We often assume our footprints are always designed correctly, or that the design rules we assign will never produce an assembly error. With SMD components, there are problems that can arise as components sit on solder paste during reflow. During assembly, excessively thick regions of solder paste are not always better for component assembly because components can float or shift during reflow.
What can be done to stop this type of component shifting during reflow? There are both design and assembly problems that can lead to floating, so the solution requires examining design data and assembly practices.
SMD Components Can Float on Solder Paste
When SMD components are being put into reflow soldering during assembly, the components must first be mounted onto solder paste on the PCB. Solder paste can be selectively deposited with an automated dispenser, or it can be deposited through a stencil. In either case, the SMD parts sit on top of the solder paste, which will then form a strong joint during reflow.
Depending on the amount of solder paste that is placed on the board, the component could float and shift during reflow. This is commonly seen as tombstoning or slight skews in reflow soldering, where components can rotate or stand on-end. The other problem that can occur is shifting or tilting of components. Components that are taller can have greater shifting or tilting due to floating on solder paste. These include:
- Connectors and pin headers
- Testpoints or board-to-board pin connectors
- Tall radial capacitors
- Narrow SMD ICs
These components can sit tall on the PCB, but may not have the bottom-heavy characteristic needed to prevent shifting on a thick pool of solder paste. Depending on the following factors, floating and tilting might occur in some components.
Excessive Stencil Thickness
Stencil thickness can result in excessive solder paste application across the entire PCB. If this occurs, tilting and shifting of components might occur anywhere in the PCB. Note that just because paste is thick it does not mean components are guaranteed to shift. But if shifting and tilting appears to happen randomly in many places in a PCB, then stencil thickness may be the culprit.
Depending on the stencil size and aperture opening, solder filets could be mismatched across pins. An example is shown below, where excessive solder paste was deposited on the end pins of an SOIC component. For this component, floating did not occur because the solder paste on the inner pins was not excessive.
Pads Are Too Small
If land pads for SMD components are too small, there will not be room to form a flattened solder filet on the component, despite the use of an appropriately thin stencil. The result is pressure from the solder paste on the component as it solidifies and forms intermetallics. If the pad size can be increased, then the extra solder can wet outwards with just enough solder underneath the part to form the intermetallic layer.
This is a component footprint design issue. If the PCB design cannot be changed, the solder volume may need to be reduced further with a thinner stencil or with smaller apertures. Generally, the land patterns and package sizing pairings defined in IPC/JEDEC standards are intended to ensure reliable soldering to landing pads such that compliant components with low profiles are less likely to exhibit this problem.
Apply the correct paste masks in your PCB footprints.
Board Warping During Reflow
Finally, if the above points are inspected and no problems are found, it is possible that the PCB may be warping during reflow. Warpage can result in thinner boards, in specialty materials with high CTE, or in physically larger planar boards. If warpage occurs, the board can twist or bend, and this can lead to slight component shifts.
For moderate-case-size components and low-profile SMD passives, this is most likely not such a problem unless they have very excessive solder paste. For tall components or tall connectors (such as pin headers or pogo pins), these top heavy components can shift more on solder paste. When warpage is present, there should be some fixture used to prevent component shifts or maintain planarity of the board during reflow.
When All Else Fails, Use a Fixture
If all the above points have been addressed, and there is still a problem with floating components that results in tilting, then a fixture may be in order. Fixtures can be fabricated and applied to a reflow tray that will hold tall components in place as the board passes through reflow. These fixtures are normally custom fabricated from sheet metal.
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