Short circuits are one of the most common defects in surface-mount PCB assembly using soldering processes like reflow soldering. They can lead to field failures and impair product function. Detecting shorts early using the right inspection methods is crucial for quality and reliability.
This article discusses ways to test for and identify different types of solder-related short circuits in SMT boards. It covers shorts between pads, traces, pins, component leads/terminations as well various inline and off-line electrical testing techniques with pros and cons. Recommendations are provided for choosing the optimal inspection strategy based on defect criticality, volume and cost considerations.
Introduction to Detection of SMT Shorts
A short circuit or ‘short’ is an unintended electrical connection between two conductors due to the following typical defects in SMT soldering:
Bridging – Solder forms a conductive bridge between adjacent pads, traces, component leads etc. which should be isolated.
Insufficient Clearance – Very close spacing of conductors allows conduction through residues or humidity.
Excess Solder – Extra solder creates shorting paths between neighboring conductors.
Tombstoning – Partially lifted SMD component creates short post-soldering between leads.
Solder Balls – Conductive solder spheres/splatter short adjacent exposed conductors.
Short circuits can cause:
- Increased current draw damaging circuits.
- Unwanted activation of components.
- incorrect logic levels and erratic circuit behavior.
- Intermittent contacts and reduced reliability over time.
Detecting and eliminating shorts in SMT assembly is thus critical for product function and robustness.
Test Methods for Detecting SMT Solder Shorts
Several testing approaches allow detecting solder shorts on surface mount boards:
Automatic Optical Inspection (AOI)
- Captures images of post-soldering PCBs and analyses using algorithms.
- Detects visible shorts like solder bridges.
- Limited by resolution, optics and algorithm accuracy.
- Typical in-line inspection technique.
X-Ray Inspection
- Radiographic images reveal hidden shorts like insufficient clearance.
- Requires expertise for analysis.
- Used for inspection of critical components.
- Time consuming for whole board inspection.
Flying Probe Testing
- Electrically probes test nodes on powered-up board using flying probe heads.
- Detects conductivity between shorted nets.
- Limited access to nodes in dense boards.
ICT – In-Circuit Test
- Tests board by probing test points at component pins level.
- Detects shorts between component leads and nodes.
- Fixtures limit access to dense boards.
- Significant test development effort.
Boundary Scan Testing
- Uses on-board JTAG circuitry for interconnect testing.
- Detects short circuits between components with boundary scan cells.
- Needs support from components.
- Very thorough but system level focus.
Functional Testing
- Validates board operation under simulation of actual conditions.
- Behavioral anomalies reveal faults like short circuits.
- System level focus. Software-based.
SMT Defects that Can Cause Short Circuits
Solder Bridging
- Solder forms undesired bridge between traces or pads.
- Caused by excess solder volume, improper temperature or component spacing.
- Bridging across multiple pins of a component especially problematic.
- Visual inspection or x-ray detects bridging. Electrical testing confirms.
Insufficient Clearance
- Very narrow spacing between conductors.
- Residues or condensation allows current flow under high humidity/voltage.
- Caused by poor board design tolerances.
- Detectable via x-ray or environmental stress screening.
Excess Solder
- Extra unwanted solder creates shorting paths.
- Due to issues in solder paste volume, reflow profile or component leveling.
- Visual inspection reveals excess solder shorts.
Tombstoning
- SMD components lift up on one side during reflow creating shorts between terminations.
- Due to imbalanced wetting forces from poor paste stencil/volume.
- Easily spotted under visual inspection.
Solder Balls/Splatter
- Airborne solder particles solidify into conductive spheres.
- Caused by solder slumping/splash during reflow.
- Can short pads, traces positioned above solder joints.
- Visual inspection identifies solder ball shorts.
Solutions for Preventing SMT Solder Shorts
SMT solder shorts can be minimized by:
- Maintaining optimum soldering temperature, ramp rates and times.
- Using no-clean, low residue flux to avoid bridging post-reflow.
- Having adequate spacing between conductors based on voltage levels.
- Containing solder paste volume between 0.66 to 1.5 times pad area.
- Making stencil apertures inner dimensions match pad sizes.
- Having sufficient clearance between stencil apertures to avoid solder bridging.
- Using stencils of proper thickness – 0.1 to 0.15 mm typically.
- Matching pad shapes/orientations to paste print direction to reduce tombstoning.
- Making any board cutouts over 30 mm edge to edge to prevent solder balling.
- Placing heat sinks and shields to avoid solder splash on areas prone to shorts.
Recommended Inspection Methods Based on Criticality
High Criticality – Full electrical testing – ICT, boundary scan combined with AOI and x-ray inspection. Ensures all shorts detected.
Medium Criticality – Sample electrical testing along with automated optical and x-ray inspection. Provides good defect coverage.
Low Criticality – Primarily automated optical inspection of solder joints combined with periodic x-ray inspection. Balance between cost and defect detection.
Inspection Guideline Based on Volume of Production
Low volume – Emphasis on functional testing to detect faults. Optical inspection for assembly process feedback.
Medium volume – Add sample electrical testing like flying probe inspection. Use automated optical inspection.
High volume – Continuous inline AOI of all boards. Stop and repeat electrical testing on sample boards.
Field Failures from SMT Solder Shorts
Field failures from undetected solder shorts include:
Early life failures – Present immediately on power up and testing. Cause extensive rework.
Late life failures – Develop over time in operation due to factors like vibration, corrosion, moisture ingress. Cause reliability concerns.
Intermittent failures – Cause erratic behavior only under specific conditions. Hard to reproduce and diagnose.
Latent defects – Exist without issue until triggered by an event like static voltage surge. Sporadic failures.
Thorough inspection during assembly prevents field issues escaping to customers. Follow best practices based on criticality.
Conclusion
Short circuits are difficult to completely avoid in complex, dense SMT boards but can be minimized via design guidelines and controlled process. Electrical test techniques like ICT and boundary scan provide the most comprehensive defect coverage but at higher cost. They are recommended for critical boards. For high volume boards, automated optical inspection offers a good balance between cost and test coverage. The appropriate inspection methods should be applied based on criticality of product operation and constraints of volume, cost and time. This allows maximizing defect detection and elimination to ensure reliable soldering and minimal field failures due to shorts.
FAQs
Q1. What is the easiest method to check for short circuits?
The quickest way is visual inspection of solder joints under a microscope to look for visible shorts like solder bridges, tombstoning, excess solder etc. It catches obvious shorts.
Q2. Can Diagnostic X-ray find short circuits?
Yes, x-ray inspection can help reveal hidden shorts like insufficient clearance which are not visible externally. It also validates component placement and solder defects.
Q3. What type of testing finds the highest number of shorts?
In-circuit testing using test fixtures will find the most shorts by testing conductivity between component nodes which optical or x-ray inspection cannot. Boundary scan testing also catches many shorts.
Q4. When should flying probe testing be used to find shorts?
Flying probes are best for medium volume, prototype or pilot runs where failures are unacceptable but volume is not high enough for justification of custom ICT fixtures.
Q5. Does AOI always catch solder shorts?
No, AOI can miss hidden shorts or those below its resolution limit. But it is very effective at finding visible shorts like solder bridges between pads and traces if algorithm is robust.
6 Inspection Methods For Short Circuit
1. Open the PCB design on the computer, light up the short-circuited network, and see where the nearest is,and where the easiest connect. Pay special attention to the short circuit inside the IC.
2. if it is manual welding, it is necessary to develop good habits:
A. Visually inspect the PCB board before soldering, and use a multimeter to check whether the important circuit (especially the power supply and ground) is short-circuited or not;
B. Use a multimeter to measure whether the power supply and ground are short-circuited after each chip is soldered.
C. Do not through off the soldering iron when soldering, if the solder tin is rubbed onto the soldering feet of the chip (especially surface mount components), it is not easy to find.
3. It was found that there was a short circuit. Take a board to cut the circuit (especially suitable for single/double board), after each secant, each part of the function block is energized separately and gradually eliminated.
4. Using the short-circuit positioning analysis instrument.
5. If there is a BGA chip, since all the solder joints are invisible by the chip cover, and it is a multi-layer pcb board (more than 4 layers), it is best to separate the power supply of each chip when designing, using magnetic beads or the resistor which is 0 ohms. so that the power supply is shorted to the ground, the magnetic bead is cut off and is detected and it is easy to locate a certain chip. Due to the difficulty in soldering the BGA, if it is not the machine’s automatic soldering, the adjacent power supply and ground solder balls will be short-circuited with a little care.
6. Small size surface mount capacitors must be careful when soldering, especially the power supply filter capacitor (103 or 104), the number is large, it is easy to cause the power supply and ground short circuit. Of course, sometimes the bad luck to come across the capacitor itself is short-circuited, so the best way is to check the capacitance before soldering.
