There’s nothing I love more than the feeling of a job well done. It makes all the late stage design changes, project creep, and long hours almost worth it. The problem with making PCBs is delayed gratification, because the work is never really over until a board has been proven reliable. In order to speed up the process, we’ve put together some tips to help you design for reliability so you can enjoy some rest and relaxation instead of troubleshooting fault circuits. Building a robust PCB starts with the components and materials you select for your board, from the substrate to the solder mask. These choices will inform later design decisions that can ensure your board will be manufactured correctly and operate for its full lifetime.
As if selecting components and keeping an up to date BOM wasn’t enough work, you’ll also need to consider the effects your choices will have on manufacturing. The first choice will be SMT or through-hole, which both have their own intricacies when it comes to reliability.
Through-hole components are generally more reliable when it comes to thermal or mechanical shock. This is because they run through the board and have more surface area for a physical connection to your PCB. That strong connection can be a double edged sword, as through hole elements are often made for higher power levels and generate more heat. Be sure to watch for hot spots from these parts that may stress your board.
SMT pieces have a higher risk for manufacturing defects, namely tombstoning. An example of this is when one of the two pads for an SMT resistor heats before the other during soldering. This can result in the solder on one side wetting before the other and pulling the resistor up like a tombstone. Pads often heat up at different rates because they are connected to different amounts of copper. Copper conducts heat, just like current, so try to match the thermal paths to both pads to reduce the chance of tombstoning during fabrication.
Once you’ve carefully chosen your components it’s time to consider the materials to use in your PCB. If you design high-speed boards, you’ve probably looked at the dielectric constant of your materials. This time we’re going to examine the CTE (coefficient of thermal expansion) and stiffness (modulus of elasticity) of your board. You may also want to think about what kind of solder you want to use.
Usually, when I think of the stiffness of a board, I think of rigid, flexible, or semi-flexible, but all rigid board aren’t made equal. Cores and prepegs are often made from fiberglass and resin, of which there are many types that can also be combined in different ratios. There are some less common options for cores or prepegs such as ceramics or even metals. You probably want to limit the CTE of your substrate so that it doesn’t mechanically stress your components by expanding and contracting during thermal cycles. The CTE for materials is often listed on their datasheet, so you don’t have to get a degree in materials engineering to get a handle on it. The modulus of elasticity is a little more difficult. More rigid materials will have a higher modulus of elasticity, but this value often has to be calculated. You might be able to get a mechanical engineering friend to do it, or make an educated guess. The manufacturer is often left to specify the materials, but it’s a good idea to let them know if you have concerns about stiffness or significant thermal cycling so they can work with you to choose the right substrates.
When it comes to solder, the choice is lead versus lead-free. The move to lead free has been primarily motivated by health and safety concerns, so you might be required to use lead free solder if you want your board to be accepted by certain regulators. The main thing to know here is that lead free solders require higher temperatures during solder reflow. Sometimes these temperatures may be high enough to push the breakdown temperatures of your substrate. If you’re going lead-free, make sure that all your components and materials can handle the heat.
Solder Mask and Dams
One last way to design for reliability in manufacturing is to use solder dams and tent or plug your vias with solder mask. The goal in both of these is to keep solder from wicking away from a pad to somewhere it isn’t wanted. If solder flows off the pad during reflow it can cause short circuits or leave too little solder on the actual pad to form a good connection.
Solder dams are exactly what they sound like, barriers that stop solder. Usually, these are placed around SMT pads to keep the solder on the pad. If you have tiny clearances you may not be able to use a solder dam, depending on the accuracy of your manufacturer’s solder mask.
Vias can also wick away solder, which is why you may want to tent or plug them with solder mask. In a tented via the annular ring is covered with solder mask. You don’t want to completely cover the via, as gases can escape the substrate during reflow and will need to exit the via. You can also fully plug a via with solder mask to ensure that no solder gets inside. This technique is sometimes used in very high density boards. Talk with your manufacturer before specifying either of these to make sure everything goes as smoothly as possible.
They say there’s no rest for the wicked and that misery loves company. That must be why my clients are always emailing me in the middle of the night to drag me into more scope changes. Make sure the work stops at the end of design and doesn’t involve troubleshooting intermittent connections on PCBs that weren’t manufactured for reliability. You’ll need to carefully specify through-hole or SMT components, depending on the degree of thermal and mechanical stress they’ll undergo during operation and manufacturing. The next step is to select materials with the right stiffness and CTE so your PCB doesn’t break on the first hot day of summer. Speaking of heat, you’ll also need to consider solder reflow temperatures if you need to use lead-free solder. Last but not least, stem the tide of customer complaints with solder dams and wisely tented or plugged vias.
|Tempo‘s Custom PCB Manufacturing Service|
If that seems like a lot to keep track of, that’s because it is. At Tempo Automation, the industry leader in fast, high-quality PCB prototyping and low volume production, we will work with you from day one to develop manufacturable boards with reliability as a core target.
To help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on CAD files or how to design for manufacturing reliability, contact us.