**Problem 1: PCB Short Circuit**
A short circuit is one of the most common faults that can cause a PCB to fail. There are various factors that contribute to this issue. Let’s explore them in detail.
The primary cause of PCB short circuits is improper solder pad design. In this case, changing round solder pads to oval shapes can help increase the distance between pads, which reduces the risk of short circuits.
Another cause of short circuits arises from incorrect orientation of PCB components. For example, if the pins of an SOIC are aligned parallel to the wave soldering direction, it can lead to a short circuit. To prevent this, component orientation should be adjusted so that the pins are perpendicular to the solder wave.
A third potential cause of short circuits is related to automatic insertion of bent leads. According to IPC standards, if the lead length is less than 2mm, and the angle of the bent lead is too large, there is a risk that the part may fall, leading to a short circuit. To prevent this, the distance from the solder joint to the circuit should be kept greater than 2mm.
In addition to the three reasons mentioned above, there are other factors that can cause short-circuit failures on PCB boards. These include overly large holes in the substrate, low temperature in the soldering furnace, poor solderability of the board, failure of the solder mask, and surface contamination of the board. These are common causes of failure that engineers can check by comparing them with the failure conditions and inspecting them one by one.
**Problem 2: Dark and Grainy Contacts on the PCB Board**
The issue of dark-colored or grainy solder joints on the PCB is often caused by contamination in the solder and excessive oxides mixed in the molten solder. These impurities result in a brittle solder joint structure. It’s important not to confuse this with discoloration caused by the use of low-tin solder.
Another cause could be changes in the composition of the solder used during manufacturing, resulting in high impurity content. In such cases, adding pure tin or replacing the solder is recommended. Additionally, stained glass can cause physical changes in the fiber buildup, such as separation between layers. This, however, is not due to poor solder joints but rather to excessive heating of the substrate. Therefore, reducing the preheating and soldering temperature or increasing the substrate’s speed may resolve the issue.
**Problem 3: Golden Yellow Solder Joints on the PCB**
Typically, the solder on a PCB is silver-gray, but occasionally, solder joints may appear golden-yellow. This is primarily caused by excessive temperature. To fix this, simply lowering the temperature of the soldering furnace is necessary.
**Problem 4: Environmental Impact on PCB Performance**
The structure of the PCB itself makes it susceptible to damage in unfavorable environments. Extreme temperature fluctuations, high humidity, excessive vibration, and other environmental factors can all degrade PCB performance or even render it unusable. For example, changes in ambient temperature can cause deformation of the board, leading to broken solder joints, warping of the board, or even damage to the copper traces.
In addition, moisture in the air can lead to oxidation, corrosion, and rusting of metal surfaces, such as exposed copper traces, solder joints, pads, and component leads. Dirt, dust, or debris on the board’s surface can reduce airflow, leading to overheating and performance degradation. Vibration, dropping, or bending the PCB can cause deformation and cracks, while excessive current or voltage can break down the PCB or cause rapid aging of components and traces.
**Problem 5: PCB Open Circuit**
An open circuit occurs when the trace is broken or when solder only covers the pad but not the component leads. In such cases, there is no adhesion or connection between the component and the PCB. Like short circuits, these can occur during production, soldering, or other operations. Vibration, stretching, dropping, or other mechanical deformations can destroy the traces or solder joints. Similarly, chemicals or moisture can cause solder or metal parts to wear down, leading to component lead breakage.
**Problem 6: Loose or Misplaced Components**
During the reflow soldering process, small components may float on the molten solder and eventually drift away from their intended solder joints. Misplacement or tilting of components can occur due to factors like insufficient support of the PCB, improper reflow oven settings, solder paste issues, or human error.
**Problem 7: Soldering Issues**
The following are common problems caused by poor soldering techniques:
– **Disturbed Solder Joints:** These occur when the solder moves before solidifying due to external disturbances. While similar to cold solder joints, the causes differ. This can be fixed by reheating and ensuring that the solder joint is undisturbed while cooling.
– **Cold Solder Joints:** This happens when the solder does not melt completely, resulting in rough surfaces and unreliable connections. Excessive solder may also prevent proper melting, causing cold solder joints. Reheating the joint and removing excess solder can resolve this.
– **Solder Bridges:** These occur when solder connects two leads together, forming an unintended electrical connection. This can lead to short circuits, damaging components or traces when excessive current flows through the bridge.
– **Poor Pad Wettability:** Insufficient wetting of the lead or pad can occur, as can issues with too much or too little solder. Pads may also lift due to overheating or improper soldering techniques.
**Problem 8: Human Error**
Most PCB manufacturing defects are the result of human error. In fact, incorrect production processes, improper component placement, and failure to adhere to manufacturing specifications contribute to up to 64% of avoidable defects. The likelihood of defects increases with circuit complexity and the number of production steps, such as densely packed components, multiple circuit layers, fine traces, surface-mount components, and power/ground planes.
Although every manufacturer strives to produce defect-free PCBs, numerous design and production challenges can lead to persistent problems.
Common issues include:
– Poor soldering, which can result in short circuits, open circuits, and cold solder joints;
– Misalignment of board layers, which can cause poor contact and degraded performance;
– Insufficient insulation on copper traces, leading to arcs between traces;
– Overly tight spacing between vias, which increases the risk of short circuits;
– Insufficient PCB thickness, causing bending and fracture.
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