Achieving the highest PCB yield rate

PCB Assembly Essentials Part 1: PCB Yield Calculations

According to some, perfection cannot be fully realized. For most endeavors, this seems accurate, as there is almost always room to improve. For industrial manufacturing processes, perfection is often quantified as the completion of an order with zero defective products. And although elusive, this type of perfection is certainly within the realm of possibility.

For PCBA production, achieving zero defects is recognized as attaining a 100 percent  yield rate. Creating fully assemble circuit boards ready for production typically requires a development or prototyping phase, which is an  iterative process during which the board design is continually improved until an acceptable level of design quality is achieved. For both prototyping and production, the final manufacturing stage is assembly. Therefore, the results of PCB assembly define the actual yield, which is the ratio of usable boards to the number of boards built. 

A yield rate estimation can be calculated prior to manufacturing and used for project planning. For example, it can help manufacturers predict the amount of necessary materials and estimate costs. Typically, this useful metric is determined by a PCB yield calculator. CMs employ these calculators to evaluate the quality of their processes and provide precise data that can be used to make design and/or process improvements to achieve the highest yield rate. 

Types of PCB Yield Calculators

The PCB yield, or yield rate, is a measurable quantity routinely used by CMs to evaluate the performance of their equipment and evaluate their processes. In fact, this type of process observation, analysis and updating is an element of good risk management. Prior to manufacturing, an estimate of the yield may be needed during planning or board design.

Calculating Yield Rate During Design

Arriving at an accurate value can be difficult, as there are several independent variables that can have a significant effect on this estimation. The specific components and their sources, the types of materials used, design specifications and the processes implemented by the manufacturer are all factors that can impact the actual number of usable boards produced. Nevertheless, there are methods for making a determination. These include direct calculation and the use of a third-party PCB yield calculator.  

Direct Calculation

Accurately estimating the PCB yield rate prior to manufacturing is a common research topic that has produced usable calculation methods. Most of these, as shown in [1], require that you make a number of assumptions about component reliability and the manufacturing process that will be employed to build your boards or are mathematically intensive. Two of these methods are given in Eqs. (1,2):

Process Method yield = (no. of components) x (solder yield) x (assembly yield) (1)

For this method, the solder yield is the ratio of good solder joints to total joints, and the assembly yield is a previous yield rate for the run size. 

Defect Method yield = exp[-(SD x n x 2.5) + AD x n + CD - n)] (2)

Here, n is the number of components and SD, AD and CD are the solder defect rate, assembly defect rate and component defect rate, respectively. 

The quickest method of estimating board yield is to employ a third-party PCB yield calculator. These tools are not easy to find and care must be taken here to ensure the basis for all values and any assumptions is sound and applicable to your specific design project. 

Manufacturing Yield Rate Calculators

Although estimating the yield rate during design can be helpful, manufacturing PCB yield rate calculators provide precise data. CMs typically view the yield rate from at least one of the following perspectives:

  1. Process First Pass Yield

Process first pass yield (FPY) is a control metric used to refine or tune a particular process. It measures the number of boards that successfully go through the process without defect.

  1. Board First Pass Yield

Also a process control metric, but in contrast to process FPY, board FPY measures the percentage of boards that pass through the manufacturing process, including final inspection and testing without the need for any rework and without scrap. 

  1. Total Build Yield

The number of usable and/or functional boards manufactured w.r.t. the amount of scrap material produced is known as the total build yield. Here, boards that can be corrected through rework are included. This value may be determined by the CM or the developer once the completed PCBAs are received.

Both designer and manufacturer PCB yield calculators contribute to your overall project—from planning to production. However, estimating rates is difficult and may have questionable accuracy, while yield rates determined during manufacturing are accurate and are used to continually improve the process during development which should result in the highest possible rate once prototyping is complete. Regardless of which method you use, the best way to attain the highest yield rate is by making good design decisions.

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How To Achieve the Best Yield Rate For Your Boards

During development, the yield rate is a numerical value that specifies how many of your boards are passable after assembly, inspection and/or testing. Practical steps, like the optimization guidelines listed below, can help you maximize this value. 

PCBA Yield Rate Optimization Guidelines

✅ Partner with a quality certified CM 

Your choice of manufacturer is the most important determinant of your PCB yield rate, as they will actually build your boards. To optimize the rate, you should partner with an experienced CM that employs the equipment and process to achieve the highest level of quality.

✅ Acquire and follow your CM’s DFM and DFA rules

For your board to be built, it must pass the DFM and DFA requirements of your to ensure this, never simply rely on the default of your PCB design software or make assumptions. Instead, contact your CM for their specific rules and guidelines, and add them to your design rule checker algorithm.  

✅ Verify your design

The manufacturability of your board is the best indicator of what your actual yield rate will be. Therefore, you should liberally utilize DFM checks during design and make sure all errors are cleared before sending the design to your CM.

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Tempo Automation is the industry leader in manufacturing high-quality PCBAs the fastest for prototyping or low-volume production. We employ a software-based digital thread manufacturing process that enables continual monitoring at all stages. Additionally, our component supply chain includes the most trusted and reliable suppliers to minimize the possibility of defects or bad boards.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on how to design a PCB yield calculator or use PCB yield rate calculations, contact us.

Articles in this PCB Assembly Essentials series:

Part 1: PCB Yield Calculations

Part 2: PCB Inspection Methods

Part 3: Supply Chain FMEA

References

[1] Helo, F. (1999). Decision Support System to Predict the Manufacturing Yield of Printed Circuit Board Assembly Lines.

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