Is it Time to Design Printed Circuit Boards Around the Bill of Materials

It’s not easy being a printed circuit board designer these days. Parts that we used to take for granted have gotten really hard to come by. Geopolitical trade wars and a pandemic were serious triggers for the undersupply. We really didn’t need a Japanese chip factory to burn down to make things worse. A giant cargo hauler clogging up a vital shipping artery for a week didn’t help the situation either.

The fear, uncertainty and doubt sown into the supply chain put the squeeze on purchasing managers who, in turn, did their very best to secure as much material as possible. Ordering more inventory than their forecasted requirements is a typical knee-jerk reaction for the big players. Some of the purchase orders may be defensive measures; an effort to block their competitors since they are being caught short-handed themselves.

The automakers are a vital sector of the US, German and Japanese economies. They have been busy lobbying their respective governments to pressure the chip makers with the goal of creating a sufficient supply of devices for the vehicles that they all want to build. Propping up that industry with their ruggedized devices leaves even less bandwidth for other industries. 

“Scarcity breeds scarcity...”

The mainstream chips that remain have become a strategic and tactical pawn to be overbought to ensure a supply down the road. Scarcity breeds scarcity as the big players use their clout to absorb as many chips as possible. Things will remain messy and uncertain for the PCB Designer who has to accommodate the supply chain crisis.

Lead times are one of the key elements in forecasting requirements. What I’ve heard from my contacts in the chip houses is that they are facing a lot of uncertainty in this environment. If everyone is trying to get a larger allocation than they require, what is the true demand? It’s not like they can announce a new fab and, presto, more chips. It takes chips to make chips and the companies that make equipment for the chip makers are in the same boat that we are. It’s a vicious cycle. 

Figure 1. Image Credit: Nikkei Asia - Typical chip lead times along with the extraordinary situation right now.

It’s not only the lead time but also the confidence in those numbers as they continue to grow. We know it’s bad but we don’t see any light at the end of the tunnel yet. When you don’t know what cards you hold and you don’t even know how many cards are in the deck, it is difficult to plan ahead with any certainty. Do you commit to paying a higher price to ensure a supply six or even three months out? These are tactical problems but they become technical problems of the highest urgency once we find a viable path forward.

Hurry up and wait? I think we need something better than that. Getting away from traditional program management in favor of agile program management might be a better solution for uncertain times like these. Maybe you’ve heard of Agile methodology. It’s more of a software development thing but can be applied to hardware as well. Instead of projects that have concrete start and end dates, Agile relies on getting everyone together, setting a goal around a set of parameters and executing on that plan in a sprint. Learn and repeat.

Design Pre-Use, A New Paradigm for a New Reality

In this case, the parameters are the line items that make it into the bill of materials. The full set of items has a lot of redundancy and only those that are most likely to be available go into the product. Every chip is a layout unto itself and a library of chip layouts are generated as you go. Some circuit elements may never be incorporated at the board level but you have them if you need them.

Instead of cataloging design reuse modules, you’re designing sub circuits for everything. Even if it is a one-off on the board, it can be created as a module in order to free it from the constraints of the reference designators. The idea is to keep your options open by having a sub-layout for everything you might need.

Modular schematics may not be as eco-friendly since each chip will use a full page or pages leaving you with some white space. Some of us still work from paper schematics, especially during placement and as prop for the final design reviews. In the interim, there are people proving out the sub-circuits to reduce the technical risk while the tactical risks are sorted out.

This is more about simulation than actual bench work but it is possible to generate PCBs for the sub-circuits where more data is required. That is, assuming you can get your hands on some sample quantities of your projected components. Various functional pieces can be cobbled together on the test bench to get a better characterization prior to the final form-factor.

What you want is a collection of circuits in which you have confidence that they will perform as required. Then, pick and choose the sub-circuits so that you can complete the Bill of Materials and deliver your board within the budget and on schedule. The single break-out board would become an interchangeable system of multiple printed circuit boards.

Multiple Footprints for an At-Risk Component

In order to alleviate uncertainty, it may be possible to have a single footprint that supports two different components. It is not an ideal solution and it will want to be distilled down to a single footprint on the next iteration. This is the key. You’re working with a dynamic universe of components and need to be able to pull a board together quickly. 

Figure 2. Image Credit: Author - U27 and U46 are crystals of the same frequency but using two different types of packaging. They are mutually exclusive stuffing options depending on what component is available.

Interposers are another way to keep your options open. The concept is simple; there is a footprint on the bottom of the substrate and a different one on the top. The two are connected such that an old board can learn a new trick or at least accommodate a second source component that is not the same footprint.

Rushing a board out may not be an optimal solution but rest assured, using an agile methodology means doing one iteration after another. Fail quickly and move forward with what you know for the next time. Managing this risk is a team sport that requires solid communication. Interesting times call for more evolved solutions.