The concept of “Bridge” is ubiquitous in daily life, serving as a vital means of connection, facilitating travel and convenience by shortening distances. However, in electronic engineering, particularly in the PCB soldering process, “Bridge” may not always be desirable. In the realm of electronic engineering, the term “Bridge” refers to solder bridge, a common PCB soldering defect. While minor instances may impair device performance, more severe cases can lead to product malfunction and even pose safety hazards.
What is Solder Bridge
Solder bridge refers to the unintentional connection of two or more distinct circuit paths or pads with solder, resulting in excess conductivity and short circuits between circuits. The image below illustrates a clear solder bridging defect, where two pins that should be separate are soldered together.
How Solder Bridges Form
When discussing the formation of solder bridges, many people mistakenly believe that they only occur during manual soldering. However, the reality is different; solder bridges can occur in both manual and automated production processes. Typically, we consider three main reasons for the formation of solder bridges: design flaws, process defects, and solder paste quality issues.
In the modern electronics manufacturing field, where there is a demand for high performance and compact size, the extensive use of fine pitch and high pin-count components is common. These components are highly sensitive to solder paste volume and viscosity, where even minor discrepancies can lead to solder bridges. Additionally, inadequate spacing between adjacent pads or mismatched sizes between pins and pads during the design phase can also contribute to solder bridges.
Furthermore, some manufacturers may produce solder bridge defects due to limited production capabilities. For instance, in traditional processes, PCB stencils are used for solder paste printing. This involves a stainless-steel plate with holes through which solder paste is applied onto the PCB. During this process, several potential risks may lead to solder bridges: 1. Oversized holes causing excess solder paste; 2. Slow separation between the template and PCB leading to excessive solder paste stretching; 3. Incorrect parameter settings resulting in poor alignment between the stencil and PCB.
Improper soldering techniques can also easily lead to solder bridge defects. For example, manual soldering involving the use of a soldering iron can result in solder material spreading between pads due to slight hand tremors or excessively steep soldering angles, rather than being accurately applied to individual pads. In the reflow soldering process, the preheating stage is critical. If the temperature is too high, solder paste may splatter, leading to its appearance between two pins. If preheating is insufficient, the solder paste may flow disorderly.
Precautions to Avoid Solder Bridges
Designers must adhere strictly to PCB design rules to ensure sufficient spacing between components, minimizing the risk of solder bridging during the soldering process. Additionally, careful consideration of package types, such as QFN and BGA, is essential, as pinouts on the underside of these packages can increase the likelihood of solder bridge.
Manufacturers should rigorously control the amount of solder paste applied, avoiding excessive or insufficient application, and strongly recommend SPI inspection after solder paste printing. Furthermore, a thorough review of production documentation before manufacturing is crucial, as these documents are ultimately input into equipment for automated processes. Additionally, after multiple production runs, inspection of reflow soldering equipment is necessary to prevent temperature discrepancies. Finally, upon completion of production, employ AOI inspection.
For users seeking PCB assembly services, it is imperative to scrutinize the materials used by the manufacturer, the facilities available, and the quality management measures to mitigate solder bridge issues. For example, specifying a trusted solder paste brand, ensuring the availability of advanced equipment such as solder paste printers, AOI, and X-Ray machines, and verifying adherence to IPC standards are essential considerations.
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