PCB assembly defects are irritating, and they can easily derail a project if not identified during assembly. The two common defects that arise on leadless components in a PCBA are open circuits and short circuits. The various causes of short circuit defects often get the most attention, but open circuits can also be very challenging. Shorts are much easier to distinguish specifically as a defect compared to open defects, including on leadless components.
Leadless components can have open circuits, but they can have a variety of causes that arise during manufacturing and soldering. They can also be difficult to identify through probing because the pads are often below the component package.
Some Causes of Opens on Leadless Parts
Warping of the PCB during reflow can cause lifting of a component during soldering. There are many causes of temporary or permanent warpage that can arise due to choices made in the design phase. If the board is designed such that there is danger of warping, fixation may be needed during reflow soldering in order to hold the board shape nearly flat and thus prevent lifting of components.
Over time, component leads or solder pads on the PCB can become contaminated. This could be due to exposure to moisture, noxious chemicals, or solvents. If a component lead becomes contaminated, then solder may have a more difficult time wetting onto the component lead and would be unable to form a strong bond. This might create a latent defect that only appears at certain temperatures, or it could result in an open circuit due to no solder adhering to the component.
Oxidation is the most common form of defect that will impact BGA soldering.
When soldering leadless components like BGAs or LGAs, short circuits are usually caused by too much solder, worn apertures in stencils, or lack of solder mask damming between pads. Therefore, it makes sense that the opposite problem (open circuits) could be caused by insufficient solder being applied through a stencil or in an automated paste dispenser.
You may need to adjust your solder paste dispenser volume.
A problem related to insufficient solder could arise when via-in-pad is used in the design. Via-in-pad is common in high-density leadless components of all types, ranging from processors in fine-pitch BGA packages to SMD modules in LGA packages. When via-in-pad is used, but the required filling/plugging data was implemented, some solder can wick to the back side of the PCB during reflow. This could reduce the available solder under the leadless component.
Head and Pillow
This defect is particular to BGA packages; it results when part of the ball separates during soldering and does not form a uniform joint. The result is one part of the ball sitting on top of the lower part of the pad. This can present an intermittent open circuit defect. When the package heats up, the package substrate and/or PCB can expand and separate the contact between the package and the PCB.
Head-in-pillow below a BGA package. [Source]
Excess Solder in Die Pads
The final defect that can lead to open circuits is excess solder in die-attached pads. This is primarily a problem in QFN packages, some SMD modules, and some LGAs. BGA packages do not have a die-attached pad.
If there is too much solder paste on the die pad, the solder could lift up the component during reflow. The result is a weak connection or open connection on other pins in the package. This is one reason you may see a PCB footprint for a QFN with some sectioning of the die pad using strips of solder mask. Defining these apertures is very simple and can be applied when creating the PCB footprint, or it can be used in the PCB layout.
Solder mask opening example for the die pad on a QFN package. The paste mask aperture could be made smaller if component lifting is creating open circuits.
How to Diagnose These Defects
There are two issues to consider when these defects arise repeatedly during production: diagnosis of the actual defect before shipping the product, as well as diagnosis of the root cause of the defect. Identification of the defect is performed by one of the standard mechanisms during PCB assembly:
- AOI (for QFNs) or X-ray inspection (for BGAs/LGAs)
- Bed of nails testing (continuity tests)
- In-circuit testing or on-the-line automated functional testing
Once these defects are identified during inspection, it’s up to the assembly process engineering team to make corrections to the process so that these defects can be prevented. Some additional investigation would be needed to determine what is the root cause of each type of defect. Some steps that can be taken are summarized in the table below.
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