Bridging During PCB Board Assembly Manufacturing
The process of pcb board assembly manufacturing requires the soldering of copper pads and component leads to create a functional circuit. A common problem is solder bridging which occurs when the surface of one or more pads is not properly wetted by the molten solder. A bridge can cause a short circuit which is dangerous for the device and its users. This issue is easily prevented by using proper PCB design techniques and manufacturing procedures.
Solder Masking – A solid layer of solder mask provides a barrier between the pads and the underlying metal, which helps prevent the formation of solder bridges. Ideally, solder mask should be applied prior to soldering. The color of the solder mask should also be a consideration; green solder mask offers the best contrast between the pad and lead and is therefore ideal for spotting problems early on.
Stencil Thickness – Using a stencil that is too thick can increase the chance of off contact printing which contributes to solder bridging. Changing the thickness of the stencil and aperture can reduce this risk. Pad Spacing – Pads that are spaced too closely increase the likelihood of solder bridging. Ensuring that minimum spacing rules are followed is critical.
Lead Length for Through-Hole Components – The correct lead length is crucial to preventing solder bridging. The length needed will vary depending on the board thickness, component size and the type of soldering used (wave or selective). A good contract manufacturer will be able to help you determine the appropriate lead length for your project.
Prevent Solder Bridging During PCB Board Assembly Manufacturing
Solder Paste Application – Applying too much solder paste increases the chances of bridging. This can be corrected by adjusting the aperture width and area ratio, or by using a thinner stencil which will deposit less solder paste. Using solder mask dams between fine pitch pads can also prevent bridging, as these act as isolation walls to keep the solder away from unintended areas.
Inadequate Solder Reflow – The temperature profile and reflow settings must be set appropriately to ensure that the solder paste melts and flows correctly. Using the recommended settings will avoid incomplete melting and insufficient separation between pads and leads, which contribute to bridging.
Other Factors – Conclusion: Flux residue and other contaminants on the pad or stencil can interfere with the wetting of the solder to the pad, contributing to bridging. Regular cleaning is essential. Solder bridging can also occur when the pad is too high or too low for its intended use. It is important to use a CAD/CAM software that allows you to optimize the pad height, if possible. In addition, using fiducial marks to accurately locate components reduces the chance of misplaced parts which can lead to bridging. It is also a good idea to work with a contract manufacturer that prioritizes quality and is willing to walk you through their production processes. Ask them how they develop their solder paste stencils and what kind of board inspection they perform.
