Mask the pins to keep solder where it belongs.
There was a time when the bison ran free on the plains and power components were easy to design with. Everything, with the exception of an exotic few, used either the TO-220 or TO-3 packages. Even when surface mount came along and cut the bison off from their grazing lands, most power components came in some derivative of the TO-220, with bent leads.
That’s no longer the case. Today, power components come in those TO-220 derivatives, SO-8 packages, QFNs, and down to 0.3mm pitch wafer-scale µBGAs. It’s madness.
The advantage of all that chaos is it gives more flexibility for sourcing and sizing of components. Which, of course, brings a few more potential issues.
Take the example in FIGURE 1. The footprints were originally created for a package with four 1.27mm (0.05") pitch leads on one side and a big heat slug on the other. The component selected is a variant in an SO-8 package. It’s not an uncommon occurrence.
Figure 1. A large open copper pad will impede a quality solder joint.
As long as pins 5 to 8 share the same internal connection, there isn’t anything electrically wrong here. However, with that large open copper pad on top, it’s going to be very difficult to get a good solder joint. The solder will spread out, and you’ll likely end up with mechanically inadequate joints.
The fix is pretty easy. Just add solder mask to separate the pins. Make the mask openings the same size as if the pins were on individual pads. There is no need to cover the whole pad with solder mask; just surround the pins so solder will stay where it’s needed. The mockup in FIGURE 2 illustrates what it would look like.
Figure 2. The same design, with solder mask added to separate the pins.
Do the same with the solder paste layer. Unless the component has a heat slug underneath, make the paste layer block the big open area.
Reflow will still take a bit of extra care, due to the thermal mass of the large solid pad, but, assembly houses are prepared to deal with such thermal challenges. Unmasked pad areas, as in the original footprint here, have to be corrected, before the board gets to manufacturing.
is marketing manager and chief technology champion at Screaming Circuits (screamingcircuits.com);