Tips for Better PCB Layout
While Milwaukee Electronics' Design Engineering Group can help your team with everything from circuit design to PCB layout to actual product design, we find that many of our customers like our ability to fill gaps on their team. If you prefer to keep PCB layout in-house we'll be happy to share our design guidelines with your team. We also created this list of common mistakes we see in PCB board layout to make it easy for your designers to eliminate them:
Vias in Pads
Poorly designed vias-in-pads are the number one cause of bad prototypes that we see. In many cases, it is unavoidable as discrete components. QFNs and fine pitch BGAs often require via-in-pad. But if done poorly, solder capillary action sucks the solder through the via and down to the other side of the board. Small parts can tombstone, large parts end up with poor mechanical connections and BGA balls can be sucked off the component. If via-in-pad must be used, cap it and/or mask it. Capping the component side is better than capping the bottom because it reduces the chance of outgassing and voids. At the least, make it as small as possible. For BGAs, your only option is to fill and plate over the vias. For QFNs, you can also cap or tent the vias.
What may look like adequate parts spacing to a designer, often causes headaches on the production floor. Parts placed too close to the edge of the PC board may impede automated handling process. Parts placed too closely together may create solderability issues. IPC Design Guidelines suggest minimum spacing options.
Part Library Mismatch
CAD system parts libraries often contain land patterns that are close to that needed for the actual component, but just enough off to create issues. For example, parts with uneven solder pads cause headaches on a fairly regular basis when matched with PCB land patterns that don't also follow the uneven dimensions. The surface tension of the molten solder will act on the bigger surface area of the pad and cause the part to shift during reflow. That can lead to reliability issues. In some cases, it can also lead to shorts with nearby components or mechanical structures. Sometimes there is variation between metric and English measurement systems in parts libraries. That difference can be irrelevant in a part with a few leads but significant in a part with a large number of leads. Care should also be taken with use of IPC padstacks and 3-pin component pin numbering.
Issues with High Current Devices
The science of cooling has changed. We're seeing more and more tiny components needing advanced power dissipation techniques. This includes high-current devices such as the MCP1726 1-Amp regulator in 3mm x 3mm package, CMLDM7484 dual 450 mA MOSFET at 1.7mm x 1.7mm).
With projector bulbs, just sticking a fan next to the bulb wasn't good enough. Tiny power components, like the MCP1726 regulator, have a similar issue. You can't simply place a heat sink on them and call it good. Instead, the cooling system must have thermal planes, thermal vias and other layout considerations. Some, like the CMLDM7484, a dual MOSFET from Central Semiconductor, in a 1.7mm x 1.7mm package, require use of aluminum or ceramic core PCBs to survive its maximum power dissipation. Using the PCB for cooling can be a lot more complex than cooling with heatsinks and fans.
Layout inconsistency is one trend that drives tombstoning issues. For example, when a small trace is going to one pad and a large trace is going to another pad, the large trace will act as a heat sink. The smaller pad melts first and surface tension causes the large trace's side to pop up. Inner copper layers under one pad, but not another, can also cause the same effect. Larger thermal mass components can also cause this. Another common cause of tombstoning is a thick solder mask. Good mask registration will help. Using a board vendor that has tight tolerances and delivers a thin flat mask surface will also help. In some cases, with the smallest parts, you may want to keep the soldermask off the pads by using non-soldermask defined (NSMD) pads.