The number of vias represented in TI example land patterns located near the end of the data sheets should be considered as an example starting point. Not all applications require vias. The need for vias depends on the amount of power the device dissipates. If the board routing becomes too challenging to employ the use of thermal vias, contact your local TI representative for further guidance. The center thermal pad, however, should always be soldered to the board for mechanical strength and reliability. For thermally challenging applications, TI recommends that the thermal vias be placed on a pitch of approximately 1,0 mm. Per standard PCB manufacturing capabilities, 0,3 mm diameter drill holes are recommended as a starting point, but a smaller via offers less risk of solder volume loss. On applications where solder volume loss through the vias is of concern, plugging or tenting can be used to achieve a repeatable process.

Vias that are plugged eliminate the risk of solder volume loss. If plugging the via is not economically suitable, tenting also can offer benefit. Tenting from the back or topside of the board can cause processing issues with some printed circuit board manufacturers due to chemistry entrapment during the plating process. In the case of plugging or tenting from the backside of the board, the voiding potential can increase due to the air trapped within the tented via. Check with your board fabrication site for recommendations.

Another solder-mask tenting technique is to tent from the top of the board. The via solder-mask diameter must be 0,1 mm larger than the via hole diameter when tenting. Another variation is to create a cross-hatch pattern of solder mask to create a predictable area of coverage. Trials have shown that via tenting from the top is less likely to produce random voids between the exposed pad and the PCB pad. See Figure 3-4 for x-ray results of various via tenting structures.

For finishes that use an organic solder preserve (OSP), not tenting any vias from the top or bottom of board has shown repeatable performance in soldering. In addition, optimizing the profile to achieve maximum flux activity during the critical reflow stage reduces the amount of voiding seen. Trials have shown that an excessive soak time, which activates the majority of the flux before the melting temperature of the alloy, results in large voids. Check with your solder paste manufacturer if an alternative profile can offer more flux activity through the critical melting phase of the alloy used.

Figure 3-4 X-Ray Images for Reference