When we refer to component skew, we are likely referring to two instances: lateral skew or rotation, where the component sits flat and is oriented at an angle, or tombstoning. The form designers are probably most familiar with is tombstoning, and there are plenty of pictures online where passives are standing on their edge due to very uneven soldering temperature. This is probably the least common type of skew in real situations, and orientation skew is much more common in real boards.
Two Kinds of Component Skew
No matter which type of skew we are referring to in SMD components, skew is caused by uneven distribution of surface tension in solder during PCB assembly. When the surface tension is uneven during soldering, the component will be pulled by the imbalanced forces, leading to skew. In the most extreme cases, this will lead to tombstoning, where a component is pulled upright along its edge. The uneven surface tension originates from uneven temperature across all solder pads in a given component.
The table below summarizes some causes and solutions of each type of skew.
The reflow profile is something that is more specialized for manufacturers, and the designer has no control over the reflow profile. In fact, even if you specify a reflow profile to deal with one at-risk component, you will still need to rely on your assembler to determine the appropriate reflow profile that minimizes reflow passes and ensures reliable assembly of all components.
At the design level, there are two simple things a designer can do to help reduce skew before sending a board to manufacturing.
The use of thermal reliefs is most often associated with soldering of through-hole components when the through-hole pads are connected to large planes. The idea is to ensure strong solder joints are formed by maintaining a uniform heat distribution between the leads on the through-hole component.
Although this is most commonly associated with through-hole components, applying a thermal connection is important if an SMD component is being soldered to a large copper fill. The small spokes on the thermal relief will prevent heat diffusion from the pad into the copper fill. This will decrease any temperature difference across the two pads and will help prevent tombstoning.
Thermal relief connection applied to a large SMD capacitor.
Break Up the Pad/Paste Mask
If you examine most component libraries, you’ll find that the paste mask layer in a component footprint will be set to the same size and shape as the solder pads. When components have an asymmetric shape and a pad that is much larger than all other pads (like a die-attached pad), the larger amount of solder on this pad could create skew during reflow.
This would arise because the smaller pads could reflow and solidify before the larger central pad. The result is uneven surface tension on the component leads, leading to skew. Here, there are two simple solutions:
- Apply a negative paste mask expansion that reduces the amount of required solder paste
- Use a smaller set of land pads and smaller paste mask regions to apply solder paste
A common IC package where component skew can occur is in D2PAK components. In a D2PAK component, the larger pad in the package would require a lot of solder to ensure a strong bond to the SMD landing pad.
Approach #2 is shown for a D2PAK package in the image below. In this example D2PAK, the large pad region in the center of the component package is broken up into smaller pads. These are solder mask defined pads with connection back to GND through the array of vias in the central region of the component. Even with this array of vias, breaking up the large pad into smaller sections as solder mask defined pads will hold solder on the pad region and will prevent wicking to the back side of the board.
Modified land pattern for a D2PAK component.
Such a solution could be used in a QFN footprint, although this is not typically done as a designer would likely just plug the via and tent the opposite side of the board. However, this would not eliminate the potential for component shifts during production.
As we can see in the example below, the component has been successfully soldered onto the central pad region without skew using the above land pattern. There is also minimal lateral shifting of the component’s position on the solder pads.
Successful soldering of a D2PAK component.
Who Needs to Worry About Skew?
Most often, this will be a problem that designers should care about when they move a design into volume production. The reasoning for this has to do with the probability of rework versus the cost of reworked or scrapped boards.
First, in short run prototyping, the reflow profile is being programmed fresh for each prototyping run; there is no chance for drift from these settings. Also, the probability for extreme skew to occur is generally low anyways, so it is not likely to occur in a prototyping run. If it does occur, it may only be a couple of boards, which could be reworked by hand. Because the risk factor is so low, we don’t generally implement the paste mask methods shown above in prototyping.
The issue is different in high volume. Although the probability of skew or tombstoning may be low, that could translate into 10’s of thousands of boards in high volume. Those scrap and rework costs add up quickly, so the designer and assembler need to work together to ensure design data is optimal for high-volume manufacturing. This is also important for companies manufacturing in multiple facilities: the designer needs to have these fixes in their design data to ensure they can manufacture the board anywhere.
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