Solder Paste Stencil
Typically, the signal pads should have a standoff height of 2 to 3 mils after assembly. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from connecting. To help prevent this, the solder stencil opening should be broken into a series of smaller openings and should cover between 50 and 75% of the pad area.
This means that when you lay out your pcb, you need to look carefully at the solder paste layer for your QFN components. If the solder paste layer in the CAD package part library just follows the copper pad pattern or the solder mask opening, you may need to customize the CAD package part library to avoid leading yourself into trouble.
To better illustrate the proper way to make your solder paste stencil for QFN parts, I went to our back room and took a couple of photos of good and bad solder paste stencil practices.
Figure 2. A worst-case stencil opening (top) with recommended practice of segmenting the opening (bottom)]
Figure 2, top half, illustrates what a just about worst-case stencil would look like. Actual size for the part is 7 x 7 mm. Note how much surface area that the center pad has compared to the row of side cutouts. With most SMT components, it is standard procedure to reduce the size of the paste cutout area in the stencil. In a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil are as shown in the bottom half of figure 2.
If you just reduce the paste opening aperture, providing one smaller opening, but don't segment it, you may end up with a part that is still too high in the middle to assure good contact on the signal pads and is also unstable and will likely tilt to one side. With leaded solder, a single 50% sized opening may work because of the wicking properties of lead-based solder. Since lead-free solder does not wick as well, it is very unlikely to work in a RoHS process. In both cases, the most consistently reliable method is to segment the stencil pattern.
The recommended practice is to segment the solder paste opening in the center of the QFN. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of high-centering and other problems associated with large paste areas, such as out gassing and spattering.
This will give good solder distribution with little chance of high-centering or outgassing problems. When you reduce or segment the solder paste stencil pattern, do not do the same with the solder mask. Assembly works best with an even distribution of solder. Masking part of the pad area will work against good even paste distribution.
Specialized Copper Pad
Some parts, especially high-frequency parts, such as the MC13192 ZigBee part, require a segmented copper pad under the QFN. If this is the case, it is important to segment the solder paste stencil to match the custom pad. It is fairly common practice to use a standard full-size square opening and assume that surface tension will end up distributing the solder in the right places. For best reliability and buildability, make sure that the openings match your copper layer underneath the stencil openings. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.
The first run of the design described above properly used a segmented copper layer, but had a full-size opening for the stencil. Sure enough, that wide opening allowed too much solder on the copper. The problem was further exacerbated when the solder migrated off of the bare board areas and on to the copper pad segments. Fixing the stencil fixed the problem.
With larger QFN parts, the opposite problem can occur in the center pad area. When the square opening for the solder paste stencil is fully open on a larger part say 10 x 10 mm or larger the paste squeegee may deform and actually scoop too much of the paste out of the opening.
This can lead to uneven paste and solder voids. Both are potential reliability problems. The solution is the same. Segment the stencil opening to create an even paste distribution.
The QFN form factor delivers a number of advantages over other SMT package form factors. It is generally a smaller part and, with the center pad, can have better grounding and thermal properties.
These advantages are partially offset by layout and assembly difficulties. By following a few simple guidelines, you can use the parts with confidence. Check the layout guidelines in the component applications notes.
Segment your solder stencil opening for the center pad. Make a custom component library for your CAD package if you need to. Then Design away.
About the author
Duane Benson is a marketing manager at Screaming Circuits. He started his career in the early-eighties designing with the Z80, CDP1802 and 8088 and moved to product marketing in 1989 as the first product manager for In Focus Systems. This combination of electronic design and marketing gives him a unique perspective on the electronics engineer as a business consumer. He can be reached at email@example.com.