LM3673

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SNVS434L – JULY 2006 – REVISED OCTOBER 2015

11.2 Layout Example

Figure 28. LM3673 DSBGA Top Layer Board Layout

11.3 DSBGA Package Assembly and Use

Use of the DSBGA package requires specialized board layout, precision mounting and careful re-flow

techniques, as detailed in Texas Instruments Application Note 1112 DSBGA Wafer Level Chip Scale Package

(SNVA009). Refer to the section Surface Mount Technology (SMD) Assembly Considerations. For best results in

assembly, alignment ordinals on the PC board must be used to facilitate placement of the device. The pad style

used with DSBGA package must be the NSMD (non-solder mask defined) type. This means that the solder-mask

opening is larger than the pad size. This prevents a lip that otherwise forms if the solder-mask and pad overlap,

from holding the device off the surface of the board and interfering with mounting. See Application Note 1112

(SNVA009) for specific instructions how to do this. The 5-pin package used for LM3673 has 300-micron solder

balls and requires 10.82 mils pads for mounting on the circuit board. The trace to each pad must enter the pad

with a 90° entry angle to prevent debris from being caught in deep corners. Initially, the trace to each pad must

be 7 mil wide, for a section approximately 7 mil long or longer, as a thermal relief. Then each trace must neck up

or down to its optimal width. The important criteria is symmetry. This ensures the solder bumps on the LM3673

re-flow evenly and that the device solders level to the board. In particular, special attention must be paid to the

pads for bumps A1 and A3, because VIN and GND are typically connected to large copper planes, inadequate

thermal relief can result in late or inadequate re-flow of these bumps.

The DSBGA package is optimized for the smallest possible size in applications with red or infrared opaque

cases. Because the DSBGA package lacks the plastic encapsulation characteristic of larger devices, it is

vulnerable to light. Backside metallization and/or epoxy coating, along with front-side shading by the printed

circuit board, reduce this sensitivity. However, the package has exposed die edges. In particular, DSBGA

devices are sensitive to light, in the red and infrared range, shining on the package’s exposed die edges.

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