My dad had a favorite expression that he quoted to me all the time while I was growing up: “If you want a job done right, do it yourself.” Admittedly, there are a few problems with this kind of thinking. There are plenty of instances I can think of where a task is best left to an expert. But this expression has some value too—mainly the importance of taking ownership of a task to get the job done right. This is especially true when designing a printed circuit board so that it can be manufactured without problems.
Design for manufacturability (DFM), is a design process concept that has been around for many years. When PCB design was still largely a matter of doing your part of the job and then “throwing-it-over-the-wall” to the next person, it was common to push DFM problems on to someone else. But that has all changed as technology has increased and time to market has decreased. It has become imperative in PCB design to plan ahead and take ownership for DFM, even before the design has gotten to the layout stage.
Let’s take a look at some of the PCB DFM guidelines that you should be considering, and how those considerations can help you choose a contract manufacturer to partner with for your next PCBA prototype.
DFM: Friend or Foe?
For PCB design, and especially layout, DFM can be a frustrating thing to deal with. You want to place components and route circuitry for the shortest possible paths as well as the best possible circuit performance. Often, the requirements of DFM will force you to change that “perfect” placement or tight routing pattern to get the correct spacing in.
But those frustrating spacings are there for a reason—to make sure that your board will be manufactured without any errors. Here are a few examples of the kinds of errors that you should avoid:
- Component placement errors: Components that are placed too close to each other, or at the wrong rotation or location in relation to each other, can cause problems during the soldering process.
- Incorrect land pattern pad sizes: Pads that are the wrong size or not placed correctly for the part that will be soldered to it can cause manufacturing problems. For small, two-pin passive components, this could cause unequal heating during soldering, resulting in the part pulling and standing up in a condition called “tombstoning.”
- Insufficient clearance between parts, mechanical objects, and the board edge: These kinds of errors can cause problems in both automated assembly processes as well as any manual rework that might be required on the finished board.
- Copper and solder mask slivers: Narrow wedges of isolated copper or solder mask can sometimes float from their locations and reattach elsewhere on the board. If this happens with copper, it may end up shorting other nets together, while floating solder mask may expose copper that should normally be covered with solder mask.
DFM requirements do add another layer of work to your job of PCB design, but it is very important. You should do everything you can to find and correct DFM errors. The good news is that you have an ally in all of this—your contract manufacturer.
What Your CM Should be Telling You About DFM
There are different levels of adherence to DFM requirements based on what stage your PCB prototype is in. Especially for early proof-of-concept boards, it’s possible there are some tradeoffs that can be made when it comes to DFM requirements to reduce the amount of time needed to produce your prototype. Your CM should be able to help you to understand all of what is needed for your specific PCB design.
Your CM should be discussing the following with you regarding DFM requirements for your prototype PCB:
- What stage is your prototype in? It may be more beneficial for you to wait to incorporate a full set of DFM requirements into the board until you are preparing it for a regular full production run, especially if you already know ahead of time that the prototype version will change significantly from the production version. In these cases, you need to be able to rely on the manufacturing wizards of your CM that the DFM problems in your prototype design will be handled on the production line.
- What critical DFM issues need addressing? Your CM should request the layout for your prototype prior to building it so that any DFM issues will be identified by the CM’s engineering group.
- How will DFM changes affect the timing of your prototype build? If there is time to make your CM’s recommended corrections, then those changes can be made before the prototype is built. If not, then those changes will be documented for the full production builds that will come later. Usually, there is enough time for the CM to address the most critical DFM issues to help with the prototype’s manufacturing. If the CM’s engineering group designs the layout themselves, then they will have the freedom to add the needed DFM features in while they design.
It all comes down to how much time is allocated for the layout of the prototype. Your CM should clearly communicate to you after reviewing your design what changes are needed, how much time those changes will take, and what the tradeoffs are to get you the prototype as quickly as possible.
PCB DFM Guidelines: Considerations for a Successful Prototype
To get the best results in a prototype build, you need to choose a CM that is fully versed in and ready to help you with DFM requirements. Not all CMs are set up for this, and in the case of “garage shop” CMs, they might not have the capabilities to support any DFM engineering at all. For success, make sure to work with a CM that:
- Completely understands all DFM requirements and will work with you during the design phase to help you incorporate those requirements.
- Has the engineering staff and facilities to review your design, recommend DFM changes, and even make those changes, if that is in your best interest.
- Has the manufacturing proficiency to work with prototype designs that have DFM problems in them to get your product built correctly and returned to you quickly.
At VSE, we have the engineering prowess to work with you through any and all DFM issues, and even incorporate DFM corrections on your design as needed. We understand, though, that your highest priority with a prototype build may be to get the project finished first and then correct any problems later. Our experienced manufacturing department has the experience and depth to work through all of these kinds of problems to get a working, usable prototype back into your hands as quickly as possible.
Where other quick-turn prototype manufacturers will do their best to push through what they’ve been given so that they can be rewarded with continual re-spins of a design, our business model is significantly different. We want to partner together with you on all phases of the design—DFM, design for test, and obsolescent component analysis, so that we can drastically reduce the number of re-spins of your design. Our reward is in successfully developing a highly manufacturable production unit that has been vetted during the prototype validation process.
