Elementary, Mr. Watson: Anatomy of Your Component—Footprint, Part 2

Have you ever gone to a buffet hungry and looking forward to digging in? You grab the plate and start down the food line, picking things as you go. Halfway through, your plate is stacked up with food, looking very similar to the Leaning Tower of Pisa. Then you get to the good stuff at the end of the buffet, but there’s no room on your plate. At this point, you probably felt much like that with the first part of looking at our footprint, but rest assured, although your plate is already full, the good stuff is still waiting for us.

I hope you’re not full, because I’ve saved the best for our second offering.

All PCB designs have two primary functions. The first is to allow components of all different sizes and shapes with their endless functions to connect to the PCB surface. The second is to connect those components with traces and vias. Last month, the first half of the buffet accomplished the first function with a discussion of copper pads, the solder paste applied to those pads, and the solder mask that sets up some boundaries and protection.

The second half of the buffet is where the good stuff is.

Silkscreen
First, every component should have a legend, also known as a silkscreen (Figure 1), the top layer of the PCB design that holds the information for the components, which includes the reference designator and orientation or polarity markers. There’s also vital information such as board identification or details needed at specific points for tests or troubleshooting. As smaller designs are much more common, the purpose of the silkscreen has changed. Putting the components and information on some designs has become a real challenge.

watson_fig1.jpg
Figure 1: Component silkscreen (legend).

As someone who started as an electronics technician, the silkscreen was vital for troubleshooting because the technician would find the defective component, replace it, and put the board back in service. Now the entire board is swapped out partially because of how cheap PCBs are. The primary function of the silkscreen is used during the assembly inspection process. Of course, visual inspection is accomplished automatically, and the silkscreen identifiers determine if it is correct.

Time for a heart-to-heart with your fabricator.

What drives your choice of text size depends on the method your fabricator intends to use. There are three basic methods of applying silkscreen:

  1. The manual screen method with line widths greater than seven mils (0.007"). A stencil is used to apply the ink, cured in an oven.
  2. The next method is liquid photoimaging (LPI), which can be down to the line width of four mils (0.004") using liquid photoimageable resist and exposed to UV light to cure. This is the most common method.
  3. The final and most expensive method is direct legend printing (DLP), which uses an inkjet printer with acrylic ink that uses the CAD data and is then cured using UV light as it is printed.

Basic Rules for Silkscreen

Identify the critical components' orientation
Integrated circuits identify Pin-1 because the component can easily rotate. Therefore, a Pin-1 marker is a must. It will save you time, money, and from calls from your assembler. When necessary, identify polarity markers on such components as capacitors and diodes.

Keep your clearance
As we all have experienced in the past two years, a phrase in everyone's lexicon is social distancing. The same is valid with your PCB and your silkscreen. So keep the distance; do not overrun the silkscreen depending on the method of applying it; all you may get back are large ink spots and "blobs" (an engineering technical term) on your PCB.

Do not apply silkscreen to copper
Assure that you do not place the silkscreen on the bare copper surfaces, although this should be a design rule in your ECAD software. Usually this is a check done by the fabrication house, but it is just good practice not to tempt fate. If it does slip through, it results in lousy solder joints.

Assembly Data
The next item in our buffet (I didn’t forgt about that) is the assembly information (Figure 2). That provides the needed details for your assembly drawing and the mechanical verification, i.e., does your PCB fit in the enclosure with no interference, which might be important down the road?

Place the assembly information on an assigned mechanical layer.

watson_fig2.jpg

Rant: Many times during the PCB process, either on purpose or by accident, the correct information given to the fabricator is lacking. The fabrication and assembly drawing is your instructional document to create your design. We provide the bare minimum. I have never experienced the situation of a fab or assembly house complaining they got too much information. The opposite is true, though.

On the assigned mechanical layer, you will have your component's 3D model (Figure 3). So, where do they come from? Well, your first source is the component manufacturer. Many component manufacturers are finally coming up to speed (not a moment too late) to provide the 3D models. Other sources include 3D Content Central and Grab CAD. These are phenomenal resources for a wide variety of component models.

watson_fig3.jpg

That brings up a fascinating subject; even for the most commonly used components, there are varying heights for each of the components. For example, take a 0402 size chip resistor; according to Octopart.com, there are 32,312 results with 25 different heights that range from 250 µm to 12.7 mm. So, which 3D model do you use?

Before I answer that, keep in mind that to stay aligned with the IPC standards (Figure 4), there are three variations of the land patterns for each component.

  1. Density Level A: Maximum land/lead to hole relationship
  2. Density Level B: Nominal land/lead to hole relationship
  3. Density Level C: Least land/lead to hole relationship

The primary use of density is to accommodate a level of manufacturing producibility. Therefore, Level A (maximum) is the most design for manufacturing (DFM) friendly vs. Level C (least), which would be the densest and most challenging to manufacture.

Watson_fig_6_0720.jpg
Figure 4: Component density level.

Which height should we use with these three variations of the 0402 footprints? We cannot have all the different variations resulting in 75 different available footprints. Nope. Often, the heights are divided into three groups, and only the tallest for that group is used. For our 0402, the first group is from 250 µm to 360 µm, the second group is from 370 µm to 450 µm, and the final group is from 457.2 µm to 12.7 mm. So the three models used are 360 µm, 450 µm, and 12.7 mm.—the worst-case scenario, we would say, for each density condition. Each one handles the group of heights.

Placement Courtyard
The final item in your footprint is the placement courtyard, which, as the name implies, is used for placing the component on the PCB. Since you use a standard grid size for the footprint, you can use that information to calculate the total area on the PCB, called the “density feasibility study.” According to IPC, the ratio between the component area and your PCB total area should not exceed 65%. I would refer you to one of my previous columns titled “Density Feasability Putting 10 Lbs in a 5-Lb Bag.”

watson-1_1120.jpg
Figure 5: Component footprint placement courtyard and size.

Putting It All Together
We have all the needed items to carry out our PCB design in the footprint. Components get mounted using the copper pads, solder paste, and mask; the assembly information is used for the Assy DWG with the 3D model verifying no interference or mechanical issues with the enclosure. The placement courtyard is available for placement on the PCB. If you run the density feasibility, you can verify that you aren't trying to push 10 pounds of stuff into a five-pound bag.

John Watson, CID, is a customer success manager at Altium.

Download The Printed Circuit Designer’s Guide to… Design for Manufacturing by David Marrakchi. You can also view other titles in our full I-007eBooks library.

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2022

Elementary, Mr. Watson: Anatomy of Your Component—Footprint, Part 2

09-08-2022

Have you ever gone to a buffet hungry and looking forward to digging in? You grab the plate and start down the food line, picking things as you go. Halfway through, your plate is stacked up with food, looking very similar to the Leaning Tower of Pisa. Then you get to the good stuff at the end of the buffet, but there’s no room on your plate. At this point, you probably feel much like that with the first part of looking at our footprint, but rest assured, although your plate is already full, the good stuff is still waiting for us. I have saved the best for our second offering.

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Elementary, Mr. Watson: The Anatomy of Your Component—Footprint

08-17-2022

I hate to disappoint you if you expect to get everything about footprints from this modest column, but a short search online results in a long list of technical standards and books on this subject. So, I will only hit the surface of the discussion. I have often spoken about the parent-child relationship principle in PCB design. In this series, we have learned that we can see how that "relationship" is supported directly by the information in the component itself. As a short review, the parent-child relationship is where you use an input of data, material, or parts (parents) as the foundation or resource for another item or document (child).

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Elementary, Mr. Watson: The Anatomy of Your PCB Component, Part 2

07-13-2022

In the start of my series of the anatomy of a component, I discussed that the component has two major divisions. The first is information that consists of name, description, parametric information, sourcing (part choices), and the datasheet (Figure 1). Next, the component comprises symbol, PCB footprint, 3D model, and simulation models. I gave the example of the dissection of the frog, with an analogy that every part has a purpose. In the same way, each part of our component has a distinct purpose in our PCB design, including our models.

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Elementary, Mr. Watson: The Anatomy of Your PCB Component, Part 1

06-16-2022

One of the classes I dreaded the most in school each year was biology. This was because I knew it was only a matter of time before I would face the rite of passage for most high school students: dissecting a frog. It wasn’t something I ever looked forward to. We had to go through the same educational exercise and maybe with the same apprehension for most of us. But my point in bringing up the painful experiences of our high school years is, although it was difficult, I did learn a powerful lesson: Every part has a purpose.

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Elementary, Mr. Watson: The Five Pillars of Your Library, Part 5—Traceability

05-12-2022

We have reached the end of this series regarding the five pillars of the component library. We now have a robust library that provides the required resources for the ever-changing industry. Above that is having a flexible library to grow with the company. The final pillar is traceability. Why is traceability so essential and considered a pillar of our library? Read on for details.

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Elementary, Mr. Watson: The Five Pillars of your Library, Part 4—Review

04-14-2022

I trust that you have been enjoying this series on the five pillars of your library. Now that we have a single library managed using our revisioning, and we have lifecycle schemes organized so that we can easily find something in the component category, family, and subfamilies, we are now ready to look at one of our library's most vital principles and pillars: reviewable.

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Elementary, Mr. Watson: The Five Pillars of Your Library, Part 3—Architecture

03-10-2022

Before I continue with the series of the five pillars of your library, I want to do a little review. Although every library is different, the five pillars are consistent with any sound library. You place these pillars to support a specific building section in building construction. To pull one out requires the remaining ones to hold the total weight above. So, each of these supports is needed for your library to succeed. You cannot choose which of them you intend to follow; to pull just one out results in the toppling of the others.

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Elementary, Mr. Watson: The Five Pillars of Your Library, Part 2—Managed

02-17-2022

The 1972 classic movie “The Candidate” tells the story of Bill McKay (played by Robert Redford), who was running for the state senate. Although he was a long shot and an underdog, McKay ends up surprising everyone with an incredibly close win. After the concession speech of his opponent, a vast mob surrounds McKay. He fights through the crowd, trying to reach his campaign manager Marvin (played by Peter Boyle). Finally reaching him, McKay pulls Marvin aside into a hotel room, sits on the bed, and after several seconds of silence, finally asks a very intense question “So, what do we do now?” The campaign manager looks bewildered, so McKay asks the question again, “What do we do now?”

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Elementary, Mr. Watson: The Five Pillars of Your Library, Part 1

01-13-2022

I have recently had some great conversations with many of you, and the same question keeps coming up: What does it takes to have an excellent component library? So, I have decided to kick off the new year by taking a deep dive into your PCB component libraries and looking in detail at the five pillars of your library. So along with taking the tree and the decorations down and making your New Year resolutions, let's resolve to take an honest look at our component libraries and get them in order.

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2021

Elementary, Mr. Watson: PCB Data Management and Security

12-12-2021

As a grandfather of six grandchildren, one of my great joys is spending time with them. There is nothing better than spending an afternoon at the park and especially playing on the teeter-totter. It's all fun and games until grandpa gets on one side, and they try to lift me. Then the harsh reality and a teachable moment in leverage, balance, and just how heavy grandpa really is hits pretty hard.

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Elementary, Mr. Watson: We’ve Never Done It That Way Before

11-11-2021

The September edition of Design007 Magazine discussed the theme of collaborating and working with a team. In that issue, I wrote a feature article called “PCB Design Is a Team Sport.” After that edition was published, I had several follow-up questions and conversations with individuals; they agreed on the importance of teamwork but felt that it's easier said than done. It's challenging because of the inherent problem of team members accepting or handling change very well. Change it's a word that sends shivers down the spine of some. You know those sort of individuals. They're easy to identify. The ones that constantly remind everyone, "We never did it that way before." As if how we did things in the past was so much better.

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Elementary, Mr. Watson: First, Component Shortages, and Now Hot Dogs?

10-14-2021

When I considered the title for this month’s article, I seriously considered calling it "From the Frying Pan Into the Fire" because I’m sure you’ve noticed recently that the component shortage problem has only worsened—we’re now seeing other supply lines breaking down.

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Elementary Mr. Watson: PCB Design—It's a Team Sport

09-29-2021

One of the hard lessons of this past year was about the value of the team and collaboration. I have repeatedly heard how many of us have a newfound respect and appreciation for the teams we work with inside our companies. Out of necessity, we had to find new ways to collaborate.

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Elementary, Mr. Watson: The Danger of Rogue Libraries

09-16-2021

For PCB designers, the most common part of the library is the collection of components used in the PCB design process. But, I have seen some libraries have other information, including a resource area, a group of documents, standards, and articles. So basically it can have anything you want.

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Elementary, Mr. Watson: Epic Fails with Design Rules

08-12-2021

Various sciences, including physics, mathematics, chemistry, are significantly involved throughout the PCB design process, rules that can sometimes be bent but not broken. However, the rules that designers break and ignore altogether and very often are the design rules.

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Elementary, Mr. Watson: Managing Risk in PCB Design

08-05-2021

PCB design is like bungee jumping. With the complexity of a PCB design, the intricate details, and various steps, it's rather easy to make mistakes. Those mistakes, many times, do not show up until it's too late and the board has gone off to fabrication and assembly. By the way, a good rule is not to use your assembly house as your quality control team for PCB designs.

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Elementary, Mr. Watson: Time to Market, from Ludicrous Speed to Plaid

07-18-2021

Mel Brooks may have something to teach us about going "ludicrous speed" in getting our designs to the finish line. John Watson explains.

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Elementary, Mr. Watson: Trust but Verify

06-10-2021

Over many years, I have seen some elaborate PCB library systems. However, the best ones were those not based on the size but rather the quality of the information. That old axiom is definitely “not quantity but rather quality.”

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Elementary, Mr. Watson: Paying the Price To Be a PCB Designer

05-13-2021

Today, the electronics industry is flourishing with innovations and technologies. The result is that the “good” designers are left in the dust. Truthfully, our industry doesn't need more good designers; rather, we need great designers—those who can face any challenge and instead of cowering in the corner, looks at the task at hand and says, "Bring it on."

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2020

Elementary, Mr. Watson: Demystifying Bypass Capacitors

12-17-2020

As PCB designers, we work under the simple rule of cause and effect, and a PCB design can quickly become a petri dish for the butterfly effect to flourish. One of those areas that can quickly snowball into major problems is your PCB power distribution structure. When it goes wrong, it usually goes very wrong and has significant issues throughout your design.

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Elementary, Mr. Watson: Density Feasibility Putting 10 Lbs in a 5-Lb Bag

11-18-2020

Whether on a customer, a system, or a PCB level, it’s essential to understand the final objective and how you intend to get there and meet the customer need at the forefront of any project. In this column, John Watson addresses density feasibility and more.

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Elementary, Mr. Watson: Location, Location, Location

10-15-2020

When it comes to PCB design, one of the most overlooked principles is component placement. Similar to a home, the component location has a considerable impact on the quality and is the real value of a PCB design. John Watson examines five rules to follow when it comes to component placement.

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Elementary, Mr. Watson: Overcoming PCB Designs Pitfalls

09-10-2020

When starting every PCB design, the hope is that we can navigate through any pitfalls that arrive. Unfortunately, many times, issues happen that you do not handle correctly; they fall through the cracks and end up in your PCB design. John Watson explains how that is when the real problems begin.

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Elementary, Mr. Watson: How to Ruin Your PCB Design in 4 Easy Steps

08-06-2020

John Watson has seen firsthand how quickly PCB designs can “go off the rails” by not following a few simple principles. In this column, he looks at four practices that can easily ruin your PCB design.

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Elementary, Mr. Watson: PCB Components Naming Conventions

07-09-2020

How you accurately analyze and identify certain information has a direct connection to the overall success of your PCB designs. In this column, John Watson focuses on the conventional naming scheme for the schematic symbol and footprint to prevent headaches and ulcers later.

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Elementary, Mr. Watson: Collaboration in the PCB Design Process

06-11-2020

The past few months have been trying for everyone, with many of us working from home. However, there are still the underlining principles of collaboration to step into a role to finish the necessary tasks to keep a project moving forward. John Watson, CID, explains.

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Elementary, Mr. Watson: Reinventing Yourself

05-28-2020

When COVID-19 first hit, many businesses were forced to close, and we immediately saw its impact on the service industry. Whatever challenge you’re facing, John Watson emphasizes that it’s time to hit the switch on reinventing.

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Elementary, Mr. Watson: The Positive Side of COVID-19

04-16-2020

With the recent COVID-19 outbreak worldwide, most of us have been forced to reshuffle how we work, live, and play. Something like this has never happened before in our lifetimes, and it is scary and challenging, but difficult times develop resilient people. John Watson shares some of the positive things he has already noticed come out of this situation.

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Elementary, Mr. Watson: Are We There Yet?

03-12-2020

Anyone who has taken a road trip with children knows the question, “Are we there yet?” very well. This question also applies to PCB design. If you are not careful, your PCB project could easily go off track and you could lose sight of what you are doing (objective), why (motivation), how (process), and when (schedule). John Watson emphasizes the importance of these fundamental questions.

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