Understanding Solder Bridge Shorts: Bake Your Cookie and Eat It Too
I love the idea of baking. Especially around the holidays, there’s something thrilling and therapeutic in planning to bake up elaborate platters of cookies and bring them to parties. In reality, what I like is to eat cookie dough. The actual rolling or spooning out cookies onto a pan and waiting for them to bake is an exercise in both tedium and restraint. When I lose my patience, I try to cram as many cookies onto the tray as possible; of course, this results in a single, giant cookie that, begrudgingly, I have to eat all myself.
Whether you’re wallowing in the winter planning of too-many holiday cookies, or just wishing for a more convenient way to plan out your space-constraints, the goal is always going to be designing the layout of your baking sheet to avoid scrapping that whole tray of cookies. If you spent the time making them, you should earn the reward of eating them. That lesson ought to be true for a lot of things though, shouldn’t it?
Unfortunately, if I overcrowd my PCB it doesn’t just turn into a cookie. No, being impatient and not working within space constraints can be much more costly than ruining a batch of cookies. Size and complexity of design are major factors that affect PCB manufacturing costs; however, more than that, overcrowded designs can lead to malfunctions and PCB shorts. Being able to manage your design to effectively work within space constraints will save (and probably earn) you more money and time that could be better spent on other things (like, for example, eating more holiday cookies).
The most common issue with high-density designs is frequent electrical shorts An unfortunate reality is that oftentimes electrical shorts can occur despite how well you designed your PCB. Solder bridges are likely so long as you have a high-density design; however, high-density designs are often unavoidable with the increasing demands from PCBs.
When you decrease the size of pads and the distance between them, there is more solder paste in a smaller volume. That paste is more likely to spill out of the appropriate stencil defined areas and create solder bridges to neighboring pads. Like accidentally baking a giant cookie, the only way to avoid those bridges is by having buffer space between the pads that is enough to protect them from a little bit of spillover from their neighbors. It isn’t designing for failure, but rather allocating design resources into understanding shortage risk and managing it appropriately.
Overheating your PCB and components is just as bad a burning cookies, and more expensive to redo.
Other manufacturing stages can also be more difficult with higher density PCB designs. Stencil issues compound the risk of solder bridging, especially in tight arrays of small components. When there is high component density, you have the highest risk of solder paste smearing or misalignment which results in inadequate buffer space. Having buffer space enables your PCB design to prevent solder paste bridges from forming. Without that buffer space, solder bridges become more likely and encourage the occurrence of errors like shorting.
Pick and place machine accuracy also limits the density you can use in your PCB design. If the accuracy of the machine is very high, you’ll have more flexibility to pack in your components. However, also consider the tooling used for the pick and place. Placement tools can extend beyond the edge of the component, which can add difficulty to tightly-packed or small component placing.
Depending on the place I’m living, sometimes I need a stepstool to reach whatever I’ve stored in the top cupboards. This might impede my cookie-baking speed, but it won’t ultimately keep me from baking. Component height also becomes an issue if the pick and place can’t reach the location next to a taller component like in the case of resistors next to headers. Imagine if you were decorating a cookie and put giant marshmallows down, then needed to draw in delicate icing without moving the cookie. I usually eat the cookie before the decorating process, but I can’t just eat a PCB.
Variable height components make cookies as tricky as a PCB
Inspection, rework, and repair
After manufacturing, the challenges of a high-density design aren’t over. Inspection is more challenging because edges of components aren’t easily visible when they are close together like they are in high-density designs. You may be blinded from viewing solder joints and verifying standoff heights. If you have varying heights, then the components might actively block each other from view.
Finding the issues in a high-density design isn’t the entire problem either—when you have components so close together, you can encounter other processes that will slow down or can even cause errors of their own. You might have to remove other components to get at whichever component you need to fix, and even with the tiniest tweezer and most stable hands, reheating sections of the board will assuredly remelt the solder and cause slippage. You may also need to add a heat shield for nearby components since there’s less total room for heat to dissipate.
Operational requirements on spacing
Before any scenarios in which errors, bridges, or shorts occur exist, the foremost demand on component spacing is from operational requirements. There are formalized specifications for spacing: the most commonly cited is IPC-D-279, section 3.3.9, but there are also manufacturer recommendations, like this chart from OCM, often for a specific packaging, or application. Once you have varied component and packaging types, the specifications often aren’t extensive enough to provide the necessary guidelines.
The requirements for component spacing depend on the function of the board, the operating parameters, and its environment. Consider isolation requirements (based on voltage, RFI, or EMC) before you try and reduce spacing, or you’ll be shortening the life of the entire product, and possibly introducing safety issues, to save a little bit of upfront cost. You might also encounter handling and safety requirements that will determine edge spacing. Initially, that buffer seems like an easy place to cut excess, but cutting your buffer could have catastrophic impacts on the lifespan on the board.
Spilled icing is delicious. Spilled solder paste is disastrous.
On top of all the other information that you need to keep track of while designing, component spacing and being aware of solder bridge shorts may seem like a troubling task. Trust me, coming from someone who often forgets to preheat the oven before putting anything into it, I understand the difficulty of keeping track of multiple instructions. But when you are doing high-density design, use with its robust design rules checking, can make sure “keep out,” areas and other physical restrictions for components are all accounted for, and that can configure manufacturer rules to check for compatibility before designing.
[Many thanks to Altium for permission to reproduce this post]