Greetings! This month, I’m offering my top six design challenges, with some additional feedback from Jeff Reinhold, one of the owners at Monsoon Solutions.
Narrowing it down to the top six was tough, because there are so many challenges for today’s board designers—from incomplete information and footprint generation to power routing constraints. These are just some of my thoughts and is by no means a complete list of all of today’s design challenges. Every designer has his or her own challenges; what is easy for some may be a challenge to others. I will be interested in reading the different approaches to this topic. Here’s my countdown:
#6: Inaccurate or Less Than Helpful App Notes or Part PDFs
As a board designer we are constantly referring to component datasheets and app notes for information helpful to layout and design. Many of these datasheets are very short and do not have enough information regarding device layout; they may have some technical app notes and a pinout but little information other than that relative to board design. Even worse, some are over 300 pages long, and most of the information is more relevant to an EE than to a board designer, thus requiring the designer to weed through the mountain of data to glean what they really need.
In my short time as a board designer, I have seen both. Many do show layout solutions and design suggestions, but some are not really the best way to place the parts such as resistors and capacitor location suggestions. So why are these datasheets so often misleading or less than helpful? Here are some possible reasons:
Understanding that everything is application-based and the engineer writing these app notes cannot possibly cover every contingency plan or every possible application.
There may be left out or missing information. Here you should seek a second or third source to find the information you need.
Misprinted information. This one is quite common. Frequently the datasheet itself was translated from its original language and as we all know, this can result in sentences that make no sense, with some that are downright hilarious.
There could be measured data that was misinterpreted and therefore useless to anyone looking to use that information.
Due diligence on the part of the designer is always required. Know what you want, what your customer wants, and how to get it. This requires amassing information about your design’s purpose.
At Monsoon Solutions, we do this in a “kick-off” meeting with the customer to ask specific questions. We cover things like power needs, mechanical constraints, fixed components, and thermal considerations, just to name a few, Even the best laid plans do not always cover everything you need to know about your design in the kick-off meeting, and follow-up conversations are usually necessary. Do not be afraid of asking more questions (within reason) to get your design right.
I also encourage you to learn and read more. Of course, be careful not to believe everything you read, either online or in print. Testing everything you read to prove the concept to yourself sometimes means you will also have to prove it to the customer. Be ready for that.
#5: Large PMICs
A power management IC is a power IC solid-state component that distributes the required measure of voltage to all other parts, which is usually accomplished using a low on-resistance MOSFET placed between the source and the load. The PMIC controls the MOSFET and thus its resistance. The PMIC manages the turn on/off rate by timing these MOSFETS, one per rail. It’s typically used in battery operated devices such as cellphones, laptops, and portable media players to decrease the amount of space required due to limited board real estate.
About this, Jeff Reinhold said:
“The reason I put PMICs above other circuits is we typically get a lot of information on how to lay out certain circuits (not just PMICs), including impedance, length, matching, clearance, and placement info. In addition, there may be pictures of completed placement/routing and even reference boards. The more complicated the circuit, the more information we might get. Very often, the parts and connections we have don’t match the input(s) exactly. It may just be that parts are sized differently, but very often the circuit is slightly different as well.
“For anything that isn’t a large PMIC, if I follow the input well, there is a good chance I won’t have to change anything after it gets reviewed and/or simulated. If I do, it’s typically minor. That has never happened with a large PMIC (small ones are easier to get right on the first try). I had one instance where I was able to follow the reference/input very closely and I thought it would be good, but it still needed some difficult work to get it right. Adding or moving caps and trying to squeeze more copper area or vias is often not easy to do in crowded areas.”
#4: ‘Scale’ or Available Space for Design
Many times as designers, we run into restrictions on available space and board real estate issues, based on a number of things, such as the number and size of components needed for a given design. I can tell you that looking at the available space where all the components are to be placed, and then looking at the extent of the board itself, can be daunting on some very small boards; remember that most boards (without having to go to truly embedded components to minimize de-coupling caps, for instance) only have their external layers available to be populated with components.
In many cases the customer may not want any components on one side or the other, further limiting the available space. Perhaps the back side must lay flat against another board.
Components cannot be simply placed wherever they fit on a given design—for example, bypass caps that need to be as close to the power pin they are associated with. In a low frequency/DC context, a bypass cap opposes changes in the voltage line by charging or discharging. The capacitor functions like a low impedance battery that can supply small amounts of transient current. In a high frequency context, the capacitor is a low impedance path to ground that protects the IC from high-frequency noise on the power line.
Consider also trace and space limitations based on power functions: The greater the power the greater the voltage; this requires wider tracks that also eat up board area.
Assembly also eats up board space. Auto insertion devices can place components extremely accurately, but they still require enough room to operate in. In addition, parts that cannot be placed by automated placement equipment must be hand placed, requiring additional space for the technician and access to get his or her fingers (or tools) within. An example would be devices such as switches or connectors. Additional space is also required for de-bugging or reworking by the technician.
IPC has some great information on guidelines for space such as IPC 2221B—the spec that deals with design—and it has solid information on voltage spacing and other electrical considerations.
IPC-A-610: This is the generic acceptability/rejection spec and covers how hardware is to be assembled onto PCBs.
IPC 7351B: This is the land pattern spec for surface mount parts with details on pad size and spacing pertaining to PCB design.
Finally, test—namely test point access—must also be considered. There needs to be enough room to probe the test points.
#3: Information Not Initially Covered by the Customer and Changes on the Fly
This one is inevitable and generally happens after the board has gone back to the customer for part placement and/or final route review. It might be information either not shared by the customer in the kick-off meeting or information you as the designer did not ask about. Frequently, previously unforeseen things pop up and need to be dealt with. Additionally, there may be some feedback from different engineers on the project; mechanical and power engineers may have different ideas on what they would like to see or imagine they would like to see from the design. More on this topic later.
In conjunction, there may be physical changes that the customer may require, such as new components due to part availability or part obsolescence. This one is quite common as components can be hard to find and a substitute must be used. It is the board designers’ responsibility to incorporate these changes with a minimal effect and loss of time.
#2: Requests for Things that ‘Don’t Play Well Together’
What I mean by this is things like small/tight pitch BGAs with high copper for high current. Sometimes the customer wants things that are just not possible from either a design or fabrication standpoint; for example, three- or four-ounce finish with 0.003”/0.003” trace and space, or very tight pitch components with space not adequate between SMD pads to be able to have higher copper. These both happen frequently and must be leveraged to have a good design solution.
Those of you who have read my columns when I worked in board fabrication know this is something I feel strongly about and is a bit of a soapbox for me. Even 0.003”/0.003” trace and space on a half-ounce foil to start can be a fabricator limitation. Some fabricators will then ask to start on one-quarter or 3/8th-ounce foil to be able to deal with the trace and space that low, remembering that starting on even a half-ounce will require a half-mil etch compensation, taking the space down to 0.0025”. At this point, most fabricators require starting on the lighter copper weights of either quarter or 3/8 ounce. These are such light copper weights that most fabricators’ etchers can easily etch that thinness of metal without an additional etch compensation digging into the available space.
Finally, by far the most important…
#1: Reading Our Customers’ Minds
About this, Jeff Reinhold said:
“We often have many inputs to deal with. Some are more difficult to decipher than others, but if they are available, we can read them. What we can’t read is our customer’s minds. When we get right down to it, this is what our job is—create data that can be used to build a circuit board, and that meets or exceeds our customer’s expectations for how that should be implemented (i.e., read the customer’s mind). I used to say that there are 500 ways to lay out a board and all of them will work, but only one of them fits what is in the customer’s head. That is still true but now the number is probably more like 20 than 500.”
In short, good design makes a product useful. It has to satisfy certain criteria—not only the functional, but also the psychological and aesthetic. Good design emphasizes the usefulness of a product while disregarding anything that could possibly detract from it.
“Our customers invariably have some picture in their head (or collective heads) of what they think the layout should be or will look like. Not only do we have to figure out what that is, via additional conversations or use of the tools at our disposal, we must balance that with our own knowledge of best practice for the given circuit and our customer’s ideas on implementation, and then be able to create something that meets their expectations, even if it must be something different than what they thought it would be.”
Jared Spool, the American writer, researcher and usability expert, said:
“Good design when it’s done well, becomes invisible. It’s only when it’s done poorly that we notice it.”
Another favorite quote about design comes from Steve Jobs:
“Design is not just what it looks like and feels like. Design is how it works.”
But my favorite quote comes from “Wind, Sand and Stars” by Antoine de Saint-Exupéry:
“A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away.”
- Lesson From A Good Bad Design.
“The Guts of a New Machine,” The New York Times Magazine, Nov. 30, 2003.
“Land of Men,” Antoine de Saint- Exupéry, 1939.
This column originally appeared in the May 2021 issue of Design007 Magazine.