The P-51 Mustang is arguably the finest piston engine fighter aircraft to ever be built. From the first flight of the prototype in 1940, it went on to be built in vast numbers and proved itself to be a superior fighter plane. The P-51 had many innovations that contributed to its success, but it also had something else—it was created using a “design-for-production” manufacturing methodology, which yielded higher production rates at lower costs.
Built in five main sections, with each section being fitted with wiring and piping before being joined together, the Mustang was a precisely constructed aircraft designed for mass production, rather than being retrofitted for it later, as most planes of that era were. With this manufacturing precision, the plane exceeded its performance expectations, was much easier to maintain in the field than other aircraft, and could be produced at higher rates for a lower cost.
The creators of the P-51 discovered 80 years ago the value of designing a product for efficient manufacturing, and we continue to benefit from those same principles today. How we design a printed circuit board for manufacturability, for instance, can make the difference between a board that can be easily produced or one with exorbitant manufacturing costs that is prone to failure. Let’s take a closer look at some of the top design for manufacturability (DFM) issues in PCB design that can ultimately cause us a lot of pain during manufacturing if not corrected.
DFM Issues in PCB Design: The Top Five Problems
There are many design problems that if not accounted for can cause you pain in your PCB design. These can range from board materials and stackup configurations that are not compatible with the needs of high-speed design to important silkscreen information being covered by components or mounting hardware. But for design issues that cause problems with manufacturing the board, those can be especially painful. Here are a handful of those problems to look out for:
- Incorrect PCB footprints and land patterns: Surface mount land patterns, or pads, that are too large can cause their components to float out of alignment during solder reflow. These parts may cause clearance problems or even short together. Pads that are too small may not form a strong enough solder joint which may result in intermittent or total board failures at some point in the future. Pads that differ in size on small two-pinned parts like SMT capacitors and resistors, or have more metal on one then the other, may cause an imbalance in heating during solder reflow. This imbalance could cause one pad to reflow before the other and pull the part with it. This effect is known as tombstoning because the part will stand up on one pad instead laying down as it should.
- Component location and rotation: Components that are not positioned correctly for wave soldering can cause problems with poor solder joints. If a larger part proceeds a smaller part into the wave, an effect known as shadowing can cause an improper solder filet to be formed between the pins of the component and the pads on the PCB. These bad solder joints are prime candidates for breaking at some point in the future. Components that are rotated so that they go through wave solder parallel to the wave instead of perpendicular to it are also susceptible to bad solder joints.
- Component clearances: PCB components need to be placed with enough clearance between them to facilitate automated placement and test. Additionally, parts need to be placed where they can be accessed for rework, or for functions such as plugging and unplugging connectors. There also needs to be a minimum amount of clearance from placed parts to the edge of the board both to prevent damage to components when individual boards are broken out of their assembly panels and for testing purposes.
- Insufficient solder mask coverage: Small surface mount pads, such as those on fine pitched parts, need to have solder mask between them to protect them from solder bridging. If left unprotected these pads can form solder slivers between them which are difficult to find and correct due to their size, and will potentially short two pads together. These shorted pads could produce intermittent problems such as false signals being sent that can be very difficult and expensive to find.
- Errors in the bill of materials (BOM): Component errors in the BOM may result in the incorrect parts being prepared for manufacturing. If not caught in time, these parts could get assembled onto the board. In the case of parts that have a slight difference in values or tolerances, this could cause intermittent problems that are difficult and expensive to find and correct.
What You Need to Resolve These Problems
Knowing the problems is the first half of the battle. Once you know what to look for, you can set up design review processes to make sure that any problems like these are discovered before you send your board out for manufacturing. It is also a great idea to understand the DFM rules for your design before you start laying the board out so that you can avoid problems like these in the first place. Here is where knowing the needs of your contract manufacturer early on can really help.
It is in the best interest of your CM to work with you up front while you are still in the layout phase of your board design. This way they can advise you on the different DFM requirements that you need to follow, and answer any DFM questions that you may have along the way so that your board is designed right for manufacturing. Also, a good CM will have engineering expertise on hand so that their answers are specific to your board’s needs and not just general guidelines.
How Your Contract Manufacturer Can Help
At VSE, we have built our business on helping PCB designers like you prepare their boards for efficient and error-free manufacturing. We have a staff of engineers that will first start with a design review of your board to check for DFM issues like those listed above before we get into manufacturing. Our component engineers will check your BOM for part availability and prices, and we will alert you to any errors that we find or part replacements that we recommend.
Likewise, our design engineers will also be looking at your board to make sure that it is ready for manufacturing, and any problems or corrective action recommendations that we find will be sent right back to you for your approval. And, of course, we invite you to work with us early on so that we can help you to work through all of these issues as you lay out your board.
