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You’ll want to make sure your boards can hit the production line by following PCB design for manufacturing guidelines
PCB design for manufacturing refers to a large set of design processes, where the goal is to ensure your PCB layout is fully manufacturable. Some people like to bifurcate DFM into design for fabrication (DFF) and design for assembly (DFA), but DFM spans all of these issues. Following important DFM guidelines will help you avoid redesigns and move to production quickly.
In today’s consumer market, getting a product out in time is key to grabbing a customer’s attention and remaining competitive. If you’re an entrepreneur about to release a new product, your time to market can give you a major advantage over your competitors. Any troubles during manufacturing will require a rerun through the manufacturing process and extend the time to market. By following the DFM guidelines for PCBs, you can reduce the chances of failure and increase your yield. Let’s take a look at the DFM guidelines every designer should know so that your next layout can pass DFM review with flying colors.
Why is DFM Required When My Design Already Passed DRC?
This is the very first question a designer will ask when given set of DFM checks along with the usual design rule checks (DRC). The difference here is that DRCs are a set of electrical rules that are intended to ensure design functionality. DRCs look for existing design issues from an electrical perspective, while DFM identifies any features in a layout that can’t be fabricated with standard PCB manufacturing processes.
DRC errors normally manifest themselves during the PCB layout phase, while DFM issues might not come up during a DRC. In other words, a given layout could pass your DRCs, but that doesn’t mean it will be manufacturable. Traditionally, manufacturers took it upon themselves to fix DFM problems in your layout, but this can create new problems as designs become more complex. As a layout designer, you should actively avoid DFM issues when creating your PCB layout rather than rely on a manufacturer.
Important PCB Design for Manufacturing Considerations
There are some DFM defects that you should check in any design. After some practice, you’ll learn to identify many of these problems quickly as you build your PCB layout.
Acid traps in a PCB are basically acute or odd angled spaces or small features in your design that can cause acid to pool during etching. The residual acid gets caught in these areas and doesn’t get washed away. As a result, the copper around these areas starts to erode, creating open or high impedance connections. With smaller and thinner trace, it is easier for residual acid to form an open circuit.
Acid trap created due to an acute angled trace routing
Some software has built-in DRCs for this particular issue. If yours doesn’t, then manually watch for potential acid traps and make sure to remove all odd trace angles on square pads.
These are free floating areas of copper on plane layers. Any copper pour that is disconnected from ground can also qualify as an island. If these areas are large enough, they might act like an antenna, causing noise and other interference within the board.
Island creation due to disconnected copper
Solder Bridges Between Pins
High pin-count ICs have tight pin pitch. Therefore, it is essential that solder mask be included between pins in the PCB layout. The solder mask can act like a dam that prevents a large blob of solder from falling between pins and creating a short.
Solder bridge between two pins on an IC. Image source: sparkfun.com.
Copper-to-Board Edge Clearance
Much of the PCB manufacturing process is automated. The copper-to-board edge clearance is the space where automated equipment grips the board and transports to other processing equipment. If there is insufficient space, then shorts can be created when current is applied to the panel during the electroplating process. The space requirement depends on the equipment being used for board transportation. Be sure to contact your fabrication house to get these specifications.
Thermal Paste Masks on Heat Sinks
Heat sinks remove heat from an electronic component through direct contact with a metal or thermally conductive paste. If the paste mask opening is too big for the heat sink, the molten solder might cause the component to float off the pad during the soldering process. This can create lots of wrong connections. Try to divide the paste mask opening into several small ones.
Cold Solder Joints or No Solder Connections
Any vias on a pad can cause solder paste to wick through the via suring assembly. This will result in a cold solder joint or an unsoldered connection. Therefore, you should check how close a via hole can be to a pad. Via-in-pad is normally use in HDI designs, otherwise the via will be connected to a pad with a small trace. If your tool doesn’t check for this, makes sure to do this check manually based on your assembly house’s requirements.
Insufficient Annular ring
An annular ring is the area of copper pad around a drilled and finished copper plated via. The primary purpose of an annular ring is to create a strong connection between a via and the copper trace. There should be enough copper around a via to form a solid connection between the copper traces and the via in a multi-layer PCB. If the drill size is larger than the size of the pad being drilled, the pad can disconnect from the via barrel or the internal signal/plane layer. You can learn more about annular ring requirements in this article.
Appropriate drill size for a given annular ring. Image source: pcbway.com.
Your PCB Design for Manufacturing Checks and Your Design Software
It is always a good idea to use a PCB design platform that provides important DFM checks. Don’t forget to contact your fabrication house and ask them about their DFM requirements. Taking care of these problems early can prevent an unnecessary redesign and improve the quality of your finished board.