The reliability and quality of printed circuit boards has never been more crucial in a modern world dependent on electronic devices: in some industries, lives depend on them.
While no manufacturing process can ever be 100% perfect, the role of a contract electronics manufacturer is to understand potential flaws and mitigate the risks. This means determining why they happen and taking proactive measures to prevent them.
A defective PCB assembly is far more than simply an inconvenience; it translates into delays, increased costs and a negative association with your business’s reputation.
Solder bridging
When solder accidentally connects two or more adjacent pads, this causes a short circuit. Forming a “bridge” between pads, pins or traces, often due to excess solder flowing onto what should be separate conductive elements, this can lead to circuit malfunction and component damage.
To prevent solder from being applied excessively, solder-resistant coating can be added to the PCB during manufacturing to protect the appropriate areas. Pads are separated during the assembly and manufacturing process by a non-conducting solder mask gap, but occasionally, an excess of solder can bridge the gap and connect the two pads. The process can be disrupted by insufficient placement registration, bad solder paste stencils, messy assembly lines and human error.
Lifting pads
Caused by mechanical stress or excessive heat during soldering, PCB lifting pads occur when the copper pad to which the component lead is attached separates from the board. This may happen due to physical or thermal issues, or a flaw in the manufacturing processes.
If lifted pads occur, the pins may not be able to make a clean connection and the component will fail. Most common on single-sided boards, pad lifting more happens when heating the surface of the board causes lower copper adhesion, although a combination of extreme heat and physical padding can also cause the issue. A solution is to use epoxy or adhesive that has high heat resistance, or utilising through-hole circuits for appropriate applications.
Tombstoning
When a component lifts from the board during the soldering process on a surface-mount PCB and resembles a tombstone, the “Stonehenge effect” can happen when one end of the component hasn’t attached properly to the PCB pad, while the other remains firmly soldered.
The soldering defect causes a passive component, like a resistor or capacitor, to stand up on one end. It occurs due to an imbalance in the solder reflow process, typically due to uneven heating, differing pad sizes, or uneven solder paste application. One side of the component bonds to its pad earlier, causing surface tension to lift the other end before it can attach.
Especially common with small components like 0402 or 0201 packages, tombstoning results in an open circuit, leading to device malfunction or failure. Preventive measures include ensuring uniform reflow profiles, balanced solder paste deposition, and precise pad design. Modern automated inspection systems often catch tombstoned parts early, but preventing it at the source is crucial for reliable electronics manufacturing and performance.
Insufficient solder
Potentially causing a weak electrical connection and reliability issues, insufficient solder can lead to poor joint integrity. It can be caused by various factors during the soldering process such as incorrect soldering techniques, inadequate heat and insufficient flux.
Poor PCB design can cause issues, such as if the pads are too close together or too small. When the traces are too compact or thin, this can also cause problems. Basic design flaws can make it challenging to get sufficient heat to the area, causing insufficient solder. Quality testing before a larger production run is important; design faults can be picked up in the early stages.
Open joints
Open joints occur when the solder hasn’t wet the surface of the board adequately, leading to a lack of electrical connection between a component lead and its corresponding pad. Poor soldering techniques, surface preparation issues, or design flaws can all cause this issue.
Causing a break in the electrical path, open joints prevent the current from flowing when the solder isn’t properly wetting both the pad and the lead, including if the solder paste is applied incorrectly, using too little or too much, or if the stencil is dirty.
Component misalignment during reflow, or warping, can also cause the issues; another common cause is heat stress, when the board has been subject to significant temperature fluctuations causing the solder joints to crack and fail. A visual inspection should reveal any gaps or breaks in the solder joints and a multimeter can be used to check for continuity across the suspected open joint, with an open circuit showing infinite resistance.
What systemic issues contribute to PCB assembly failures?
Even when the latest technology is used, any human intervention can introduce an error that can negatively impact an assembly. This can lead to flaws such as incorrect component placement or poor soldering, causing faults that may be challenging to rectify once the PCB has been produced.
Imprecise or inaccurate component placement on a PCB can disrupt its functionality, causing poor electrical connections and potentially short circuits, leading to failure further down the line. If a substandard soldering technique is applied, it can cause various issues including solder bridges, voids and cold joints, compromising the electrical integrity of the board. This can mean individual components failing, or damage to the whole assembly.
Environmental stress including factors such as moisture, corrosive elements and temperature fluctuations can also have a negative effect on the components and solder joints. Over time, environmental stress can weaken the PCB’s structural integrity.
Electrical overstress by exposing the PCB assembly to currents or voltages above its designed specification can cause gradual or immediate failure. This can damage individual components, or the complete assembly. Finally, assembly process inconsistencies including variations in temperature profiles while reflow soldering, or in the solder paste can lead to defects that will compromise the reliability of the PCB.
Why choose Sellectronics?
As trusted and reliable contract electronics manufacturing professionals, Sellectronics routinely addresses all of these areas through our rigorous PCB quality control measures to minimise the risks of assembly defects. We have put in place multiple quality assurance protocols, combined with precise assembly procedures, to deliver PCB assemblies of the highest quality and reliability. These include testing through both external and in-house strategies, such as in-circuit tests; functional test solutions; boundary scans; bespoke test fixtures; 19 inch rack testing; Labview Functional; and outsourced X-Ray testing.
For further information on our products and services, please contact us to discuss the next electronics manufacturing project that you have planned and how we can help you move it to the next level.