Sam Sattel
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What Are Tin Whiskers On a PCB, and How Can You Prevent Them?
Take a look inside all of your electronic devices, and you’ll be hard pressed to find one that uses lead in its active components, switches, relays, and board finish. But it didn’t always use to be this way. Before the enactment of the Restriction of Hazardous Substances (RoHS) directive in 2006, lead was being used in nearly all of our electronics. For good reasons, is was known for its ability to be a simple, inexpensive, and reliable way to create flexible solder joints that could withstand jolts and shock.
The use of lead was how we were making circuits boards that would go into rockets, satellites, airplanes, and other high-performance applications that were bound to get jolted around during their normal operation. But this is the past now, and in the process of working towards a lead-free future for electronics, we’ve run into a number of problems along the way, like tin whiskers. If you’ve never had this problem plague your design, then cross your fingers, because it can happen at any time.
What Are Tin Whiskers?
You might have overheard a fellow engineer talk about a tin whiskers issue, but talk is about all you’ll likely get. Why’s that? Because of tin whiskers, as much as they can plague our PCB designs and destroy them, are also almost invisible to the human eye, being up to 100 times thinner than a single strand of human hair in some instances. Check it out below.
Despite their almost invisible footprint, tin whiskers can create a bridge between the leads on electronic devices. And if you know anything about bridging, it’s that a short circuit is close behind, ready to ruin your device. While the issue of tin whiskers isn’t new, we had a relatively decent solution to them since the 1940s by using lead in our plating finishes and solder, but now we don’t, and so the problem of tin whiskers has come back into the spotlight.
The real issue with tin whiskers is that it’s completely random when or if they will infiltrate your circuit board. We can’t predict their patterns, and there are no quantitative data available that shows when a tin whisker might happen, it just does. So consider this – pure tin plating is used in nearly every electronic device around the world these days. We’re talking about the electronics used for our communications systems, financial systems, transportation systems, power systems, and a whole lot more. What happens when a random tin whisker issue decides to crop up in one of these?
You’ll get a complete and total failure of a system that society relies on today to function, all because of a random issue that we have no solution. That’s some scary stuff. This almost happened back in 2005, when the Millstone nuclear power plant in Connecticut experienced a random turn-off signal that would have shut down all of its operations. The cause? Tin whiskers.
What Causes Tin Whiskers?
Here’s the troubling part to stomach, we don’t know exactly what causes tin whiskers. There are a ton of theories out there, but all we really do know is that when lead isn’t used in electronics for plating and soldering, the risk for tin whiskers grows. A standard theory is that tin whiskers may occur when stress is exerted on the tin plating of an electronic device. Experts that agree with this theory have drilled down into the issue to spell out some factors that can cause stress on tin plating, such as:
- Residual Stress. These are stress factors that are due to the manufacturing process and plating chemistry. This is especially true for an electroplated PCB finish which causes greater stresses than other plating processes.
- Intermetallic Stress. This stress factor is related to how the diffusion of a substrate material into tin plating can alter the structure of the tin plating. Over time, this stress can cause tin whisker issues to arise.
- Mechanical Stress. There’s also the simple mechanical stress applied to a circuit board during its typical production process. Whether that’s a screw being used or a board being accidentally scratched during handling.
Whichever theory you agree with for the cause of tin whiskers, what makes them so dangerous is that after tin plating is stressed, tin whiskers can actually grow. This isn’t just a static object; it’s something that can lay dormant and unseen on day one after manufacturing and a year down the road you’re board is completely ruined when tin whiskers are all over the place.
When whiskers do grow, they’ll usually follow a path between two conductors where the whisker will fuse, creating a short circuit between a path or producing a false signal and device failure. There are even some instances where a whisker fusing a link between conductors can form a kind of conductive plasma that can carry over 200 amps of current! If a whisker doesn’t happen to form a bath between two conductors, then it will likely break and fall onto a trace on your board, disrupting the normal operation of your electrical circuit.
What You Can Do About It
After reading everything above, you might be thinking that solving a tin whisker problem is near impossible, but it’s not. If you’re designing electronics that need to meet RoHS requirements, then you’ve got the power to make choices that can lessen the chances of tin whiskers becoming a problem. Let’s take a look at the top three solutions that you as a designer can make to prevent tin whiskers from showing up on your PCB.
#1 – Refinish Your Manufactured PCB
This is the most passive and simplest solution of the three, so let’s start here. If you are going to be using a pure tin plating finish on your PCB, then you need to make sure that your manufacturer refinishes your board with a hot-solder dip to help reduce the risk of tin whiskers. Some manufacturers will offer this as an option if you’re having a lead-free device manufactured. Be sure to confirm that yours does as well and take advantage of it.
#2 – Avoid Using Pure Tin Plated Components
If you’re responsible for purchasing the physical parts, then avoid buying components with pure tin plating. This doesn’t mean that your parts need to be plated with lead, as there are plenty of other platings combine tin and a secondary metal to reduce the risk of tin whisker growth.
#3 – Consider a Different PCB Finish
Your last and final line of defense against tin whiskers is to consider using a plating finish on your PCB that isn’t pure tin. Each has their advantages and disadvantages to consider, let’s consider the top three at your disposal.
- Lead Free HASL
This finish has been the go-to plating used by the PCB manufacturing industry and works by submerging your PCB in a molten pot of tin alloy and then blowing excess solder away with hot air. Lead Free HASL is known for its low cost, superb shelf life, and ease of availability. However, you might experience other issues when using this finish, like solder bridging, thermal shock, or uneven surfaces.
This plating uses a water-based organic compound that bonds to the copper on your board to protect it during soldering. The advantage of OSP is its naturally lead-free nature, cost effectiveness, and simplicity of application by your manufacturer. However, be aware that OSP has a short shelf life, you can’t measure the thickness of it, and it’s not recommended for use with through-hole components.
The nickel in this plating finish provides a barrier of protection between your board’s surface and the solder for components, while the gold protects the nickel during long-term storage. ENIG offers some great advantages as being a lead-free solution, having a long shelf life, and working well with through-hole components. However, be aware that ENIG is more expensive than other plating finishes, isn’t reworkable if mistakes are made, and can result in signal losses.
It’s a Balancing Act
It’s pretty clear why we’ve demanded a lead-free future from our electronic devices. All of those gadgets that end up in a landfill can lead some nasty stuff leaching into our environment. And not only that, lead is a known neurotoxin that can affect brain development in both children and adults. So as far as the environment and our personal health go, restricting the use of lead in electronics seems like the obvious choice.
But where’s the balance here? By restricting the use of lead through RoHS and other directives, have we also doomed our electronic design to eventual failure at a random and unknown date down the road? The good news is that you as a designer now have three strategies at your disposal to lessen the risk of tin whiskers destroying your design. Be sure to have your board refinished with a hot-solder dip, purchase parts that are not pure tin plated, and consider choosing a different finish like Lead Free HASL, OSP, or ENIG.
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