PCB yield, at its core, is a measure of manufacturing efficiency in the production of printed circuit board assemblies (PCBAs). It tells you how many good products you get compared to how many you started with.
Simplistically, as defined in the reference, the term "PCB yield" is used for the ratio of good assemblies at the end of a process to the number of assemblies originally started, expressed as a percentage.
Understanding the Ratio
Think of it like baking a batch of cookies. If you put 100 cookies in the oven and 95 come out perfectly, your yield is 95/100, or 95%. In the context of PCBs:
- Number of assemblies originally started: This is the total quantity of boards that entered the assembly process at the beginning.
- Number of good assemblies at the end: This is the count of boards that passed all final quality checks and are deemed acceptable products.
- The Ratio: (Good Assemblies / Started Assemblies)
- The Percentage: [(Good Assemblies / Started Assemblies) * 100]%
Formula:
Yield (%) = ($\frac{\text{Number of Good Assemblies}}{\text{Number of Started Assemblies}}$) * 100
The Nuance: Rework
While the basic definition is straightforward, the reference highlights an important point: "In practice, an apparently high yield can disguise the fact that substantial rework has had to be under taken in order to rectify faults."
This means a high yield percentage doesn't always tell the whole story about process quality. If many boards required repairs (rework) to become "good assemblies," the true underlying process performance might be lower than the final yield percentage suggests. High rework adds costs, time, and can potentially impact long-term reliability, even if the final yield looks good on paper.
Why PCB Yield Matters
High PCB yield is a critical goal in electronics manufacturing because it directly impacts:
- Cost: Lower yield means more scrap, rework, and wasted materials and labor.
- Efficiency: A process with high yield runs smoothly with fewer interruptions for fixing issues.
- Quality: High yield often correlates with stable, reliable processes that produce consistently good products.
- Throughput: Getting more good boards from the same starting quantity increases production output.
Achieving and maintaining high yield is a continuous effort involving quality control, process optimization, and addressing the root causes of defects.
