My dad’s favorite expression was, “If you want a job done right, do it yourself.” That doesn’t mean there won’t be times when we will need the help of others, but it does highlight the importance of ensuring that a job is done right. This is especially true when designing a printed circuit board for error-free manufacturing.
Design for manufacturability, or DFM, is a design process methodology that has been around for many years. When PCB layout designers were more isolated in their jobs, it was common to push DFM problems onto the next department in line, typically manufacturing. But workflows like that have changed as technology has evolved and time to market has decreased. It has become imperative in PCB design to plan and take ownership of DFM, even before the design reaches the layout stage.
Let’s look at some of the PCB DFM guidelines that will be crucial to you and how those considerations can help you choose a contract manufacturer to partner with for your next PCBA prototype.
Common PCB DFM Guidelines for Checking
||Duplicate drill locations, spacing to the board edge, and features|
||Assembly spacing, alignment for wave soldering, testability|
||Spacing for solder shorts, board edge clearance, thermal imbalance|
||Thermal pads on thru-holes, keepout area violations, isolated copper|
||Missing clearances, missing mask between pads|
||Silkscreen in holes or under parts, missing reference designators|
DFM: Friend or Foe?
For circuit board layout, PCB DFM guidelines can be frustrating. The electrical needs of the design often conflict with the manufacturability requirements. For instance, components should be placed close together for the best possible circuit performance, but those tight spaces often violate the PCB DFM guidelines. It is not unusual for designers to change that “perfect” placement or precise routing pattern to achieve the required DFM clearances.
However, those frustrating spacings are there for a reason—to ensure your board will be manufactured without any errors. Here are a few examples of the kinds of errors that must be avoided:
- Component placement errors: Components placed too close to each other or at the wrong rotation or location can cause problems during the soldering process.
- Incorrect land pattern pad sizes: Pads that are the wrong size or not located correctly for the part to be soldered to, can cause manufacturing problems. Small, two-pin passive components with incorrect pad sizes can suffer from unequal heating during soldering, resulting in the part standing up on one pad in a condition called “tombstoning.”
- Insufficient clearance between parts, mechanical objects, and the board edge: These errors can cause problems for automated assembly processes and 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 will add another layer of work to your job as a PCB designer, but they are essential. You should do everything possible to find and correct DFM errors in a layout and avoid those same situations in future designs. 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 circuit board design is in. Especially for early proof-of-concept boards, such as PCB prototypes, some tradeoffs can be made regarding DFM requirements to reduce the time needed to produce your prototype. 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 prepare it for a regular production run, especially if you already know that the prototype version will change significantly from the production version.
- What critical DFM issues need addressing? Your CM should request the layout for your prototype before building it so that the CM’s engineering group can conduct a design review and identify any DFM issues.
- How will DFM changes affect the timing of your prototype build? If the schedule allows, the CM will make its recommended corrections to the design before the prototype is built. If not, the CM will document these changes to be incorporated into the regular 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, they will be free to add the needed DFM corrections and enhancements while designing.
It all comes down to how much time is allocated for the layout of the prototype. After reviewing your design, your CM should clearly communicate with you what changes are needed, how long 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 with DFM problems to get your product built correctly and returned to you quickly.
At VSE, we have the engineering prowess to work with you through any DFM issues and even incorporate DFM corrections on your design as needed. However, We understand that your highest priority with a prototype build may be to finish the project 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 make do with what they have and continually charge customers for re-spins of a design, our business model is significantly different. We want to partner with you on all phases of the design, DFM, test, and obsolescent component analysis to 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.