How to Design for Ease of PCB Fabrication

May 22, 2019 Cadence PCB Solutions

Fabricated PCB without components

 

My father-in-law was not a learned man, at least with respect to formal education. However, he had a capacity that many do not possess to teach himself. After being thrust into the role of provider for a large family at an early age, he dropped out of school to learn a trade that would provide adequate income to support his family. Eventually, he became a great mechanic and some say the best automobile body repairman in the area. Although, I learned virtually everything about working on cars from him I also learned something that was even more valuable to me in my career as a designer of electrical circuits: the difference between good and bad designs.

A good design is when the designer incorporates who will be using the product and how it will be built into the process. Usability is one of the primary criteria by which your design is judged and manufacturability determines its quality, ability to perform its function reliably, or if it can be built at all. For PCBs, there are two parts to the manufacturing process: fabrication, during which the physical architecture or PCB layout is constructed, and assembly, where the components are mounted on the board. Both of these are important; however, assembly cannot be done until successful fabrication is achieved and the choices you make during layout are directly proportional to how well your board is built and how much it will cost.

First, let’s take a look at fabrication in depth. This will provide insight that can then be used to inform a strategy to design for ease of PCB fabrication and support the creation of a quality product.  

The PCB Fabrication Process

Although a contract manufacturer (CM) is most often engaged to construct your boards, an understanding of the PCB manufacturing steps can provide enlightenment and an understanding of the constraints within which your CM must work. The entire process can be broken into three distinct stages: fabrication, component procurement, and assembly.

Although these three stages are distinct, they are interrelated and many design choices and decisions impact all of the three. For example, for each component to be mounted during assembly, the footprint for the pads etched on the board during fabrication, must exactly match the component package type acquired. Below, the main steps in the fabrication process are listed along with some of the issues that may arise and threaten your board’s manufacturability or quality.

 

PCB Fabrication Steps and Issues

Steps

Description

Manufacturability Issues

Image creation

Here the images for the outer and inner layers are created on the board.

Image misalignment can cause problems for drilling vias and mounting holes.

Etching (inner layers)

Unwanted copper is removed and desired copper area is left only on traces and pads.

That are many trace issues that can occur if etching is done improperly. For example, unconnected traces will prevent signal flow which in turn halts fabrication until corrected. Traces without adequate spacing may prevent solder masking or if left uncorrected cause shorts.

Stackup

During this step, the board layers are aligned and pressed.

Alignment is a major issue here, as well.

Drilling holes

These can be plated through holes (PTHs), non-plated through holes, or vias, like buried, blind or through-hole.

Alignment is an issue as well, but there are spacing and clearance issues that can impact solder masking. Additionally, aspect-ratio is important as it determines what type of drilling can be used.

Etching (outer layers)

Outer layers typically have full copper coverage and etching is used to remove the excess and expose traces and pads.

In addition to spacing and clearances between pads and traces, there is also board edge clearance to contend with.

Hole plating

The conductive material for plated through holes (PTHs) and other vias is performed here.

Hole annular ring clearance can be an issue here. Another consideration is tenting, where holes are covered instead of left open. A major problem here is outgassing that can happen during assembly.  

Solder masking

Solder mask is the covering (usually green) used everywhere on the surface except over traces and pads that protects the board.

This covering guards against contaminants and prevents assembly problems like solder bridging that can lead to shorts and damage.

Silkscreen printing

The silkscreen provides reference information. This includes pin 1 indication for integrated chips (ICs), reference numbers for components, logos, etc.

Silkscreen errors will not prevent your board from being fabricated; however, it can hamper the assembly process and may negatively affect operation.

Finish application

This step is primarily to provide protection for copper areas of the board.

This step is sometimes omitted and does not impact board fabrication. However, it may impact your board’s operational lifetime.

 

Many of the issues listed above will cause the fabrication of your board to be delayed until corrected, while others may cause problems for assembly or your board’s operation in the field. Nevertheless, correcting these issues must be done during the initial or a subsequent design stage, prior to fabrication. Depending on the particular issue and when it is discovered and corrected, significant loss of time and additional costs can be incurred.

The Process of Design for Ease of PCB Fabrication

As discussed above, having errors with your design can cause issues. And, if discovered after fabrication, correcting these can be a major problem requiring extensive repair. In fact, you will probably have to have completely new boards fabricated in addition to doing redesign.

 

Repair crew fixes circuit board

Repairing PCB errors after fabrication

 

Fortunately, these issues and problems can be avoided by employing a strategy for design for ease of PCB fabrication. This strategy is built upon design for manufacturing (DFM). By following the guidelines listed you can significantly impact whether and how your boards are built.

 

Guidelines for DFM Usage to Ease PCB Fabrication

  • Set DRC constraints according to CM DFM

The first rule of effective DFM usage is that the guidelines must be applicable for the equipment used by the CM that is actually going to build the boards. Although there are some general defaults that may be broadly applied in some cases, it is advisable not to rely upon these as most CMs will furnish you with rules and tolerances. In some cases, you can download a DRC or rules file that you can upload into your PCB design package. Why manually enter rules into your system when you can get the rules in a format that can be used by the tools? Ask your PCB manufacturer.

 

  • Clear all DRC violations before sending for fabrication

Prior to submitting your files for fabrication, you should correct any DRC violations as it

is likely that design changes will be required to correct these errors.

 

  • Make any required design changes before fabrication

Once you submit your design files to your CM, they will most likely perform a DRC check themselves. If there are still issues or errors, make the corrections prior to fabrication to avoid the creation of useless boards.

 

In order to create good designs, it is imperative that you consider your board’s manufacturing. Doing so, not only will ensure that your PCBs can be made, but should also help to make sure your customers or clients are satisfied with the resulting product.

The best way to design for ease of PCB fabrication is to employ an advanced real-time DFM checking; such as is available from the industry premier PCB design and analysis software provider, Cadence. Cadence Allegro's PCB editor tools enable in-design decision-making and faster routing for better designs that will not require excessive back and forth with your CM to be built well.                         

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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