Question: During batch testing of a batch of smart home PCBs, it was found that 10% of the boards have a short circuit between the power and ground. The circuit design has been checked, and there is clearly no short circuit. Why does this issue occur during mass production?

Answer: For batch PCB short circuits, besides obvious line bridging, there are two easily overlooked "hidden pitfalls": solder bead residue and via burrs. These issues are highly concealed and difficult to detect during sample testing, but they can emerge intensively during mass production.

Here’s a real case: A client produced a batch of smart door lock PCBs. During design, the spacing between the power and ground was 0.3mm, meeting safety standards. However, after mass production, a large number of boards experienced a short circuit between power and ground. Upon observation under a microscope, the culprit was found to be via burrs. The manufacturer’s drill bit was severely worn during drilling, causing tiny copper burrs to form on the edges of the vias. These burrs happened to bridge the power and ground traces, resulting in a short circuit.

To resolve batch short circuit issues, besides routine circuit checks, it’s essential to focus on investigating these two hidden pitfalls:

**Hidden Pitfall 1: Solder Bead Residue, the Invisible "Short Circuit Killer"**  

Solder beads are tiny droplets of solder that splatter when solder paste melts during reflow soldering. With a diameter of only 0.05–0.1mm, they are difficult to detect with the naked eye but can cause short circuits between adjacent traces. In mass production, solder bead residue mainly occurs due to two reasons:

*   **Improper solder paste printing:** Oversized stencil openings or uneven printing pressure can lead to excessive solder paste, which splatters during reflow to form solder beads. Solution: Optimize stencil opening dimensions, ensuring the opening width does not exceed half the trace spacing. Control printing pressure between 5–8 PSI to ensure even coverage of solder paste on pads.

*   **Insufficient PCB surface cleanliness:** Residual oil or dust on the PCB surface during production can attract solder paste, forming beads during reflow. Solution: Require the manufacturer to perform ultrasonic cleaning after production to remove surface impurities.

**Inspection Tip:** Use a magnifying glass or microscope to inspect densely routed areas, especially traces with spacing below 0.2mm, for tiny solder bead residue. During batch testing, besides using a multimeter, employ an insulation resistance tester to apply a 500V high voltage between power and ground and measure the insulation resistance. An insulation resistance below 100MΩ likely indicates solder bead-related short circuits.

**Hidden Pitfall 2: Via Burrs, the "Invisible Bomb" in Drilling**  

Via burrs are fine copper strands produced on the edges of via walls during drilling. These strands can protrude and bridge adjacent traces, causing short circuits. In mass production, via burrs mainly occur due to two reasons:

*   **Worn or low-quality drill bits:** Excessive use of drill bits blunts the cutting edges, leading to burr formation. Solution: Require the manufacturer to replace drill bits every 5,000 holes and use high-quality tungsten steel drill bits.

*   **Improper drilling parameters:** Excessive drilling speed or pressure can tear the via walls, creating burrs. Solution: Optimize drilling parameters, such as controlling spindle speed to 30,000–40,000 RPM and feed rate to 0.1–0.2mm/s, to ensure smooth via walls.

**Inspection Tip:** Focus on inspecting densely packed via areas, especially micro-vias with diameters below 0.3mm. Use a microscope to examine via wall edges for protruding copper burrs. Solution: Require the manufacturer to perform deburring after drilling, using chemical or mechanical methods to ensure smooth, burr-free via walls.

Finally, here’s a handy tip for preventing batch short circuits: During design, add isolation slots or isolation pads between power and ground traces. Even if tiny solder beads or burrs are present, they won’t be able to bridge across the isolation slots, significantly reducing the probability of batch short circuits.