TI recommends the use of paste when mounting nFBGAs. The use of paste offers the following advantages:

• It acts as a flux to aid wetting of the solder ball to the PCB land.
• The adhesive properties of the paste will hold the component in place during reflow.
• It helps compensate for minor variations in the planarity of the solder balls.
• Paste contributes to the final volume of solder in the joint, and thus allows this volume to be varied to give an optimum joint.

Paste selection is normally driven by overall system assembly requirements. In general, the “no clean” compositions are preferred due to the difficulty in cleaning under the mounted component. Most assembly operations have found that no changes in existing pastes are required by the addition of nFBGA, but due to the large variety of board designs and tolerances, it is not possible to say this will be true for any specific application.

Nearly as critical as paste selection is stencil design. A proactive approach to stencil design can pay large dividends in assembly yields and lower costs. In general, nFBGA packages are special cases of BGA packages, and the general design guidelines for BGA package assembly applies to them as well. There are some excellent papers on BGA assembly, so only a brief overview of issues especially important to nFBGA packages is presented here.

The typical stencil hole diameter should be the same size as the land area, and 100-120-μm-thick stencils have been found to give the best results. Good release and a consistent amount of solder paste and shapes are critical, especially as ball pitches decrease. The use of metal squeegee blades, or at the very least, high durometer polyblades, is important in achieving this. Paste viscosity and consistency during screening are some variables that require close control.