The bridges in our transportation network serve as some of the most incredible examples of structural engineering and testament to a city’s progress and sophistication. On an individual level, building bridges between people encourages societal growth and healthy interpersonal relationships. We’ve all heard the idiom, “take care not to burn your bridges.” Bridges provide essential services in many aspects of our lives but are never desirable in the manufacturing of printed circuit boards.
When manufacturing printed circuit boards a defect can occur when the metal from two different nets inadvertently gets shorted or bridged together with solder. This particular defect can be small and difficult to find, but the short circuit it creates causes big problems. These can include intermittent connections, false test results, or traces and component leads that burn through. We’re going to take a closer look at this problem and discuss some PCB design methods on how to prevent solder bridging—so your bridges don’t burn you.
Some of the Causes for Solder Bridging
Solder bridging can occur during printed circuit board assembly for a variety of reasons. Here are some of the top causes:
The position of parts on the board, both during design and then again during manufacturing, can have a big impact on how the board is soldered:
- Component footprints need to be located on the board during PCB design for the assembly soldering process. For instance, wave soldered boards need to have surface mount discrete parts placed perpendicular to the direction of the board, traveling through the wave so their pins solder evenly.
- During assembly, the physical parts must be placed correctly on their footprints to avoid uneven soldering due to the wrong lead to pad size relationship.
Incorrect Solder Mask Definition
Solder mask is designed to prevent solder from bridging between pads, but if it isn’t defined correctly it may not provide the protection expected of it:
- Surface mount pads that are larger than they should be could end up being too close to each other for sufficient solder mask coverage between them.
- Solder mask shapes in a pad that are defined larger than they should be may not provide enough solder mask coverage to protect the pad from bridging.
- Fine pitch pads on large pin-count devices can cause problems if the solder mask is defined too narrowly or missing altogether between the pads.
- If the solder mask is insufficiently applied on the board during assembly, it can also leave the spaces between pads unprotected.
Incorrect solder paste stencil specifications
Solder paste is often applied with a stencil. If the stencil is made incorrectly, the solder paste will not be applied in the correct proportions and could easily bridge from one pad to another:
- If the surface mount pads on the board are smaller than what they should be, the solder stencil apertures for those pads will deposit too much paste on the pad.
- If the stencil apertures were made too large for their associated pads, then again, too much solder paste will be applied.
- If the stencil itself is too thick, then unnecessary amounts of solder will be applied to each pad.
Solder paste stencil placement during assembly
How the solder stencil is positioned during PCB assembly can also have an impact on how the parts are soldered:
- If the stencil is not sealed against the bare board during printing, it can result in too much paste being applied or the paste not conforming to the desired pad shape for successful soldering.
- If the stencil is not aligned correctly with the board, the solder paste can seep out and bridge to other pads.
Many problems that lead to solder bridges being formed during assembly, can be mitigated with good PCB design methods and controls.
How to Prevent Solder Bridging in Your PCB Design
Solder bridging can be a result of problems with the component footprints used in PCB design, as well as how those parts are placed on the board. Here are some design recommendations that can help:
Use Correctly Built Footprints in Your PCB Libraries:
|Surface mount pad sizes||Be careful to use industry specified pad sizes on your PCB footprints to ensure that a good solder joint can be formed.|
|Solder paste shapes||A surface mount pad must have a correctly sized solder paste shape defined for it to create the right sized aperture in the solder stencil.|
|Solder mask shapes||Make sure that each pad in a PCB footprint has the correct sized solder mask definition built into it for full solder mask protection.|
|Pad to pad spacing||Footprint pads should have enough spacing between them for the even application of solder mask between the pads.|
|Use the correct footprint||Make sure to use the correct footprint land pattern for the physical component that will be assembled on the board.|
Use the Correct Spacing and Rotations When Placing Parts:
|Industry standards||Follow the appropriate industry standards for component spacing and rotations during parts placement. Components that are to be wave soldered must be placed correctly for the direction that the board travels through the wave.|
|Design rules||Use the design rules and constraints built into your PCB design tools. Usually, these rules can be set up to check for component spacing as well as other design for manufacturing (DFM) requirements.|
Good PCB design methodologies will reduce the amount of solder bridging that can happen during assembly, but there is still the possibility of it occurring. Here is where using an experienced PCB contract manufacturer will pay off.
Solder Bridging Detection and Correction
Even with all the best precautions, there is always the chance that solder bridging will still happen during PCB assembly. For this reason, a good assembly house will use a variety of methods for detecting any solder defects. These methods include standard visual inspection, automatic optical inspection equipment (AOI), and even X-ray inspections. Once any solder bridges are found, it is a relatively simple matter for the rework technicians to correct the problem.
At VSE we have been building circuit boards for over 30 years and understand how to detect and correct manufacturing defects like solder bridging, as well as how to design PCBs to avoid those problems. Our engineering staff routinely collaborates with designers like you to set up design systems with the correct rules to avoid defects. In the same manner, our assembly technicians are trained to find and correct any soldering problems that occur during production.