The Economics of AOI and AXI for EMS Providers -- The intelligent use of AOI and AXI in the PCB assembly process can be valuable in increasing throughput, improving yield and enhancing profitability.

The electronic manufacturing services (EMS) industry is an extremely competitive business. Margins are razor thin. Continual pressure exists to drive down costs, while the manufacturing process gets more challenging. Asset utilization is one key to success in this market. The more efficiently an EMS provider uses its assets, the more profitable its business. In this environment, manufacturing as much product as quickly as possible is critical because it reduces work in process, spreads out fixed costs and reduces inventory levels. Another key to success is to have the process as defect-free as possible, as each defect slows down the process and costs money to repair.

For years, EMS providers have strived to improve their manufacturing processes such that visual inspections and in-circuit tests (ICT) are no longer required or are used sparingly. However, no machine or process is perfect, so test continues to be part of the standard manufacturing process. This fact is especially true today as printed circuit boards (PCBs) increase in complexity and become harder to manufacture.

With today's manufacturing trends toward smaller components and limited test access, traditional test approaches such as visual inspection and ICT are becoming less effective. As a result, automated optical inspection (AOI) and automated x-ray inspection (AXI) are becoming standard test methods.

AOI and AXI can provide significant benefits when used in different stages of the PCB assembly process. These benefits include faster time-to-market, faster cycle time, higher yields and reduced rework (Figure 1). The stages where AOI and AXI can be used include prototype inspections, initial production, production changeovers and production inspection.

Prototype Inspection

After the new PCB is designed, a prototype board is built. This prototype may be a one-off, or several can be built. Subsequent prototype builds may also be required.

When a failure occurs on the prototype board, the designers must determine if the failure is related to the board's design or manufacture. On simple boards, this question is fairly easy to resolve. However, on boards with hidden solder joints or more complex boards, determining the exact nature of the problem can be difficult.

In this situation, AOI and AXI can help improve the debug process. Both AOI and AXI can have test programs generated quickly after prototype build. During the prototype phase, AOI can verify that the correct components are being used and that the components are actually on the board. AOI can also locate visible solder joint defects on the boards. AXI can validate the quality of solder joints and locate shorts, opens, insufficient solder and other solder-related defects. This verification can be done with both visible solder joints and hidden solder joints used in components such as ball grid arrays (BGAs) and chip-scale packages (CSPs).

The benefits of using AOI or AXI during the prototype test stage are driven by the yields, the number of boards being produced, the fault spectrum of the boards being manufactured, and the impact of improving time-to-market. The lower the yield, the more likely that AOI and AXI can have a large impact. The more boards that are produced, the more difficulty exists in finding defects. Thus, boards that are difficult to build or that are more complex can benefit from AOI or AXI prototype test inspection.

For complex and difficult boards, AOI and AXI can be used for structural test (ST) during the prototype stage, so ICT or visual inspection is sometimes not required to find defects. Thus, EMS providers can efficiently ensure high yields from the prototype manufacturing step, and less time will be spent debugging and "fingerpointing" the prototype boards. In addition, if ICT fixtures and programs are used, the prototype test costs can be reduced.

By far, improved time-to-market is the biggest impact of using AOI or AXI during the prototype stage. By eliminating the prototype debug bottleneck, companies have brought products to market faster by up to several weeks. In today's competitive environment, huge economic impacts may result from only a few days improvement in time-to-market.

Initial Production and Production Changeovers

After the prototype stage is complete, the boards start into production. AOI and AXI can provide two benefits during this stage, depending on the situation.

In the first situation, the original equipment manufacturer (OEM) is under tremendous time-to-market pressure and starts production before ICT and functional test are ready. One example is leading-edge notebooks, where earlier introduction can result in large increases in sales volume and where margins drop quickly after introduction. In this case, AOI and AXI can detect defects early and thus reduce the number of boards being reworked at ICT and functional test.

Some manufacturers produce thousands or even tens of thousands of boards without having completed tests. Using AOI or AXI in this situation minimizes the chance that extensive and expensive rework of the initial production runs will be needed.

In addition, the use of AOI and AXI in this situation can make the manufacturing process more efficient by reducing rework costs and by providing feedback that can be used to improve yields.

The second case applies to higher mix EMS providers. During a changeover to a new board type, one or a few boards are built on the line. After these initial boards are built, the manufacturer must review the first-off boards to ensure that all the right components are placed, no solder defects exist, no components are missing and no extra components have been placed. The manufacturing line may be stopped for a few minutes, or it may be stopped for several hours to validate that the board is built correctly.

In this case, visual inspection can be used to validate the solder joints, and AOI can review the solder joints and validate the correct components and presence/absence. The higher the mix and the longer the line is down, the bigger the economic impact of reducing this downtime with AOI. Because production costs are fixed, reducing the downtime can improve the inventory turns of the manufacturing line.

Production Inspection

During production, AOI and AXI can catch defects before ICT and functional test. AOI and AXI can reduce or eliminate boards accumulating at ICT and functional test and also reduce the number of retests at these stages. The savings from this approach are due to reduced diagnostic and repair costs and also more efficient use of assets. By eliminating redundant coverage, ICT fixtures can be simplified, resulting in faster ICT test times and reduced ICT fixture and program costs. For boards with limited or no test access, such as cell-phones, AOI and AXI have become a standard test approach replacing ICT.

SMS Technologies, a southern California contract electronics manufacturer, acquired a pair of AOI systems to improve assembled PCB yields. SMS assembles PCBs for many different customers, so its production lots are relatively small; no more than 4,000 PCBs in a lot. Thus, much visual inspection is needed to assure that the PCBs are correct.

SMS found that the efficiency of visual inspection was not adequate to detect problems on small runs related to solder defects and missing and incorrect parts. The company concluded that the only effective way to correct this problem was automated optical inspection.

"We added AOI systems to our surface-mount assembly lines because too many defects were getting through," said Allen Stein, SMS' director of manufacturing. "AOI inspection has definitely improved both our quality and yield."

SMS added AXI to its production line when more of its customers began to include BGAs and microBGAs in their assemblies. AXI was the only way the company could effectively ensure that its surface-mount process was adequate for these packages.

Besides reducing rework at ICT and functional test, another benefit of having higher test coverage earlier in the process is more efficient use of assets. Celestica has stated that using AXI with ICT and functional test on complex boards "helps shorten the overall-time-in-process by a factor of three."1 This test strategy reduced the product cycle time and the work-in-process and increased Celestica's asset utilization.

Product reliability can also be impacted by AOI and AXI. Several articles have reported that adding test coverage can improve shipped product reliability.2,3 More test coverage means less chance of latent defects being shipped to customers. The implication is that adding AOI and AXI can reduce field failures and warranty costs, while improving customer satisfaction.

AOI and AXI also provide real-time process feedback. Both test techniques pinpoint the exact locations of defects, and both tests can be run in-line. With real-time monitoring and feedback, the manufacturer can minimize the defects coming out of the process.

Combining AOI and AXI

One increasingly popular test technique is combining AOI and AXI. This inspection method has additional positive economic impact, as the coverage and benefits of these two complementary test approaches are combined. The AXI augments the AOI test coverage by adding test to those devices with hidden solder joints, such as BGAs. The benefits are higher overall coverage and a test approach that can deal with the hidden joints.

AOI/AXI Benefits for OEMs

Besides EMS providers, many large telecom OEMs have stated that they use AOI, AXI or both in different stages of the PCB assembly process. For example, Siemens reported using AXI in structural test and catching 90 percent of all defects.4

Nortel Networks also documented its use of AXI for prototype test.5 The benefits reported include: elimination of ICT prototype fixtures and programs; dramatically improved yields when compared to previous prototype debug approaches; reduced tension between the design and manufacturing teams; reduced costs due to the elimination of the prototype ICT testing; and faster time-to-market because of the faster prototype turns. Nortel found that this approach reduced defects by as much as 50 percent at the ICT and functional test stages.

The Costs of AOI and AXI

AOI and AXI systems range in cost from $50,000 to $450,000. In addition, operators are required to program these systems; more complex systems require engineers to program them. Thus, the costs of purchasing and operating either system can be quite high.

The key question is: Does the benefit of the system justify its cost? The answer depends on the situation. If yields are very high and rework and scrap costs are low, then the system is likely not justified. If the boards have low yields or rework and scrap costs are high, then the economics can be extremely strong.

The key factors for the economic analysis include the board yields at different test stages, board volume, cost of repairs, scrap rate and the fault spectrum of the defects. With this information, managers can easily combine the system cost with the economic benefits to determine a return-on-investment calculation. However, the more defects that occur and the costlier they are to repair, the bigger the economic impact of using AOI and AXI.

Conclusion

With the trend toward greater PCB complexity, visual inspection and ICT have become less effective test approaches. Reworking complex boards at functional test or system test is very expensive. Therefore, manufacturers have adopted alternate test techniques to locate defects earlier in the assembly process. The result has been a dramatic increase in the use of AOI and AXI.

For EMS providers, the benefits of using AOI and AXI can include faster prototype turns, faster time-to-market, reduced line down-time, reduced bonepile boards, faster product cycle time, reduced field failures and reduced rework at ICT and functional test. The exact economic impact depends on the product, fault spectrum and specific situation.

The EMS industry is extremely competitive and efficiently uses assets. The intelligent use of AOI or AXI can be a valuable tool in optimizing the PCB assembly process.

References

1. Verma, A., et al. (August 2000). Complementary test strategies on high-complexity boards. Circuits Assembly.

2. Engelmaier, W., et al. (December 2000). Using AXI to ensure solder joint reliability. Circuits Assembly.

3. Bonham, S., et al. (November 1997). No single test strategy. Circuits Assembly.

4. Shell, T., et al. (April 1999). X-ray inspection finds defects. Quality.

5. Messier, M. (1998). Improving the inspection process for limited access PCBs. SMT.

Colin Charette is vice president of marketing with Photon Dynamics, Electronics Division, Aliso Viejo, CA; e-mail: [e-mail].

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