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What Is Solder Bridging On a PCB?

Sam Sattel

eagle-academy

What Is Solder Bridging On a PCB, and How Can You Prevent It?

If there’s one thing sure about electronics design, it’s that everything keeps on getting smaller and smaller. Just look at the wearable industry these days. We’re cramming in electronics that used to be housed in a giant desktop tower into something that can be strapped to your wrist. And while this process of constant miniaturization might seem like a miracle to some, for engineers and manufacturers it can be a nightmare.

There’s always that nagging stress of wondering how you’ll fit all of those components into package sizes that keep shrinking. And then hoping you get a board back without any soldering issues. Of the many manufacturing issues that are caused by these shrinking package sizes, solder bridging takes the top of the list. But what is it exactly, and how can you prevent it from happening in your design? Let’s find out.

Solder Bridging 101

Solder bridging is just one of the many soldering issues that can occur on your PCB during its manufacturing journey. And as its name suggests, it takes place when two or more pads become connected through an excessive application of solder, creating a bridge. Unlike other soldering issues which can be easy to identify, like the tombstoning of a component, identifying a solder bridge isn’t as easy to catch. This issue can be microscopic in size, but even the smallest solder bridge that goes undetected on your PCB can lead to some nasty results, like a short circuit or a trace/component burning up.

solder-bridge

A close up of a solder bridge being formed between two of the pins on an IC. (Image source)

So what keeps solder from being excessively applied to your board in the first place? That’s the job of a solder mask layer. This solder resist coating gets added to your PCB during the manufacturing process and is used to protect specific areas of your PCB that should not have solder applied. For example, the space between two pads on an integrated circuit will be designated with a solder mask, so the only solder you’ll get is on the two pads, but not between them, which could cause a solder bridge.

soldermask-pcb

The green color on any PCB is a dead giveaway of solder mask being applied, although it comes in a variety of other colors. (Image source)

Solder Bridging Causes

Many different conditions can lead to solder bridging occurring on your PCB design during manufacturing. Some of these are related to the equipment and processes of the manufacturer, and others are related to the decisions you made during your design process. Some of the most common causes for solder bridging includes:

  • Using too much solder on SMT pads because of an incorrect stencil specification.
  • Having a bad seal between a stencil and bare board during the printing process.
  • Designing solder pads that are too big in relation to the gap between pads.
  • Placing components imprecisely or having a reduced component lead to pad size relationship.
  • Having an insufficient layer of solder resistance applied between the pads on your board.

The good news about solder bridging is that there are a number of preventative measures you can take during your design process to stop this issue from cropping up. Let’s take a look at how mask reliefs, solder dams, and mask defined pads can help to burn those bridges.

#1 – Double Check Your Design Rules for Mask Reliefs

A solder mask relief is defined as an area on your board layout that doesn’t need a solder mask application. This is most commonly designated as a thin outline surrounding surface mount pads, through-hole pads, test points, and vias. Check out the image below, and you’ll see the solder mask relief highlighted in green.

soldermask-relief

Solder mask relief surrounding three pads, keeping solder from each component separated. (Image source)

When setting up your design rules at the start of your project, we always recommend setting a relief between .003” to .008” for all of your SMT components. However, you might need to adjust this relief value if your pads are too close together. If you apply a solder mask relief that leaves no solder mask between each pad, then you’re chances of causing a solder bridge become all the more likely (more on this in tip #2).

One quick side note. What color solder mask you choose to use is also going to influence how extensive your mask reliefs need to be. Here’s a handy chart that breaks it down by color:

Solder Mask Color Minimum Solder Mask Relief
Green .003”
Clear .003”
Red .003”
Blue .003”
Black .004”
White .004”
Yellow .004”
Orange .004”

#2 – Always Leave a Solder Mask Dam Between Pads

Building on tip #1, you always need to ensure that you have a solder mask dam between each of your SMT pads. A solder mask dam acts as isolation wall between the application of solder on each of your pads. Think of it as a physical dam. If you don’t have a dam keeping solder contained on a pad, then it can easily spill over onto another pad, causing an unintended solder bridge.

solder-mask-clearance

A side perspective of two pads and a solder mask dam separating them on a PCB. (Image source)

As a general guideline, we always recommend setting up solder mask dams between each of the pads on your SMT components. This becomes increasingly important the smaller and more compact your boards get, especially for Integrated Circuits (ICs). As a bare minimum, this dam size should always be maintained at .004” (2 mils), otherwise, it becomes very challenging for your manufacturer to isolate solder between pads.

#3 – Use Solder Mask Defined Pads For Tight Pitched Areas

There might be times when you know you need a solder mask dam between two SMT pads, but you just don’t have enough room to maintain a barrier and mask relief. You might run into this kind of problem when working with very tightly spaced pads on a Ball Grid Array (BGA) or Land Grid Array (LGA).

If this is the case, then consider using a solder mask defined pad which provides a solder mask relief that’s the same size as the copper pad it is protecting. If you do end up going this route, you’ll likely need to make a note in your fabrication files telling your manufacturer to leave the solder mask defined pad as is without applying your standard mask clearances.

soldermask-defined-pads

Here you can see the difference between a Non-Solder Mask Defined Pad (Left), and a Solder Mask Defined Pad (Right). (Image source)

Burn Those Bridges

There you are, three surefire ways to make sure that solder bridging doesn’t break your design when it comes time for manufacturing. As we’ve seen, solder mask plays an invaluable role of keeping your components protected and isolated from one another during the application of solder. And with some healthy boundaries between two pads in the form of solder dams and mask reliefs, you’ll be well on your way towards burning those solder bridges before they ever get a chance to be built.

Ready to design and manufacture your first professional PCB? Try Autodesk EAGLE for free today!

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