Custom PCB Design Helpful Tips

Congratulations!  You have gone through the process of developing your own printed circuit boards using one of the many CAD tools available.  This could be from any of the very popular software packages such as Altium, Pads, Eagle, or Ki-CAD or from an outside vendor you have contracted with to provide a PCB design for you.  If you are an electrical engineer, much of this could be common knowledge to you, but we have helped many engineers with much of this same information.  So whether you are new to PCB layout or a professional, please keep reading to make sure you haven't missed something!

IMPORTANT TO NOTE:  We are not able to advise on your actual electrical design and application of your custom PCB, that is outside the scope of this tutorial.  Electrical engineering design flaws are outside our ability to advise or control from a manufacturing perspective.  In other words, if you make mistakes in your electrical design, i.e. your schematic, that is on you as the designer.  We can only fabricate or assemble to what you have called out.  The information provided below is purely based on manufacturing and assembly requirements and what should be included as a bare minimum with a design package for a smooth and successful PCB production and/or assembly run.  

No two CAD package are exactly alike, but they all share several traits that carry across most tools.  Namely, the ability to capture electronic schematic data and create a PCB layout, this is it's main function.  While different packages have different design capabilities, what they all must do at the end is output those design files into the proper files for PCB fabrication, Gerber and NC Drill files! 

Gerber files show the multiple layers of the custom PCB along with the board outline, individual traces, pads, vias, silkscreens, etc.  Gerber files can be extracted in a few different formats, so it's important that you are utilizing 274X file format.  Most CAD software packages should default to this standard, but it's good to verify!   

The second file type you need to export are called NC Drill files.  These files specifically show the drill hole locations, sizes, and whether they are plated or non-plated (PTH OR NPTH).  Both file types are critical for proper PCB fabrication.  Gerber and Drill files should be put together into a single zip folder when sending for quotation purposes.

Gerber files will contain the two-dimensional layer data needed to make your custom circuit board.  This includes all copper layers, solder mask, solder paste, silk screen, and PCB design outline.  Each of these layers is important for both fabrication and later assembly.  Make sure all of this data has been properly outputted from your CAD program into your custom PCB design data files.

If you are new to PCB layout, here are some common mistakes we see in customer supplied files to look for in your data package:

• No fabrication notes, layer stack up information, or overall thickness specification.
• No drill chart ( not critical, but helpful)
• Board outline layer is missing.  This shows the actual custom PCB board dimensions for length and width.
• Solder paste files are missing (especially important for assembly down the road)
• Silkscreen layer is missing or incomplete (important for circuit board assembly or troubleshooting circuit after assembly)
• Silkscreen layer has characters covering exposed pads or intersecting characters between component reference designators.
• Silkscreen layer is missing important information.  For example:  diode cathode orientation, tantalum capacitor polarity, pin 1 designation for IC's and other active components, along with missing reference designators.
• Customers who have "hand drawn" traces.  Use of an auto-router is best practice for beginners.  Traces should bend at 45 degree angles only when necessary for optimal circuit performance.  This does not include interconnecting between layers with vias or other application like differential pairs.

The most common issue is customer's lack of even suppling the appropriate drill files for their custom PCB.  Your CAD package should easily generate both a Plated Through Hole (PTH) and Non-Plated Through Hole (NPTH) file for fabrication.  Even if one of these files is empty, it's still good practice to generate everything, this way you develop good habits and avoid future project issues that could be overlooked down the road on your next PCB layout.

If there is something that is missing from a large majority of submitted projects, it's almost always fabrication notes and a proper layer stack up.  What are these and why should you consider including them in your PCB design? 

Gerber files, while containing all the layer and trace information, still lack a great amount of detail.  That is where fab notes come into play to fill in the gaps for the fabricator.  Fabrication notes are used to call out the details for final board specifications that cannot be determined by just looking at the two-dimensional PCB layout. They can include things such as material type, plating requirements, and other helpful information.  We cover this in further depth in the next section.

Stack up information shows the breakdown between all the layers of your PCB layout and the target final thickness.  This would include core and prepreg thicknesses along with copper weights for each layer.  This information can be very critical to some customers and we can work with you to make sure your stack up requirements meet your desired intentions in electrical design.

In most simplistic 2-layer boards, fabrication notes are not overly critical as a standard .062" thick FR4 is going to be utilized in about 99% of cases.  However, if your PCB design has more than two layers or a has a specific thickness and/or copper requirements, then you need to put some consideration into making a fabrication notes and layer stack up section in your layout to be included in the gerber file information or as a separate "READ ME" text as a bare minimum. This is important because many of the things listed below can have a direct impact on pricing for your custom PCB fabrication and helps insure your quote is accurate up front.  

Here are some common fab notations to include:

• Proper PCB board materials to utilize for construction.
• Final PCB board thickness and acceptable tolerance.
• Copper weight requirements.
• Drill location and size tolerances.
• Flatness specifications for finished product.
• Plating requirements:  HASL, ENIG, etc.
• Solder mask and silkscreen color (green/white is industry standard)
• Controlled impedance requirements
• Blind/Buried Via or Via in pad requirements.
• Via Fill requirements.
• Full layer stack up showing individual layer and core thickness along with corresponding copper weights and dielectric requirements for each layer.

This list is not all inclusive and there can be other requirements your circuit would demand for proper electrical function.  Please be sure to reach out with any questions with regards to your project.

Yes, you can include this in your PCB design, but we would advise you to let your assembler decide the best way to array a board when necessary.  The reason being every assembly house will have it's own way of wanting to process your custom PCB project.  This could mean preferences in how the boards are spaced in the array, the overall array size, tooling hole size, rail sizing, or even how the array will break out after assembly.  

The good news is this is something that can be done in the Computer Aided Manufacturing process, more commonly referred to as CAM, prior to fabrication at no extra charge.  To put this in the most simple terms, CAM software is used to turn your two dimensional CAD gerber files into a machine language used for automated manufacturing.  The most important part here is this, we will help guide you in what will work best for both fabrication and assembly of your PCB layout to help deliver the overall best pricing!

Fiducials are used for properly triangulating part location placement data in assembly.  If you have a complex assembly (more than 50 parts per side or small/fine pitch components) you should consider adding these into your PCB layout.  You only need to add two minimum, three preferred.  They don't need to be large, 1.0mm diameter is sufficient for just about any design and the value they add back to your assembler is enormous.  Anything with very tight tolerances for placement (Large QFP, QFN, or any BGA) should utilize them in circuit to increase board to board accuracy in an array assembly.

If you don't have the ability to go back and add them we can still help.  If we are doing the fabrication and assembly for you then we will add them into the rails of the array during CAM when the boards go to fab.  There is no added fee for this and it will be seamless for the customer as your circuit design remains unchanged.