Printed Circuit Board Defects: Dealing with PCB Component Warpage
Like many engineers, I was raised on the “Stars,” Star Trek, Star Wars, Stargate. I’m still occasionally surprised to realize how much these fictional worlds, my goals, and my vocabulary. For an entire semester, I didn’t realize that phasers, the ray guns of Star Trek, and phasors, the handy mathematical notation, were completely unrelated, even through some obscure nerd culture and even spelled differently! “Warp” is another that I haven’t quite come to terms with. A PCB manufacturer once explained to me that he thought we were having an issue with a package warping, and I was convinced he didn’t have a clue what he was talking about. I read a lot of science blogs, if warp drives existed and were that tiny, I would most certainly know.
Component warping can results from a variety of causes. Mechanical mishandling occasionally causes bending. It’s even possible, though rare, to have poor manufacturing result in outgassing. This can create a bubble inside packaging or force a casing askew. However, the most common cause is thermal issues. Rework may cause warping in your components during reflow processing, or a thermal mismatch between the packaging and solder can cause warping when materials experience thermal expansion at different rates.
Heating during reflow is one of the most causes of component warping.
Where PCB Component Warpage Occurs
Occasionally, you’ll receive components with warped packaging, either bent or otherwise not perfectly flat, due to some effect during manufacturing or shipping. Depending on the severity of the warping, it may be difficult to tell there’s anything wrong before you start using the parts. Unfortunately, once you’ve incorporated those components onto your board, you’ll be hard pressed to prove whether the warping occurred before or after your processing and handling. This is one reason why it’s good practice to produce a few samples with regular parts before you manufacture the entire batch. If there are issues, it’s much easier to track down and rectify when you only have a few boards.
Ideally, your PCB will be produced with uniform laminates and good pressing procedures. That should give you a very planar surface, and a consistent surface to apply solder paste for component soldering during reflow. If the thickness of your PCB varies over the surface of the board, or the board is twisted or warped, then the additional warping of a component can compound the total number of defects you’ll see in the finished product. Even perfect components can end up tilted, with poor quality solder joints if the board surface is uneven, so you should verify the board surface planarity if you start having electrical opens that aren’t quite matching up from board to board.
How Warpage Affects Your PCB
The impact of component warping may be small enough that you’ll never notice, or it might cause immediate electrical issues. In my opinion, the worst case is when it weakens a solder joint enough to cause a premature, but impossible to detect, failure.
Most often, these electrical issues will show up in ball grid arrays, where components have a large surface area to be affected by warping. The effects you’ll see vary on whether the warpage curves the casing up or down, though you might be unlucky enough to get both.
When the warpage increases distance between the PCB and casing, there are a couple possible outcomes. In some cases, the solder ball will “drop,” staying low to the PCB rather than connected to the component. That results in an open circuit. Otherwise, the solder ball will stretch to make the connection. You see a circuit, but the solder in the join is thinned, and sometimes weirdly shaped, making the joint will be less reliable over time.
The Surface Mount Technology Association has a great presentation about warpage, and if you check the images on page 6, you can see these issues in ball joints.
If the surface warps down, usually with corners and edges slumping during reflow, then you’ll suddenly have too much solder under your component. It will often squish off of the pad, bridging to other solder pads and shorting them together, like you see in the image below.
The impacts of warping are much worse as you decrease the pitch of the pads. This might be obvious, but it’s good to hear regardless. The smaller area for solder to wick over means it’s easier for solder to overflow, and you’ll have less solder to work with if joints are being stretched.
With a warped component, solder can stretch, breaking a connection, or spill over onto other pads in ball grid arrays, shorting connections together.
Best Practices For Mitigating Warpage
Fortunately, there are a few options to mitigate warping. First, use solder mask defined pads because non-solder mask defined pads will have much lower molten solder height. That’s because molten solder doesn’t have as much wicking area to spread out over. You can also adjust the process materials and temperatures, often bringing temperatures down, or reducing the thermal mismatch between a leadless solder and components can dramatically improve your outcomes. If you have corners slumping during reflow, you can use spacers to support them until they’ve cooled.
However, if the process materials and temperatures can’t be adjusted, you may have to choose a new material for the component packaging. RoHS policies increase the difficulty of having no warpage at all. This is because leadless devices are both more sensitive to issues with warping and flatness and more likely to have them.
Tracking parts and manufacturing outcomes will help tremendously if your company wants to avoid the same issues showing up on different products. Altium’s BOM Management Tools and library tools like Altium Vault allow you to manage your suppliers and unify data across teams on processing results for different components. You can even incorporate design rules that keep low tolerance parts out of high risk product designs.
If you’re ready to get started, Altium’s representatives can help you warp speed ahead. Contact an expert at Altium today.