It is often recommended that PCB assembly end with cleaning of any residues on or around components. However, this is sometimes easier said than done. Some components that are soldered with no-clean flux will have very low clearances, and as a result, their low clearances will result in accumulation of flux residues beneath the component package. Most assemblers may be content to leave these residues alone as it is impossible to access them below a low-clearance or zero-clearance component.
The question then becomes: will these residues result in failure of the system or component? It is known that flux residues can break down, even if not fully cured. The leftover uncured residues can create the same problem as residual cured residues. Therefore, there are some simple design for assembly steps a designer could consider when working with zero-clearance components.
Residues Accumulate Below Low-Clearance Components
Solder flux is important because it aids in soldering/desoldering processes by eliminating oxide films on the surface of the soldered metals. Solder flux also aids wetting of solder onto surfaces such that it provides more uniform cover and adherence to the soldered surfaces. These are important functions of solder paste, even with low-clearance or zero-clearance components.
There can be a problem with no-clean flux residues that accumulate below these components, such as below LQFPs, QFNs, or CSPs. These components normally have a die-attached pad on the bottom side (normally connected to a ground plane), known as a bottom-terminated component.
Leadless packages can have uncured flux accumulation below the die-attached pad.
At a high level, problems with flux residues arise in the following set of steps:
- Uncured flux accumulates below the component during soldering
- Flux residue causes a failure due to moisture accumulation, voiding, or breakdown
- The failure is traced back to the component, which is then desoldered
- After desoldering, uncured flux is found and cleaned
- The component is replaced and the process repeats!
In some cases, such as with very low voltages or in dry environments, flux residue below a component will not be a reliability issue. With some newer power components that have very low clearances, this will be a problem as the flux could break down due to exposure to high voltage, causing a short.
Therefore, to help prevent these problems, ground vias should be used in the die-attached pad to provide venting for solder paste through the back side of the PCB. This is commonly shown in many footprints for flat-pack or CSP components, such as the leadless package shown above.
The via array performs two functions. First, it allows outgassing to occur, which will aid curing of the flux during a reflow cycle or during manual assembly. Second, these act as thermal vias that carry heat away from the die and conduct it directly into an internal ground plane.
QFN package example with vias on the internal pad.
These vias should be untented and unfilled/unplated if you want to take advantage of venting below the component in a leadless package such as that shown above. While this will cause some solder to flow to the back side of the board, it is best if other components are kept clear of this region so as to prevent any shorts on the back side.
If venting cannot be used below the component, such as when component densities on the opposite side are high, there are three options for helping prevent these accumulation of uncured flux.
Alternative #1 - Increase the Pre-soak Time
During reflow soldering, it’s possible to increase the soak time so that more of the flux cures during a single reflow cycle. This will not remove flux below a zero-clearance component, but it will help cure it and help eliminate the reliability challenges listed above.
Alternative #2 - Solder Paste Printing
Instead of a stencil and squeegee, selectively print solder paste onto the component pads. With this process, you can control the amount of solder applied to a component’s SMD pads, as well as where it is applied. This would reduce the region where any uncured flux might accumulate, as well as preventing a buildup of cured flux around the component edge that would trap uncured flux in the die-attached pad region.
Printing solder paste can be used to control the amount being dispensed on pads for no-clearance components.
No-clean flux may be one of the most unfortunately-named products ever conceived. As has been pointed out in another article, there are many instances where no-clean flux actually requires cleaning due to its leftover residues, and particularly their electrical properties. In the event that you need to remove leftover no-clean flux residues, or prevent them entirely below no-clearance components, talk to your assembler to see what can be done. Also consider the design options listed above to prevent flux residue accumulation.
Whatever your assembly requirements, or if you need venting below zero clearance components, make sure you use the industry’s best CAD tools in OrCAD from Cadence to prepare your PCB design for volume assembly. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.q