Using Circuit Simulation for Maximizing Yield in PCB Manufacturing

Not too long ago was Thanksgiving, and like all Thanksgiving celebrations before it there was plenty of food. Part of my holiday fun is in helping the cooks by taste testing a dish, and I do my best to be the first in line for this job. I try to explain that my motivation is only to make certain that everyone will get the best tasting food possible, but everyone knows that I am lying and in fact I just want the extra food for myself. No matter what though, testing, analyzing, and simulation is an extremely important part to the success of any project.

In PCB design, the goal is to produce a circuit board that can be manufactured as error free as possible. There are many ways that designers do this including following good design for manufacturability rules (DFM) in their design, and carefully checking everything for errors. Another important part of this process is to make sure that the components being used will work as expected, and that means validating them using circuit simulation. Here’s a little more on what I’m talking about.

Error Free Manufacturing Depends on a Clean Bill of Materials

To get the highest yields in manufacturing, there are several aspects of the design that must be completed successfully. The primary aspect of the design that must be correct is its basic functionality. If the board won’t work correctly the way it was designed, then there will be no reason to manufacture it.

The next important aspect is to physically design the board for manufacturability (DFM). A board that hasn’t been laid out with good DFM practices will cause many problems when it is assembled. For instance, component placement that isn’t optimized for the board’s soldering process could result in unacceptable solder joints. These bad solder joints may eventually cause intermittent circuit failures which in turn would require reworking the PCB, or outright scrapping the board. Equally important is designing the board for test (DFT). If the board isn’t designed for automated test, the manufacturer will either have to test it manually or re-design the board. In either case, this adds a lot of time and expense into the manufacturing of the board.

The bill of materials (BOM) is another critical area of the design that if not done correctly, can cause manufacturing delays and problems. An incorrect or incomplete PCB BOM is often the problem that manufacturers highlight the most as being their greatest nemesis. Here are some of the concerns reported by manufacturers that they have dealt with in problematic BOMs:

• Price: Components are sometimes specified without regard to price, and are more expensive than suitable alternatives.

• Status: Components may have an end of life (EOL) or not recommended for new design status, making them undesirable for future builds.

• Availability: Sometimes parts ares specified that simply aren’t available for purchase.

• Correct Part Numbers: Careless BOM creation often results in part numbers that don’t match the component descriptions.

• Reference Designators: Sometimes the reference designators in the BOM doesn't match what is on the board. This often is the result of eleventh hour design changes without a BOM update.

• Missing Data & Information: Missing parts, missing reference designators, missing part numbers or descriptions, all of these problems can render a BOM unusable.

From this it is obvious how important it is to have a good, accurate, and clean bill of materials in order to maximize your PCB manufacturing yields. Here is where the circuit simulation becomes important.

For error free manufacturing, you need a clean bill of materials

How Circuit Simulation Can Help with Maximizing Yield in PCB Manufacturing

Once the functionality of the circuit has been decided, the next step is to capture the schematic using the appropriate components. Here is where circuit simulation can save you a lot of prototype expenses by proving the viability of the circuit before you complete the design. At the same time, circuit simulation will validate the components you have chosen as being correct for the circuit.

At this point of the schematic development is when components are usually looked at for part procurement. If it is discovered that any of the chosen parts poses a problem due to their price, availability, or status, those components will be substituted with different parts. This traditional cycle of part substitutions is where component problems can be introduced to the design. Even though the circuit has already gone through circuit simulation, the substituted parts may now invalidate those original simulations. Not every part will work as a direct substitution as minor variations in tolerances or other specifications can impact the performance of the circuit.

It is crucial therefore that the design engineer be part of the component substitution process. Just because a substituted part may make more sense as far as price and availability goes, the design will still be considered nonmanufacturable if the circuitry won’t perform as intended. With the design engineer part of the process to produce a clean bill of materials however, additional circuit simulations can be run that will verify whether or not the part substitutions will work. These simulations to validate new components in the design are an important part of increasing the manufacturing yields of the PCB.

Take the guesswork out of component selection with circuit simulation

Taking the Guesswork Out of Component Selection in PCB Design

Whether it’s being able to statistically predict component failures in designs to adjust tolerances and maximize overall yield, or determining sensitivity analysis and identifying critical components wherein a failure would take down the entire circuit, simulation can provide you the data you need. Ensuring critical components won’t fail, and understanding how to adjust your designs when what you’ve made is going to fail at some point will prove highly cost effective for your production runs. 

For the PCB designer, the best thing that you can do to help yourself produce a clean bill of materials is to use design tools that give you all of the versatility that you need for the job. Not only do you need a schematic capture tool that can handle any size and complexity of design, but you also need tools that allow you to make part choices based on real-time component information and state of the art circuit simulation tools. This way you can research and select the best choices in components and simulate the circuit as you are developing it.

Not every EDA vendor out there can give you all of this functionality in one suite of tools, but you can find exactly what you need from Cadence. Their collection of high performance PCB design tools will give you the library, schematic capture, circuit simulation, and PCB layout capabilities that can handle any design. PSpice is a feature rich circuit simulator that has proven itself over the years to be engineers choice for circuit design. It is fully integrated into the Cadence schematic capture tools and will help you in designing PCBs with the highest manufacturing yields.

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