A Google search quickly shows that some see first time yield as purely a measure of whether a unit passes a test. Others view it as something being built without needing any touchup or repair at all. The reality is that both are important. The base cost of any electronics product is determined by the material, labor and equipment cost. To ensure that it is built correctly, many products go through several inspections and/or test processes. Each one of these adds cost, but it is clearly important to build a working product. The first time yield of a printed circuit board assembly (PCBA) will be discussed, but it really applies to anything.
First time yield is a measure of the total number of units that pass a test without requiring rework to the total number of units built. It typically does not include any touch-ups done during the manufacturing process, but anytime additional rework is required, cost is added, and steps should be made to minimize process flaws.
Manufacturing Process Overview
Every step in the electronic board assembly manufacturing process is critical. A mistake made at any stage can cause errors that may cause manufacturing defects and performance errors. The following is a brief description of the various steps of a typical PCB assembly process.
Parts procurement – ordering the parts to build the PCB. It is important to order parts through proper channels to ensure genuine parts are being procured that will work
Receiving – this step ensures that the part ordered was received and is then labelled and stocked
Kitting parts – gathering all parts required to build a PCBA
SMT programming – using the latest released PCB and bill of materials (BOM) documentation to program how parts will be placed on a PCB
Solder paste application – applying solder paste to a PCB; this is done by a process like a T-shirt silkscreen over a stencil specifically designed for the PCB
SMT parts placement – specialized equipment is used to automatically place parts on the PCB
Reflow – the PCB with all parts placed is run through an oven which melts the solder and solders the parts to the PCB
Through-hole parts installation – completion of the PCB assembly by installing any parts that couldn’t be placed with an SMT process
Cleaning – cleaning any flux residues from the board
Testing – testing the final PCB assembly to ensure proper operation; at ACDi, this could be a flying probe test or a functional test typically supplied by our customers. It is important for the test to be sufficient to ensure the proper operation of the product. The goal is for the product to work. If the product makes it to an assembler or even a customer before issues are found, much higher costs occur.
With all these steps to build a PCB, there are many chances for something to go wrong. Each step has checks and balances in place to build the best possible product. To fully verify every step would add significant cost, but to take shortcuts would cause test failures at the end of the process, causing significant expense to find and repair the issue. To devise a solution with the best balance, a set of standardized processes are developed which minimizes excess costs added in each stage while continuing to build a solid product. For this to work, any mistake found is fed back to earlier steps in the process to determine what went wrong and correct it.
Quality inspection of the assembly is done in several steps. Any time these inspections can be automated, variance in human interpretation and skill can be minimized. Automated inspections include solder paste inspection to ensure sufficient coverage, electronic parts verification to ensure proper parts are being placed and optical inspection to check proper parts placement, polarity and solder connections. Additionally, any component that does not have visible solder connections are X-rayed.
The first time yield of the first-time build of a product is a good baseline to track. Improvements in the process for a particular product should be made until a target first time yield is achieved. While a 100% first time yield would be the ideal goal, it may add too much cost to a product, so depending on the complexity of the actual PCB and the ease to fix any issues, a target first time yield of 98-99% may be a more desirable goal.
100 PCB assemblies go all the way through the manufacturing process. Of the 100 boards, 80 pass and 20 fail. The first time yield is 80%. The boards are troubleshot and are found that all 20 failures have a problem with pins on U1 not being properly soldered. Feedback about the specific failure is provided to the manufacturing engineer. He/she checks on why it was missed at automated optical inspection (AOI) and corrects that. The manufacturing engineer also finds that sufficient solder was not applied because of a fault in the stencil. This is also corrected. The next 100 boards are run with these fixes in place and all 100 pass for a first time yield of 100%.
In this example, it could have taken several days to find the issue, then the rework on each board could have taken 15 minutes per board considering the material flow. It is entirely possible that it could have taken an average 30 minutes per board (of 100 boards) to correct these issues. At a nominal $100/hr., this would add a cost of $50/board if steps were not taken to correct the issues. This could cause the contract manufacturer its competitiveness/profitability OR the product to be too expensive.
100 PCB assemblies go all the way through the manufacturing process. All 100 fail. The first time yield is 0%. This type of failure is easier to troubleshoot because the same failure is likely causing all the issues. After troubleshooting, it is found that the customer made a simple change to the BOM and omitted a critical component, causing it not to be placed. This feedback was provided to the customer who provided an updated corrected BOM. The part was installed, and all boards passed. All subsequent builds pass at 100% first time yield.
In this example, the problem clearly must be corrected. While the initial costs to find and correct the issue may have also been $50/board (like example #1), the problem will not be repeated.
In actual practice, several issues may arise in early builds. It is important for feedback to flow back through the process to correct all issues.
FTP on a first-time build should be used as a baseline only. Corrective steps will be done through feedback and process corrections to improve it to the target first time yield. Once that targeted first time yield is achieved, it should be tracked on all subsequent builds to quickly identify any new issues that may arise.
Maintaining a high first time yield is important for several reasons:
- A high first time yield keeps costs down for the manufacturer and ultimately the final customer
- It also makes the manufacturer more efficient, allowing a higher throughput of products or additional products to be built
If you have any questions about first time yield, or how ACDi’s quality management system can improve your electronic device’s first time yield. Contact us for a conversation.