PCB DFM Guidelines

Design for manufacturability, or DFM, is a design process methodology that has been around for many years. When PCB layout designers were more isolated, pushing DFM problems down the line was common, typically in manufacturing. However, 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 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 (CM) to partner with for your next PCBA prototype.

PCB DFM Guidelines Checklist
Holes	Duplicate drill locations, spacing to the board edge, design features
Components	Assembly spacing, alignment for wave soldering, testability
Traces	Spacing for solder shorts, board edge clearance, thermal imbalance
Planes	Thermal pads on thru-holes, keepout area violations, isolated copper
Solder Mask	Missing clearances, missing mask between pads
Silkscreen	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 frequently 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 board production proceeds without any errors. Here are a few examples of DFM errors to look out for:

• Component placement – 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 soldering 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 (i.e., “tombstoning.”)

 

• Insufficient clearance between parts, mechanical objects, and the board edge – These errors can frustrate automated assembly processes and manual rework.

• 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 masks may expose copper unintentionally.

What Your CM Should Be Telling You About DFM

There are different levels of adherence to DFM requirements based on the stage of your circuit board design. Some tradeoffs regarding DFM requirements are permissible, especially for early proof-of-concept boards, such as PCB prototypes. A 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 to wait to incorporate a complete 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? A 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 the recommended corrections to the design before the prototype production commences. If not, the CM will document these changes for incorporation into the standard production builds.

PCB DFM Guidelines: Considerations for a Successful Prototype

At VSE, we have the engineering prowess to work with you through any DFM issues and even incorporate PCB DFM guidelines into your design. Our experienced manufacturing department has the experience and depth to work through all of these problems to get a working, usable prototype back into your hands as quickly as possible. VSE has helped realize life-saving and life-changing devices alongside our valued manufacturing partners for over forty years.