solder connection interface between the PCB and the package. With the
correct footprint, the packages will self-align when subjected to a solder reflow process. The purpose is to minimize the stress
on the package after board mounting. The MMA8491Q accelerometers use the QFN package. This section describes suggested
methods of soldering and mounting these devices to the PCB for consumer applications.
6.1 Overview of soldering considerations
The information provided here is based on experiments executed on QFN devices. They do not represent exact conditions
present at a customer site. Hence, information herein should be used as guidance only and process and design optimizations
are recommended to develop an application specific solution. It should be noted that with the proper PCB footprint and solder
stencil designs, the package will self-align during the solder reflow process.
6.2 Halogen content
This package is designed to be Halogen Free, exceeding most industry and customer standards. Halogen Free means that no
homogeneous material within the assembly package shall contain chlorine (Cl) in excess of 700 ppm or 0.07% weight/weight or
bromine (Br) in excess of 900 ppm or 0.09% weight/weight.
6.3 PCB mounting recommendations
1. Do not solder down Exposed Pad (EP) under the package to minimize board mounting stress impact to product
performance.
2. PCB landing pad is 0.675 mm x 0.325 mm as shown in Figure 12.
3. Solder mask opening = PCB land pad edge + 0.2 mm larger all around.
4. Stencil opening size is 0.625 mm x 0.3 mm.
5. Stencil thickness is 100 or 125 μm.
6. The solder mask should not cover any of the PCB landing pads, as shown in Figure 12.
7. No additional via nor metal pattern underneath package on the top of the PCB layer.
8. Do not place any components or vias within 2 mm of the package land area. This may cause additional package stress
if it is too close to the package land area.
9. Signal traces connected to pads should be as symmetric as possible. Put dummy traces on NC pads, to have same
length of exposed trace for all pads.
10. Use a standard pick and place process and equipment. Do not use a hand soldering process.
11. Customers are advised to be cautious about the proximity of screw down holes to the sensor, and the location of any
press fit to the assembled PCB when in an enclosure. It is important that the assembled PCB remain flat after
assembly to keep electronic operation of the device optimal.
12. The PCB should be rated for the multiple lead-free reflow condition with max 260°C temperature.
13. Freescale sensors are compliant with Restrictions on Hazardous Substances (RoHS), having halide free molding
compound (green) and lead-free terminations. These terminations are compatible with tin-lead (Sn-Pb) as well as tin-
silver-copper (Sn-Ag-Cu) solder paste soldering processes. Reflow profiles applicable to those processes can be used
successfully for soldering the devices.
