Flying Probe Testers Lower PCBA Costs


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With traditional bed of nails testers (ICT and MDA), loss of nodal access and high fixturing costs can be prohibitive. AOI and AXI cannot verify component values or operation. Recent advances in flying probe technology have made this a viable solution to address the problem of production bottlenecks and low yields in final or functional test. I will analyze the financial benefits of implementing flying probe systems in a board manufacturing process.

By Bob Steel, Acculogic

Electronics manufacturers are faced with many challenges in producing high-quality boards today: denser placement of smaller parts, increasing interconnect density, loss of electrical access, to name a few. These factors contribute to lower yields after board assembly.

Traditional approaches to find these faults rely on bed of nails testers (ICT and MDA); however, with loss of nodal access, these approaches are less effective. In high-mix/low-volume environments, fixturing costs can be prohibitive. AOI is a possible solution, but it cannot verify component values or operation, or check hidden solder joints. AXI can check hidden solder connections, but it cannot check component values or operation. Both AOI and AXI can result in high false call rates, requiring human operators to verify failures.

As a result, many companies have settled on a process flow of moving directly from assembly to functional or final test (FT), with some manual visual inspection. Out of necessity, companies are looking for better solutions because production goals and quality levels are not being met.

Recent advances in flying probe test technology have made this a viable solution to address the problem of production bottlenecks and low yields in final or functional test. The capital outlay required to purchase a system will be given close scrutiny by management. The purpose of this article is to analyze the financial benefits of implementing flying probe systems in a board manufacturing process.

Current Test Regime

The first step is to model your current process flow after assembly. Half the battle is understanding details of the process and collecting data: What steps are involved after the boards leave the assembly line? What is the time per step and the labor rates involved? What are the yields at each step and the types of faults found? Find out the average number of faults per board. This is important at functional or system test because after a fault is found, troubleshooting is done and the repair made. The board then must be retested, and if another fault exists, the process has to be repeated. Once this data is collected, the cost for the current process can be computed.

To simplify a sample analysis for "Company B," let's look at the impact for one board at functional test only, and then examine additional cost-savings areas. The following data was collected: board volume per year = 20,000; first pass yield (FPY) = 70%; test time = 4 min.; troubleshooting time = 15 min.; and repair time = 5 min. Average number of faults per board = 2. The labor rates are: functional test technician = $25 per hour and the repair operator = $15 per hour, an operator for the flying probe tester = $10 per hour.

Current process results in the following numbers. Test time first pass = 20,000 × 4 min = 80,000 min. 30% of boards need troubleshot = 6000 × 15 min = 90,000 min. Retest repaired boards = 6000 × 4 min = 24,000 min. Find second fault, troubleshoot = 6000 × 15 min = 90,000 min. Second retest = 6,000 × 4 min = 24,000 min. Total test time = 308,000 min.

Test with Flying Probe

The next step is to model the process using a flying probe test system. What is the impact at functional test with a flying probe step inserted after board assembly? The fault coverage on this board is 95%. This can be verified by having the test system provider program the board and run a coverage report, or it can be estimated by knowing the faults found at functional test and estimating the coverage by knowing what faults should be found at flying probe test. The flying probe test time on this board is 3 minutes.

The first pass yield at functional test will improve to 98.5% in this example, and the average number of faults per board will drop to 1. This translates to the following times at functional. Test time first pass = 20,000 × 4 min = 80,000 min. 1.5% of boards need troubleshoot = 300 × 15 min = 4,500 min. Retest repaired boards = 300 × 4 min = 1,200 min. Total time for test with flying prober added = 85,700 min. Repair times at FT are not factored in since we catch these faults earlier in the process, and assume the same repair times at flying probe test. The resultant cost savings for this one board = 222,300 min or 3,705 hours at $25 = $92,625.

The loading on the system, assuming two shifts per day and seven productive hours per shift = 37.1%. There is still approximately 63% capacity left, so let us add more boards. Estimate coverage, test times and yields at functional test for more boards. Assuming similar boards, the loading at 90% of capacity will result in a savings of $224,697 per year. The hours saved in functional test = 8,988 hours per year.

Headcount reductions can be made at functional test since the hours worked per technician per year is about 1,750 (250 work days per year and 7 work hours per day). This allows Company B to reduce headcount in the factory or move these people to another job, and eliminate the bottleneck at FT.

At this point we look at simple payback time, which is the initial capital outlay divided by the savings per year (Table 1). Return on capital investment (ROI) in flying probers can be achieved within 1.5 years. Other factors make for an even better payback time: operator reductions; higher yields going into system or final test, resulting in lower cost of troubleshooting and retest in this area. Additional boards can be put on the flying probe system by testing on third shift. Bone pile boards can be run across the prober and many can be recovered.

Conclusion

This article is not an exhaustive analysis of the full impact that a flying probe system could have on cost of manufacturing. Corporate taxes need to be factored in, and for a multiple year analysis, discounted cash flows need to be considered to compute the return on investment, so the purchase of the flying probe System can be compared to other capital purchases. Visit www.acculogic.com for an Excel spreadsheet that provides a detailed ROI analysis of the cost savings that can be realized by using a flying probe system in your manufacturing facility.

This example on reduced FT cost for Company B demonstrates that a flying probe system can reduce overall cost of manufacturing PCBs in high-mix and low- to medium-volume environments. SMT

Bob Steel, North American sales manager, Acculogic, www.acculogic.com.

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